~CABLEWAYS.~--Cableways arranged to span the work and if the area is wide

to travel across the work at right angles to the span will handle

concrete, concrete materials, forms, steel and supplies with great

economy. They are particularly suitable for bridge and dam work, filter

and reservoir work, building foundations and low buildings. The

arrangement of a cableway plant for bridge work is described in Chapter

XVII. A cableway of 800 ft. clear span on fixed towers 45 ft. high will

cost complete from $4,500 to $5,000, and will handle 200 cu. yds. of

concrete per 10-hour day. To put the cableway on traveling towers will

cost about $1,000 more. In constructing the Pittsburg filtration work

four traveling cableways from 250 to 600 ft. span were used. The towers

were from 50 to 60 ft. in height and each traveled on a 5-rail track.

The cableways were self-propelling. With conditions favorable each

cableway delivered 300 cu. yds. of concrete per day. A cableway plant

for heavy fortification work is described in Chapter XI.

~BELT CONVEYORS.~--Belt conveyors may be used successfully for handling

both concrete materials and mixed concrete. For handling wet concrete

the slope must be quite flat, and the belt must be provided with some

means of cleaning off the sticky mortar paste. In several cases rotating

brushes stationed at the end of the belt, where it turns over the tail

pulley, have worked successfully; these brushes sweep the belt clean.

Except for the cleaning device the ordinary arrangement of belt conveyor

for dry materials serves for concrete.

In constructing a large gas works at Astoria, Long Island, near New York

city, belt conveyors were used to handle both the sand, gravel and

cement bags and the mixed concrete. The belt for handling sand and

gravel is shown by Fig. 13. A derrick operating a clam-shell unloaded

the sand and gravel into a small hopper, discharging into dump cars

operated by a "dinky" up an incline, passing over sand and gravel

storage bins. A 20-in. belt conveyor ran horizontally 105 ft. under the

bins, then up an incline of 3.4 ft. in 125 ft. to feeding hoppers over

the mixers. This conveyor received alternately sand and gravel by chute

from the storage bins and bags of cement loaded by hand, and carried

them to the feeding bins and mixer platform. The speed of the belt was

350 ft. per minute, and it required 6 h.p. to operate it when carrying

100 tons per hour. The mixing was done in two Smith mixers, which turned

out 70 cu. yds. or 35 cu. yds. each per hour. The mixed concrete was

delivered onto a 50-ft. 24-in. belt conveyor traveling at a speed of 400

ft. per minute and dumping through a chute into cars. Only 1 h.p. was

required to run the concrete conveyor. A rotating brush was used to keep

the belt clean at the dumping end. It will be noted that only a small

amount of power is required for operation.

[Illustration: Fig. 13.--Belt Conveyor Transporting Sand and Gravel.]

~CHUTES.~--Chutes of wood or iron are among the simplest and most

efficient means of moving the cement, sand and stone and the mixed

concrete when the ground levels permit such devices.

Bags of cement if given a start in casting will slide down a steel or

very smooth wooden chute with a slope of 1 ft. in 5 or 6 ft. A wooden

trough 12 ins. deep and 24 ins. wide with boards dressed on the inside

may be used. When the inclination is steep and the fall is great, some

device is necessary to diminish the velocity of descent; the following

is an example of such a device which was successfully employed in a

chute of the above dimensions, 400 ft. long and having a drop of 110 ft.

This chute had a maximum inclination of 45° and its lower end curved to

a horizontal tangent, running into the storehouse. Near the bottom of

the chute a horizontal strip was nailed across the upper edges and to it

was nailed the upper end of a 20 ft., 1×12-in. board, the lower end of

which rested on the bottom of the chute. Several pieces of timber spiked

to the upper side loaded the lower end of this board. The cement bag in

descending wedged itself into the angle between the chute and the board

and lifted the latter, the spring of the board and the weight at the

lower end offering enough resistance to cut down the velocity. After the

chute had been in use for some time and had worn smooth it was found

necessary to add two more brakes to check the bags.

Broken stone will slide down a steel or steel lined chute with a slope

of 1 in 3 or 4 ft. if given a start in casting. Damp sand will not slide

down a chute with a slope of 1½ in 1.

A wet cement grout will flow down a smooth plank chute, with a slope of

1 in 4 ft., and wet concrete will move on the same slope; comparatively

dry concrete requires a slope of nearly 1 in 1, or 45°, to secure free

movement. Mr. W. J. Douglas gives the following examples of conveying

concrete by chute, prefaced by the statement that his experience

indicates that concrete can thus be conveyed considerable distances

without material injury if proper precautions are taken.

In the first case a semi-circular steel trough about 2 ft. wide and 1

ft. deep and 15 ft. long set on a slope of 45° was used. A lift gate of

sheet steel was set in the chute about 2 ft. from the upper end. The

concrete was allowed to accumulate behind this gate until a wheelbarrow

load was had, when the batch was let loose by lifting the gate and was

discharged into barrows at the bottom. In another case a vertical chute

15 ft. long, consisting of a 15-in. square box with a canvas end, was

used. The concrete was dumped into the chute in batches of about 8 cu.

ft.; two men at the bottom "cut down" the pile with hoes to keep it from

coning and causing separation of the stone. In a third case a continuous

mixer fed into a sheet iron lined rectangular chute about 2½ ft. wide

and 1 ft. deep, with a vertical drop of 60 ft. on a slope of 1 in 1, or

45°. A gate was fixed in the chute 2 ft. from the top and at the bottom

the chute fed into a pyramidal hopper 3 ft. square at the top, 1 ft.

square at the bottom and 4½ ft. deep. This hopper was provided with a

bottom gate and was set on legs so that its top was about 10 ft. above

ground. As the concrete filled in the hopper was raised and the chute

cut off. The hopper was kept full all the time and was discharged by

bottom gate and spout into wheelbarrows. In a fourth case the apparatus

shown by the sketch, Fig. 14, was used. The continuous mixer discharged

onto an 18-in. rubber conveyor belt on conical rollers and 18 ft. long.

The inner end of the conveyor frame was carried on the ground at the

edge of the pit and the outer end was supported by ropes from the top of

a gallows frame standing on the pit bottom. The belt discharged over end

into a vertical steel chute 12 ins. in diameter and 8 ft. long; this

chute was fastened to the conveyor frame. Encircling and overlapping the

12-in. chute was a second slightly larger chute suspended by means of

two ropes from the gallows frame. The bottom of this second chute was

kept about 6 ins. below the top edges of a pyramidal hopper like the one

described above. In operation the chutes and the hopper were kept filled

with concrete so that the only drop of the concrete was 3 ft. from the

conveyor belt into the topmost chute.

[Illustration: Fig. 14.--Belt Conveyor and Chute for Handling Concrete.]

Concrete may be handled in long flat chutes by stationing men along the

chute with shovels which they work like paddles to keep the mixture

moving. In one case concrete was so handled in a chute 200 ft. long

having a slope of 1 in 10 ft. The chute was a V-shaped trough made of

1×12-in. boards in sections 16 ft. long. The men paddling were stationed

10 ft. apart, so that with wages at $1.50 per day the cost would be 1½

cts. per cu. yd. for every 10 ft. the concrete was conveyed. In

connection with this particular work we are informed that a Eureka

continuous mixer was used. The gravel was dumped near the mixer and a

team hitched to a drag scraper delivered the gravel alongside the mixer.

Four men shoveled the gravel into the measuring hopper, but only two men

worked at a time, shoveling for a period of 15 minutes and then resting

for a corresponding period while the other two men worked. In this

manner the four men shoveled enough gravel to make 100 cu. yds. of

concrete per day. A fifth man opened the cement bags and kept the cement

hopper filled.

~METHODS OF CHARGING MIXERS.~--By charging is meant the process of

delivering raw materials from stock into the mixer. Several methods are

practiced and will be considered in the following order: (1) By gravity

from overhead bins; (2) by wheelbarrow or hand cart (a) to charging

chute and (b) to elevating charging hoppers; (3) by charging cars

operated by cable or other means; (4) by shoveling directly into mixer;

(5) by derricks or other hoists.

~Charging by Gravity from Overhead Bins.~--Chuting the sand and stone from

overhead bins to the charging hopper is a simple, rapid and economical

method of charging mixers. The bottoms of the bins should always be high

enough above the charging floor to give ample head room for men to move

about erect, and the length of chute may be anything reasonable more

than this that conditions such as the side hill delivery of material may

necessitate. When the mixer is located to one side of the bins the slope

of the chute will have to be watched. Broken stone or pebbles will move

on a comparatively flat slope but sand, particularly if damp, requires a

steep chute. The measuring hopper is best kept entirely independent of

the mixer so that it can be filled with a new charge while the mixer is

turning and discharging the preceding batch. One man can attend the sand

and cement chutes if they be conveniently arranged, and one man can open

and empty the cement bags if they be stacked close at hand. A third man

will level off the sand and stone in the measuring hopper and help in

the chuting. A gang of this size will easily measure up a charge every 2

minutes when no delays occur.

[Illustration: Fig. 15.--Side Hill Mixing Plant.]

A number of plants charging by gravity from overhead bins are described

in succeeding chapters and are referenced in the index. As a general

example a side hill plant of conventional construction is shown by Fig.

15. The trestle work was made of 12×12-in. timbers and was approximately

40 ft. in height. Three tracks occupy the top platform. Under each track

was a material bin; one on each side for gravel and a middle bin for

sand. The sand bin was divided by a partition into two compartments.

These bins discharged into two measuring hoppers one gravel bin and one

compartment of the sand bin into each hopper. Two cement chutes from the

top platform provided for the delivery of the cement to the mixers,

either directly from cars or from the cement storage house. The mixing

was done in two Smith No. 5 mixers, one under each measuring hopper, and

these mixers discharged by chutes into buckets on flat cars. Thus the

concrete materials brought directly from a siding in car load lots to

the top of the platform were handled entirely by gravity to the cars

delivering the mixed concrete to the work. The gang operating the mixing

plant, with the wages paid, was composed as follows: 1 foreman and

engineer at $3 per day, 1 fireman at $2 per day and 15 laborers at $1.50

per day. With this gang the two mixers turned out 400 cu. yds. of

concrete per day and, frequently, 800 cu. yds. in 24 hours. Taking these

figures the labor cost from raw materials in cars on the platform to

mixed concrete in cars on the delivery track was as follows:

1 foreman and engineer at $3 $ 3.00

1 fireman at $2 2.00

15 laborers at $1.50 22.50

-----

Total labor $27.50

Assuming 400 cu. yds. output, this gives a cost of $27.50 ÷ 400 = 6.875

cts. per cu. yd.

~Charging with Wheelbarrows.~--The economics of wheelbarrow haulage are

discussed in some detail in Chapter III. For machine mixer work the

problem of loading, transporting and dumping is complicated by the

greater rapidity with which the mixing is done and by the necessity,

usually, of using inclines to reach the charging hopper level. The

incline cuts down the output of the wheelers or in other words makes

necessary a larger gang to handle the same amount of material.

Conditions being the same, the height of the charging chute of the mixer

determines the height of incline and the size of the charging gang, so

that a mixer with a high charging level costs more to charge with

wheelbarrows than does one with a low charging level. Exact figures of

the increased cost of a few feet extra elevation of the wheelbarrow

incline are not available, but some idea may be had from a brief

calculation. The materials for a cubic yard of concrete will weigh about

3,700 lbs., so that to raise the materials for 100 cu. yds. of concrete,

including weight of barrows, 1 ft. calls for about 400,000 ft. lbs. of

work. A man will do about 800,000 ft. lbs. of useful work in a day, so

that each foot of additional height of incline means an additional

half-day's work for one man.

Wheeling to elevating charging hoppers obviates the use of inclines.

Figure 19 shows a mixer equipped with such a hopper, and the arrangement

provided for other makes of mixer is much similar. When the hopper is

lowered ready to receive its load its top edge over which the

wheelbarrows are dumped is from 12 to 14 ins. above ground level. The

wheeling is all done on the level. The elevating bucket is operated by

the mixer engine and is usually detachable. Where mixers have to be

moved frequently, requiring the erection and moving of the incline each

time, an elevating charging hopper is particularly useful; it can be

hoisted clear of the ground and moved with the mixer, so that it is

ready to use the moment that the mixer is set at its new station.

While the ordinary wheelbarrow is generally used for charging, better

work can be done under some conditions by using special charging barrows

of larger capacity and dumping from the end and ahead of the wheel. Two

forms of charging barrow are shown by Figs. 16 and 17. The Acme barrow

will hold 4 cu. ft. and the Ransome barrow is made in 3 to 6 cu. ft.

capacities. Where inclines are necessary these barrows can often be

hauled up the incline by power. A sprocket chain in the plane of the

incline and operated by the mixer engine is an excellent arrangement. A

prong riveted to the rear face of the barrow and projecting downward is

"caught into" the chain, which pulls the barrow to the top, the man

following to dump and return for another load.

[Illustration: Fig. 16.--Forward Dump Charging Barrow, Sterling

Wheelbarrow Co.]

[Illustration: Fig. 17.--Forward Dump Charging Barrow, Ransome Concrete

Machinery Co.]

~Charging with Cars.~--Cars moved by cable, team or hand are a

particularly economic charging device when the mixer is located a little

distance from the stock piles or bins. Either separate cars for cement,

sand and stone, each holding the proper amount of its material for a

batch, can be used, or a single car containing enough of all three

materials for a batch. The last arrangement is ordinarily more

economical in time and labor, and in plant required. In either case the

car serves as the measuring hopper, there being no further proportioning

of the materials after they have been loaded into the car, and it must

be arranged for measuring. Usually all that is necessary, where one car

is used, is to mark the levels on the sides to which it is to be filled

with sand and then stone; the car is run to the sand stock and filled to

the level marked for sand and then to the stone stock and filled to the

level marked for stone. The cement may be added to the charge either

before or after it is run to the mixer as convenience in storing the

cement stock dictates. Instead of having marks to show the proper

proportions of sand and stone, the car is sometimes divided into two

compartments, one for each material and each holding the proper

proportion of its material when level full. This arrangement makes

proper proportioning somewhat more certain, since the men charging the

car cannot over-run the marks. In case separate cars are used for each

material, they are simply filled level full or to mark, and dumped in

succession into the feeding hopper. Trestle and car plant construction

and costs are given in a preceding section.

~Charging by Shoveling.~--Charging by shoveling directly into the mixer is

seldom practiced except in street work with continuous mixers or in

charging gravity mixers of the trough type. Shoveling is not an economic

method of handling materials where the work involves carrying in

shovels, and it is only in a few classes of concrete work or in

isolated, exceptional cases that charging with shovels does not involve

carrying. The amount of material that men will load with shovels is

given in Chapter III, and the reader who wishes a full discussion of the

subject is referred to Gillette and Hauer, "Earth Excavation and

Embankments; Methods and Cost."

In charging continuous mixers with shovels the usual practice for mixers

without automatic feed devices is to work from a continuous stock pile

of sand, stone and cement spread in layers in the proper proportions.

