STRUCTURES 667
Impervious Concrete Mixtures--Star Stetten Cement--Medusa Waterproofing
Compound--Novoid Waterproofing Compound--Impermeable Coatings and
Washes: Bituminous Coatings; Szerelmey Stone Liquid Wash; Sylvester
Wash; Sylvester Mortars; Hydrolithic Coating; Cement Mortar Coatings;
Oil and Paraffine Washes--Impermeable Diaphragms; Long Island R. R.
Subway; New York Rapid Transit Subway.
Concrete Construction Methods and Cost
CHAPTER I.
METHODS AND COST OF SELECTING AND PREPARING MATERIALS FOR CONCRETE.
Concrete is an artificial stone produced by mixing cement mortar with
broken stone, gravel, broken slag, cinders or other similar fragmentary
materials. The component parts are therefore hydraulic cement, sand and
the broken stone or other coarse material commonly designated as the
aggregate.
CEMENT.
At least a score of varieties of hydraulic cement are listed in the
classifications of cement technologists. The constructing engineer and
contractor recognize only three varieties: Portland cement, natural
cement and slag or puzzolan cement. All concrete used in engineering
work is made of either Portland, natural or slag cement, and the great
bulk of all concrete is made of Portland cement. Only these three
varieties of cement are, therefore, considered here and they only in
their aspects having relation to the economics of construction work. For
a full discussion of the chemical and physical properties of hydraulic
cements and for the methods of determining these properties by tests,
the reader is referred to "Practical Cement Testing," by W. Purves
Taylor.
~PORTLAND CEMENT.~--Portland cement is the best of the hydraulic cements.
Being made from a rigidly controlled artificial mixture of lime, silica
and alumina the product of the best mills is a remarkably strong,
uniform and stable material. It is suitable for all classes of concrete
work and is the only variety of hydraulic cement allowable for
reinforced concrete or for plain concrete having to endure hard wear or
to be used where strength, density and durability of high degree are
demanded.
~NATURAL CEMENT.~--Natural cement differs from Portland cement in degree
only. It is made by calcining and grinding a limestone rock containing
naturally enough clayey matter (silica and alumina) to make a cement
that will harden under water. Owing to the imperfection and irregularity
of the natural rock mixture, natural cement is weaker and less uniform
than Portland cement. Natural cement concrete is suitable for work in
which great unit strength or uniformity of quality is not essential. It
is never used for reinforced work.
~SLAG CEMENT.~--Slag cement has a strength approaching very closely that
of Portland cement, but as it will not stand exposure to the air slag
cement concrete is suitable for use only under water. Slag cement is
made by grinding together slaked lime and granulated blast furnace slag.
~SIZE AND WEIGHT OF BARRELS OF CEMENT.~--The commercial unit of
measurement of cement is the barrel; the unit of shipment is the bag. A
barrel of Portland cement contains 380 lbs. of cement, and the barrel
itself weighs 20 lbs.; there are four bags (cloth or paper sacks) of
cement to the barrel, and the regulation cloth sack weighs 1½ lbs.
The size of cement barrels varies, due to the differences in weight of
cement and to differences in compacting the cement into the barrel. A
light burned Portland cement weighs 100 lbs. per struck bushel; a heavy
burned Portland cement weighs 118 to 125 lbs. per struck bushel. The
number of cubic feet of packed Portland cement in a barrel ranges from 3
to 3½. Natural cements are lighter than Portland cement. A barrel of
Louisville, Akron, Utica or other Western natural cement contains 265
lbs. of cement and weighs 15 lbs. itself; a barrel of Rosendale or other
Eastern cement contains 300 lbs. of cement and the barrel itself weighs
20 lbs. There are 3-¾ cu. ft. in a barrel of Louisville cement. Usually
there are three bags to a barrel of natural cement.
As stated above, the usual shipping unit for cement is the bag, but
cement is often bought in barrels or, for large works, in bulk. When
bought in cloth bags, a charge is made of 10 cts. each for the bags,
but on return of the bags a credit of 8 to 10 cts. each is allowed.
Cement bought in barrels costs 10 cts. more per barrel than in bulk, and
cement ordered in paper bags costs 5 cts. more per barrel than in bulk.
Cement is usually bought in cloth sacks which are returned, but to get
the advantage of this method of purchase the user must have an accurate
system for preserving, checking up and shipping the bags.
Where any considerable amount of cement is to be used the contractor
will find that it will pay to erect a small bag house or to close off a
room at the mixing plant. Provide the enclosure with a locked door and
with a small window into which the bags are required to be thrown as
fast as emptied. One trustworthy man is given the key and the task of
counting up the empty bags each day to see that they check with the bags
of cement used. The following rule for packing and shipping is given by
Gilbreth.[A]
[Footnote A: "Field System," Frank B. Gilbreth. Myron C. Clark
Publishing Co., New York and Chicago.]
"Pack cement bags laid flat, one on top of the other, in piles of 50.
They can then be counted easily. Freight must be prepaid when cement
bags are returned and bills of lading must be obtained in duplicate or
credit cannot be obtained on shipment."
The volumes given above are for cement compacted in the barrel. When the
cement is emptied and shoveled into boxes it measures from 20 to 30 per
cent more than when packed in the barrel. The following table compiled
from tests made for the Boston Transit Commission, Mr. Howard Carson,
Chief Engineer, in 1896, shows the variation in volume of cement
measured loose and packed in barrels:
Per cent
Brand Vol. Barrel Vol. Packed Vol. Loose Increase
Portland. cu. ft. cu. ft. cu. ft. in bulk
Giant 3.5 3.35 4.17 25
Atlas 3.45 3.21 3.75 18
Saylors 3.25 3.15 4.05 30
Alsen 3.22 3.16 4.19 33
Dyckerhoff 3.12 3.03 4.00 33
Mr. Clarence M. Foster is authority for the statement that Utica cement
barrels measure 16-1/4 ins. across at the heads, 19½ ins. across the
bilge, and 25-3/4 ins. in length under heads, and contain 3.77 cu. ft.
When 265 lbs. of Utica natural hydraulic cement are packed in a barrel
it fills it within 2½ ins. of the top and occupies 3.45 cu. ft., and
this is therefore the volume of a barrel of Utica hydraulic cement
packed tight.
In comparative tests made of the weights and volumes of various brands
of cements at Chicago in 1903, the following figures were secured:
Vol. per Weight per Weight per
bbl., cu. ft. bbl., lbs. cu. ft.
Brand. Loose. Gross. Net. Loose, lbs.
Dyckerhoff 4.47 395 369.5 83
Atlas 4.45 401 381 85.5
Alpha 4.37 400.5 381 86.5
Puzzolan 4.84 375 353.5 73.5
Steel 4.96 345 322.5 67.5
Hilton 4.64 393 370.5 79.5
~SPECIFICATIONS AND TESTING~--The great bulk of cement used in
construction work is bought on specification. The various government
bureaus, state and city works departments, railway companies, and most
public service corporations have their own specifications. Standard
specifications are also put forward by several of the national
engineering societies, and one of these or the personal specification of
the engineer is used for individual works. Buying cement to
specification necessitates testing to determine that the material
purchased meets the specified requirements. For a complete discussion of
the methods of conducting such tests the reader is referred to
"Practical Cement Testing" by W. Purves Taylor.
According to this authority a field testing laboratory will cost for
equipment $250 to $350. Such a laboratory can be operated by two or
three men at a salary charge of from $100 to $200 per month. Two men
will test on an average four samples per day and each additional man
will test four more samples. The cost of testing will range from $3 to
$5 per sample, which is roughly equivalent to 3 cts. per barrel of
cement, or from 3 to 5 cts. per cubic yard of concrete. These figures
are for field laboratory work reasonably well conducted under ordinarily
favorable conditions. In large laboratories the cost per sample will run
somewhat lower.
SAND.
Sand constitutes from 1/3 to 1/2 of the volume of concrete; when a large
amount of concrete is to be made a contractor cannot, therefore, afford
to guess at his source of sand supply. A long haul over poor roads can
easily make the sand cost more than the stone per cubic yard of
concrete.
~PROPERTIES OF GOOD SAND.~--Engineers commonly specify that sand for
concrete shall be clean and sharp, and silicious in character. Neither
sharpness nor excessive cleanliness is worth seeking after if it
involves much expense. Tests show conclusively that sand with rounded
grains makes quite as strong a mortar, other things being equal, as does
sand with angular grains. The admixture with sand of a considerable
percentage of loam or clay is also not the unmixed evil it has been
supposed to be. Myron S. Falk records[B] a number of elaborate
experiments on this point. These experiments demonstrate conclusively
that loam and clay in sand to the amount of 10 to 15 per cent. result in
no material reduction in the strength of mortars made with this sand as
compared with mortars made with the same sand after washing. There can
be no doubt but that for much concrete work the expense entailed in
washing sand is an unnecessary one.
[Footnote B: "Cements, Mortars and Concretes" By Myron S. Falk. Myron C.
Clark Publishing Co., Chicago, Ill.]
The only substitute for natural sand for concrete, that need be
considered practically, is pulverized stone, either the dust and fine
screenings produced in crushing rock or an artificial sand made by
reducing suitable rocks to powder. As a conclusion from the records of
numerous tests, M. S. Falk says: "It may be concluded that rock
screenings may be substituted for sand, either in mortar or concrete,
without any loss of strength resulting. This is important commercially,
for it precludes the necessity of screening the dust from crushed rock
and avoids, at the same time, the cost of procuring a natural sand to
take its place."
The principal danger in using stone dust is failure to secure the proper
balance of different size grains. This is also an important matter in
the choice of natural sands. Sand composed of a mixture of grains
ranging from fine to coarse gives uniformly stronger mortars than does
sand with grains of nearly one size, and as between a coarse and a fine
sand of one size of grains the coarse sand gives the stronger mortar.
Further data on the effect of size of grains on the utility of sand for
concrete are given in Chapter II, in the section on Voids in Sand, and
for those who wish to study in detail, the test data on this and the
other matters referred to here, the authors recommend "Cements, Mortars
and Concretes; Their Physical Properties," by Myron S. Falk.
~COST OF SAND.~--A very common price for sand in cities is $1 per cu. yd.,
delivered at the work. It may be noted here that as sand is often sold
by the load instead of the cubic yard, it is wise to have a written
agreement defining the size of a load. Where the contractor gets his
sand from the pit its cost will be the cost of excavating and loading at
the pit, the cost of hauling in wagons, the cost of freight and
rehandling it if necessary, and the cost of washing, added together.
An energetic man working under a good foreman will load 20 cu. yds. of
sand into wagons per 10-hour day; with a poor foreman or when laborers
are scarce, it is not safe to count on more than 15 cu. yds. per day.
With wages at $1.50 per day this will make the cost of loading 10 cts.
per cubic yard. The cost of hauling will include the cost of lost team
time and dumping, which will average about 5 cts. per cubic yard. With 1
cu. yd. loads, wages of team 35 cts. per hour, and speed of travel 2½
miles per hour, the cost of hauling proper is ½ ct. per 100 ft., or 27
cts. per mile. Assuming a mile haul, the cost of sand delivered based on
the above figures will be 10 cts. + 5 cts. + ½ ct. per 100 ft. = 15 +
27 cts. = 42 cts. per cu. yd. Freight rates can always be secured and it
is usually safe to estimate the weight on a basis of 2,700 lbs. per
cubic yard. For a full discussion of the cost of excavating sand and
other earths the reader is referred to "Earth Excavation and
Embankments; Methods and Cost," by Halbert P. Gillette and Daniel J.
Hauer.
~METHODS AND COST OF WASHING SAND.~--When the available sand carries
considerable percentages of loam or clay and the specifications require
that clean sand shall be used, washing is necessary. The best and
cheapest method of performing this task will depend upon the local
conditions and the amount of sand to be washed.
~Washing With Hose.~--When the quantity of sand to be washed does not
exceed 15 to 30 cu. yds. per day the simplest method, perhaps, is to use
a hose. Build a wooden tank or box, 8 ft. wide and 15 ft. long, the
bottom having a slope of 8 ins. in the 15 ft. The sides should be about
8 ins. high at the lower end and rise gradually to 3 ft. in height at
the upper end. Close the lower end of the tank with a board gate about 6
ins. in height and sliding in grooves so that it can be removed. Dump
about 3 cu. yds. of sand into the upper end of the tank and play a
¾-in. hose stream of water on it, the hose man standing at the lower
end of the tank. The water and sand flow down the inclined bottom of the
tank where the sand remains and the dirt flows over the gate and off
with the water. It takes about an hour to wash a 3-cu. yd. batch, and by
building a pair of tanks so that the hose man can shift from one to the
other, washing can proceed continuously and one man will wash 30 cu.
yds. per 10-hour day at a cost, with wages at $1.50, of 5 cts. per cubic
yard. The sand, of course, has to be shoveled from the tank and this
will cost about 10 cts. per cubic yard, making 15 cts. per cubic yard
for washing and shoveling, and to this must be added any extra hauling
and, if the water is pumped, the cost of pumping which may amount to 10
cts. per cubic yard for coal and wages. Altogether a cost of from 15 to
30 cts. per cubic yard may be figured for washing sand with a hose.
[Illustration: Fig. 1.--Plan and Elevation of Two-Hopper Ejector Sand
Washing Plant.]
[Illustration: Fig. 2.--Plan and Elevation of Four-Hopper Ejector Sand
Washing-Plant.]
~Washing With Sand Ejectors.~--When large quantities of sand are to be
washed use may be made of the sand ejector system, commonly employed in
washing filter sand at large water filtration plants; water under
pressure is required. In this system the dirty sand is delivered into a
conical or pyramidal hopper, from the bottom of which it is drawn by an
ejector and delivered mixed with water into a second similar hopper;
here the water and dirt overflow the top of the hopper, while the sand
settles and is again ejected into a third hopper or to the stock pile or
bins. The system may consist of anywhere from two to six hoppers. Figure
1 shows a two-hopper lay-out and Fig. 2 shows a four-hopper lay-out. In
the first plant the washed sand is delivered into bins so arranged, as
will be seen, that the bins are virtually a third washing hopper. The
clean sand is chuted from these bins directly into cars or wagons. In
the second plant the clean sand is ejected into a trough which leads it
into buckets handled by a derrick. The details of one of the washing
hoppers for the plant shown by Fig. 1 are illustrated by Fig. 3.
[Illustration: Fig. 3.--Details of Washing Hopper and Ejector for Plant
Shown by Fig. 1.]
At filter plants the dirty sand is delivered mixed with water to the
first hopper by means of ejectors stationed in the filters and
discharging through pipes to the washers. When, as would usually be the
case in contract work, the sand is delivered comparatively dry to the
first hopper, this hopper must be provided with a sprinkler pipe to wet
the sand. In studying the ejector washing plants illustrated it should
be borne in mind that for concrete work they would not need to be of
such permanent construction as for filter plants, the washers would be
mounted on timber frames, underground piping would be done away with,
etc.; at best, however, such plants are expensive and will be warranted
only when the amount of sand to be washed is large.
The usual assumption of water-works engineers is that the volume of
water required for washing filter sand is 15 times the volume of the
sand washed. At the Albany, N. Y., filters the sand passes through five
ejectors at the rate of 3 to 5 cu. yds. per hour and takes 4,000
gallons of water per cubic yard. One man shovels sand into the washer
and two take it away. Based on an output of 32 cu. yds. in 10 hours, Mr.
Allen Hazen estimates the cost of washing as follows:
3 men, at $2 per day $6.00
110,000 gallons of water, at $0.05 5.50
------
Total, 32 cu. yds., at 36 cts. $11.50
~Washing With Tank Washers.~--Figure 4 shows a sand washer used in
constructing a concrete lock at Springdale, Pa., in the United States
government improvement work on the Allegheny river. The device consisted
of a circular tank 9 ft. in diameter and 7 ft. high, provided with a
sloping false bottom perforated with 1-in. holes, through which water
was forced as indicated. A 7½×5×6-in. pump with a 3-in. discharge pipe
was used to force water into the tank, and the rotating paddles were
operated by a 7 h.p. engine. This apparatus washed a batch of 14 cu.
yds. in from 1 to 2 hours at a cost of 7 cts. per cubic yard. The sand
contained much fine coal and silt. The above data are given by Mr. W. H.
Roper.
[Illustration: Fig. 4.--Details of Tank Washer Used at Springdale, Pa.]
[Illustration: Fig. 5.--Details of Tank Washer Used at Yonkers, N. Y.]
[Illustration: Fig. 6.--Details of Rotating Tank Sand Washer Used at
Hudson, N. Y.]
Another form of tank washer, designed by Mr. Allen Hazen, for washing
bank sand at Yonkers, N. Y., is shown by Fig. 5. This apparatus
consisted of a 10×2½×2½ ft. wooden box, with a 6-in. pipe entering one
end at the bottom and there branching into three 3-in. pipes, extending
along the bottom and capped at the ends. The undersides of the 3-in.
pipes were pierced with ½-in. holes 6 ins. apart, through which water
under pressure was discharged into the box. Sand was shoveled into the
box at one end and the upward currents of water raised the fine and
dirty particles until they escaped through the waste troughs. When the
box became filled with sand a sliding door at one end was opened and the
batch discharged. The operation was continuous as long as sand was
shoveled into the box; by manipulating the door the sand could be made
to run out with a very small percentage of water. Sand containing 7 per
cent of dirt was thus washed so that it contained only 0.6 per cent
dirt. The washer handled 200 cu. yds. of sand in 10 hours. The above
data are given by F. H. Stephenson.
A somewhat more elaborate form of tank washer than either of those
described is shown by Fig. 6. This apparatus was used by Mr. Geo. A.
Soper for washing filter sand at Hudson, N. Y. The dirty sand was
shoveled into a sort of hopper, from which it was fed by a hose stream
into an inclined cylinder, along which it traveled and was discharged
into a wooden trough provided with a screw conveyor and closed at both
ends. The water overflowing the sides of the trough carried away the
dirt and the clean sand was delivered by the screw to the bucket
elevator which hoisted it to a platform, from which it was taken by
barrows to the stock pile. A 4-h.p. engine with a 5-h.p. boiler operated
the cylinder, screw, elevator and pump. Four men operated the washer and
handled 32 cu. yds. of sand per day; with wages at $1.50 the cost of
washing was 20 cts. per cubic yard.
[Illustration: Fig. 7.--Arrangement of Sand Washing Plant at Lynchburg,
Va.]
In constructing a concrete block dam at Lynchburg, Va., sand containing
from 15 to 30 per cent. of loam, clay and vegetable matter was washed
to a cleanliness of 2 to 5 per cent of such matter by the device shown
by Fig. 7. A small creek was diverted, as shown, into a wooden flume
terminating in two sand tanks; by means of the swinging gate the flow
was passed through either tank as desired. The sand was hauled by wagon
and shoveled into the upper end of the flume; the current carried it
down into one of the tanks washing the dirt loose and carrying it off
with the overflow over the end of the tank while the sand settled in the
tank. When one tank was full the flow was diverted into the other tank
and the sand in the first tank was shoveled out, loaded into wagons, and
hauled to the stock pile. As built this washer handled about 30 cu. yds.
of sand per 10-hour day, but the tanks were built too small for the
flume, which could readily handle 75 cu. yds. per day with no larger
working force. This force consisted of three men at $1.50 per day,
making the cost, for a 30 cu. yd. output, 15 cts. per cu. yd. for
washing.
None of the figures given above includes the cost of handling the sand
to and from the washer. When this involves much extra loading and
hauling, it amounts to a considerable expense, and in any plan for
washing sand the contractor should figure, with exceeding care, the
extra handling due to the necessity of washing.
AGGREGATES.
The aggregates commonly used in making concrete are broken or crushed
stone, gravel, slag and cinders. Slag and cinders make a concrete that
weighs considerably less than stone or gravel mixtures, and being the
products of combustion are commonly supposed to make a specially fire
resisting concrete; their use is, therefore, confined very closely to
fireproof building work and, in fact, to floor construction for such
buildings. Slag and cinder concretes are for this reason given minor
consideration in this volume.
~BROKEN STONE.~--Stone produced by crushing any of the harder and tougher
varieties of rock is suitable for concrete. Perhaps the best stone is
produced by crushing trap rock. Crushed trap besides being hard and
tough is angular and has an excellent fracture surface for holding
cement; it also withstands heat better than most stone. Next to trap
the hard, tough, crystalline limestones make perhaps the best all around
concrete material; cement adheres to limestone better than to any other
rock. Limestone, however, calcines when subjected to fire and is,
therefore, objected to by many engineers for building construction. The
harder and denser sandstones, mica-schists, granites and syanites make
good stone for concrete and occasionally shale and slate may be used.
~GRAVEL.~--Gravel makes one of the best possible aggregates for concrete.
The conditions under which gravel is produced by nature make it
reasonably certain that only the tougher and harder rocks enter into its
composition; the rounded shapes of the component particles permit gravel
to be more closely tamped than broken stone and give less danger of
voids from bridging; the mixture is also generally a fairly well
balanced composition of fine and coarse particles. The surfaces of the
particles being generally smooth give perhaps a poorer bond with the
cement than most broken stone. In the matter of strength the most recent
tests show that there is very little choice between gravel and broken
stone concrete.
~SLAG AND CINDERS.~--The slag used for concrete aggregate is iron blast
furnace slag crushed to proper size. Cinders for aggregate are steam
boiler cinders; they are best with the fine ashes screened out and
should not contain more than 15 per cent. of unburned coal.
~BALANCED AGGREGATE.~--With the aggregate, as with the sand for concrete,
the best results, other things being equal, will be secured by using a
well-balanced mixture of coarse and fine particles. Usually the product
of a rock crusher is fairly well balanced except for the very fine
material. There is nearly always a deficiency of this, which, as
explained in a succeeding section, has to be supplied by adding sand.
Usually, also, the engineer accepts the crusher product coarser than
screenings as being well enough balanced for concrete work, but this is
not always the case. Engineers occasionally demand an artificial mixture
of varying proportions of different size stones and may even go so far
as to require gravel to be screened and reproportioned. This artificial
grading of the aggregate adds to the cost of the concrete in some
proportion which must be determined for each individual case.
~SIZE OF AGGREGATE.~--The size of aggregate to be used depends upon the
massiveness of the structure, its purpose, and whether or not it is
reinforced. It is seldom that aggregate larger than will pass a 3-in.
ring is used and this only in very massive work. The more usual size is
2½ ins. For reinforced concrete 1¼ ins. is about the maximum size
allowed and in building work 1-in. aggregate is most commonly used. Same
constructors use no aggregate larger than ¾ in. in reinforced building
work, and others require that for that portion of the concrete coming
directly in contact with the reinforcement the aggregate shall not
exceed ¼ to ½ in. The great bulk of concrete work is done with aggregate
smaller than 2 ins., and as a general thing where the massiveness of the
structure will allow of much larger sizes it will be more economic to
use rubble concrete. (See Chapter VI.)
~COST OF AGGREGATE.~--The locality in which the work is done determines
the cost of the aggregate. Concerns producing broken stone or screened
and washed gravel for concrete are to be found within shipping distance
in most sections of the country so that these materials may be purchased
in any amount desired. The cost will then be the market price of the
material f. o. b. cars at plant plus the freight rates and the cost of
unloading and haulage to the stock piles. If the contractor uses a local
stone or gravel the aggregate cost will be, for stone the costs of
quarrying and crushing and transportation, and, for gravel, the cost of
excavation, screening, washing and transportation.