The shoveling is done in such a manner that each shovelful contains a

mixture of cement, sand and stone, and so that the rate of delivery to

the mixer is as uniform as possible. In charging mixers having automatic

feed devices the sand and stone are simply shoveled into the sand and

stone hoppers, whence they are fed automatically to the mixer. In

charging gravity mixers by shoveling the method is essentially the same;

the cement, sand and stone properly proportioned are spread in layers on

the shoveling board at the head of the mixer and the mixture then

shoveled into the mixer. In both of these cases mixing is performed to a

certain extent by the shoveling, and in both the provision of the

combination stock pile from which the men work involves labor which

comes within the meaning of the term charging as we have used it here.

Examples of street work in which the mixers were charged by shoveling

are given in Chapter XIV.

~Charging with Derricks.~--When the stock piles are located close to the

mixer and the plant is fixed or is not frequently moved derricks can be

used economically for charging, particularly if the mixer be elevated so

that inclines become expensive. The following mode of operation will be

found to work well: Set the derrick so that its boom "covers" the sand

and stone piles and the mixer, and provide it with three buckets so that

there will always be one bucket at the stone pile and another at the

sand pile while the third is being handled. The derrick swinging from

the mixer, where it has discharged a bucket, drops the empty bucket at

the stone pile and picks up the bucket standing there, which has

received its proper charge of stone, and swings it to the sand pile and

drops it to get its charge of sand. Here it picks up the bucket standing

at the sand pile and which has its charges of both stone and sand, and

swings it to the mixer. By this arrangement the work of the derrick and

of the men filling the buckets is practically continuous. The buckets

can be provided with marks on the inside to show the proper points to

which to fill the stone and the sand or a partition may be riveted in

making a compartment for sand and another for stone. A special

charging-bucket that is arranged with a wheel and detachable handles

which permit it to be handled like a wheelbarrow is shown by Fig. 18.

This bucket can be used to advantage where the stock piles are too far

from the mixer for the derrick to reach both, the bucket being loaded

and wheeled to within reach of the derrick.

[Illustration: Fig. 18.--Charging Bucket With Wheel and Detachable

Handle.]

~TYPES OF MIXERS.~--There are two types of concrete mixing machines or

concrete mixers as they are more commonly called: (1) Batch mixers and

(2) continuous mixers. In mixers of the first type a charge of cement,

sand, aggregate and water is put into the machine which mixes and

discharges the batch before taking in another charge; charging, mixing

and discharging is done in batches. In continuous mixers the cement

sand, stone and water are charged into the machine in a continuous

stream and the mixed concrete is discharged in another continuous

stream. While all concrete mixers are either batch or continuous mixers,

it is common practice because of their distinctive character to separate

gravity mixers, whether batch or continuous, into a third type. In

gravity mixers the concrete materials are made to mingle by falling

through specially constructed troughs, or tubes, or hoppers. We shall

describe mixers in this chapter as (1) batch mixers, (2) continuous

mixers, and (3) gravity mixers. No attempt will be made, however, to

describe all or even all the leading mixers of each type; a

representative mixer or two of each type will be described, enough to

give an indication of the range of practice, and the reader referred to

manufacturers' literature for further information.

~Batch Mixers.~--Batch mixers are made in two principal forms which may be

designated as tilting and non-tilting mixers. In the first form the

mixer drum is tilted as one would tilt a bucket of water to discharge

the batch. In non-tilting mixers the mixer drum remains in one position,

the batch being discharged by special mechanism which dips it out a

portion at a time. In both forms the charge is put into the mixer as a

unit and kept confined as a unit during the time of mixing, which may be

any period wished by the operator.

[Illustration: Fig. 19.--Chicago Improved Cube Concrete Mixer with

Elevating Charging Hopper.]

_Chicago Improved Cube Tilting Mixer._--Figure 19 shows the improved

cube mixer made by the Municipal Engineering & Contracting Co., Chicago,

Ill. The drum consists of a cubical box with rounded corners and edges.

This box has hollow gudgeons at two diagonally opposite corners and

these gudgeons are open as shown to provide for charging and

discharging. The box is rotated by gears meshing with a circumferential

rack midway between gudgeons and another set of gears operate to tilt

the mixer. The inside of the box is smooth, there being no deflectors,

as its shape is such as to fold the batch repeatedly and thus accomplish

the mixing.

[Illustration: Fig. 20.--Ransome Concrete Mixer.]

_Ransome Non-Tilting Mixer._--Figure 20 shows a representative

non-tilting mixer made by the Ransome Concrete Machinery Co., Dunellen,

N. J. It consists of a cylindrical drum riding on rollers and rotated by

a train of gears meshing with circumferential racks on the drum. The

drum has a circular opening at each end; a charging chute enters one

opening and a tilting discharge chute may be thrown into or out of the

opposite opening. The cylindrical shell of the drum is provided inside

with steel plate deflectors, which plow through and pick up and drop the

concrete mixture as the drum revolves. The shape and arrangement of the

deflectors are such that the batch is shifted back and forth axially

across the mixer. To discharge the batch the discharge chute is tilted

so that its end projects into the mixer, in which position the material

picked up by the deflectors drops back onto the chute and runs out. The

discharge chute being independent of the mixing drum it can be thrown

into and out of discharge position at will without stopping the rotation

of the drum, and so can discharge any part or all of the batch at once.

The top edge of the charging chute ranges from 30½ to 38 ins. in height

above the top of the frame, varying with the size of the mixer.

[Illustration: Fig. 21.--Smith Concrete Mixer.]

_Smith Tilting Mixer._--Figure 21 shows a tilting mixer, known as the

Smith mixer, made by the Contractors' Supply & Equipment Co., Chicago,

Ill. The drum consists of two truncated cones with their large ends

fastened together and their small ends open for receiving the charge and

discharge of the batch. The drum is operated by a train of gears meshing

into a rack at mid-length where the cones join. In addition there is

another set of gears which tilt the drum to make the concrete flow out

of the discharge end. The inside of the drum is provided with steel

plate deflectors, which plow through and pick and drop the concrete

mixture shifting it back and forth axially in the process.

~Continuous Mixers.~--Continuous mixers are those in which the cement,

sand and stone are fed to the charging hopper in a continuous stream and

the mixed concrete is discharged in another continuous stream. They are

built in two principal forms. In one form the cement, sand and stone

properly proportioned are shoveled directly into the mixing drum. In the

other form these materials are dumped into separate charging hoppers and

are automatically fed into the mixing drum in any relative proportions

desired. One form of continuous mixer with automatic feed is described

in the succeeding paragraph and another form is described in Chapter

XIV. The continuous mixer without automatic feed consists simply of a

trough with a rotating paddle shaft and its driving mechanism. The

charging, the mixing and the discharging are done in what is virtually a

succession of very small batches.

[Illustration: Fig. 22.--Eureka Automatic Feed Continuous Mixer.]

_Eureka Automatic Feed Mixer._--Figure 22 shows the construction of the

continuous mixer built by the Eureka Machine Co., Lansing, Mich. The

cement bin and feeder is the small one in the foreground. There is a

pocketed cylinder revolving between concave plates, opening into the

hopper above, from which the pockets in the feeder are filled, and

discharging directly into the mixing trough below. Back of this is

shown the feeder for sand or gravel up to 2-in. screen size. This is a

pocketed cylinder similar to that used in the cement feeder, except that

it is larger, and instead of being provided on the discharge side with a

concave plate, is surmounted by a roller, held by springs. This serves

to cut off the excessive flow of material, but provides sufficient

flexibility to allow the rough coarse material to be fed through the

machine without its catching. The feeder for crushed stone is a similar

construction on larger lines, to handle material up to 3-in. size. These

several feeders can be set to give any desired mixture. On any material

fit to be used in concrete, they will measure with an error of less than

5 per cent., an agitator being provided in the sand bin to prevent damp

sand from bridging over the feeder, and preventing its action. The mixer

consists of a trough, with a square shaft, on which are mounted 37

mixing paddles, which are slipped on in rotation, so as to form

practically a continuous conveyor, but as each paddle is distinct, and

is shaped like the mold board of a plow, the material, as it passes from

one to the next, is turned over and stirred. Water is sprayed into the

mass at the center of the trough. The result is a dry mix, followed by a

wet mix. The mixing trough is made of heavy gage steel, well reinforced,

and practically indestructible. To take care of the discharge of

material while changing wheelbarrows, a hood is provided on the

discharge end of the machine, which can be lowered, and will hold about

a wheelbarrow load.

~Gravity Mixers.~--Gravity mixers are constructed in two general forms.

The first form is a trough whose bottom or sides or both are provided

with pegs, deflectors or other devices for giving the material a zig-zag

motion as it flows down the trough. The second form consists of a series

of hoppers set one above the other so that the batch is spilled from one

into the next and is thus mixed.

The chief advantage claimed for gravity mixers is that no power is

required to operate them. This is obviously so only in the sense that

gravity mixers have no power-operated moving mechanism, and the fact

should not be overestimated. The cost of power used in the actual

performance of mixing is a very small item. The distance between feed

and discharge levels is always greater for gravity mixers than for

machine mixers, and the power required to raise the concrete materials

the excess height may easily be greater than the power required to

operate a machine mixer. On the other hand the simplicity of the gravity

mixer insures low maintenance costs.

_Gilbreth Trough Mixer._--Figure 23 shows the construction of one of the

best known makes of gravity mixers of the trough form. In operation the

cement, sand and stone in the proper proportions are spread in

superimposed layers on a shoveling board at hopper level and are then

shoveled as evenly as possible into the hopper. From the hopper the

materials flow down the trough, receiving the water about half way down,

and are mixed by being cut and turned by the pins and deflectors. The

trough of the mixer is about 10 ft. long.

[Illustration: Fig. 23.--Gilbreth Gravity Mixer, Trough Form.]

[Illustration: Fig. 24.--Hains Gravity Mixer, Fixed Hopper Form.]

_Hains Gravity Mixer._--The form of gravity mixer made by the Hains

Concrete Mixer Co., Washington, D. C., is shown by Figs. 24 and 25. The

charge passes through the hoppers in succession. Considering first the

stationary plant, shown by Fig. 24, the four hoppers at the top have a

combined capacity of one of the lower hoppers. Each top hopper is

charged with cement, sand and stone in the order named and in the proper

proportions. Water is then dashed over the tops of the filled hoppers

and they are dumped simultaneously into the hopper next below. This

hopper is then discharged into the next and so on to the bottom.

Meanwhile the four top hoppers have been charged with materials for

another batch. It will be observed that (1) the concrete is mixed in

separate batches and (2) the ingredients making a batch are accurately

proportioned and begin to be mixed for the whole batch at once. The best

arrangement is to have the top of the hopper tower carry sand and stone

bins which chute directly into the top hoppers. In the telescopic mixer

shown by Fig. 25 the purpose has been to provide a mixer which, hung

from a derrick or cableway, will receive a charge of raw materials at

stock pile and deliver a batch of mixed concrete to the work, the

operation of mixing being performed during the hoist to the work. By

providing two mixers so that one can be charged while the other is being

hoisted continuous operation is secured. The following are records of

operation of stationary gravity mixers of this type.

[Illustration: Fig. 25.--Hains Gravity Mixer, Telescoping Hopper Form.]

In building a dock at Baltimore, Md., a plant consisting of two large

hoppers and four charging hoppers with sand and stone bins above was

used. One man at each large conical hopper tending the gates and two men

charging the four pyramidal hoppers composed the mixer gang. A scow load

of sand and another of stone were moored alongside the work and a

clam-shell bucket dredge loaded the material from these barges into the

mixer bins. Each batch was 25 cu. ft. of 1-2-5 concrete rammed in place.

The men at the upper hoppers would empty a sack of cement in each, and

then by opening gates in the bottom of the bins above, allow the

necessary amounts of sand and stone to flow in, marks having been

previously made on the sides of the hoppers to show the correct

proportion of each of the ingredients. The amount of water found by

experience to be necessary, would then be dashed into the hoppers, and

the charges allowed to run into the first cone hopper below. Refilling

would begin at the top while the men were caring for the first charge in

the lower hoppers. The process was thus continuous. The concrete was

chuted directly into place from the bottom hopper. The record of output

was 110 batches per 10-hour day. Wages of common labor were $1.50 per

day. The labor cost per cubic yard of concrete in place was 35 cts.

In constructing the Cedar Grove reservoir at Newark, N. J., a Hains

mixer made the following records of output:

Cu. yds.

Best output per 10-hour day 403

Average daily output for best month 302

Average daily output for whole job 225

The stone, sand and cement were all raised by bucket elevators to the

top of the high wooden tower that supported the bins and mixer. There

were 10 men operating the mixer so that (exclusive of power, interest

and depreciation) the labor cost of mixing averaged only 7 cts. per cu.

yd.; during one month it was as low as 5 cts. per cu. yd. This does not

include delivering the materials to the men at the mixer, nor does it

include conveying the concrete away and placing it. The work was done by

contract.

~OUTPUT OF MIXERS.~--With a good mixer the output depends upon the methods

of conveying the materials to and from the mixer. Most makers of mixers

publish capacities of their machines in batches or cubic yards output

per hour; these figures may generally be taken as stating nearly the

maximum output possible. Considering batch mixers, as being the type

most commonly used, it may be assumed that where the work is well

organized and no delay occurs in delivering the materials to the mixer

that a batch every 2 minutes, or 300 batches in 10 hours, will be

averaged, and there are a few records of a batch every 1½ minutes.

To illustrate to how great an extent the output of a mixer depends on

the methods adopted in handling the materials to and from the mixer we

compare two actual cases that came under the authors' observation. The

mixers used were of the same size and make. In one case the stone was

shoveled into the charging hopper by four men and the sand and cement

were delivered in barrows by four other men; six men took the concrete

away in wheelbarrows. The output of the mixer was one batch every 5

minutes, or 120 batches, or 60 cu. yds., in 10 hours. In the other case

the sand and the stone were chuted directly into the charging hopper

from overhead bins and the mixer discharged into one-batch buckets on

cars. The output of the mixer was one batch every 2 minutes, or 300

batches in 10 hours. In the first case the capacity of the mixer was

limited by the ability of a gang of workable size to get the raw

materials to and the mixed concrete away from the mixer. In the second

case the capacity was limited only by the amount of mixing deemed

necessary.

While the necessity of rapid charging of a mixer to secure its best

output is generally realized it is often forgotten that the rapidity of

discharge is also a factor of importance. The size of the conveyor by

which the concrete is removed affects the time of discharge. By timing a

string of wheelbarrows in line the authors have found that it takes

about 7 seconds to fill each barrow; as a rule slight delays will

increase this time to 10 seconds. With a load of 1 cu. ft. per barrow it

requires 13 barrow loads to take away a ½ cu. yd. batch. This makes the

time of discharging a batch 130 seconds, or say 2 minutes. The same

mixer discharging into a batch size bucket will discharge in 15 to 20

seconds, saving at least 1½ minutes in discharging each batch.

~MIXER EFFICIENCY.~--Various attempts have been made to rate the

efficiency of concrete mixers. In all cases a percentage basis of

comparison has been adopted; arbitrary values are assigned to the

several functions of a mixer, such as 40 per cent. for perfect mixing,

10 per cent. for time of mixing and 25 per cent. for control of water,

the total being 100 per cent., and each mixer analyzed and given a

rating according as it is considered to approach the full value of any

function. Such percentage ratings are unscientific and misleading; they

present definite figures for what are mere arbitrary determinations. The

values assigned to the several functions are purely arbitrary in the

first place, and in the second place the decision as to how near those

values any mixer approaches are matters of personal judgment.

_The most efficient mixer is the one that gives the maximum product of

standard quality at the least cost for production._

This rule recognizes the fact that in practical construction different

standards of quality are accepted for different kinds of work. No

engineer demands, for example, the same quality of mixture for a

pavement base that he does for a reinforced concrete girder. If mixer A

turns out concrete of a quality suitable for pavement base cheaper than

does mixer B, then it is the more efficient mixer for the purpose, even

though mixer B will make the superior quality of concrete required for a

reinforced girder while mixer A will not. This method of determining

efficiency holds accurate for any standard of quality that may be

demanded.

CHAPTER V.

METHODS AND COST OF DEPOSITING CONCRETE UNDER WATER AND OF SUBAQUEOUS

GROUTING.

Mixed concrete if emptied loose and allowed to sink through water is

destroyed; the cement paste is washed away and the sand and stone settle

onto the bottom more or less segregated and practically without

cementing value. In fact, if concrete is deposited with the utmost care

in closed buckets and there is any current to speak of a considerable

portion of cement is certain to wash out of the deposited mass. Even in

almost still water some of the cement will rise to the surface and

appear as a sort of milky scum, commonly called _laitance_. Placing

concrete under water, therefore, involves the distinctive task of

providing means to prevent the washing action of the water. It is also

distinguished from work done in air by the fact that it cannot be

compacted by ramming, but the main problem is that of preventing wash

during and after placing.

~DEPOSITING IN CLOSED BUCKETS.~--Special buckets for depositing concrete

under water are made by several manufacturers of concrete buckets. These

buckets vary in detail but are all similar in having doors to close the

concrete away from the water and, generally, in being bottom dumping.

The bucket shown by Fig. 26 was designed by Mr. John F. O'Rourke, and is

built by the Cockburn Barrow & Machine Co., of Jersey City, N. J. This

bucket was used in depositing the concrete for the City Island Bridge

foundations described in Chapter XII and also in a number of other

works. It consists of a nearly cubical shell of steel open at top and

bottom, and having heavy timbers rivetted around the bottom edges. The

open top has two flat flap doors. Two similar doors hinged about midway

of the sides close to form a V-shaped hopper bottom inside the shell and

serve when open, to close the openings in the sides of the shell. In

loading the bucket the bottom doors are drawn inward and upward by the

chains and held by a temporary key. The loaded bucket is then lifted by

the bail and the key removed, since when suspended the pull on the bail

holds the chains taut and the doors closed. As soon as the bucket rests

on the bottom the pull of the concrete on the doors slides the bail down

and the doors swing downward and back discharging the concrete. The

timbers around the bottom edges keep the bucket from sinking into the

deposited concrete, and the doors and shell exclude all water from the

batch until it is finally in place.

[Illustration: Fig. 26.--O'Rourke Bucket fur Depositing Concrete Under

Water.]

The subaqueous concrete bucket shown by Figs. 27 and 28 is made by the

Cyclopean Iron Works Co., Jersey City, N. J. Fig. 27 shows the bucket

suspended full ready for lowering; the cover is closed and latched and

the bail is held vertical by the tag line catch A. Other points to be

noted are the eccentric pivoting of the bail, the latch unlocking lever

and roller B and C, and the stop D. In the position shown the

bucket is lowered through the water and when at the proper depth just

above bottom the tag line is given a sharp pull, uncatching the bail.

The body of the bucket turns bottom side up, revolving on the bail

pivots, and just as the revolution is completed the bail engages the

roller C on the latch unlocking lever and swings the lever enough to

unlatch the top and allow it to swing down as shown by Fig. 28 and

release the concrete. The stop D keeps the body of the bucket from

swinging beyond the vertical in dumping.

[Illustration: Fig. 27.--Cyclopean Bucket for Depositing Concrete Under

Water (Closed Position).]

[Illustration: Fig. 28.--Cyclopean Bucket for Depositing Concrete Under

Water (Open Position).]

Figures 29 and 30 show the subaqueous concrete bucket made by the G. L.

Stuebner Iron Works, Long Island City, N. Y., essentially the same

bucket, omitting the cover and with a peaked bail, is used for work in

air. For subaqueous work the safety hooks A are lifted from the angles

B and wired to the bail in the position shown by the dotted lines, and

a tag line is attached to the handle bar C. The bucket being filled

and the cover placed is lowered through the water to the bottom and then

discharged by a pull on the tag line.

~DEPOSITING IN BAGS.~--Two methods of depositing concrete in bags are

available to the engineer; one method is to employ a bag of heavy tight

woven material, from which the concrete is emptied at the bottom, the

bag serving like the buckets previously described simply as means of

conveyance, and the other method is to use bags of paper or loose woven

gunnysack which are left in the work, the idea being that the paper will

soften or the cement will ooze out through the openings in the cloth

sufficiently to bond the separate bagfuls into a practically solid mass.

[Illustration: Fig. 29.--Stuebner Bucket for Depositing Concrete Under

Water (Closed Position).]

[Illustration: Fig. 30.--Stuebner Bucket for Depositing Concrete Under

Water (Open Position).]

[Illustration: Fig. 31.--Bag for Depositing Concrete Under Water.]

[Illustration: Fig. 32.--Form for Molding Footing for Block Concrete

Breakwater.]

The bag shown by Fig. 31 was used to deposit concrete for leveling up a

rough rock bottom and so provide a footing for a concrete block pier

constructed in 1902 at Peterhead, N. B., by Mr. William Shield, M. Inst.

C. E. Careful longitudinal profiles were taken of the rock bottom one at

each edge of the footing. Side forms were then made in 20-ft. sections

as shown by Fig. 32; the lagging boards being cut to fit the determined

profile and the top of the longitudinal piece being flush with the top

of the proposed footing. The concrete was filled in between the side

forms and leveled off by the T-rail straight-edge. In placing the side

forms the longitudinal pieces were placed by divers who were given the

proper elevations by level rods having 10 to 15-ft. extension pieces to

raise the targets above the water surface. When leveled the side pieces

were anchor-bolted as shown to the rock, the anchor-bolts being wedged

into the holes to permit future removal. The concrete was then lowered

in the bag shown by Fig. 31, the divers assisting in guiding the bag to

position. The mouth of the bag being tied by one turn of a line having

loops through which a wooden key is slipped to hold the line tight, a

sharp tug on the tripping rope loosens the key and empties the bag. The

bags used on this work had a capacity of 2¼ cu. ft. To permit the

removal of the side forms after the concrete had hardened, a strip of

jute sacking was spread against the lagging boards with a flap

extending 15 to 18 ins. under the concrete. The forms were removed by

divers who loosened the anchor bolt wedges.

In placing small amounts of concrete for bridge foundations in Nova

Scotia, bags, made of rough brown paper were used to hold the concrete.

Each bag held about 1 cu. ft. The bags were made up quickly and dropped

into the water one after the other so that the following one was

deposited before the cement escaped from the former one. The paper was

immediately destroyed by submersion and concrete remained. The bags cost

$1.35 per hundred or 35 cts. per cu. yd. of concrete. Concrete was thus

deposited in 18 ft. of water without a diver.

[Illustration: Fig. 33.--Steel Tremie for Depositing Concrete Under

Water.]

~DEPOSITING THROUGH A TREMIE.~--A tremie consists of a tube of wood or,

better, of sheet metal, which reaches from above the surface to the

bottom of the water; it is operated by filling the tube with concrete

and keeping it full by successive additions while allowing the concrete

to flow out gradually at the bottom by raising the tube slightly to

provide the necessary opening. A good example of a sheet steel tremie is

shown by Fig. 33. This tremie was used by Mr. Wm. H. Ward in

constructing the Harvard Bridge foundations and numerous other

subaqueous structures of concrete. In these works the tube was suspended

from a derrick. Wheelbarrows filled the tube and hopper with concrete

and kept them full; the derrick raised the tube a few inches and swung

it gently so as to move it slowly over the area to be filled. Care being

taken to keep the tube at one height, the concrete was readily deposited

in even layers. Concrete thus deposited in 18 ft. of water was found to

be level and solid on pumping the pit dry.

[Illustration: Fig. 34.--Tremie and Traveler Used at Charlestown, Mass.,

Bridge.]

Another method of handling a tremie was employed in constructing the

foundations for the Charlestown Bridge at Boston, Mass. Foundation piles

were driven and sawed off under water. A frame was built above water and

supported by a curbing attached to certain piles in the outer rows of

the foundation reserved for this purpose. In this frame the vertical

members were Wakefield sheet-piling plank, spaced 6 to 10 ft. apart, and

connected by three lines of double waling bolted to the verticals at

three different heights. This frame was lowered to the bottom so as to

enclose the bearing piles. The posts or verticals were then driven, one

by one, into the bottom, the frame being flexible enough to permit this.

The spaces between the posts or verticals were then filled by

sheet-piling and the frame was bolted to the curbing piles. This curbing

afterward supported the traveler used in laying the concrete. Thus a

coffer dam was formed to receive the concrete as shown in Fig. 34. The

1-2-5 concrete was deposited up to within 5½ ft. of the mean low water

level, the last foot being laid after water was pumped out. The tremie

used to deposit the concrete was a tube 14 ins. in diameter at the

bottom and 11 ins. at the neck, with a hopper at the top. It was made in

removable sections, with outside flanges, and was suspended by a

differential hoist from a truck moving laterally on a traveler, Fig. 34.

The foot of the chute rested on the bottom until filled with concrete;

then the chute was slowly raised and the concrete allowed to run but

into a conical heap, more concrete being dumped into the hopper. As the

truck moved across the traveler a ridge of concrete was made; then the

traveler was moved forward and another parallel ridge was made. The best

results were obtained when the layers were 2½ ft. thick, but layers up

to 6 ft. thick were laid. If the layer was too thick, or uneven, or if

the chute was moved or raised too quickly, the charge in the tube was

"lost." This was objectionable because the charging of the chute anew

resulted in "washing" the cement more or less out of the concrete until

the chute was again filled. To reduce this objection the contractor was

directed to dump some neat cement into the tube before filling with

concrete. A canvass piston was devised which could be pushed ahead of

the concrete when filling the chute. It consisted of two truncated cones

of canvass, one flaring downward to force the water ahead, and the other

flaring upward to hold the concrete. The canvass was stiffened and held

against the sides of the chute by longitudinal ribs of spring steel

wire; the waist was filled by a thick block of wood to which all the

springs were attached; and to this block were connected additional steel

guides to prevent overturning and a rope to regulate the descent. Very

little water forced its way past this piston and it was a success, but

as the cost was considerable and a piston was lost each time, its use

was abandoned as the evil to be avoided did not justify the outlay.

The chute worked best when the concrete was mixed not quite wet enough

to be plastic. If mixed too wet the charge was liable to be "lost," and

if dry it would choke the chute. An excess of gravel permitted water to

ascend in the tube; and an excess of sand tended to check the flow of

concrete.

In constructing the piers for a masonry arch bridge in France in 1888

much the same method was followed, except that a wooden tremie 16 ins.

square made in detachable sections was used. This tremie had a hopper

top and was also provided with a removable cap or cover for the bottom

end, the latter device being intended to keep the water out of the tube

and prevent "washing" the first charge of concrete. The piers were

constructed by first driving piles and sawing them off several feet

above the bottom but below water level, and then filling them nearly to

their tops with broken stone. An open box caisson was then sunk onto the

stone and embracing the pile tops and then filled around the outside

with more broken stone. The caisson was then filled with concrete

through the tremie which was handled by a traveling crane. The crane was

mounted and traveled transversely of the pier on a platform which in

turn moved along tracks laid lengthwise of the caisson. The tube was

gradually filled with concrete and lowered, the detachable bottom of the

tube was then removed, allowing the concrete to run out. The tube was

first moved across the caisson and then downstream and back across the

caisson, and this operation repeated until a 16-in. layer was completed.

The tube was then raised 16 ins. and the operations repeated to form

another layer. There was almost no _laitance_. From 90 to 100 cu. yds.

were deposited daily.

Still another example of tremie work is furnished by the task of

depositing a large mass of concrete under water in the construction of

the Nussdorf Lock at Vienna. This lock has a total width of 92 ft. over

all, and is 49.2 ft. clear inside. The excavation, which was carried to

a depth of 26.24 ft. below water level, was made full width, between

sheet piling, and the bottom was filled in with rammed sand and gravel,

forming a kind of invert with its upper surface horizontal in the middle

and sloping upwards a trifle at both sides. A mass of concrete having a

total thickness of 13.12 ft. was built on this foundation in the center

where the upper surfaces were 13.12 ft. below the water level. Concrete

walls were carried up at the sides of the lock to a height of 3.28 ft.;

these walls were 8.2 ft. thick. The methods used in placing the concrete

were as follows: Three longitudinal rows of piles were driven on each

side of the axis of the lock, these piles supporting a 6-rail track

about 7 ft. above the water level. Three carriages spanning the full

width of the lock transversely moved on this track. Each carriage had

three trolleys, one in each of the main panels of the transverse pile

bends. These trolleys each carried a vertical telescopic tube, by means

of which the concrete was deposited at the bottom of the lock. These

tubes or chutes were of different lengths in the three carriages; the

first ones deposited the concrete up to a level of 23 ft. below the

surface; the next set deposited the concrete between that level and 19.7

ft., and the last set completed the subaqueous work up to the final

height of 16.4 ft. below the surface. The tops of the tubes were level

with a transverse track extending the full length of the carriage. The

ends of these tracks just cleared the outside rows of piles, which, on

one side of the lock, supported a distribution track parallel to the

axis of the lock. Dump cars running on this distribution track delivered

the concrete to smaller dump cars on the carriage tracks, and in turn

these smaller cars dumped into either of these chutes on each carriage.

The carriages were moved from end to end of the lock, the whole area of

the lock coming under the nine chutes, inasmuch as each chute moved

one-third the length of the carriage. The concrete was deposited in

three horizontal layers 3.28 ft. thick, the layers being built in

comparatively narrow banks, so that the different layers would key

together and form a corrugated mass. The chutes were shortened as the

concrete was deposited, three layers being placed successively. The main

body of the bottom and the side walls were built by this method, and

then the water was pumped out and a 2.3 ft. layer of concrete rammed

over the bottom and completed with a finished surface 9 ft. thick.

~GROUTING SUBMERGED STONE.~--Masses of gravel, broken or rubble stone

deposited under water may be cemented into virtually a solid concrete by

charging the interstices with grout forced through pipes from the

surface. Mr. H. F. White gives the following records of grouting

submerged gravel:

In experiment No. 1 a reservoir 10 ft. square was filled to a depth of

18 ins. with clean gravel ballast (1½ to 2-in. size) submerged in water.