~SCREENED OR CRUSHER-RUN STONE FOR CONCRETE.~--Formerly engineers almost
universally demanded that broken stone for concrete should have all the
finer particles screened out. This practice has been modified to some
considerable extent in recent years by using all the crusher product
both coarse and fine, or, as it is commonly expressed, by using
run-of-crusher stone. The comparative merits of screened and crusher-run
stone for concrete work are questions of comparative economy and
convenience. The fine stone dust and chips produced in crushing stone
are not, as was once thought, deleterious; they simply take the place of
so much of the sand which would, were the stone screened, be required to
balance the sand and stone mixture. It is seldom that the proportion of
chips and dust produced in crushing stone is large enough to replace the
sand constituent entirely; some sand has nearly always to be added to
run-of-crusher stone and it is in determining the amount of this
addition that uncertainty lies. The proportions of dust and chips in
crushed stone vary with the kind of stone and with the kind of crusher
used. Furthermore, when run-of-crusher stone is chuted from the crusher
into a bin or pile the screenings and the coarse stones segregate.
Examination of a crusher-run stone pile will show a cone-shaped heart of
fine material enclosed by a shell of coarser stone, consequently when
this pile of stone is taken from to make concrete a uniform mixture of
fine and coarse particles is not secured, the material taken from the
outside of the pile will be mostly coarse and that from the inside
mostly fine. This segregation combined with the natural variation in the
crusher product makes the task of adding sand and producing a balanced
sand and stone mixture one of extreme uncertainty and some difficulty
unless considerable expenditure is made in testing and reproportioning.
When the product of the crusher is screened the task of proportioning
the sand to the stone is a straightforward operation, and the screened
out chips and dust can be used as a portion of the sand if desired. The
only saving, then, in using crusher-run stone direct is the very small
one of not having to screen out the fine material. The conclusion must
be that the economy of unscreened stone for concrete is a very doubtful
quantity, and that the risk of irregularity in unscreened stone mixtures
is a serious one. The engineer's specifications will generally determine
for the contractor whether he is to use screened or crusher-run stone,
but these same specifications will not guarantee the regularity of the
resulting concrete mixture; this will be the contractor's burden and if
the engineer's inspection is rigid and the crusher-run product runs
uneven for the reasons given above it will be a burden of considerable
expense. The contractor will do well to know his product or to know his
man before bidding less or even as little on crusher-run as on screened
stone concrete.
~COST OF QUARRYING AND CRUSHING STONE.~--The following examples of the
cost of quarrying and crushing stone are fairly representative of the
conditions which would prevail on ordinary contract work. In quarrying
and crushing New Jersey trap rock with gyratory crushers the following
was the cost of producing 200 cu. yds. per day:
Per day. Per cu. yd.
3 drillers at $2.75 $ 8.25 $0.041
3 helpers at $1.75 5.25 0.026
10 men barring out and sledging 15.00 0.075
14 men loading carts 21.00 0.105
4 cart horses 6.00 0.030
2 cart drivers 3.00 0.015
2 men dumping carts and feeding crusher 3.00 0.015
1 fireman for drill boiler 2.50 0.013
1 engineman for crusher 3.00 0.015
1 blacksmith 3.00 0.015
1 blacksmith helper 2.00 0.010
1 foreman 5.00 0.025
2 tons coal at $3.50 7.00 0.035
150 lbs. 40% dynamite at 15 cts. 22.50 0.113
------ -------
Total $106.50 $0.533
The quarry face worked was 12 to 18 ft., and the stone was crushed to
2-in. size. Owing to the seamy character of the rock it was broken by
blasting into comparatively small pieces requiring very little sledging.
The stone was loaded into one-horse dump carts, the driver taking one
cart to the crusher while the other was being loaded. The haul was 100
ft. The carts were dumped into an inclined chute leading to a No. 5
Gates crusher. The stone was elevated by a bucket elevator and screened.
All stone larger than 2 ins. was returned through a chute to a No. 3
Gates crusher for recrushing. The cost given above does not include
interest, depreciation, and repairs; these items would add about $8 to
$10 more per day or 4 to 5 cts. per cubic yard.
In quarrying limestone, where the face of the quarry was only 5 to 6 ft.
high, and where the amount of stripping was small, one steam drill was
used. This drill received its steam from the same boiler that supplied
the crusher engine. The drill averaged 60 ft. of hole drilled per 10-hr.
day, but was poorly handled and frequently laid off for repairs. The
cost of quarrying and crushing was as follows:
Quarry.
1 driller $ 2.50
1 helper 1.50
1 man stripping 1.50
4 men quarrying 6.00
1 blacksmith 2.50
1/8 ton coal at $3 1.00
Repairs to drill .60
Hose, drill steel and interest on plant .90
24 lbs. dynamite 3.60
------
Total $20.10
Crusher.
1 engineman $ 2.50
2 men feeding crusher 3.50
6 men wheeling 9.00
1 bin man 1.50
1 general foreman 3.00
1/3 ton coal at $3 1.00
1 gallon oil .25
Repairs to crusher 1.00
Repairs to engine and boiler 1.00
Interest on plant 1.00
------
Total $23.75
Summary:
Per day. Per. cu. yd.
Quarrying $20.10 $0.37
Crushing 23.75 0.39
------ -----
Total for 60 cu. yds. $43.85 $0.76
The "4 men quarrying" barred out and sledged the stone to sizes that
would enter a 9×16-in. jaw crusher. The "6 men wheeling" delivered the
stone in wheelbarrows to the crusher platform, the run plank being never
longer than 150 ft. Two men fed the stone into the crusher, and a
bin-man helped load the wagons from the bin, and kept tally of the
loads. The stone was measured loose in the wagons, and it was found that
the average load was 1½ cu. yds., weighing 2,400 lbs. per cu. yd. There
were 40 wagon loads, or 60 cu. yds. crushed per 10-hr. day, although on
some days as high as 75 cu. yds. were crushed. The stone was screened
through a rotary screen, 9 ft. long, having three sizes of openings,
½-in., 1¼-in. and 2¼-in. The output was 16% of the smallest size, 24% of
the middle size, and 60% of the large size. All tailings over 2½ ins. in
size were recrushed.
It will be noticed that the interest on the plant is quite an important
item. This is due to the fact that, year in and year out, a quarrying
and crushing plant seldom averages more than 100 days actually worked
per year, and the total charge for interest must be distributed over
these 100 days, and not over 300 days as is so commonly and erroneously
done. The cost of stripping the earth off the rock is often considerably
in excess of the above given cost, and each case must be estimated
separately. Quarry rental or royalty is usually not in excess of 5 cts.
per cu. yd., and frequently much less. The dynamite used was 40%, and
the cost of electric exploders is included in the cost given. Where a
higher quarry face is used the cost of drilling and the cost of
explosives per cu. yd. is less. Exclusive of quarry rent and heavy
stripping costs, a contractor should be able to quarry and crush
limestone or sandstone for not more than 75 cts. per cu. yd., or 62 cts.
per ton of 2,000 lbs., wages and conditions being as above given.
The labor cost of erecting bins and installing a 9×16 jaw crusher,
elevator, etc., averages about $75, including hauling the plant two or
three miles, and dismantling the plant when work is finished.
The following is a record of the cost of crushing stone and cobbles on
four jobs at Newton, Mass., in 1891. On jobs A and B the stone was
quarried and crushed; on jobs C and D cobblestones were crushed. A
9×15-in. Farrel-Marsondon crusher was used, stone being fed in by two
laborers. A rotary screen having ½, 1 and 2½-in. openings delivered the
stone into bins having four compartments, the last receiving the
"tailings" which had failed to pass through the screen. The broken stone
was measured in carts as they left the bin, but several cart loads were
weighed, giving the following weights per cubic foot of broken stone:
-----------Size.--------------
½-in. 1-in. 2½-ins. Tailings.
lbs. lbs. lbs. lbs.
Greenish trap rock, "A" 95.8 84.3 88.3 91.0
Conglomerate, "B" 101.0 87.7 94.4 ....
Cobblestones, "C" and "D" 102.5 98.0 99.6 ....
A one-horse cart held 26 to 28 cu. ft. (average 1 cu. yd.) of broken
stone; a two-horse cart, 40 to 42 cu. ft., at the crusher.
---------------------Job.-------------
A. B. C. D.
Hours run 412 144 101 198
Short tons per hour 9.0 11.2 15.7 12.1
Cu. yds. per hour 7.7 8.9 11.8 9.0
Per cent of tailings 31.8 29.3 17.5 20.5
Per cent of 2½-in. stone 51.3 51.9 57.0 55.1
Per cent of 1-in. stone 10.2 .... .... ....
Per cent of ½-in. stone or dust 6.7 18.8 25.5 23.4
---------------------Job.-------------
A. B. C. D.
Explosives, coal for drill and repairs $0.084 $0.018 .... ....
Labor steam drilling 0.092 .... .... ....
Labor hand drilling .... 0.249 .... ....
Sharpening tools 0.069 0.023 .... ....
Sledging stone for crusher 0.279 0.420 .... ....
Loading carts 0.098 0.127 .... $0.144
Carting to crusher 0.072 0.062 $0.314 0.098
Feeding crusher 0.053 0.053 0.033 0.065
Engineer of crusher 0.031 0.038 0.029 0.036
Coal for crusher 0.079 0.050 0.047 0.044
Repairs to crusher 0.041 .... .... 0.011
Moving portable crusher .... 0.023 .... 0.019
Watchman ($1.75 a day) .... 0.053 0.022 0.030
------ ------ ------ ------
Total cost per cu. yd. $0.898 $1.116 $0.445 $0.447
Total cost per short ton 0.745 0.885 0.330 0.372
Note.--"A" was trap rock; "B" was conglomerate rock; "C" and
"D" were trap and granite cobblestones. Common laborers on jobs
"A" and "D" were paid $1.75 per 9-hr. day; on jobs "B" and "C,"
$1.50 per 9-hr. day; two-horse cart and driver, $5 per day;
blacksmith, $2.50; engineer on crusher, $2 on job "A," $2.25 on
"B," $2.00 on "C," $2.50 on "D"; steam driller received $3, and
helper $1.75 a day; foreman, $3 a day. Coal was $5.25 per short
ton. Forcite powder, 11-1/3 cts. per lb.
For a full discussion of quarrying and crushing methods and costs and
for descriptions of crushing machinery and plants the reader is referred
to "Rock Excavation; Methods and Cost," by Halbert P. Gillette.
~SCREENING AND WASHING GRAVEL.~--Handwork is resorted to in screening
gravel only when the amount to be screened is small and when it is
simply required to separate the fine sand without sorting the coarser
material into sizes. The gravel is shoveled against a portable inclined
screen through which the sand drops while the pebbles slide down and
accumulate at the bottom. The cost of screening by hand is the cost of
shoveling the gravel against the screen divided by the number of cubic
yards of saved material. In screening gravel for sand the richer the
gravel is in fine material the cheaper will be the cost per cubic yard
for screening; on the contrary in screening gravel for the pebbles the
less sand there is in the gravel the cheaper will be the cost per cubic
yard for screening. The cost of shoveling divided by the number of cubic
yards shoveled is the cost of screening only when both the sand and the
coarser material are saved. Tests made in the pit will enable the
contractor to estimate how many cubic yards of gravel must be shoveled
to get a cubic yard of sand or pebbles. An energetic man will shovel
about 25 cu. yds. of gravel against a screen per 10-hour day and keep
the screened material cleared away, providing no carrying is necessary.
A mechanical arrangement capable of handling a considerably larger
yardage of material is shown by Fig. 8. Two men and a team are required.