A 2-in. gas pipe rested on the gravel and was surmounted with a funnel.

A 1:1 Portland grout was poured in. After 21 days set the water was

drawn off, and it was found that the grout had permeated the ballast

for a space of 8 ft. square at the bottom and 6 ft. square at the top,

leaving a small pile of pure cement mortar 6 ins. high about the base of

the pipe; 16 cu. ft. of cement and 16 cu. ft. of sand concreted 100 cu.

yds. of ballast. In experiment No. 2, under the same conditions, a grout

made of 1 part lime, 1 part surki (puzzulana or trass) and 1 part sand,

was found to have spread over the entire bottom, 10 ft. square, rising 5

ins. on the sides, and making the concreted mass about 3½ ft. square at

the top; 25 cu. ft. of the dry materials concreted 100 cu. ft. of

ballast. In experiment No. 3 the ballast was 2½ ft. deep. A grout (using

8 cu. ft. of each ingredient) made as in experiment No. 2 covered the

bottom, rose 14 ins. on the sides and made a top surface 4½ ft. square;

32 cu. ft. of the dry materials grouted 100 cu. ft. of ballast. In

experiment No. 4 the ballast was of bats and pieces 3 or 4 ins. in size

laid 7 ft. deep. A grout made as in experiment No. 2 (using 88 cu. ft.

of each ingredient) concreted the whole mass to a depth of 6 ft. up the

sides, and 2½ ft. square at the pipe on the surface of the ballast. Mr.

White says that a grout containing more than 1 part of sand to 1 of

Portland cement will not run freely through a 2-in. pipe, as the sand

settles out and chokes the pipe. Even with 1:1 grout it must be

constantly stirred and a steady flow into the pipe maintained. The

lime-trass grout does not give the same trouble.

Mr. W. R. Knipple describes the work of grouting rubble stone and gravel

for the base of the Hermitage Breakwater. This breakwater is 525 ft.

long, 50 ft. wide at base and 42 ft. wide at top, and 68 ft. high, was

built on the island of Jersey. Where earth (from 0 to 8½ ft. deep)

overlaid the granite rock, it was dredged and the trench filled in with

rubble stones and gravel until a level foundation was secured. Cement

grout was then forced into this filling through pipe placed 8 to 10 ft.

apart. The grouting was done in sections 12½ ft. long, from 7 to 10 days

being taken to complete each. Upon this foundation concrete blocks,

4×4×9 to 12 ft., were laid in courses inclined at an angle of 68°. The

first four courses were laid by divers, the blocks being stacked dry two

courses high at a time. The joints below water were calked by divers and

above water by masons, and a section was then grouted. When two courses

had been laid and grouted, two more courses were laid and grouted in

turn, and so on. In places, grouting was done in 50 ft. of water. The

grout should be a thick paste; a 30-ft. column of grout will balance a

60-ft. column of water.

CHAPTER VI.

METHODS AND COST OF MAKING AND USING RUBBLE AND ASPHALTIC CONCRETE.

Two kinds of concrete which vary in composition and character from the

common standard mixtures of cement, sand and broken aggregate are

extensively employed in engineering construction. These are rubble

concrete and asphaltic concrete.

~RUBBLE CONCRETE.~--In constructing massive walls and slabs a reduction in

cost may often (not always) be obtained by introducing large stones into

the concrete. Concrete of this character is called rubble concrete, and

the percentage of rubble stone contained varies from a few per cent. to,

in some cases, over half of the volume. The saving effected comes partly

from the reduction in the cement required per cubic yard of concrete and

partly from the saving in crushing.

The saving in cement may be readily figured if the composition of the

concrete and the volume of the added rubble stones be known. A 1-2½-5

concrete requires according to Table X in Chapter II 1.13 bbls. of

cement per cubic yard. Assuming a barrel of cement to make 3.65 cu. ft.

of paste, we have 3.65 × 1.13 = 4.12 cu. ft. of cement paste per cubic

yard of 1-2½-5 concrete. This means that about 15 per cent. of the

volume of the concrete structure is cement. If rubble stone be

introduced to 50 per cent. of the volume, then the structure has about

7½ per cent. of its volume of cement. It is of interest to note in this

connection that rubble masonry composed of 65 per cent. stone and 35 per

cent. of 1-2½ mortar would have some 11½ per cent. of its volume made up

of cement.

The saving in crushing is not so simple a determination. Generally

speaking, the fact that a considerable volume of the concrete is

composed of what, we will call uncrushed stone, means a saving in the

stone constituent of one structure amounting to what it would have cost

to break up and screen this volume of uncrushed stone, but there are

exceptions. For example, the anchorages of the Manhattan Bridge over the

East River at New York city were specified to be of rubble concrete,

doubtless because the designer believed rubble concrete to be cheaper

than plain concrete. In this case an economic mistake was made, for all

the rubble stone used had to be quarried up the Hudson River, loaded

onto and shipped by barges to the site and then unloaded and handled to

the work using derricks. Now this repeated handling of large, irregular

rubble stones is expensive. Crushed stone as we have shown in Chapter IV

can be unloaded from boats at a very low cost by means of clam shells.

It can be transported on a belt conveyor, elevated by bucket conveyer,

mixed with sand and cement and delivered to the work all with very

little manual labor when the installation of a very efficient plant is

justified by the magnitude of the job. Large rubble stones cannot be

handled so cheaply or with so great rapidity as crushed stone; the work

may be so expensive, due to repeated handlings, as to offset the cost of

crushing as well as the extra cost of cement in plain concrete. On the

other hand, the cost of quarrying rock suitable for rubble concrete is

no greater than the cost of quarrying it for crushing--it is generally

less because the stone does not have to be broken so small--so that when

the cost of getting the quarried rock to the crusher and the crushed

stone into the concrete comes about the same as getting the quarried

stone into the structure it is absurd practice to require crushing. To

go back then to our first thought, the question whether or not saving

results from the use of rubble concrete, is a separate problem in

engineering economics for each structure.

In planning rubble concrete work the form of the rubble stones as they

come from the quarry deserves consideration. Stones that have flat beds

like many sandstones and limestones can be laid upon layers of dry

concrete and have the vertical interstices filled with dry concrete by

tamping. It requires a sloppy concrete to thoroughly embed stones which

break out irregularly. In the following examples of rubble concrete work

the reader will find structures varying widely enough in character and

in the percentages of rubble used to cover most ordinary conditions of

such work.

Where the rubble stones are very large it is now customary to use the

term "cyclopean masonry" instead of rubble concrete. Many engineers who

have not studied the economics of the subject believe that the use of

massive blocks of stone bedded in concrete necessarily gives the

cheapest form of masonry. We have already indicated conditions where

ordinary concrete is cheaper than rubble concrete. We may add that if

the quarry yields a rock that breaks up naturally into small sized

blocks, it is the height of economic folly to specify large sized

cyclopean blocks. Nevertheless this blunder has been frequently made in

the recent past.

[Illustration: Fig. 35.--Diagram Cross-Section of Rubble Concrete Dam,

Chattahoochee River.]

~Chattahoochee River Dam.~--The roll-way portion, 680 ft. long, of the dam

for the Atlanta Water & Electric Power Co., shown in section by Fig. 35,

was built of a hearting of rubble concrete with a fine concrete facing

and a rubble rear wall. The facing, 12 ins. thick of 1-2-4 concrete,

gave a smooth surface for the top and face of the dam, while the rubble

rear wall enabled back forms to be dispensed with and, it was

considered, made a more impervious masonry. The concrete matrix for the

core was a 1-2-5 stone mixture made very wet. The rubble stones, some as

large as 4 cu. yds., were bedded in the concrete by dropping them a few

yards from a derrick and "working" them with bars; a well formed stone

was readily settled 6 ins. into a 10-in. bed of concrete. The volume of

rubble was from 33 to 45 per cent. of the total volume of the masonry.

The 1-2-4 concrete facing was brought up together with the rubble core,

using face forms and templates to get the proper profile. The work was

done by contract and the average was 5,500 cu. yds. of concrete placed

per month.

[Illustration: Fig. 36.--Cross-Section of Barossa Dam of Rubble

Concrete.]

~Barossa Dam, South Australia.~--The Barossa Dam for the water-works for

Gawler, South Australia, is an arch with a radius of 200 ft., and an arc

length on top of 422 ft.; its height above the bed of the stream is 95

ft. Figure 36 is a cross-section of the dam at the center. The dam

contains 17,975 cu. yds. of rubble concrete in the proportions of 2,215

cu. yds. of rubble stone to 15,760 cu. yds. of concrete; thus about 12.3

per cent. of the dam was of rubble. The concrete was mixed by weight of

1 part cement, 1½ parts sand, and a varying proportion of aggregate

composed of 4½ parts 1¼ to 2-in. stone, 2 parts ½ to 1¼-in. stone and 1

part 1/8 to ½-in. stone or screenings. The sand was one-half river sand

and one-half crusher sand. The following shows the amounts by weight of

the several materials for each of the several classes of concrete per

cubic yard:

-------------Stone----------

Class. Excess Mortar. 1¼-2. ½-1¼. 1/8-½. Sand. Cement.

A 7.5% 1,500 661½ 333¼ 804 434

B 12.5 1,433-1/3 637 318 858½ 463

C 12.5 1,434 637 318½ 859 474

D 15 1,402 623 312 884 484

[Illustration: Fig. 37.--Apparatus Used for Weighing Concrete Materials

at Barossa Dam.]

The average composition of the concrete was 1-1½-3½. Its cost per cubic

yard in place including rubble was 38s 9d per cu. yd. or about $9.30. In

proportioning the mixture on the work use was made of the device shown

by Fig. 37 to weigh the aggregate. The measuring car is pushed back

under the stone hopper chute until the wheels drop into shallow notches

in the balanced track rails; stone is then admitted until the lead

weight begins to rise, when the car is pushed forward and dumps

automatically as indicated.

~Other Rubble Concrete Dams.~--Rubble concrete containing from 55 to 60

per cent. rubble was used in constructing the Boonton Dam at Boonton, N.

J. The stones used measured from 1 to 2½ cu. yds. each; the concrete was

made so wet that when the stones were dropped into it, it flowed into

every crevice. The materials were all delivered on cars, from which they

were delivered to the dam by derricks provided with bull-wheels. On the

dam there were 4 laborers and 1 mason to each derrick, and this gang

dumped the concrete and joggled the rubble stones into it. Records of

125 cu. yds. per 10 hours, with one derrick, were made. With 35

derricks, 20 of which were laying masonry and 15 either passing

materials or being moved, as much as 21,000 cu. yds. of masonry were

laid in one month. The amount of cement per cubic yard of masonry is

variously stated to have been 0.6 to 0.75 bbl. The stone was granite.

The Spier Falls Dam on the upper Hudson River was built of rubble

concrete containing about 33 per cent. rubble stone. The concrete was a

1-2½-5 mixture, and the engineer states that about 1 bbl. of cement was

used per cubic yard of rubble concrete. This high percentage of cement

may be accounted for by the fact that there was a considerable amount of

rubble masonry in cement mortar included in the total. The stones and

concrete were delivered along the dam by cableways and stiff-leg

derricks set on the downstream sloping face of the dam delivered them

from the cableways into place. There were two laborers to each mason

employed in placing the materials, wages being 15 and 35 cts. per hour,

respectively. The labor cost of placing the materials was 60 cts. per

cubic yard of masonry. The stone was granite.

Granite rubble laid in layers on beds of concrete and filled between

with concrete was used in constructing the Hemet Dam in California. The

concrete was a 1-3-6 mixture, and was thoroughly tamped under and

between the stones. For face work the stones were roughly scabbled to

shape and laid in mortar. The stone was taken from the quarry 400 ft.

away and delivered directly on the dam by cableways; here two derricks

handled the stone into place, the dam being only 246 ft. arc length on

top, though it was 122½ ft. high. The cableways would take a 10-ton

load; stones could be taken from the quarry, hoisted 150 ft. and

delivered to the work in 40 to 60 seconds. Common labor at $1.75 per day

was used for all masonry except facing, where masons at $3.50 were

employed. Cement cost delivered $5 per barrel, of which from $1 to $1.50

per barrel was the cost of hauling 23 miles by team over roads having 18

per cent. grades in places. Sand was taken from the stream bed and

delivered to the work by bucket conveyor. "Under favorable conditions

some of the masonry was put in for as low as $4 per cu. yd." There were

31,100 cu. yds. of masonry in the dam, which required 20,000 bbls. of

cement, or 0.64 bbl. per cubic yard.

The following novel method of making rubble concrete was employed in

enlarging two old dams and in constructing two new dams for a small

water-works. The available time was short, the amount of work was too

small and too scattered to justify the installation of a stone crusher,

and suitable gravel was not at hand. Sufficient small boulders in old

walls, and borrow pits and on surface of fields were available, and were

used with thin Portland cement mortar. One part of Alpha or Lehigh

cement and three parts sand were mixed dry at first and then wet with

just enough water to make the resulting mortar flow by gravity. This

mortar was shoveled into the forms continuously by one set of men while

other men were throwing into the mortar in the forms the boulders which

were cleaned and broken so as not to be more than 7 ins. long. In

general the performance was continuous. Three mortar beds were placed

parallel with, and against, one side of the forms, with spaces of about

4 ft. between the ends of the beds. The boulders were dumped on the

opposite side of the forms. Two men shoveled in all the mortar and did

nothing else. While they were emptying one bed the mortar was being

mixed in the preceding bed by two other men and the materials placed in

the third bed by still others. Another gang was continually throwing in

the boulders and small stones and still another was breaking stone. One

man should keep the mortar well stirred while the bed is being emptied.

About 20 men were necessary to do all parts of the work. The forms were

of 2-in. planed plank tongued and grooved. Especial pains were taken to

make the forms tight, and all leaks that appeared were quickly stopped

with dry cement. Some pains were taken to prevent a flat side of large

stones from coming in direct contact with the forms, but round boulders

and small stones needed no care to prevent their showing in the finished

work.

[Illustration: Fig. 38.--Bridge Abutment of Rubble Concrete.]

In conclusion it is interesting to note, perhaps, the earliest use of

rubble concrete for dam construction in this country in constructing the

Boyd's Corner Dam on the Croton River near New York. This dam was begun

in 1867 and for a time rubble concrete was used, but was finally

discontinued, due to the impression that it might not be watertight. The

specifications called for dry concrete to be thoroughly rammed in

between the rubble stones, and to give room for this ramming the

contractor was not permitted to lay any two stones closer together than

12 ins. As a result not more than 33 per cent. of the concrete was

rubble.

~Abutment for Railway Bridge.~--Figure 38 shows a bridge abutment built of

rubble concrete at a cost of about $4.50 per cu. yd. The concrete was a

1-2½-4½ mixture laid in 4-in. layers. On each layer were laid large

rubble stones bedded flat and spaced to give 6-in. vertical joints; the

vertical joints were filled with concrete by ramming and then another

layer of concrete placed and so on. A force of 28 men and a foreman

averaged 40 cu. yds. of rubble concrete per day. The following is the

itemized cost per cubic yard, not including forms, for 278 cu. yds:

Item. Per Cu. Yd.