The team is attached to the scraper by means of the rope passing through
the pulley at the top of the incline. The scraper is loaded in the usual
manner, hauled up the incline until its wheels are stopped by blocks and
then the team is backed up to slacken the rope and permit the scraper to
tip and dump its load. The trip holding the scraper while dumping is
operated from the ground. The scraper load falls onto an inclined
screen which takes out the sand and delivers the pebbles into the wagon.
By erecting bins to catch the sand and pebbles this same arrangement
could be made continuous in operation.
[Illustration: Fig. 8.--Device for Excavating and Screening Gravel and
Loading Wagons.]
[Illustration: Fig. 9.--Gravel Washing Plant of 120 to 130 Cu. Yds., Per
Hour Capacity.]
In commercial gravel mining, the gravel is usually sorted into several
sizes and generally it is washed as well as screened. Where the pebbles
run into larger sizes a crushing plant is also usually installed to
reduce the large stones. Works producing several hundred cubic yards of
screened and washed gravel per day require a plant of larger size and
greater cost than even a very large piece of concrete work will warrant,
so that only general mention will be made here of such plants. The
commercial sizes of gravel are usually 2-in., 1-in., ½-in. and ¼-in.,
down to sand. No very detailed costs of producing gravel by these
commercial plants are available. At the plant of the Lake Shore &
Michigan Southern Ry., where gravel is screened and washed for ballast,
the gravel is passed over a 2-in., a ¾-in., a ¼-in. and a 1/8-in. screen
in turn and the fine sand is saved. About 2,000 tons are handled per
day; the washed gravel, 2-in. to 1/8-in. sizes, represents from 40 to 65
per cent. of the raw gravel and costs from 23 to 30 cts. per cu. yd.,
for excavation, screening and washing. The drawings of Fig. 9 show a
gravel washing plant having a capacity of 120 to 130 cu. yds. per hour,
operated by the Stewart-Peck Sand Co., of Kansas City, Mo. Where washing
alone is necessary a plant of one or two washer units like those here
shown could be installed without excessive cost by a contractor at any
point where water is available. Each washer unit consists of two
hexagonal troughs 18 ins. in diameter and 18 ft. long. A shaft carrying
blades set spirally is rotated in each trough to agitate the gravel and
force it along; each trough also has a fall of 6 ins. toward its
receiving end. The two troughs are inclosed in a tank or box and above
and between them is a 5-in. pipe having ¾-in. holes 3 ins. apart so
arranged that the streams are directed into the troughs. The water and
dirt pass off at the lower end of the troughs while the gravel is fed by
the screws into a chute discharging into a bucket elevator, which in
turn feeds into a storage bin. The gravel to be washed runs from 2 ins.
to 1/8-in. in size; it is excavated by steam shovel and loaded into 1½
cu. yd. dump cars, three of which are hauled by a mule to the washers,
where the load is dumped into the troughs. The plant having a capacity
of 120 to 130 cu. yds. per hour cost $25,000, including pump and an
8-in. pipe line a mile long. A 100-hp. engine operates the plant, and 20
men are needed for all purposes. This plant produces washed gravel at a
profit for 40 cts. per cu. yd.
CHAPTER II.
THEORY AND PRACTICE OF PROPORTIONING CONCRETE.
American engineers proportion concrete mixtures by measure, thus a 1-3-5
concrete is one composed of 1 volume of cement, 3 volumes of sand and 5
volumes of aggregate. In Continental Europe concrete is commonly
proportioned by weight and there have been prominent advocates of this
practice among American engineers. It is not evident how such a change
in prevailing American practice would be of practical advantage. Aside
from the fact that it is seldom convenient to weigh the ingredients of
each batch, sand, stone and gravel are by no means constant in specific
gravity, so that the greater exactness of proportioning by weight is not
apparent. In this volume only incidental attention is given to
gravimetric methods of proportioning concrete.
~VOIDS.~--Both the sand and the aggregates employed for concrete contain
voids. The amount of this void space depends upon a number of
conditions. As the task of proportioning concrete consists in so
proportioning the several materials that all void spaces are filled with
finer material the conditions influencing the proportion of voids in
sand and aggregates must be known.
~Voids in Sand.~--The two conditions exerting the greatest influence on
the proportion of voids in sand are the presence of moisture and the
size of the grains of which the sand is composed.
TABLE I.--SHOWING EFFECT OF ADDITIONS OF DIFFERENT PERCENTAGES OF
MOISTURE ON VOLUME OF SAND.
Per cent of water in sand 0 0.5 1 2 3 5 10
Weight per cu. yd. of fine Lbs. Lbs. Lbs. Lbs. Lbs. Lbs. Lbs.
sand and water 3,457 2,206 2,085 2,044 2,037 2,035 2,133
Weight per cu. yd. of coarse
sand and water 2,551 2,466 2,380 2,122 2,058 2,070 2,200
The volume of sand is greatly affected by the presence of varying
percentages of moisture in the sand. A dry loose sand that has 45 per
cent. voids if mixed with 5 per cent. by weight of water will swell,
unless tamped, to such an extent that its voids may be 57 per cent. The
same sand if saturated with water until it becomes a thin paste may show
only 37½ per cent. voids after the sand has settled. Table I shows the
results of tests made by Feret, the French experimenter. Two kinds of
sand were used, a very fine sand and a coarse sand. They were measured
in a box that held 2 cu. ft. and was 8 ins. deep, the sand being
shoveled into the box but not tamped or shaken. After measuring and
weighing the dry sand 0.5 per cent. by weight of water was added and the
sand was mixed and shoveled back into the box again and then weighed.
These operations were repeated with varying percentages of water up to
10 per cent. It will be noted that the weight of mixed water and sand is
given; to ascertain the exact weight of dry sand in any mixture, divide
the weight given in the table by 100 per cent. plus the given tabular
per cent.; thus the weight of dry, fine sand in a 5 per cent. mixture is
2,035 ÷ 1.5 = 1,98 lbs. per cu. yd. The voids in the dry sand were 45
per cent. and in the sand with 5 per cent. moisture they were 56.7 per
cent. Pouring water onto loose, dry sand compacts it. By mixing fine
sand and water to a thin paste and allowing it to settle, it was found
that the sand occupied 11 per cent. less space than when measured dry.
The voids in fine sand, having a specific gravity of 2.65, were
determined by measurement in a quart measure and found to be as follows:
Sand not packed, per cent. voids 44½
Sand shaken to refusal, per cent. voids 35
Sand saturated with water, per cent. voids 37½
Another series of tests made by Mr. H. P. Boardman, using Chicago sand
having 34 to 40 per cent. voids, showed the following results:
Water added, per cent. 2 4 6 8 10
Resulting per cent. increase 17.6 22 19.5 16.6 15.6
Mr. Wm. B. Fuller found by tests that a dry sand, having 34 per cent.
voids, shrunk 9.6 per cent. in volume upon thorough tamping until it had
27 per cent. voids. The same sand moistened with 6 per cent. water and
loose had 44 per cent. voids, which was reduced to 31 per cent. by
ramming. The same sand saturated with water had 33 per cent. voids and
by thorough ramming its volume was reduced 8½ per cent. until the sand
had only 26¼ per cent. voids. Further experiments might be quoted and
will be found recorded in several general treatises on concrete, but
these are enough to demonstrate conclusively that any theory of the
quantity of cement in mortar to be correct must take into account the
effect of moisture on the voids in sand.
The effect of the size and the shape of the component grains on the
amount of voids in sand is considerable. Feret's experiments are
conclusive on these points, and they alone will be followed here. Taking
for convenience three sizes of sand Feret mixed them in all the varying
proportions possible with a total of 10 parts; there were 66 mixtures.
The sizes used were: Large (L), sand composed of grains passing a
sieve of 5 meshes per linear inch and retained on a sieve of 15 meshes
per linear inch; medium (M>), sand passing a sieve of 15 meshes and
retained on a sieve of 50 meshes per linear inch, and fine (F), sand
passing a 50-mesh sieve. With a dry sand whose grains have a specific
gravity of 2.65, the weight of a cubic yard of either the fine, or the
medium, or the large size, was 2,190 lbs., which is equivalent to 51 per
cent. voids. The greatest weight of mixture, 2,840 lbs. per cu. yd., was
an L_{6}M_{0}F_{4} mixture, that is, one composed of six parts
large, no parts medium and 4 parts fine; this mixture was the densest of
the 66 mixtures made, having 36 per cent. voids. It will be noted that
the common opinion that the densest mixture is obtained by a mixture of
gradually increasing sizes of grains is incorrect; there must be enough
difference in the size of the grains to provide voids so large that the
smaller grains will enter them and not wedge the larger grains apart.
Turning now to the shape of the grains, the tests showed that rounded
grains give less voids than angular grains. Using sand having a
composition of L_{5}M_{3}F_{2} Feret got the following results:
--Per cent. Voids--
Kind of Grains. Shaken. Unshaken.
Natural sand, rounded grains 25.6 35.9
Crushed quartzite, angular grains 27.4 42.1
Crushed shells, flat grains 31.8 44.3
Residue of quartzite, flat grains 34.6 47.5
The sand was shaken until no further settlement occurred. It is plain
from these data on the effect of size and shape of grains on voids why
it is that discrepancies exist in the published data on voids in dry
sand. An idea of the wide variation in the granulometric composition of
different sands is given by Table II. Table III shows the voids as
determined for sands from different localities in the United States.
TABLE II.--SHOWING GRANULOMETRIC COMPOSITIONS OF DIFFERENT SANDS.
Held by a Sieve. A B C E
No. 10 35.3%
No. 20 32.1 12.8% 4.2% 11%
No. 30 14.6 49.0 12.5 14
No. 40 ... ... 44.4 ...
No. 50 9.6 29.3 ... 53
No. 100 4.9 5.7 ... ...
No. 200 2.0 2.3 ... ...
----- ----- ----- -----
Voids 33% 39% 41.7% 31%
NOTE.--A, is a "fine gravel" (containing 8% clay) used at
Philadelphia. B, Delaware River sand. C, St. Mary's River sand.
D, Green River, Ky., sand, "clean and sharp."
TABLE III.--SHOWING MEASURED VOIDS IN SAND FROM DIFFERENT LOCALITIES.
Percent
Locality. Authority. Voids. Remarks.
Ohio River W. M. Hall 31 Washed
Sandusky, O. C. E. Sherman 40 Lake
Franklin Co., O. C. E. Sherman 40 Bank
Sandusky Bay, O. S. B. Newberry 32.3 ......
St. Louis, Mo. H. H. Henby 34.3 Miss. River
Sault Ste. Marie H. von Schon 41.7 River
Chicago, Ill. H. P. Broadman 34 to 40 ......
Philadelphia, Pa 39 Del. River
Mass. Coast 31 to 34 ......
Boston, Mass Geo. Kimball 33 Clean
Cow Bay, L. I. Myron S. Falk 40½ ......
Little Falls, N. J. W. B. Fuller 45.6 ......
Canton, Ill. G. W. Chandler 30 Clean
~Voids in Broken Stone and Gravel.~--The percentage of voids in broken
stone varies with the nature of the stone: whether it is broken by hand
or by crushers; with the kind of crusher used, and upon whether it is
screened or crusher-run product. The voids in broken stone seldom
exceed 52 per cent. even when the fragments are of uniform size and the
stone is shoveled loose into the measuring box. The following records of
actual determinations of voids in broken stone cover a sufficiently wide
range of conditions to show about the limits of variation.
The following are results of tests made by Mr. A. N. Johnson, State
Engineer of Illinois, to determine the variation in voids in crushed
stone due to variation in size and to method of loading into the
measuring box. The percentage of voids was determined by weighing the
amount of water added to fill the box:
Method of Per cent.
Size. Loading. of Voids.
3 in. 20-ft. drop 41.8
3 in. 15-ft drop 46.8
3 in. 15-ft. drop 47.2
3 in. Shovels 48.7
1½ in. 20-ft. drop 42.5
1½ in. 15-ft. drop 46.8
1½ in. 15-ft. drop 46.8
1½ in. Shovels 50.5
¾ in. 20-ft. drop 39.4
¾ in. 15-ft. drop 42.7
¾ in. 15-ft. drop 41.5
¾ in. 15-ft. drop 41.8
¾ in. Shovels 45.2
¾ in. Shovels 44.6
3/8 in. Shovels 41.0
3/8 in. Shovels 40.6
3/8 in. Shovels 41.0
The table shows clearly the effect on voids of compacting the stone by
dropping it; it also shows for the ¾-in. and the 3/8-in. stone loaded by
shovels how uniformly the percentages of voids run for stone of one size
only. Dropping the stone 20 ft. reduced the voids some 12 to 15 per
cent. as compared with shoveling.
TABLE IV.--SHOWING DETERMINED PERCENTAGES OF VOIDS IN BROKEN STONE FROM
VARIOUS COMMON ROCKS.
--------------------+--------+------------------------------------------------
| Percent|
Authority. | Voids.| Remarks.
--------------------+--------+------------------------------------------------
Sabin | 49.0 | Limestone, crusher run after screening out
| | 1/8-in. and under.
" | 44.0 | Limsetone (1 part screenings mixed with
| | 6 parts broken stone).
Wm. M. Black | 46.5 | Screened and washed, 2-ins. and under.
J. J. R. Croes | 47.5 | Gneiss, after screening out ¼-in. and under.
S. B. Newberry | 47.0 | Chiefly about egg size.
H. P. Broadman |39 to 42| Chicago limestone, crusher run.
" |48 to 52| " " screened into sizes.
Wm. M. Hall | 48.0 | Green River limestone, 2½-ins. and smaller
| | dust screened out.
" | 50.0 | Hudson River trap, 2½-ins. and smaller,
| | dust screened out.
Wm. B. Fuller | 47.6 | New Jersey trap, crusher run, 1/6 to 2.1 in.
Geo. A. Kimball | 49.5 | Roxbury conglomerate, ½ to 2½ ins.
Myron S. Falk | 48.0 | Limestone, ½ to 3 ins.
W. H. Henby | 43.0 | " 2-in size.
" | 46.0 | " 1½-in size
Feret | 53.4 | Stone, 1.6 to 2.4 ins.
" | 51.7 | " 0.8 to 1.6 in.
" | 52.1 | " 0.4 to 0.8 in.
A. W. Dow | 45.3 | Bluestone, 89% being 1½ to 2½ ins.
" | 45.3 | " 90% being 1/6 to 1½ in.
Taylor and Thompson | 54.5 | Trap, hard, 1 to 2½ ins.
" | 54.5 | " " ½ to 1 in.
" | 45.0 | " " 0 to 2½ in.
" | 51.2 | " soft, ¾ to 2 ins.
G. W. Chandler | 40.0 | Canton, Ill.
Emile Low | 39.0 | Buffalo limestone, crusher run, dust in.
C. M. Saville | 46.0 | Crushed cobblestone, screened into sizes.
--------------------+--------+-----------------------------------------------
TABLE V.--SHOWING PERCENTAGES OF VOIDS IN GRAVEL AND BROKEN STONE OF
DIFFERENT GRANULOMETRIC COMPOSITIONS.
/--Per cent Voids in--\
Passing a ring of 2.4" 1.6" 0.8" Round Broken
Held by a ring 1.6" 0.8" 0.4" Pebbles. Stone.
Parts 1 0 0 40.0 53.4
" 0 1 0 38.8 51.7
" 0 0 1 41.7 52.1
" 1 1 0 35.8 50.5
" 1 0 1 35.6 47.1
" 0 1 1 37.9 40.5
" 1 1 1 35.5 47.8
" 4 1 1 34.5 49.2
" 1 4 1 36.6 49.4
" 1 1 4 38.1 48.6
" 8 0 2 34.1 ....
Table IV gives the voids in broken stone as determined by various
engineers; it requires no explanation. Table V, taken from Feret's
tests, shows the effect of changes in granulometric composition on the
amount of voids in both broken stone and gravel. Considering the column
giving voids in stone it is to be noted first how nearly equal the voids
are for stone of uniform size whatever that size be. As was the case
with sand a mixture of coarse and fine particles gives the fewest voids;
for stone an L_{1}M_{0}F_{1} mixture and for gravel an L_{8}M_{0}F_{2}
mixture. Tamping reduces the voids in broken stone. Mr. Geo. W. Rafter
gives the voids in clean, hand-broken limestone passing a 2½-in. ring as
43 per cent. after being lightly shaken and 37½ per cent. after being
rammed. Generally speaking heavy ramming will reduce the voids in loose
stone about 20 per cent.
It is rare that gravel has less than 30 per cent. or more than 45 per
cent. voids. If the pebbles vary considerably in size so that the small
fit in between the large, the voids may be as low as 30 per cent. but if
the pebbles are tolerably uniform in size the voids will approach 45 per
cent. Table V shows the effect of granulometric composition on the voids
in gravel as determined by Feret. Mr. H. Von Schon gives the following
granulometric analysis of a gravel having 34.1 per cent. voids:
Retained on 1-in. ring, per cent. 10.70
Retained on 3/8-in. ring, per cent. 23.65
Retained on No. 4 sieve, per cent. 8.70
Retained on No. 10 sieve, per cent. 17.14
Retained on No. 20 sieve, per cent. 21.76
Retained on No. 30 sieve, per cent. 6.49
Retained on No. 40 sieve, per cent. 5.96
Passed a No. 40 sieve, per cent. 5.59
Passed a 1½-in ring, per cent. 100.00
As mixtures of broken stone and gravel are often used the following
determinations of voids in such mixtures are given. The following
determinations were made by Mr. Wm. M. Hall for mixtures of blue
limestone and Ohio River washed gravel:
Per cent. Per cent. Per cent.
Stone. Gravel. Voids in Mix
100 with 0 48
80 " 20 44
70 " 30 41
60 " 40 38½
50 " 50 36
0 " 100 35
The dust was screened from the stone all of which passed a 2½-in. ring;
the gravel all passed a 1½-in. screen. Using the same sizes of gravel
and Hudson River trap rock, the results were:
Per cent. Per cent. Per cent.
Trap. Gravel. Voids in Mix.
100 with 0 50
60 " 40 38½
50 " 50 36
0 " 100 35
The weight of a cubic foot of loose gravel or stone is not an accurate
index of the percentage of voids unless the specific gravity is known.
Pure quartz weighs 165 lbs., per cu. ft., hence broken quartz having 40
per cent. voids weighs 165 × .60 = 99 lbs. per cu. ft. Few gravels are
entirely quartz, and many contain stone having a greater specific
gravity like some traps or a less specific gravity like some shales and
sandstone. Tables VI and VII give the specific gravities of common
stones and minerals and Table VIII gives the weights corresponding to
different percentages of voids for different specific gravities.
TABLE VI.--SPECIFIC GRAVITY OF STONE. (Condensed from Merrill's "Stones
for Building.")
Trap, Boston, Mass. 2.78
" Duluth, Minn. 2.8 to 3.0
" Jersey City, N. J. 3.03
" Staten Island, N. Y. 2.86
Gneiss, Madison Ave., N. Y. 2.92
Granite, New London, Conn. 2.66
" Greenwich, Conn. 2.84
" Vinalhaven, Me. 2.66
" Quincy, Mass. 2.66
" Barre, Vt. 2.65
Limestone, Joliet, Ill. 2.56
" Quincy, Ill. 2.51 to 2.57
Limestone, (oolitic) Bedford, Ind. 2.25 to 2.45
" Marquette, Mich. 2.34
" Glens Falls, N.Y. 2.70
" Lake Champlain, N. Y. 2.75
Sandstone, Portland, Conn. 2.64
" Haverstraw, N. Y. 2.13
" Medina, N. Y. 2.41
" Potsdam, N. Y. 2.60
" (grit) Berea, O. 2.12
TABLE VII.--SPECIFIC GRAVITY OF COMMON MINERALS AND ROCKS.
Apatite 2.92-3.25
Basalt 3.01
Calcite, CaCO_{3} 2.5-2.73
Cassiterite, SnO_{2} 6.4-7.1
Cerrusite, PbCO_{3} 6.46-6.48
Chalcopyrite, CuFeS_{2} 4.1-4.3
Coal, anthracite 1.3-1.84
Coal, bituminous 1.2-1.5
Diabase 2.6-3.03
Diorite 2.92
Dolomite, CaMg (CO_{3})² 2.8-2.9
Felspar 2.44-2.78
Felsite 2.65
Galena, Pbs 7.25-7.77
Garnet 3.15-4.31
Gneiss 2.62-2.92
Granite 2.55-2.86
Gypsum 2.3-3.28
Halite (salt) NaCl 2.1-2.56
Hematite, Fe_{2}O_{3} 4.5-5.3
Hornblende 3.05-3.47
Limonite, Fe_{3}O_{4} (OH)^{6} 3.6-4.0
Limestone 2.35-2.87
Magnetite, Fe_{3}O_{4} 4.9-5.2
Marble 2.08-2.85
Mica 2.75-3.1
Mica Schist 2.5-2.9
Olivine 3.33-3.5
Porphyry 2.5-2.6
Pyrite, FeS_{2} 4.83-5.2
Quartz, SiO_{2} 2.5-2.8
Quartzite 2.6-2.7
Sandstone 2.0-2.78
" Medina 2.4
" Ohio 2.2
" Slaty 1.82
Shale 2.4-2.8
Slate 2.5-2.8
Sphalerite, ZnS 3.9-4.2
Stibnite, Sb_{2}S_{3} 4.5-4.6
Syenite 2.27-2.65
Talc 2.56-2.8
Trap 2.6-3.0
TABLE VIII.--SHOWING WEIGHT OF STONE WITH DIFFERENT PERCENTAGES OF VOIDS
FOR DIFFERENT SPECIFIC GRAVITIES.
Weight Weight
in Lbs. in Lbs. Weight in Lbs. per cu. yd.
Specific per per when Voids are
Gravity. cu. ft. cu. yd. 30% 35% 40% 45% 50%
1.0 62.355 1,684 1,178 1,094 1,010 926 842
2.0 124.7 3,367 2,357 2,187 2,020 1,852 1,684
2.1 130.9 3,536 2,475 2,298 2,121 1,945 1,768
2.2 137.2 3,704 2,593 2,408 2,222 2,037 1,852
2.3 143.4 3,872 2,711 2,517 2,323 2,130 1,936
2.4 149.7 4,041 2,828 2,626 2,424 2,222 2,020
2.5 155.9 4,209 2,946 2,736 2,525 2,315 2,105
2.6 162.1 4,377 3,064 2,845 2,626 2,408 2,189
2.7 168.4 4,546 3,182 2,955 2,727 2,500 2,273
2.8 174.6 4,714 3,300 3,064 2,828 2,593 2,357
2.9 180.9 4,882 3,418 3,174 2,929 2,685 2,441
3.0 187.1 5,051 3,536 3,283 3,030 2,778 2,526
3.1 193.3 5,219 3,653 3,392 3,131 2,871 2,609
3.2 199.5 5,388 3,771 3,502 3,232 2,963 2,694
3.3 205.8 5,556 3,889 3,611 3,333 3,056 2,778
3.4 212.0 5,724 4,007 3,721 3,434 3,148 2,862
3.5 218.3 5,893 4,125 3,830 3,535 3,241 2,947
In buying broken stone by the cubic yard it should be remembered that
hauling in a wagon compacts the stone by shaking it down and reduces the
volume. Table IX shows the results of tests made by the Illinois Highway
Commission to determine the settlement of crushed stone in wagon loads
for different lengths of haul. The road over which the tests were made
was a macadam road, not particularly smooth, but might be considered as
an average road surface. The wagon used was one with a dump bottom
supported by chains, which were drawn as tight as possible, so as to
reduce the sag to a minimum. It will be noticed that about 50 per cent.
of the settlement occurs within the first 100 ft., and 75 per cent. of
the settlement in the first 200 ft. Almost all of the settlement occurs
during the first half mile, as the tests showed practically no
additional settlement for distances beyond. Some of the wagons were
loaded from the ground with shovels, others were loaded from bins, the
stone having a 15-ft. drop, which compacted the stone a little more than
where loaded with shovels, so that there was somewhat less settlement.