0.82 bbls. cement, at $2.60 $2.14

0.22 cu. yd. sand, at $1.00 0.22

0.52 cu. yd. broken stone, at $0.94 0.49

0.38 cu. yd. rubble stone, at $0.63 0.24

Water 0.07

Labor, at 15 cts. per hour 1.19

Foreman 0.09

-------

Total $4.44

~Some English Data on Rubble Concrete.~--Railway work, under Mr. John

Strain, in Scotland and Spain, involved the building of abutments, piers

and arches of rubble concrete. The concrete was made of 1 part cement to

5 parts of ballast, the ballast consisting of broken stone or slag and

sand mixed in proportions determined by experiment. The materials were

mixed by turning with shovels 4 times dry, then 4 times more during the

addition of water through a rose nozzle. A bed of concrete 6 ins. thick

was first laid, and on this a layer of rubble stones, no two stones

being nearer together than 3 ins., nor nearer the forms than 3 ins. The

stones were rammed and probed around with a trowel to leave no spaces.

Over each layer of rubble, concrete was spread to a depth of 6 ins. The

forms or molds for piers for a viaduct were simply large open boxes, the

four sides of which could be taken apart. The depth of the boxes was

uniform, and they were numbered from the top down, so, that, knowing the

height of a given pier, the proper box for the base could be selected.

As each box was filled, the next one smaller in size was swung into

place with a derrick. The following bridge piers for the Tharsis &

Calanas Railway were built:

Length Height No. Cu. Yds. Weeks

of of of in to

Name. Bridge. Piers. Spans. Piers. Build.

Ft. Ft.

Tamujoso River 435 28 12 1,737 14½

Oraque 423 31 11 1,590 15

Cascabelero 480 30 to 80 10 2,680 21

No. 16 294 28 to 50 7 1,046 16½

Tiesa 165 16 to 23 8 420 4

It is stated that the construction of some of these piers in ordinary

masonry would have taken four times as long. The rock available for

rubble did not yield large blocks, consequently the percentage of pure

concrete in the piers was large, averaging 70 per cent. In one case,

where the stones were smaller than usual, the percentage of concrete was

76½ per cent. In other work the percentage has been as low as 55 per

cent., and in still other work where a rubble face work was used the

percentage of concrete has been 40 per cent.

In these piers the average quantities of materials per cubic yard of

rubble concrete were:

448 lbs. (0.178 cu. yd.) cement.

0.36 cu. yd. sand.

0.68 cu. yd. broken stone (measured loose in piles).

0.30 cu. yd. rubble (measured solid).

Several railway bridge piers and abutments in Scotland are cited. In one

of these, large rubble stones of irregular size and weighing 2 tons each

were set inside the forms, 3 ins. away from the plank and 3 ins. from

one another. The gang to each derrick was: 1 derrick man and 1 boy, 1

mason and 10 laborers, and about one-quarter of the time of 1 carpenter

and his helper raising the forms. For bridges of 400 cu. yds., the

progress was 12 to 15 cu. yds. a day. The forms were left in place 10

days.

To chip off a few inches from the face of a concrete abutment that was

too far out, required the work of 1 quarryman 5 days per cu. yd. of

solid concrete chipped off.

Concrete was used for a skew arch over the River Dochart, on the Killin

Railway, Scotland. There were 5 arches, each of 30 ft. span on the

square or 42 ft. on the skew, the skew being 45°. The piers were of

rubble concrete. The concrete in the arch was wheeled 300 ft. on a

trestle, and dumped onto the centers. It was rammed in 6-in. layers,

which were laid corresponding to the courses of arch stones. As the

layers approached the crown of the arch, some difficulty was experienced

in keeping the surfaces perpendicular. Each arch was completed in a day.

In a paper by John W. Steven, in Proc. Inst. C. E., the following is

given:

Rubble Per Cent.

Concrete Concrete of Rubble

Per Per in Rubble

Cu. Yd. Cu. Yd. Concrete.

Ardrossan Harbor $6.00 $5.00 20.0

Irvine Branch 7.00 3.68 63.6

Calanas & Tharsis Ry 7.08 3.43 30.3

Mr. Martin Murphy describes some bridge foundations in Nova Scotia.

Rubble concrete was used in some of the piers. The rubble concrete

consisted of 1 part cement, 2 parts sand, 1 part clean gravel, and 5

parts of large stones weighing 20 lbs. each and upwards. The sand,

cement and gravel were turned three times dry and three times wet, and

put into the forms. The rubble stones were bedded in the concrete by

hand, being set on end, 2 or 3 ins. apart. No rubble stones were placed

within 6 ins. of the forms, thus leaving a face of plain concrete; and

the rubble stones were not carried higher than 18 ins. below the top of

the pier. One cubic yard of this rubble concrete required 0.8 to 0.9

bbl. of cement.

~ASPHALT CONCRETE.~--Asphalt or tar concrete in which steam cinders or

broken stone or gravel and sand are mixed with asphaltum or tar instead

of cement paste are used to some extent in lining reservoirs,

constructing mill floors, etc. Such mixtures differ in degree only from

the mixtures used for asphalt street paving, for discussion of which the

various books on paving and asphalts should be consulted. The two

examples of asphalt concrete work given here are fairly representative

of the mixtures and methods employed for concrete work as distinguished

from asphalt work.

~Slope Paving for Earth Dam.~--Mr. Robert B. Stanton describes a small log

dam faced upstream with earth, upon which was laid an asphalt concrete

lining to make it water tight. The stone was broken to 2-in. pieces,

all the fines being left in and sufficient fine material added to fill

the voids. The stone was heated and mixed in pans or kettles from a

street paving outfit; and the asphaltum paste, composed of 4 parts

California refined asphaltum and 1 part crude petroleum, was boiled in

another kettle. The boiling hot paste was poured with ladles over the

hot stone, and the whole mixed over the fire with shovels and hoes. The

asphalt concrete was taken away in hot iron wheelbarrows, placed in a

4-in. layer rammed and ironed with hot irons. The concrete was laid in

strips 4 to 6 ft. wide, the edges being coated with hot paste. After the

whole reservoir was lined, it was painted with the asphalt paste, boiled

much longer, until when cold it was hard and stone was broken to 2-in.

pieces, all the fines being left in and sufficient fine material added

to fill the voids. The stone was heated and mixed in pans or kettles

from a street paving outfit; and the asphaltum paste, composed of 4

parts California refined asphaltum and 1 part crude petroleum, was

boiled in another kettle. The boiling hot paste was poured with ladles

over the hot stone, and the whole mixed over the fire with shovels and

hoes. The asphalt concrete was taken away in hot iron wheelbarrows,

placed in a 4-in. layer rammed and ironed with hot irons. The concrete

was laid in strips 4 to 6 ft. wide, the edges being coated with hot

paste. After the whole reservoir was lined, it was painted with the

asphalt paste, boiled much longer, until when cold it was hard and

brittle, breaking like glass under the hammer. This paste was put on

very hot and ironed down. It should not be more than {1/8}-in. thick or

it will "creep" on slopes of 1½ to 1. After two hot summers and one cold

winter there was not a single crack anywhere in the lining. A mixture of

sand and asphalt will creep on slopes of 1½ to 1, but asphalt concrete

will not. With asphalt at $20 a ton, and labor at $2 a day, the cost was

15 cts. a sq. ft. for 4-in. asphalt concrete. On a high slope Mr.

Stanton recommends making slight berms every 6 ft. to support the

concrete and prevent creeping. Asphalt concrete resists the wear of wind

and water that cuts away granite and iron.

~Base for Mill Floor.~--In constructing 17,784 sq. ft. of tar concrete

base for a mill floor, Mr. C. H. Chadsey used a sand, broken stone and

tar mixture mixed in a mechanical mixer. The apparatus used and the mode

of procedure followed were as follows:

Two parallel 8-in. brick walls 26 ft. long were built 4 ft. apart and 2½

ft. high to form a furnace. On these walls at one end was set a 4×6×2

ft. steel plate tar heating tank. Next to this tank for a space of 4×8

ft. the walls were spanned between with steel plates. This area was used

for heating sand. Another space of 4×8 ft. was covered with 1½ in. steel

rods arranged to form a grid; this space was used for heating the broken

stones. The grid proved especially efficient, as it permitted the hot

air to pass up through the stones, while a small cleaning door at the

ground allowed the screenings which dropped through the grid to be

raked out and added to the mixture. A fire from barrel staves and refuse

wood built under the tank end was sufficient to heat the tar, sand and

stone.

For mixing the materials a Ransome mixer was selected for the reason

that heat could be supplied to the exterior of the drum by building a

wood fire underneath. This fire was maintained to prevent the mixture

from adhering to the mixing blades, and it proved quite effective,

though occasionally they would have to be cleaned with a chisel bar,

particularly when the aggregate was not sufficiently heated before being

admitted to the mixer. A little "dead oil" applied to the discharge

chute and to the shovels, wheelbarrows and other tools effectually

prevented the concrete from adhering to them.

The method of depositing the concrete was practically the same as that

used in laying cement sidewalks. Wood strips attached to stakes driven

into the ground provided templates for gaging the thickness of the base

and for leveling off the surface. The wood covering consisted of a layer

of 2-in. planks, covered by matched hardwood flooring. In placing the

planking, the base was covered with a ¼-in. layer of hot pitch, into

which the planks were pressed immediately, the last plank laid being

toe-nailed to the preceding plank just enough to keep the joint tight.

After a few minutes the planks adhered so firmly to the base that they

could be removed only with difficulty. The hardwood surface was put on

in the usual manner. The prices of materials and wages for the work were

as follows:

Pitch, bulk, per lb. $ 0.0075

Gravel, per cu. yd. 1.50

Spruce sub-floor, per M. ft. B. M. 15.00

Hardwood surface, per M. ft. B. M. 33.00

Laborers, per 10-hour day. 1.50

Foreman, per 10-hour day. 4.00

Carpenters, per 10-hour day. 2.00

At these prices and not including a small administration cost or the

cost of tools and plant, the cost of the floor consisting of 4½ ins. of

concrete, 2 ins. of spruce sub-flooring and 7/8-in. hardwood finish was

as follows per square foot:

Pitch $0.04

Gravel 0.02

Spruce, for sub-floor 0.03

Hardwood for surfacing 0.035

Labor, mixing 0.03

Labor, laying 0.015

Carpenter work 0.025

------

Total per square foot $0.195

CHAPTER VII.

METHODS AND COST OF LAYING CONCRETE IN FREEZING WEATHER.

Reinforced concrete work may be done in freezing weather if the end to

be gained warrants the extra cost. Laboratory experiments show beyond

much doubt that Portland cement concrete which does not undergo freezing

temperatures until final set has taken place, or which, if frozen before

it has set, is allowed to complete the setting process after thawing

without a second interruption by freezing, does not suffer loss of

ultimate strength or durability. These requirements for safety may be

satisfied by so treating the materials or compounding the mixture that

freezing will not occur at normal freezing temperature or else will be

delayed until the concrete has set, by so housing in the work and

artificially treating the inclosed space that its temperature never

falls as low as the freezing point, or, by letting the concrete freeze

if it will and then by suitable protection and by artificial heating

produce and maintain a thawing temperature until set has taken place.

~LOWERING THE FREEZING POINT OF THE MIXING WATER.~--Lowering the freezing

point of the mixing water is the simplest and cheapest method by which

concrete can be mixed and deposited in freezing weather. The method

consists simply in adding some substance to the water which will produce

a brine or emulsion that freezes at some temperature below 32° F.

determined by the substance added and the richness of the admixture. A

great variety of substances may be added to water to produce low

freezing brines, but in concrete work only those may be used that do

little or no injury to the strength and durability of the concrete.

Practice has definitely determined only one of these, namely, sodium

chloride or common salt, though some others have been used successfully

in isolated cases. A point to be borne in mind is that cold retards the

setting of cement and that the use of anti-freezing mixtures emphasizes

this phenomenon and its attendant disadvantages in practical

construction. The accompanying diagram, Fig. 39, based on the

experiments of Tetmajer, show the effect on the freezing point of water

by the admixtures of various substances that have been suggested for

reducing the freezing point of mortar and concrete mixtures.

[Illustration: Fig. 39.--Diagram Showing Effect on Freezing Point of

Water by Admixture of Various Substances.]

~Common Salt (Sodium Chloride).~--The substance most usually employed to

lower the freezing point of water used in concrete is common salt.

Laboratory experiments show that the addition of salt retards the

setting and probably lowers the strength of cement at short periods, but

does not, when not used to excess, injure the ultimate strength. The

amount beyond which the addition of salt begins to affect injuriously

the strength of cement is stated variously by various authorities.

Sutcliffe states that it is not safe to go beyond 7 or 8 per cent. by

weight of the water; Sabin places the safe figures at 10 per cent., and

the same figure is given by a number of other American experimenters. A

number of rules have been formulated for varying the percentage of salt

with the temperature of the atmosphere. Prof. Tetmajer's rule as stated

by Prof. J. B. Johnson, is to add 1 per cent. of salt by weight of the

water for each degree Fahrenheit below 32°. A rule quoted by many

writers is "1 lb. of salt to 18 gallons of water for a temperature of

32° F., and an increase of 1 oz. for each degree lower temperature."

This rule gives entirely inadequate amounts to be effective, the

percentage by weight of the water being about 1 per cent. The familiar

rules of enough salt to make a brine that will "float an egg" or "float

a potato" are likewise untrustworthy; they call respectively, according

to actual tests made by Mr. Sanford E. Thompson, for 15 per cent. and 11

per cent. of salt which is too much, according to the authorities quoted

above, to be used safely. In practice an arbitrary quantity of salt per

barrel of cement or per 100 lbs. of water is usually chosen. Preferably

the amount should be stated in terms of its percentage by weight of the

water, since if stated in terms of pounds per barrel of cement the

richness of the brine will vary with the richness of the concrete

mixture, its composition, etc. As examples of the percentages used in

practice, the following works may be quoted: New York Rapid Transit

Railway, 9 per cent. by weight of the water; Foster-Armstrong Piano

Works, 6 per cent. by weight of the water. In summary, it would seem

that if a rule for the use of salt is to be adopted that of Tetmajer,

which is to add 1 per cent. by weight of the water for each degree

Fahrenheit below 32°, is as logical and accurate as any. It should,

however, be accompanied by the proviso that no more than 10 per cent. by

weight of salt should be considered safe practice, and that if the frost

is too keen for this to avail some other method should be adopted or the

work stopped. It may be taken that each unit per cent. of salt added to

water reduces the freezing temperature of the brine about 1.08° F.; a 10

per cent. salt brine will therefore freeze at 32° - 11° = 21° F. The

range of efficiency of salt as a preventative of frost in mixing and

laying concrete is, obviously, quite limited.