But at the end of a half mile the density was practically the same,
whatever the method of loading. The density at the beginning and at the
end of the haul can be compared by the weight of a given volume of
crushed stone. For convenience, the weight of a cubic yard of the
material at the beginning of the haul and at the end was computed from
the known contents of a wagon.
TABLE IX.--SHOWING SETTLEMENT OF BROKEN STONE DUE TO DIFFERENT LENGTHS
OF HAUL ON ORDINARILY GOOD ROAD IN WAGONS.
[Transcriber's Note: Table split]
-----------+------------+---------------------------------------------------+
Size. | Method of | |
| Loading. | Per cent Settlement for Hauling. |
| +---------------------------------------------------+
| |100'|200'|300'|400'|500'|600'|700'|½ Mile|1 Mile |
-----------+------------+----+----+----+----+----+----+----+------+---------+
Screenings |15 ft. drop |....| ...|....|....|....|....|....| 11.5 | 11.5 |
Screenings |15 ft. drop |....| ...|....|....|....|....|....| 12.6 | 12.6 |
Screenings |15 ft. drop | 7.3| 8.3| 8.9| 9.2| 9.5|10.1|10.1| 11.2 | .... |
Screenings |15 ft. drop | 5.0| 9.6|10.2|10.2|10.4|10.4|10.4| 12.4 | .... |
| | | | | | | | | | |
===========+============+====+====+====+====+====+====+====+======+=========+
1½ inch |15 ft. drop | ...|....|....|....|....|....|....| 11.5 | 11.5[C] |
1½ inch |15 ft. drop | 5.3| 6.2| 7.1| 7.7| 7.9| 8.0| 8.3| 9.2 | .... |
1½ inch |15 ft. drop | 2.6| 3.7| 4.9| 5.3| 5.3| 5.3| 5.4| 5.4 | .... |
1½ inch |Shovels | 3.5| 4.1| 4.8| 5.3| 5.3| 5.7| 6.5| 7.25| .... |
1½ inch |Shovels | ...|....|....|....|....|....|....| 12.6 | 12.6 |
===========+============+====+====+====+====+====+====+====+======+=========+
3 inch |15 ft. drop | ...|....|....|....|....|....|....| 10.1 | 10.1 |
3 inch |15 ft. drop | 3.5| 4.2| 4.5| 4.8| 5.0| 5.0| 5.0| 6.0 | .... |
3 inch |15 ft. drop | 0.5| 2.5| 2.5| 4.1| 4.3| 4.3| 4.3| 4.9 | .... |
3 inch |Shovels | ...|....|....|....|....|....|....| 12.6 | 12.6 |
3 inch |Shovels | 5.0| 5.6| 6.5| 6.5| 6.8| 6.8| 6.8| 7.1 | .... |
------------------------+----+----+----+----+----+----+----+------+---------+
[Footnote C: Same per cent of settlement for two-mile haul.]
-----------+------------+-----------------
Size. | Method of | Weight per
| Loading. | Cu. Yd. in Lbs.
| +-----------------
| | At | At
| | start.| finish.
-----------+------------+-------+-------
Screenings |15 ft. drop | 2,518 | 2,840
Screenings |15 ft. drop | 2,518 | 2,886
Screenings |15 ft. drop | 2,450 | 2,770
Screenings |15 ft. drop | 2,425 | 2,780
===========+============+=======+========
| | |
1½ inch |15 ft. drop | 2,305 | 2,600
1½ inch |15 ft. drop | 2,380 | 2,625
1½ inch |15 ft. drop | 2,450 | 2,600
1½ inch |Shovels | 2,270 | 2,445
1½ inch |Shovels | 2,305 | 2,642
===========+============+=======+========
3 inch |15 ft. drop | 2,376 | 2,638
3 inch |15 ft. drop | 2,360 | 2,505
3 inch |15 ft. drop | 2,470 | 2,595
3 inch |Shovels | 2,270 | 2,601
3 inch |Shovels | 2,335 | 2,510
------------------------+-------+--------
~THEORY OF THE QUANTITY OF CEMENT IN MORTAR AND CONCRETE.~--All sand
contains a large percentage of voids; in 1 cu. ft. of loose sand there
is 0.3 to 0.5 cu. ft. of voids, that is, 30 to 50 per cent. of the sand
is voids. In making mortar the cement is mixed with the sand and the
flour-like particles of the cement fit in between the grains of sand
occupying a part or all of the voids. The amount of cement required in a
mortar will naturally depend upon the amount of voids in the particular
sand with which it is mixed and since a correct estimate of the number
of barrels of cement per cubic yard of mortar is very important, and
since it is not always possible to make actual mixtures before bidding,
rules based on various theories have been formulated for determining
these quantities. In this volume the rule based on the theory outlined
by one of the authors in 1901 will be followed. The following is a
discussion of the authors' theory:
When loose sand is mixed with water, its volume or bulk is increased;
subsequent jarring will decrease its volume, but still leave a net gain
of about 10 per cent.; that is, 1 cu. ft. of dry sand becomes about 1.1
cu. ft. of damp sand. Not only does this increase in the volume of the
sand occur, but, instead of increasing the voids that can be filled with
cement, there is an absolute loss in the volume of available voids. This
is due to the space occupied by the water necessary to bring the sand to
the consistency of mortar; furthermore, there is seldom a perfect
mixture of the sand and cement in practice, thus reducing the available
voids. It is safe to call this reduction in available voids about 10 per
cent.
When loose, dry Portland cement is wetted, it shrinks about 15 per cent,
in volume, behaving differently from the sand, but it never shrinks back
to quite as small a volume as it occupies when packed tightly in a
barrel. Since barrels of different brands vary widely in size, the
careful engineer or contractor will test any brand he intends using in
large quantities, in order to ascertain exactly how much cement paste
can be made. He will find a range of from 3.2 cu. ft. to 3.8 cu. ft. per
barrel of Portland cement. Obviously the larger barrel may be cheaper
though its price is higher. Specifications often state the number of
cubic feet that will be allowed per barrel in mixing the concrete
ingredients, so that any rule or formula to be of practical value must
contain a factor to allow for the specified size of the barrel, and
another factor to allow for the actual number of cubic feet of paste
that a barrel will yield--the two being usually quite different.
The deduction of a rational, practical formula for computing the
quantity of cement required for a given mixture will now be given, based
upon the facts above outlined.
Let p = number of cu. ft. cement paste per bbl., as determined
by actual test.
n = number of cu. ft. of cement per bbl., as specified in
the specifications.
s = parts of sand (by volume) to one part of cement, as
specified.
g = parts of gravel or broken stone (by volume) to one
part of cement, as specified.
v = percentage of voids in the dry sand, as determined
by test.
V = percentage of voids in the gravel or stone, as determined
by test.
Then, in a mortar of 1 part cement to s parts sand, we have:
n s = cu. ft. of dry sand to 1 bbl. of cement.
n s v = " " " voids in the dry sand.
0.9 n s v = " " " available voids in the wet sand.
1.1 n s = " " " wet sand.
p - 0.9 n s v = " " " cement paste in excess of the voids.
Therefore:
1.1 n s + (p - 0.9 n s v) = cu. ft. of mortar per bbl.
Therefore:
27 27
N = ------------------------- = --------------------
1.1 n s + (p - 0.9 n s v) p + n s (1.1 - 0.9 v)
N being the number of barrels of cement per cu. yd. of mortar.
When the mortar is made so lean that there is not enough cement paste to
fill the voids in the sand, the formula becomes:
27
N = -------
1.1 n s
A similar line of reasoning will give us a rational formula for
determining the quantity of cement in concrete; but there is one point
of difference between sand and gravel (or broken stone), namely, that
the gravel does not swell materially in volume when mixed with water.
However, a certain amount of water is required to wet the surface of the
pebbles, and this water reduces the available voids, that is, the voids
that can be filled by the mortar. With this in mind, the following
deduction is clear, using the nomenclature and symbols above given:
ng = cu. ft. of dry gravel (or stone).
ng V = " " " voids in dry gravel.
0.9 ng V = " " " "available voids" in the wet gravel.
p + n s (1.1 - 0.9 v) - 0.9 ng V = excess of mortar over the available
voids in the wet gravel.
ng + p + n s (1.1 - 0.9 v) - 0.9 ng V = cu. ft. of concrete from
1 bbl. cement.
27
N = -----------------------------------------------
p + n s (1.1 - 0.9v) + ng (1 - 0.9V)
N being the number of barrels of cement required to make 1 cu. yd. of
concrete.
This formula is rational and perfectly general. Other experimenters may
find it desirable to use constants slightly different from the 1.1 and
the 0.9, for fine sands swell more than coarse sands, and hold more
water.
The reader must bear in mind that when the voids in the sand exceed the
cement paste, and when the available voids in the gravel (or stone)
exceed the mortar, the formula becomes:
27
N = ------
ng
These formulas give the amounts of cement in mortars and concretes
compacted in place. Tables X to XIII are based upon the foregoing
theory, and will be found to check satisfactorily with actual tests.
In using these tables remember that the proportion of cement to sand is
by volume, and not by weight. If the specifications state that a barrel
of cement shall be considered to hold 4 cu. ft., for example, and that
the mortar shall be 1 part cement to 2 parts sand, then 2 barrel of
cement is mixed with 8 cu. ft. of sand, regardless of what is the actual
size of the barrel, and regardless of how much cement paste can be made
with a barrel of cement. If the specifications fail to state what the
size of a barrel will be, then the contractor is left to guess.
TABLE X.--BARRELS OF PORTLAND CEMENT PER CUBIC YARD OF MORTAR.
(Voids in sand being 35%, and 1 bbl. cement yielding 3.65 cu. ft. of
cement paste.)
----------------------------------+------+-------+------+-------+------+------
Proportion of Cement to Sand |1 to 1|1 to 1½|1 to 2|1 to 2½|1 to 3|1 to 4
----------------------------------+------+-------+------+-------+------+------
| Bbls.| Bbls. | Bbls.| Bbls. | Bbls.| Bbls.
Barrel specified to be 3.5 cu. ft.| 4.22 | 3.49 | 2.97 | 2.57 | 2.28 | 1.76
" " " 3.8 | 4.09 | 3.33 | 2.81 | 2.45 | 2.16 | 1.62
" " " 4.0 | 4.00 | 3.24 | 2.73 | 2.36 | 2.08 | 1.54
" " " 4.4 | 3.81 | 3.07 | 2.57 | 2.27 | 2.00 | 1.40
+------+-------+------+-------+------+------
Cu. yds. sand per cu. yd. mortar | 0.6 | 0.7 | 0.8 | 0.9 | 1.0 | 1.0
----------------------------------+------+-------+------+-------+------+------
TABLE XI.--BARRELS OF PORTLAND CEMENT PER CUBIC YARD OF MORTAR.
(Voids in sand being 45%, and 1 bbl. cement yielding 3.4 cu. ft. of
cement paste.)
----------------------------------+------+-------+------+-------+------+------
Proportion of Cement to Sand |1 to 1|1 to 1½|1 to 2|1 to 2½|1 to 3|1 to 4
----------------------------------+------+-------+------+-------+------+------
| Bbls.| Bbls. | Bbls.| Bbls. | Bbls.| Bbls.