~HEATING CONCRETE MATERIALS.~--Heating the sand, stone and mixing water

acts both to hasten the setting and to lengthen the time before the

mixture becomes cold enough to freeze. At temperatures not greatly below

freezing the combined effects are sufficient to ensure the setting of

the concrete before it can freeze. More specific data of efficiency are

difficult to arrive at. There are no test data that show how long it

takes a concrete mixture at a certain temperature to lose its heat and

become cold enough to freeze at any specific temperature of the

surrounding air, and a theoretical calculation of this period is so

beset with difficulties as to be impracticable. Strength tests of

concrete made with heated materials have shown clearly enough that the

heating has no effect worth mentioning on either strength or

durability. Either the water, the sand, the aggregate or all three may

be heated; usually the cement is not heated but it may be if desired.

~Portable Heaters.~--An ordinary half cylinder of sheet steel set on the

ground like an arch is the simplest form of sand heater. A wood fire is

built under the arch and the sand to be heated is heaped on the top and

sides. The efficiency of this device may be improved by closing one end

of the arch and adding a short chimney stack, but even the very crude

arrangement of sheets of corrugated iron bent to an arc will do good

service where the quantities handled are small. This form of heater may

be used for stone or gravel in the same manner as for sand. It is

inexpensive, simple to operate and requires only waste wood for fuel,

but unless it is fired with exceeding care the sand in contact with the

metal will be burned. The drawings of Fig. 40 show the construction of a

portable heater for sand, stone and water used in constructing concrete

culverts on the New York Central & Hudson River Railroad. This device

weighs 1,200 lbs., and costs about $50.

[Illustration: Fig. 40.--Portable Sand, Stone and Water Heater.]

~Heating in Stationary Bins.~--The following arrangement for heating sand

and gravel in large quantities in bins was employed in constructing the

Foster-Armstrong Piano Works at Rochester, N. Y. The daily consumption

of sand and gravel on this work was about 50 cu. yds. and 100 cu. yds.,

respectively. To provide storage for the sand and gravel, a bin 16 ft.

square in projected plan was constructed with vertical sides and a

sloping bottom as illustrated in Fig. 41. This bin was divided by a

vertical partition into a large compartment for gravel and a small

compartment for sand and was provided with two grates of boiler tubes

arranged as shown. These grates caused V-shaped cavities to be formed

beneath in the gravel and sand. Into these cavities penetrated through

one end of the bin 6-in. pipes from a hot air furnace and 1-in. pipes

from a steam boiler. The hot air pipes merely pass through the wall but

the steam pipes continue nearly to the opposite side of the bin and are

provided with open crosses at intervals along their length. In addition

to the conduits described there is a small pipe for steam located below

and near the bottom of the bin. The hot air pipes connected with a small

furnace and air was forced through them by a Sturtevant No. 6 blower.

The steam pipes connected with the boiler of a steam heating system

installed to keep the buildings warm during construction.

[Illustration: Fig. 41.--Bin Arrangement for Heating Sand and Stone.]

~Other Examples of Heating Materials.~--In the construction of the power

plant of the Billings (Mont.) Water Power Co., practically all of the

concrete work above the main floor level was put in during weather so

cold that it was necessary to heat both the gravel and water used. A

sand heater was constructed of four 15-ft. lengths of 15-in. cast iron

pipe, two in series and the two sets placed side by side. This gave a

total length of 30 ft. for heating, making it possible to use the gravel

from alternate ends and rendering the heating process continuous. The

gravel was dumped directly on the heater, thus avoiding the additional

expense of handling it a second time. The heater pipes were laid

somewhat slanting, the fire being built in the lower end. A 10-ft. flue

furnished sufficient draft for all occasions. With this arrangement it

was possible to heat the gravel to a temperature of 80° or 90° F. even

during the coldest weather. Steam for heating the water was available

from the plant. The temperature at which the concrete was placed in the

forms was kept between 65° and 75° F. This was regulated by the man on

the mixer platform by varying the temperature of the water to suit the

conditions of the gravel. When the ingredients were heated in this

manner it was found advisable to mix the concrete "sloppy," using even

more water than would be commonly used in the so-called "sloppy"

concrete. No difficulty was experienced with temperature cracks if the

concrete, when placed, was not above 75° F. All cracks of this nature

which did appear were of no consequence, as they never extended more

than ½ in. below the surface. The concrete was placed in as large masses

as possible. It was covered nights with sacks and canvas and, when the

walls were less than 3 ft. in width, the outside of the forms was lagged

with tar paper. An air space was always left between the surface of the

concrete and the covering. Under these conditions there was sufficient

heat in the mass to prevent its freezing for several days, which was

ample time for permanent setting.

During the construction in 1902 of the Wachusett Dam at Clinton, Mass.,

for the Metropolitan Water Works Commission the following procedures

were followed in laying concrete in freezing weather: After November 15

all masonry was laid in Portland cement, and after November 28 the sand

and water were heated and salt added in the proportion of 4 lbs. per

barrel of cement. The sand was heated in a bin, 16½×15½×10 ft. deep,

provided with about 20 coils of 2-in. pipe, passing around the inside of

the bin. The sand, which was dumped in the top of the bin and drawn from

the bottom, remained there long enough to become warm. The salt for each

batch of mortar was dissolved in the water which was heated by steam;

steam was also used to thaw ice from the stone masonry. The laying of

masonry was not started on mornings when the temperature was lower than

18° F. above zero, and not even with this temperature unless the day was

clear and higher temperature expected. At the close of each day the

masonry built was covered with canvas.

In the construction of dams for Huronian Company's power development in

Canada a large part of the concrete work in dams, and also in power

house foundations, was done in winter, with the temperature varying from

a few degrees of frost to 15 degrees below zero, and on several

occasions much lower. No difficulty was found in securing good concrete

work, the only precaution taken being to heat the mixing water by

turning a ¾-in. steam pipe into the water barrel supplying the mixer,

and, during the process of mixing, to use a jet of live steam in the

mixer, keeping the cylinder closed by wooden coverings during the

process of mixing. No attempt was made to heat sand or stone. In all the

winter work care was taken to use only cement which would attain its

initial set in not more than 65 minutes.

In constructing a concrete arch bridge at Plano, Ill., the sand and

gravel were heated previous to mixing and the mixed concrete after

placing was kept from freezing by playing a steam jet from a hose

connected with the boiler of the mixer on the surface of the concrete

until it was certain that initial set had taken place. Readings taken

with thermometers showed that in no instance did the temperature of the

concrete fall below 32° F. within a period of 10 or 12 hours after

placing.

From experience gained in doing miscellaneous railway work in cold

weather Mr. L. J. Hotchkiss gives the following:

"For thin reinforced walls, it is not safe to rely on heating the water

alone or even the water and sand, but the stone also must be heated and

the concrete when it goes into the forms should be steaming hot. For

mass walls the stone need not be heated except in very cold weather.

Where concrete is mixed in small quantities the water can be heated by a

wood fire, and if a wood fire be kept burning over night on top of the

piles of stone and sand a considerable quantity can be heated. The fire

can be kept going during the day and moved back on the pile as the

heated material is used. This plan requires a quantity of fuel which in

most cases is prohibitive and is not sufficient to supply a power mixer.

For general use steam is far better.

"A convenient method is to build a long wooden box 8 or 10 in. square

with numerous holes bored in its sides. This is laid on the ground,

connected with a steam pipe and covered with sand, stone or gravel. The

steam escaping through the holes in the box will heat over night a pile

of sand, or sand and gravel, 8 or 10 ft. high. Perforated pipes can be

substituted for boxes. Material can be heated more rapidly if the steam

be allowed to escape in the pile than if it is confined in pipes which

are not perforated. Crushed stone requires much more heat than sand or

sand and gravel mixed because of the greater volume of air spaces. In

many cases material which has already been unloaded must be heated. The

expense of putting steam boxes or pipes under it is considerable. To

avoid this one or more steam jets may be used, the end of the jet pipe

being pushed several feet into the pile of material. If the jets are

connected up with steam hose they are easily moved from place to place.

It is difficult to heat stone in this way except in moderate weather.

"On mass work and at such temperatures as are met with in this latitude

(Chicago, Ill.) it is not usually necessary to protect concrete which

has been placed hot except in the top of the form. This can be done by

covering the top of the form with canvas and running a jet of steam

under it. If canvas is not available boards and straw or manure answer

the purpose. If heat is kept on for 36 hours after completion, this is

sufficient, except in unusually cold weather. The above treatment is all

that is required for reinforced retaining walls of ordinary height. But

where box culverts or arches carrying heavy loads must be placed in

service as soon as possible, the only safe way is to keep the main part

of the structure warm until the concrete is thoroughly hardened. Forms

for these structures can be closed at the ends and stoves or salamanders

kept going inside, or steam heat may be used. The outside may be covered

with canvas or boards, and straw and steam jets run underneath. After

the concrete has set enough to permit the removal of the outer forms of

box culverts, fires may be built near the side walls and the concrete

seasoned rapidly. Where structures need not be loaded until after the

arrival of warm weather, heat may be applied for 36 hours, and the

centering left in place until the concrete has hardened. Careful

inspection of winter concrete should be made before loads are applied.

In this connection it may be noted that concrete which has been partly

seasoned and then frozen, closely resembles thoroughly seasoned

concrete. Pieces broken off with a smooth fracture through all the

stones and showing no frost marks, when thawed out, can be broken with

the hands."

In building Portland cement concrete foundations for the West End St.

Ry., Boston, and the Brooklyn Heights R. R., much of the work was done

in winter. A large watertight tank was constructed, of such size that

three skips or boxes of stone could be lowered into it. The tank was

filled with water, and a jet of steam kept the water hot in the coldest

weather. The broken stone was heated through to the temperature of the

water in a few minutes. One of the stone boxes was then hoisted out, and

dumped on one side of the mixing machine, and then run through the

machine with sand, cement and water. The concrete was wheeled to place

without delay and rammed in 12-in. layers. The heat was retained until

the cement was set. In severely cold weather the sand was heated and the

mixing water also. A covering of hay or gunnysacks may be used.

~COVERING AND HOUSING THE WORK.~--Methods of covering concrete to protect

it from light frosts such as may occur over night will suggest

themselves to all; sacking, shavings, straw, etc., may all be used.

Covering wall forms with tar paper nailed to the studding so as to form

with the lagging a cellular covering is an excellent device and will

serve in very cold weather if the sand and stone have been heated. From

these simple precautions the methods used may range to the elaborate

systems of housing described in the following paragraphs.

~Method of Housing in Dam, Chaudiere Falls, Quebec.~--In constructing a

dam for the water power plant at Chaudiere Falls, P. Q., the work was

housed in. The wing dam and its end piers aggregated about 250 ft. in

length by about 20 ft. in width. A house 100 ft. long and 24 ft. wide

was constructed in sections about 10 ft. square connected by cleats with

bolts and nuts. This house was put up over the wing dam. It was 20 ft.

high to the eaves, with a pitched roof, and the ends were closed up; in

the roof on the forebay side were hatchways with sliding doors along the

whole length. Small entrance doors for the workmen were provided in the

ends of the building. The house was heated by a number of cylindrical

sheet-iron stoves about 18 ins. in diameter by 24 ins. high, burning

coke; thermometers placed at different points in the shed gave warning

to stop work when the temperature fell below freezing, which, however,

rarely occurred. Mixing boards were located in the shed, and concrete,

sand and broken stone were supplied in skipfuls by guy derricks located

in the forebay, which passed the material through the hatchways in the

roof, the proper hatchway being opened for the purpose and quickly

closed. The mortar was first mixed on a board, and then a skip-load of

stone was dumped into the middle of the batch and the whole well mixed.

The water was made lukewarm by introducing a steam-jet into several

casks which were kept full. The sand was heated outside in the forebay

on an ordinary sand heater. The broken stone was heated in piles by a

steam-jet; a pipe line on the ground was made up of short lengths of

straight pipe alternating with T-sections--turned up. The stone was

piled 3 to 4 ft. deep over the pipe and a little steam turned into the

pipe. Several such piles kept going all the time supplied enough stone

for the work; the stone was never overheated, and was moist enough not

to dry out the mortar when mixed with it. In this manner the concreting

was successfully carried on and the wing dam built high enough to keep

high water out of the forebay.

Some danger from freezing was also encountered the next season, when the

last part of the wing dam was being constructed. This work was done when

the temperature was close to freezing, and it became necessary to keep

the freshly placed concrete warm over night. This was done by covering

the work loosely with canvas, under which the nozzle of a steam hose was

introduced. By keeping a little steam going all night the concrete was

easily kept above freezing temperature.

[Illustration: Fig. 42.--Canvas Curtain for Enclosing Open Walls.]

[Illustration: Fig. 43.--Sketch Showing Method of Applying Curtains to

Open Walls.]

~Method of Housing in Building Work.~--The following method of housing in

building work is used by Mr. E. L. Ransome. The feature of the system is

that the enclosing structure is made up of a combination of portable

units which can be used over and over again in different jobs. The

construction is best explained in connection with sketches.

Figure 43 shows a first floor wall column with the wall girder

surmounting it and the connecting floor system. It will be seen that the

open sides are enclosed by canvas curtains and the floor slab is covered

with wood shutters. The curtains are composed of separate pieces so

devised that they may be attached to each other by means of snaps and

eyes; one of these curtain units is shown by Fig. 42. Referring now to

Fig. 43, the curtain A is held by the tying-rings to a continuous

string piece B, the upper portion or flap D being held down by a

metal bar or other heavy object so as to lap over the floor covers E.

The lower edge of the curtain is attached to the string piece C. The

sketch has been made to show how the curtain adjusts itself to irregular

projections such as the supports for a wall girder form; to prevent the

curtain tearing on such projections it is well to cover or wrap the

rough edges with burlap, bagging or other convenient material. The

details of the wooden floor covers are shown by Fig. 44; they are

constructed so as to give a hollow space between them and the floor and

holes are left in the floor slab as at H, Fig. 43, to permit the warm

air from below to enter this hollow space. This warm air is provided by

heating the enclosed story of the building by any convenient adequate

means. In constructing factory buildings, 50×200 ft. in plan at

Rochester, N. Y., Mr. Ransome used a line of 3/4 to 3/8-in. steam pipe

located at floor level and running around all four sides and a similar

line running lengthwise of the building at the center, these pipes

discharging live steam through openings into the enclosed space. In

addition to the steam piping 10 braziers in which coke fires were kept

were scattered around the floor. This equipment kept the enclosed story,

50×100 ft.×13 ft. high, at a temperature of 80° F. and at temperature of

about 40° F. between the floor top and its board covering. The work was

not stopped at any time because of cold and the temperatures outside

ranged from zero to 10° above.

[Illustration: Fig. 44.--Portable Wooden Panels for Covering Floors.]

CHAPTER VIII.

METHODS AND COST OF FINISHING CONCRETE SURFACES.

Good design in concrete as well as in steel, masonry and wood, requires

that the structure shall be good to look at. This means that the

proportions must be good and that the surface finish must be pleasing.

Good proportions are a matter of design but a pleasing surface finish is

a matter of construction. Many, perhaps the majority of, concrete

structures do not have a pleasing surface finish; the surface is

irregular, uneven in texture, and stained or discolored or of lifeless

hue. The reasons for these faults and the possible means of remedying

them are matters that concern the construction engineer and the

contractor.

Imperfections in the surface of concrete are due to one or more of the

following causes: (1) Imperfectly made forms; (2) imperfectly mixed

concrete; (3) carelessly placed concrete; (4) use of forms with dirt or

cement adhering to the boards; (5) efflorescence and discoloration of

the surface after the forms are removed.

~IMPERFECTLY MADE FORMS.~--In well mixed and placed concrete the film of

cement paste which flushes to the surface will take the impress of every

flaw in the surface of the forms. It will even show the grain marks in

well dressed lumber. From this it will be seen how very difficult it is

so to mold concrete that the surface will not bear evidence of the mold

used. The task is impracticable of perfect accomplishment and the degree

of perfection to which it can be carried depends upon the workmanship

expended in form construction. Forms with a smooth and even surface are

difficult and expensive to secure. It is impracticable in the first

place to secure lagging boards dressed to exact thickness and in the

second place it is impracticable to secure perfect carpenter work;

joints cannot be got perfectly close and a nail omitted here or there

leaves a board free to warp. From this point on the use of imperfectly

sized lumber and careless carpentry can go to almost any degree of

roughness in the form work. Only approximately smooth and unmarked

concrete surfaces can be secured in plain wooden forms and this only

with the very best kind of form construction. So much for the

limitations of form work in the matter of securing surface finish. These

limitations may be reduced in various ways. Joint marks may be

eliminated wholly or partly by pointing the joints with clay or mortar

or by pasting strips of paper or cloth over them, or the whole surface

of the lagging can be papered; by the use of oiled paper there will be

little trouble from the paper sticking. Grain marks and surface

imperfections can be reduced by oiling the lumber so as to fill the

pores or by first oiling and then filling the coat of oil with fine sand

blown or cast against the boards.

The preceding remarks are of course based on the assumption that as

nearly as possible a smooth and even surface finish is desired. If

something less than this is sufficient, and in many cases it is, form

produced surface defects become negligible in the proportion that they

do not exceed the standards of roughness and irregularity considered

permissible by the engineer and these standards are individual with the

engineer; what one considers excessive roughness and irregularity

another may consider as amply even and smooth. The point to be kept in

mind is that beyond a certain state of evenness and regularity form

produced surfaces are impracticable to obtain, because to construct

forms of the necessary perfection to obtain them costs far more than it

does to employ special supplementary finishing processes.

Surface blemishes due to dirt or cement adhering to the form boards have

no excuse if the engineer or contractor cares to avoid them. It is a

simple matter to keep the lagging clean and free from such

accumulations.

~IMPERFECT MIXING AND PLACING.~--Imperfectly mixed and placed concrete

gives irregularly colored, pitted and honeycombed surfaces with here a

patch of smooth mortar and there a patch of exposed stone. Careful

mixing and placing will avoid this defect, or all chance of it may be

eliminated by using surface coatings of special mixtures. There is no

great difficulty, however, in obtaining a reasonably homogeneous surface

with concrete; the task merely requires that the mixing shall be

reasonably uniform and homogeneous and that in placing this mixture the

spading next to the lagging shall be done in such a way as to pull the

coarse stones back and flush the mortar to the surface. Spading forks

are excellent for this purpose. A better tool is a special spade made

with a perforated blade; this special spade costs $3.

~EFFLORESCENCE.~--Efflorescence is the term applied to the whitish or

yellowish accumulations which often appear on concrete surfaces.

"Whitewash" is another name given to these blotches. Efflorescence is

due to certain salts leaching out of the concrete and accumulating into

thin layers where the water evaporates on the surface. These salts are

most probably sulphates of calcium and magnesium, both of which are

contained in many cements and both of which are slightly soluble in

water. Efflorescence is very erratic in its appearance. Some concretes

never exhibit it; in some it may not appear for several years, and in

others it shows soon after construction and may appear in great

quantities. The most effective way to prevent efflorescence would

naturally be to use cements entirely free from sulphates, chlorides or

whatever other soluble salts are the cause of the phenomenon, but the

likelihood of engineers resorting to the trouble of such selection,

except in rare instances, is not great, even if they knew what cements

to select, so that other means must be sought. The most common place for

efflorescence to appear in walls is at the horizontal junction of two

days' work or where a coping is placed after the main body of the wall

has been completed. The reason of this seems to be that the salt

solutions seep down through the concrete until they strike the nearly

impervious film of cement that forms on the top surface of the old

concrete before the new is added, and then they follow along this

impervious film to the face of the wall. The authors have suggested that

this cause might be remedied by ending the day's work by a layer whose

top has a slight slope down toward the rear of the wall or perhaps by

placing all the concrete in similarly sloping layers. Mr. C. H.

Cartlidge is authority for the statement that this leaching at joints

can be largely done away with by the simple process of washing the top

surface of concrete which has been allowed to set over night by

scrubbing it with wire brushes in conjunction with thorough flushing

with a hose. But efflorescence frequently appears on the faces of walls

built without construction joints and in which a wet concrete is puddled

in and not tamped in layers, and here other means are obviously

essential. Waterproofing the surface of the wall should be effective so

long as the waterproofing lasts; indeed one of the claims made for some

of these waterproofing compounds is that efflorescence is prevented. The

various waterproofing mixtures capable of such use will be found

described in Chapter XXV. Failing in any or all of these methods of

preventing efflorescence the engineer must resort to remedial measures.

The saline coating must be scraped, or chipped, or better, washed away

with acids.

Efflorescence was removed from a concrete bridge at Washington, D. C, by

using hydrochloric (muriatic) acid and common scrubbing brushes; 30

gals. of acid and 36 scrubbing brushes were used to clean 250 sq. yds.

of concrete. The acid was diluted with 4 or 5 parts water to 1 of acid;

water was constantly played with a hose on the concrete while being

cleaned to prevent penetration of the acid. One house-front cleaner and

5 laborers were employed, and the total cost was $150, or 60 cts. per

sq. yd. This high cost was due to the difficulty of cleaning the

balustrades. It is thought that the cost of cleaning the spandrels and

wing walls did not exceed 20 cts. per sq. yd. The cleaning was perfectly

satisfactory. An experiment was made with wire brushes without acid, but

the cost was $2.40 per sq. yd. The flour removed by the wire brushes was

found by analysis to be silicate of lime. Acetic acid was tried in place

of muriatic, but required more scrubbing.

~SPADED AND TROWELED FINISHES.~--With wet-concrete and ordinarily good

form construction a reasonably good surface appearance can be obtained

by spading and troweling. For doing the spading a common gardener's hoe,

straightened out so that the blade is nearly in line with the handle

will do good work. The blade of the tool is pushed down next to the

lagging and the stone pulled back giving the grout opportunity to flush

to the face. Troweling, that is troweling without grout wash, requires,

of course, that the concrete be stripped before it has become too hard

to be worked. As troweling is seldom required except for tops of copings

and corners it is generally practicable to bare the concrete while it is

still fairly green. In this condition the edges of copings, etc., can be

rounded by edging tools such as cement sidewalk workers use.

~PLASTER AND STUCCO FINISH.~--The ordinary concrete surface with a

film-like cement covering will not hold plaster or stucco. To get proper

adhesion the concrete surface must be scrubbed, treated with acid or

tooled before the plaster or stucco is applied and this makes an

expensive finish since either of the preliminary treatments constitutes

a good finish by itself. When a coarse grained facing is made of very

dry mixtures, as described in a succeeding section, it has been made to

hold plaster very well on inside work. In general plaster and stucco

finishes can be classed as uncertain even when the concrete surface has

been prepared to take them, and when the concrete has not been so

prepared such finishes can be classed as absolutely unreliable.

~MORTAR AND CEMENT FACING.~--Where a surface finish of fine texture or of

some special color or composition is desired the concrete is often faced

with a coat of mortar or, sometimes, neat cement paste or grout. Mortar

facing is laid from 1 to 2 ins. thick, usually 1½-ins., the mortar being

a 1-1, 1-2 or 1-3 mixture and of cement and ordinary sand where no

special color or texture is sought. This facing often receives a future

special finish as described in succeeding sections, but it is more

usually used as left by the forms or at best with only a troweling or

brushing with grout. Engineers nearly always require that the mortar

facing and the concrete backing shall be constructed simultaneously.

This is accomplished by using facing forms, two kinds of which are shown

by Figs. 45 and 46. In use the sheet steel plates are placed on edge the

proper distance back of the lagging and the space between them and the

lagging is filled with the facing mortar. The concrete backing is then

filled in to the height of the plate, which is then lifted vertically

and the backing and facing thoroughly bonded by tamping them together.

The form shown by Fig. 46, though somewhat the more expensive, is the

preferable one, since the attached ribs keep the plate its exact

distance from the lagging without any watching by the men, while the

flare at the top facilitates filling. The facing mortar has to be rather

carefully mixed; it must be wet enough to work easily and completely

into the narrow space and yet not be so soft that in tamping the backing

the stones are easily forced through it. Also since the facing cannot

proceed faster than the backing the mortar has to be mixed in small

batches so that it is always fresh. A cubic yard of mortar will make 216

sq. ft. of 1½-in. facing. Cement facing is seldom made more than 1 in.

thick. If placed as a paste the process is essentially the same as for

placing mortar. When grout is used a form is not used; place and tamp

the concrete in 6 to 8-in layers, then shove a common gardener's spade

down between the concrete and the lagging and pull back the concrete

about an inch and pour the opening full of grout and withdraw the spade.

If this work is carefully done there will be very few stones showing

when the forms are removed. When stiff pastes or mortars are used the

contractor often places the facing by plastering the lagging just ahead

of the concreting; this process requires constant watching to see that

the plaster coat does not slough or peel off before it is backed up with

concrete.

[Illustration: Fig. 45.--Form for Applying Cement Facing (Massachusetts

Highway Commission).]

[Illustration: Fig. 46.--Form for Applying Cement Facing (Illinois

Central R. R.).]

~SPECIAL FACING MIXTURES FOR MINIMIZING FORM MARKS.~--The ordinary facing

mixture of mortar or cement is so fine grained and plastic that it

readily takes the impress of every irregularity in the form lagging;

where a particularly good finish is desired this makes necessary

subsequent finishing treatments. To avoid these subsequent treatments

and at the same time to reduce the form marks, special facing mixtures,

which will not take the imprint of and which will minimize rather than

exaggerate every imperfection in the forms, have been used with very

considerable success in the concrete work done for the various Chicago,

Ill., parks. The mixture used consists usually of 1 part cement, 3 parts

fine limestone screenings and 3 parts ¾-in. crushed limestone; these

materials are mixed quite dry so no mortar will flush to the surface

when rammed hard. With moderately good form work the imprint of the

joints is hardly noticeable and grain marks do not show at all. For thin

building walls the special mixture is used throughout the wall, but for

more massive structures it is used only for the facing.

~GROUT WASHES.~--Grout finishes serve only to fill the small pits and

pores in the surface coating; cavities or joint lines, if any exist,

must be removed by plastering or rubbing before the grout is applied or

else by applying the grout by rubbing. In ordinary work the grout is

applied with a brush after the holes have been plastered and the joint

marks rubbed down. The grout to be applied with a brush should be about

the consistency of whitewash; a 1 cement 2 sand mixture is a good one.

Where a more perfect finish of dark color is desired the grout of neat

cement and lampblack in equal parts may be applied as follows: Two coats

with a brush, the second coat after the first has dried, and one coat by

sweeping with a small broom. The broom marks give a slightly rough

surface. Instead of a liquid grout a stiff grout or semi-liquid mortar

applied with a trowel or float can be used. In this case the grout

should be applied in a very thin coat and troweled or floated so that

only the pores are filled and no body of mortar left on the surface or

else it will scale off. A more expensive but very superior grout finish

is obtained by rubbing and scouring the wet grout into the surface with

cement mortar bricks, carborundum bricks, or such like abrasive

materials. A 1 cement 1 sand mortar brick, with a handle molded into it,

and having about the dimensions of an ordinary building brick makes a

good tool for rubbing down joint marks as well as for applying grout.

[Illustration: Fig. 47.--Concrete Baluster Finished by Scrubbing and

Washing.]

~FINISHING BY SCRUBBING AND WASHING.~--A successful finish for concrete

structures consists in removing the forms while the concrete is green

and then scrubbing the surface with a brush and water until the film of

cement is removed and the clean sand or stone left exposed. This method

has been chiefly used in concrete work done by the city of Philadelphia,

Pa., Mr. Henry M. Quimby, Bridge Engineer. Figure 47 shows an example of

scrubbed finish, but of course the texture or color of the surface will

vary with the character of the face mixture and the hue of the sand or

chips used. Warm tones can be secured by the use of crushed brick or red

gravel; a dark colored stone with light sand gives a color much

resembling granite; fine gravel or coarse sand gives a texture like

sandstone. In much of this work done in Philadelphia a mixture composed

of 1 part cement, 2 parts bank sand and 3 parts crushed and cleaned

black, slaty shale from 3/8 to 1/4 in. in size, has been used with good

results both in appearance and in durability. The scrubbing is done

with an ordinary house scrubbing brush at the same time flushing the

concrete with water from a sponge or bucket or, preferably, from a hose.

In general the washing is done on the day following the placing of the

concrete but the proper time depends upon the rapidity with which the

concrete sets. In warm weather 24 hours after placing is generally about

right, but in cold weather 48 hours may be required, and in very cold

weather the concrete has been left to set a week and the scrubbing has

been successful. With the concrete in just the proper condition a few

turns of the brush with plenty of water will clean away the cement, but

if a little too hard wire brushes must be used and if still harder a

scouring brick or an ordinary brick with sand is necessary to cut the

cement film. The process requires that the forms shall be so constructed

that the lagging can be removed when the concrete has reached the proper

age for treatment. Mr. Quimby sets the studs 8 to 12 ins. from the face

and braces the lagging boards against them by cleats nailed so as to be

easily loosened. His practice is to use boards in one width the full

depth of the course and to nail a triangular bead strip to the face at

each edge. These bead strips mark the joints between courses as shown by

Fig. 48. When a "board" is taken off it is cleaned and oiled and reset

for a new course by inserting the bottom bead strip in the half

indentation left by the top bead in the concrete. This is, of course,

for work of such size that one course is a day's work of concreting. In

such work, two carpenters with perhaps one helper will remove a course

of "boards" say 100 ft. long in from 4 to 8 hours. While forms of the

kind described cost more to construct there is a saving by repeated

re-use of the lagging boards. The indentations or beads marking the

courses serve perfectly to conceal the construction joints. The cost of

scrubbing varies with the hardness of the concrete; when in just the

right condition for effective work one man can scrub 100 sq. ft. in an

hour; on the other hand it has taken one man a whole day to scrub and

scour the same area when the concrete was allowed to get hard.

[Illustration: Fig. 48.--Concrete Abutment with Scrubbed Finish and

Course Marks.]