Barrel specified to be 3.5 cu. ft.| 4.62 | 3.80 | 3.25 | 2.84 | 2.35 | 1.76
" " " 3.8 " | 4.32 | 3.61 | 3.10 | 2.72 | 2.16 | 1.62
" " " 4.0 " | 4.19 | 3.46 | 3.00 | 2.64 | 2.05 | 1.54
" " " 4.4 " | 3.94 | 3.34 | 2.90 | 2.57 | 1.86 | 1.40
+------+-------+------+-------+------+------
Cu. yds. sand per cu. yd. mortar | 0.6 | 0.8 | 0.9 | 1.0 | 1.0 | 1.0
----------------------------------+------+-------+------+-------+------+------
If the specifications call for proportions by weight, assume a Portland
barrel to contain 380 lbs. of cement, and test the actual weight of a
cubic foot of the sand to be used. Sand varies extremely in weight, due
both to the variation in the per cent. of voids, and to the variation in
the kind of minerals of which the sand is composed. A quartz sand having
35 per cent. voids weighs 107 lbs. per cu. ft.; but a quartz sand
having 45 per cent. voids weighs only 91 lbs. per cu. ft. If the weight
of the sand must be guessed at, assume 100 lbs. per cu. ft. If the
specifications require a mixture of 1 cement to 2 of sand by weight, we
will have 380 lbs. (or 1 bbl.) of cement mixed with 2 × 380, or 760 lbs.
of sand; and if the sand weighs 90 lbs. per cu. ft., we shall have 760 ÷
90, or 8.44 cu. ft. of sand to every barrel of cement. In order to use
the tables above given, we may specify our own size of barrel; let us
say 4 cu. ft.; then 8.44 ÷ 4 gives 2.11 parts of sand by volume to 1
part of cement. Without material error we may call this a 1 to 2 mortar,
and use the tables, remembering that our barrel is now "specified to be"
4 cu. ft. If we have a brand of cement that yields 3.4 cu. ft. of paste
per bbl., and sand having 45 per cent. voids, we find that approximately
3 bbls. of cement per cu. yd. of mortar will be required.
TABLE XII.--INGREDIENTS IN 1 CUBIC YARD OF CONCRETE.
(Sand voids, 40%; stone voids, 45%; Portland cement barrel yielding 3.65
cu. ft. paste. Barrel specified to be 3.8 cu. ft.)
--------------------------------+-------+-------+-------+-------+-------+------
| 1:2:4 | 1:2:5 | 1:2:6 | 1:2½:5| 1:2½:6| 1:3:4
--------------------------------+-------+-------+-------+-------+-------+------
Bbls. cement per cu. yd. concr't| 1.46 | 1.30 | 1.18 | 1.13 | 1.00 | 1.25
Cu. yds. sand " " | 0.41 | 0.36 | 0.33 | 0.40 | 0.35 | 0.53
Cu. yds. stone " " | 0.82 | 0.90 | 1.00 | 0.80 | 0.84 | 0.71
--------------------------------+-------+-------+-------+-------+-------+------
Proportions by Volume. | 1:3:5 | 1:3:6 | 1:3:7 | 1:4:7 | 1:4:8 | 1:4:9
--------------------------------+-------+-------+-------+-------+-------+------
Bbls. cement per cu. yd. concr't| 1.13 | 1.05 | 0.96 | 0.82 | 0.77 | 0.73
Cu. yds. sand " " | 0.48 | 0.44 | 0.40 | 0.46 | 0.43 | 0.41
Cu. yds. stone " " | 0.80 | 0.88 | 0.93 | 0.80 | 0.86 | 0.92
--------------------------------+-------+-------+-------+-------+-------+------
NOTE.--This table is to be used where cement is measured packed
in the barrel, for the ordinary barrel holds 3.8 cu. ft.
It should be evident from the foregoing discussions that no table can be
made, and no rule can be formulated that will yield accurate results
unless the brand of cement is tested and the percentage of voids in the
sand determined. This being so the sensible plan is to use the tables
merely as a rough guide, and, where the quantity of cement to be used is
very large, to make a few batches of mortar using the available brands
of cement and sand in the proportions specified. Ten dollars spent in
this way may save a thousand, even on a comparatively small job, by
showing what cement and sand to select.
It will be seen that Tables XII and XIII can be condensed into the
following rule:
_Add together the number of parts and divide this sum into ten, the
quotient will be approximately the number of barrels of cement per cubic
yard._
TABLE XIII.--INGREDIENTS IN 1 CUBIC YARD OF CONCRETE.
(Sand voids, 40%; stone voids, 45%; Portland cement barrel yielding 3.65
cu. ft. of paste. Barrel specified to be 4.4 cu. ft.)
--------------------------------+------+------+------+------+------+-----
Proportions by Volume. |1:2:4 |1:2:5 |1:2:6 |1:2½:5|1:2½:6|1:3:4
--------------------------------+------+------+------+------+------+-----
Bbls. cement per cu. yd. concr't| 1.30 | 1.16 | 1.00 | 1.07 | 0.96 | 1.08
Cu. yds. sand " " | 0.42 | 0.38 | 0.33 | 0.44 | 0.40 | 0.53
Cu. yds. stone " " | 0.84 | 0.95 | 1.00 | 0.88 | 0.95 | 0.71
--------------------------------+------+------+------+------+------+-----
Proportions by Volume. |1:3:5 |1:3:6 |1:3:7 |1:4:7 |1:4:8 |1:4:9
--------------------------------+------+------+------+------+------+-----
Bbls. cement per cu. yd. concr't| 0.96 | 0.90 | 0.82 | 0.75 | 0.68 | 0.64
Cu. yds. sand " " | 0.47 | 0.44 | 0.40 | 0.49 | 0.44 | 0.42
Cu. yds. stone " " | 0.78 | 0.88 | 0.93 | 0.86 | 0.88 | 0.95
--------------------------------+------+------+------+------+------+-----
NOTE.--This table is to be used when the cement is measured
loose, after dumping it into a box, for under such conditions a
barrel of cement yields 4.4 cu. ft. of loose cement.
Thus for a 1:2:5 concrete, the sum of the parts is 1 + 2 + 5, which is
8; then 10 ÷ 8 is 1.25 bbls., which is approximately equal to the 1.30
bbls. given in the table. Neither is this rule nor are the tables
applicable if a different size of cement barrel is specified, or if the
voids in the sand or stone differ materially from 40 per cent. to 45 per
cent. respectively. There are such innumerable combinations of varying
voids, and varying sizes of barrel, that the authors do not deem it
worth while to give other tables. The following amounts of cement per
cubic yard of mortar were determined by test:
----------------+------+------+------+------+------+------+------+------+------
Authority | Neat.|1 to 1|1 to 2|1 to 3|1 to 4|1 to 5|1 to 6|1 to 7|1 to 8
----------------+-------------+------+------+------+------+------+------+------
| Bbls.| Bbls.| Bbls.| Bbls.| Bbls.| Bbls.| Bbls.| Bbls.| Bbls.
Sabin | 7.40 | 4.17 | 2.84 | 2.06 | 1.62 | 1.33 | 1.14 | .... | ....
W. B. Fuller | 8.02 | 4.58 | 3.09 | 2.30 | 1.80 | 1.48 | 1.23 | 1.11 | 1.00
H. P. Boardman. | 7.40 | 4.50 | 3.18 | 2.35 | .... | .... | .... | .... | ....
----------------+------+------+------+------+------+------+------+------+------
The proportions were by barrels of cement to barrels of sand, and Sabin
called a 380-lb. barrel 3.65 cu. ft., whereas Fuller called a 380-lb.
barrel 3.80 cu. ft.; and Boardman called a 380-lb. barrel 3.5 cu. ft.
Sabin used a sand having 38 per cent. voids; Fuller used a sand having
45 per cent. voids; and Boardman used a sand having 38 per cent. voids.
It will be seen that the cement used by Sabin yielded 3.65 cu. ft. of
cement paste per bbl. (i. e. 27 ÷ 7.4), whereas the (Atlas) cement used
by Fuller yielded 3.4 cu. ft. of cement paste per bbl. Sabin found that
a barrel of cement measured 4.37 cu. ft. when dumped and measured loose.
Mr. Boardman states a barrel (380 lbs., net) of Lehigh Portland cement
yields 3.65 cu. ft. of cement paste; and that a barrel (265 lbs., net)
of Louisville natural cement yields 3.0 cu. ft. of cement paste.
Mr. J. J. R. Croes, M. Am. Soc. C. E., states that 1 bbl. of Rosendale
cement and 2 bbl. of sand (8 cu. ft.) make 9.7 cu. ft. of mortar, the
extreme variations from this average being 7 per cent.
Frequently concrete is made by mixing one volume of cement with a given
number of volumes of pit gravel; no sand being used other than the sand
that is found naturally mixed with the gravel. In such cases the cement
rarely increases the bulk of the gravel, hence Table XIV will give the
approximate amount of cement, assuming 1 cu. yd. of gravel per cubic
yard of concrete.
TABLE XIV.--SHOWING BARRELS OF CEMENT PER CUBIC YARD OF VARIOUS MIXTURES
OF CEMENT AND PIT GRAVEL.
---------+------------------------------------------------------------
Spc. Vol.|Barrels of Cement per Cubic Yard of Concrete for Mixtures of
of bbl. +-------+-------+-------+-------+-------+-------+------------
cu. ft. | 1-5 | 1-6 | 1-7 | 1-8 | 1-9 | 1-10 | 1-12
---------+-------+-------+-------+-------+-------+-------+------------
3.8 | 1.41 | 1.18 | 1.01 | 0.874 | 0.789 | 0.71 | 0.59
4.4 | 1.25 | 1.02 | 0.875 | 0.766 | 0.681 | 0.61 | 0.51
---------+-------+-------+-------+-------+-------+-------+------------
~PERCENTAGE OF WATER IN CONCRETE.~--Tests show that dry mixtures when
carefully deposited and well tamped produce the stronger concrete. This
superiority of dry mixtures it must be observed presupposes careful
deposition and thorough tamping, and these are tasks which are difficult
to have accomplished properly in actual construction work and which, if
accomplished properly, require time and labor. Wet mixtures readily flow
into the corners and angles of the forms and between and around the
reinforcing bars with only a small amount of puddling and slicing and
are, therefore, nearly always used because of the time and labor saved
in depositing and tamping. The following rule by which to determine the
percentage of water by weight for any given mixture of mortar for wet
concrete will be found satisfactory:
_Multiply the parts of sand by 8, add 24 to the product, and divide the
total by the sum of the parts of sand and cement._
For example if the percentage of water is required for a 1-3 mortar:
(3 × 8) + 24
------------ = 12.
4
Hence the water should be 12 per cent. of the combined weight of cement
and sand. For a 1-1 mortar the rule gives 16 per cent.; for a 1-2 mortar
it gives 13½ per cent., and for a 1-6 mortar it gives 10.3 per cent.
To calculate the amount of water per cubic yard of 1-3-6 concrete for
example the procedure would be as follows: By the above rule a 1-3
mortar requires
(3 × 8) + 24
------------ = 12 per cent. water.
4
A 1-3-6 concrete, according to Table XII, contains 1.05 bbls. cement and
0.44 cu. yd. sand. Cement weighs 380 lbs. per barrel, hence 1.05 bbls.
would weigh 380 × 1.05 = 399 lbs. Sand weighs 2,700 lbs. per cu. yd.,
hence 0.44 cu. yd. of sand would weigh 2,700 × 0.44 = 1,188 lbs. The
combined weight of the cement and sand would thus be 399 + 1,188 = 1,587
lbs. and 12 per cent. of 1.587 lbs. is 190 lbs. of water. Water weighs
8.355 lbs. per gallon, hence 190 × 8.355 = 23 gallons of water per cubic
yard of 1-3-6 concrete.