~FINISHING BY ETCHING WITH ACID.~--The acid etched or acid wash process of

finishing concrete consists in first washing the surface with an acid

preparation to remove the surface cement and expose the sand and stone,

then with an alkaline solution to remove all free acid, and finally,

with clear water in sufficient volume to cleanse and flush the surface

thoroughly. The work can be done at any time after the forms are removed

and does not require skilled labor; any man with enough judgment to

determine when the etching has progressed far enough can do the work.

This process has been very extensively used in Chicago by the South Park

Commission, Mr. Linn White, Engineer. In this work the concrete is faced

with a mixture of cement, sand and stone chips, any stone being used

that is not affected by acid. Limestone is excluded. Where some color is

desired the facing can be mixed with mineral pigments or with colored

sand or stone chips. This acid wash process has been patented, the

patentees being represented by Mr. J. K. Irvine, Sioux City, Ia.

~TOOLING CONCRETE SURFACES.~--Concrete surfaces may be bush-hammered or

otherwise tool finished like natural stone, exactly the same methods and

tools being used. Tooling must wait, however, until the concrete has

become fairly hard. As the result of his experience in tooling some

43,000 sq. ft. of concrete, Mr. W. J. Douglas states that the concrete

should be at least 30 days old and, preferably, 60 days old, if

possible, when bush-hammered. There is a great variation in the costs

given for tooling concrete. Mr. C. R. Neher states that a concrete face

can be bush-hammered by an ordinary laborer at the rate of 100 sq. ft.

in 10 hours or at a cost of 1½ cts. per square foot with wages at 15

cts. per hour. Mr. E. L. Ransome states that bush-hammering costs from

1½ to 2½ cts. per square foot, wages of common laborers being 15 cts.

per hour. The walls of the Pacific Borax Co.'s factory at Bayonne, N.

J., were dressed by hand at the rate of 100 to 200 sq. ft. per man per

day; using pneumatic hammer one man was able to dress from 300 to 600

sq. ft. per day. In constructing the Harvard Stadium the walls were

dressed with pneumatic hammers fitted with a tool with a saw-tooth

cutting blade like an ice chopper. Men timed by one of the authors on a

visit to this work were dressing wall surface at the rate of 50 sq. ft.

per hour, but the contractor stated that the average work per man per

day was 200 sq. ft. Common laborers were employed. The average cost of

bush-hammering some 43,000 sq. ft. of plain and ornamental blocks for

the Connecticut Avenue Bridge at Washington, D. C, was 26 cts. per

square foot. Both pneumatic tools and hand tooling were employed and the

work of both is lumped in the above cost, but hand tooling cost about

twice as much as machine tooling. The work was done by high-priced men,

foremen stone cutters at $5 per day and stone cutters at $4 per day.

Moreover a grade of work equal to the best bush-hammered stone work was

demanded. Full details of the cost of this work are given in Chapter

XVII. Mr. H. M. Quimby states that the cost of tooling concrete runs

from 3 cts. to 12 cts. per square foot, according to the character and

extent of the work and the equipment.

~GRAVEL OR PEBBLE SURFACE FINISH.~--An effective variation of the ordinary

stone concrete surface is secured by using an aggregate of rounded

pebbles of nearly uniform size and by scrubbing or etching remove the

cement enough to leave the pebbles about half exposed at the surface. In

constructing a bridge at Washington, D. C, the concrete was a 1-2-5

gravel mixture of 1½ to 2-in. pebbles for the spandrels and arch ring

face and of 1-in. pebbles for the parapet walls. The forms were removed

while the concrete was still green and the cement scrubbed from around

the faces and sides of the pebbles using wire brushes and water. Tests

showed that at 12 hours age the concrete was not hard enough to prevent

the pebbles from being brushed loose and that at 36 hours age it was too

hard to permit the mortar to be scrubbed away without excessive labor;

the best results were obtained when the concrete was about 24 hours old.

~COLORED FACING.~--Where occasion calls for concrete of a color or tint

other than is obtained by the use of the ordinary materials either an

aggregate of a color suitable for the purpose may be used or the mixture

may be colored by the addition of some mineral pigment. The first method

is by all odds the preferable one; it gives a color which will endure

for all time and it in no way injures the strength or durability of the

concrete. Mineral pigments may be secured from any of several well-known

firms who make them for coloring concrete, and they may be had in almost

every shade. Directions for using these colors can be had from the

makers. All but a very few of these mineral colors injure the strength

and durability of the concrete if used in amounts sufficient to produce

the desired color and all of them fade in time. The best method of

producing a colored mortar or concrete facing is to mix the cement with

screenings produced by crushing a natural stone of the desired color.

CHAPTER IX.

METHODS AND COST OF FORM CONSTRUCTION.

Concrete being a plastic material when deposited requires molds or forms

to give it the shape required and to maintain it in that shape until it

has hardened to sufficient strength to require no exterior support. The

material used in constructing forms is wood. Beyond the use of metal

molds for building blocks for sewer construction and for ornamental and

a few architectural shapes, iron and steel are used in form construction

only as ties and clamps to hold parts of wood forms together--except in

rare instances. A discussion of form construction, therefore, is

essentially a discussion of wood forms.

Before taking up this discussion, however, attention deserves to be

called to the opportunities for the development of metal forms. Lumber

is costly and is growing more scarce and costly all the time. A

substitute which can be repeatedly used and whose durability and salvage

value are great presents itself in steel if only a system of form units

can be devised which is reasonably adjustable to varying conditions.

Cylindrical steel column molds have been used to some extent and are

discussed in Chapter XIX. In Chapter XVI we describe a steel form for

side walls of a tunnel lining. In some building work done in the

northwest corrugated steel panels or sheets have been used as lagging

for floor slab centers. A number of styles of metal forms or centers for

sewer and tunnel work have been devised and used and are discussed in

Chapter XXI. Despite this considerable use of metal for special forms

nothing approaching its general use like wood has been attempted, and

the field lies wide open for invention.

The economics of form construction deserve the most serious attention of

the engineer and contractor. It is seldom that form work, outside of

very massive foundation construction, costs less than 50 cts. per cubic

yard of concrete in place, and it is not unusual in the more complex

structures for it to cost $5 per cubic yard of concrete in place. These

costs include the cost of materials and of framing, handling and

removing the forms but they do not embrace extremely high or low costs.

It is evident without further demonstration that time spent in planning

economic form construction for any considerable job of concrete work is

time spent profitably.

In the following sections we review the general considerations which

enter into all form work. Specific details of construction and specific

costs of form work are given in succeeding chapters where each class of

concrete work is discussed separately. This chapter is intended

principally to familiarize the reader with general principles governing

form work.

~EFFECT OF DESIGN ON FORM WORK.~--The designing engineer can generally aid

largely in reducing the cost of form work if he will. This is

particularly true in building work in which, also, form costs run high.

By arranging his beam spacing and sizes with a little care he will

enable the contractor to use his forms over and over and thus greatly

reduce the expense for lumber. In the same way columns may be made of

dimensions which will avoid frequent remaking of column forms. Panel

recesses in walls may be made the thickness of a board or plank, instead

of some odd depth that will require a special thickness of lumber, or

beams may be made of such size that certain dimension widths of lumber

can be used without splitting. In general, carpenter work costs more

than concrete and where a little excess concrete may be contributed to

save carpenter work it pays to contribute it. The figures given in

Chapter XIX, showing the reduction in lumber cost coming from using the

same material over a second or third time, should be studied in this

connection. The leading firms of engineering-contractors which both

design and construct reinforced concrete buildings fully realize these

opportunities and take advantage of them, but the general practitioner,

particularly if he be an architect, does not do so. The authors have

personal knowledge of one building in which a slight change in spacing

and dimensions of beams--a change that would have been of no

architectural or structural significance--would have reduced the

successful contractor's bid for the work by $10,000. The designing

engineer should hold it as a cardinal point in design that form work,

and we will add here reinforcement also, should so far as possible be

made interchangeable from bay to bay and from floor to floor.

~KIND OF LUMBER.~--The local market and the character of the work

generally determine the kind of lumber to be used for forms. The

hardwoods are out of the question for form construction because they

cost too much and are too hard to work. Among the soft woods white pine

costs too much for general use and hemlock is unreliable when exposed to

the weather. This reduces the list generally available to spruce, Norway

pine and the southern pines. Neither green nor kiln-dried lumber is so

good as partially dry stuff, since the kiln-dried lumber swells and

crushes or bulges the joints and green lumber does not swell enough to

close the joints. Forms have to withstand, temporarily, very heavy

loads, therefore, knots, shakes and rot must be watched after. The

choosing of good lumber is a simple process and the contractor who wants

to be able to rely on his forms will look after it carefully, without

going to extremes which the work does not warrant.

~FINISH AND DIMENSIONS OF LUMBER.~--Dressing the lumber serves four

important purposes: It permits the forms to be constructed more nearly

true to line and surface; it permits tighter joint construction; it

gives a smoother surface finish to the concrete, and it facilitates the

removal and cleaning of the forms. Undressed lumber may be used for the

backs of walls and abutments, for work below ground and wherever a

smooth and true surface is unimportant; there are some contractors,

however, who prefer lumber dressed on one side even for these purposes

because of the smaller cost of cleaning. For floor and wall forms the

lumber should always be dressed on one side; where the work is very

particular both sides should be dressed, and in special cases the sides

of the joists or studs against which the lagging lies may be dressed.

For ordinary work a square edge finish does well enough but for fine

face work a tongue and groove or bevel edge finish is preferable. The

tongue and groove finish gives a somewhat tighter joint on first laying

but it does not take up swelling or resist wear so well as the bevel

edge finish.

When ordering new lumber for forms the contractor will save much future

work and waste if he does it from plans. Timber cut to length and width

to go directly into the forms reduces both mill and carpenter work on

the site, and in many cases it can be so ordered if ordered from plans.

Waste is another item that is reduced by ordering from plans; with

lumber costing its present prices crop ends run into money very rapidly.

When old lumber from a previous job is to be used the contractor can

only make the best of his stock, but even here form plans will result in

saving. Sort and pile the old lumber according to sizes and make a

schedule of the quantity of each size on hand; this schedule in the

hands of the man who designs the forms and of the head carpenter will

materially reduce waste and carpenter work. It is often possible

especially in making concrete foundations for frame buildings to use

lumber for forms which is subsequently used for floor beams, etc., in

the building.

Contractors differ greatly in their ideas of the proper thickness of

lumber to use for various parts of form work. Generally speaking 1¼ to

2-in. stuff is used for wall lagging held by studding and 1-in. stuff

when built into panels; for floor lagging 1-in stuff with joists spaced

up to 24 ins. or when built into panels; for column lagging 1¼ to 2-in.

stuff; for sides of girders 1, 1¼, 1½ and 2-in. stuff are all used; and

for bottoms of girders, 1½ and 2-in. stuff. These figures are by no

means invariable as a study of the numerous examples of actual form work

given throughout this book will show.

~COMPUTATION OF FORMS.~--If the minimum amount of lumber consistent with a

given deflection is to be used in form work the sizes and spacing of the

supporting members must be actually computed for the loading. As a

practical matter of fact the amount of material used and the arrangement

of the supports are often subject to requirements of unit construction,

clearance, staging, etc., which supersede the matter of economical

adaptation of material to loading. The designing of form work is at

best, therefore, a compromise between rules of thumb and scientific

calculation. In wall work empirical methods are nearly always followed.

In girder and floor slab work, on the other hand, design is commonly

based on computation.

In the matter of loads the general practice is to assume the weight of

concrete as a liquid at some amount which it is considered will also

cover the weight of men, barrows, runways and current construction

materials. The assumed weights vary. One prominent engineering firm

assumes the load to be the dead weight of concrete as a liquid and the

load due to placing and specifies that the forms shall be designed to

carry this load without deflection. Mr. W. J. Douglas, Engineer of

Bridges, Washington, D. C, assumes for lateral thrust on wall forms that

concrete is a liquid of half its own weight, or 75 lbs. per cu. ft. Mr.

Sanford E. Thompson, Consulting Engineer, Newton Highlands, Mass.,

assumes for dead load, weight of concrete including reinforcement as 154

lbs. per cu. ft., and for live load, 75 lbs. per sq. ft. on slabs and 50

lbs. per sq. ft. in figuring beam and girder forms and struts.

The assumed safe stresses in form work may be taken somewhat higher than

is usual in timber construction, because of the temporary character of

the load. In calculating beams the safe extreme fiber stress may be

assumed at 750 lbs. per sq. in. The safe stress in pounds per square

inch for struts or posts is shown by Table XV, compiled by Mr. Sanford

E. Thompson. The sizes of struts given are those most commonly used in

form work.

TABLE XV.--SAFE STRENGTH OF TIMBER STRUTS FOR FRAME WORK.

--Dimensions of Strut.--

Length Strut. 3×4-in. 4×4-in. 6×6-in. 8×8-in.

Feet. Lbs. Lbs. Lbs. Lbs.

14 ..... 700 900 1,100

12 600 800 1,000 1,200

10 700 900 1,100 1,200

8 850 1,050 1,200 1,200

6 1,000 1,200 1,200 1,200

In using this table it must be borne in mind that bracing both ways

reduces the length of a long strut. For example, if a strut 24 ft. long

be divided into three panels by bracing the length of strut so far as

the table is concerned is 8 ft.

As stated above wall forms are rarely computed. Experience has shown

that the maximum spans of various thicknesses of lagging between

supports are: 1-in. boards, 24 ins.; 1½-in. plank, 4 ft., and 2-in.

plank, 5 ft. Studding will vary in size from 2×4 to 4×6 ins., strutted

and braced horizontally to meet conditions. Column forms, like wall

forms, are rarely computed, yokes being spaced 2 ft. apart for 1¼-in.

lagging up to 3 to 3½ ft. apart for 2-in. lagging.

Floor forms, including girder and slab forms, are computed on the basis

of a maximum deflection and not on the basis of strength. Sagging forms

are liable to rupture the beam or slab. The amount of deflection

considered allowable varies from no deflection up to 3/8 to ½ in.

Assuming the deflection, permissible thickness of the timber necessary

to carry the load is determined by the formulas:

d = 5 W l³ ÷ 384 E I (1)

and

bh³

I = --- (2)

12

Formula (1) is the familiar one for computing deflection for a beam

supported (not fixed) at the ends. Mr. Sanford F. Thompson suggests

using the constant {3/384}, which is an approximate mean between {1/384}

that for beams with fixed ends and {5/384} that for beams with ends

supported. Formula (1) then becomes

d = 3 W l³ ÷ 384 E I,

in which as above:

d = maximum deflection in inches.

W = total load on plank or joist.

l = length between supports in inches.

E = modulus of elasticity of lumber.

I = moment of inertia of cross-section.

b = breadth of lumber.

h = depth of lumber.

The deflection, d, being assumed formula (1) is solved for I, moment

of inertia. Substituting the value of I in formula (2) we can readily

estimate the size of joist or thickness of plank to use.--For spruce,

yellow pine and the other woods commonly used in form work E may be

taken equal to 1,300,000 lbs. per sq. in.