~METHODS OF MEASURING AND WEIGHING.~--The cement, sand and aggregate for
concrete mixtures are usually measured by hand, the measuring being done
either in the charging buckets or in the barrows or other receptacles
used to handle the material to the charging buckets. The process is
simple in either case when once the units of measurement are definitely
stated. This is not always the case. Some engineers require the
contractor to measure the sand and stone in the same sized barrel that
the cement comes in, in which case 1 part of sand or aggregate usually
means 3.5 cu. ft. Other engineers permit both heads of the barrel to be
knocked out for convenience in measuring the sand and stone, in which
case a barrel means 3.75 cu. ft. Still other engineers permit the cement
to be measured loose in a box, then a barrel usually means from 4 to 4.5
cu. ft. Cement is shipped either in barrels or in bags and the engineer
should specify definitely the volume at which he will allow the original
package to be counted, and also, if cement barrels are to be used in
measuring the sand and stone, he should specify what a "barrel" is to
be. When the concrete is to be mixed by hand the better practice is to
measure the sand and stone in bottomless boxes of the general type shown
by Fig. 10 and of known volume, and then specify that a bag of cement
shall be called 1 cu. ft., 0.6 cu. ft., or such other fraction of a
cubic foot as the engineer may choose. The contractor then has a
definite basis on which to estimate the quantity of cement required for
any specified mixture. The same is true if the measuring of the sand and
stone be done in barrows or in the charging bucket. The volume of the
bag or barrel of cement being specified the contractor has a definite
and simple problem to solve in measuring his materials.
[Illustration: Fig. 10.--Bottomless Box for Measuring Materials in
Proportioning Concrete.]
To avoid uncertainty and labor in measuring the cement, sand and stone
or gravel various automatic measuring devices have been designed. A
continuous mixer with automatic measuring and charging mechanism is
described in Chapter XIV. Figure 11 shows the Trump automatic measuring
device. It consists of a series of revolving cylinders, each opening
onto a "table," which revolves with the cylinders, and of a set of fixed
"knives," which, as the "tables" revolve, scrape off portions of the
material discharged from each cylinder onto its "table." The
illustration shows a set of two cylinders; for concrete work a third
cylinder is added. The three tables are set one above the other, each
with its storage cylinder, and being attached to the same spindle all
revolve together. For each table there is a knife with its own
adjusting mechanism. These knives may be adjusted at will to vary the
percentage of material scraped off.
[Illustration: Fig. 11.--Sketch Showing Trump Automatic Measuring Device
for Materials in Proportioning Concrete.]
Automatic measuring devices are most used in connection with continuous
mixers, but they may be easily adapted to batch mixers if desired. One
point to be observed is that all of these automatic devices measure the
cement loose and this must be allowed for in proportioning the mixture.
CHAPTER III.
METHODS AND COST OF MAKING AND PLACING CONCRETE BY HAND.
The making and placing of concrete by hand is divided into the following
operations: (1) Loading the barrows, buckets, carts or cars used to
transport the cement, sand and stone to the mixing board; (2)
Transporting and dumping the material; (3) Mixing the material by
turning with shovels and hoes; (4) Loading the concrete by shovels into
barrows, buckets, carts or cars; (5) Transporting the concrete to place;
(6) Dumping and spreading; (7) Ramming.
~LOADING INTO STOCK PILES.~--Stock piles should always be provided unless
there is some very good reason to the contrary. They prevent stoppage of
work through irregularities in the delivery of the material, and they
save foreman's time in watching that the material is delivered as
promptly as needed for the work immediately in hand. The location of the
stock piles should be as close to the work as possible without being in
the way of construction; forethought both in locating the piles and in
proportioning their size to the work will save the contractor money.
The stone and sand will ordinarily be delivered in wagons or cars. If
delivered in cars, effort should be made to secure delivery in flat cars
when the unloading is to be done by shoveling; this is more particularly
necessary for the broken stone. Stone can be shoveled from hopper bottom
cars only with difficulty as compared with shoveling from flat bottom
cars; the ratio is about 14 cu. yds. per day per man from hopper bottom
cars as compared with 20 cu. yds. per day per man from flat bottom cars.
When the cars can be unloaded through a trestle, hopper bottom cars
should by all means be secured for delivering the stone. If the amount
of work will justify the expense, a trestle may be built; often there is
a railway embankment which can be dug away for a short distance and the
track carried on stringers to make a dumping place, from which the stone
can be shoveled.
Sand can be dumped directly on the ground, but broken stone unless it is
very small, ¾-in. or less, should always be dumped on a well made plank
floor. A good floor is made of 2-in. plank, nailed to 4×6-in. mud sills,
spaced 3 ft. apart, and well bedded in the ground. Loose plank laid
directly on the ground settle unevenly and thus the smooth shoveling
surface which is sought is not obtained; the object of the floor is to
provide an even surface, along which a square pointed shovel can be
pushed; it is very difficult to force such a shovel into broken stone
unless it is very fine. A spading fork is a better tool than a shovel,
with which to load broken stone from piles. A man can load from 18 to 20
cu. yds. of broken stone into wheelbarrows or carts in 10 hours when
shoveling from a good floor, but he can load only 12 to 14 cu. yds. per
day when shoveling from a pile without such a floor. It is a common
thing to see stone unloaded from cars directly onto the sloping side of
a railway embankment. This makes very difficult shoveling and results in
a waste of stone. Stone can usually be delivered by a steel lined chute
directly to a flooring located at the foot of the embankment; coarse
broken stone if given a start when cast from a shovel will slide on an
iron chute having a slope as flat as 3 or 4 to 1; sand will not slide on
a slope of 1½ to 1. When chuting is not practicable it will pay often to
shovel the stone into buckets handled by a stiff-leg derrick rather than
to unload it onto the bank. Stock piles of ample storage capacity are
essential when delivery is by rail, because of the uncertainty of rail
shipments. When the contractor is taking the sand and stone direct from
pit and quarry by wagon it is not necessary to have large stock piles.
~LOADING FROM STOCK PILES.~--In loading sand into wheelbarrows or carts
with shovels a man will load 20 cu. yds. per 10-hour day if he is
energetic and is working under a good foreman. Under opposite conditions
15 cu. yds. per man per day is all that it is safe to count on. A man
shoveling from a good floor will load 20 cu. yds. of stone per 10-hour
day; this is reduced to 15 cu. yds. per day if the stone is shoveled off
the ground and to 12 cu. yds. per day if in addition the management is
poor. There are ordinarily in a cubic yard of concrete about 1 cu. yd.
of stone and 0.4 cu. yd. of sand, so that the cost of loading the
materials into barrows or carts, with wages at 15 cts. per hour and
assuming 15 cu. yds. to be a day's work, would be:
1 cu. yd. stone loaded for 10 cts.
0.4 cu. yd. sand loaded for 4 cts.
-------
Total 14 cts.
To this is to be added the cost of loading the cement. This will cost
not over 2 cts. per cu. yd. of concrete; the total cost of loading
concrete materials into barrows or carts, therefore, does not often
exceed 16 cts. per cu. yd. of concrete.
~TRANSPORTING MATERIALS TO MIXING BOARDS~--Carrying the sand and stone
from stock piles to mixing board in shovels should never be practiced.
It takes from 100 to 150 shovelfuls of stone to make 1 cu. yd.; it,
therefore, costs 50 cts. per cu. yd. to carry it 100 ft. and return
empty handed, for in walking short distances the men travel very
slowly--about 150 ft. per minute. It costs more to walk a half dozen
paces with stone carried in shovels than to wheel it in barrows.
The most common method of transporting materials from stock piles to
mixing boards is in wheelbarrows. The usual wheelbarrow load on a level
plank runway is 3 bags of cement (300 lbs) or 3 cu. ft. of sand or
stone. If a steep rise must be overcome to reach the mixing platform the
load will be reduced to 2 bags (200 lbs.) of cement or 2 cu. ft. of sand
or stone. A man wheeling a barrow travels at a rate of 200 ft. per
minute, going and coming, and loses ¾ minute each trip dumping the load,
fixing run planks, etc. An active man will do 20 to 25 per cent. more
work than this, while a very lazy man may do 20 per cent. less. With
wages at 15 cts. per hour, the cost of wheeling materials for 1 cu. yd.
of concrete may be obtained by the following rule:
_To a fixed cost of 4 cts. (for lost time) add 1 ct. for every 20 ft. of
distance away from the stock pile if there is a steep rise in the
runway, but if the runway is level, add 1 ct. for every 30 ft. distance
of haul._
Since loading the barrows, as given above, was 16 cts. per cu. yd., the
total fixed cost is 16 + 4 = 20 cts. per cu. yd., to which is added 1
ct. for every 20 or 30 ft. haul depending on the grade of the runway.
The preceding figures assume the use of plank runways for the
wheelbarrows. These should never be omitted, and the barrows wheeled
over the ground. Even a hard packed earth path in dry weather is
inferior to a plank runway and when the ground is soft or muddy the loss
in efficiency of the men is serious. Where the runway must rise to the
mixing board, give it a slope or grade seldom steeper than 1 in 8, and
if possible flatter. Make a runway on a trestle at least 18 ins. wide,
so that men will be in no danger of falling. See to it, also, that the
planks are so well supported that they do not spring down when walked
over, for a springy plank makes hard wheeling. If the planks are so long
between the "horses" or "bents" used to support them, that they spring
badly, it is usually a simple matter to nail a cleat across the
underside of the planks and stand an upright strut underneath to support
and stiffen the plank.
When two-wheeled carts of the type shown by Fig. 12 are used the runway
requires two lines of planks.
Two-wheeled carts pushed by hand have been less used for handling
concrete materials than for handling concrete, but for distances from 50
to 150 ft. from stock pile to mixing board such carts are probably
cheaper for transporting materials than are wheelbarrows. These carts
hold generally three wheelbarrow loads and they are handled by one man
practically as rapidly and easily as is a wheelbarrow.
For all distances over 50 ft. from stock pile to mixing board, it is
cheaper to haul materials in one-horse dump carts than it is in
wheelbarrows. A cart should be loaded in 4 minutes and dumped in about 1
minute, making 5 minutes lost time each round trip. It should travel at
a speed of not less than 200 ft. per minute, although it is not unusual
to see variations of 15 or 20 per cent., one way or another, from this
average, depending upon the management of the work. A one-horse cart
will readily carry enough stone and sand to make ½ cu. yd. of concrete,
if the roads are fairly hard and level; and a horse can pull this load
up a 10 per cent. (rise of 1 ft. in 10 ft.) planked roadway provided
with cleats to give a foothold. If a horse, cart and driver can be hired
for 30 cts. per hour, the cost of hauling the materials for 1 cu. yd. of
concrete is given by the following rule:
_To a fixed cost of 5 cts. (for lost time at both ends of haul) add 1
ct. for every 100 ft. of distance from stock pile to mixing board._
[Illustration: Fig. 12.--Two-Wheeled Ransome Cart for Hauling Concrete.]
Where carts are used it is possible to locate the stock piles several
hundred feet from the mixing boards without adding materially to the
cost of the concrete. It is well, however, to have the stock piles in
sight of the foreman at the mixing board, so as to insure promptness of
delivery.
~METHODS AND COST OF MIXING.~--In mixing concrete by hand the materials
are spread in superimposed layers on a mixing board and mixed together
first dry and then with water by turning them with shovels or hoes. The
number of turns, the relative arrangement of the layers, and the
sequence of operations vary in practice with the notions of the
engineer controlling the work. No one mode of procedure in hand mixing
can, therefore, be specified for general application; the following are
representative examples of practice in hand mixing:
Measure the stone in a bottomless box and spread it until its thickness
in inches equals its parts by volume. Measure the sand in a bottomless
box set on the stone and spread the sand evenly over the stone layer.
Place the cement on the sand and spread evenly. Turn the material twice
with a square pointed shovel and then turn it a third time while water
is gently sprinkled on. A fourth turn is made to mix thoroughly the
water and the concrete is then shoveled into barrows, giving it a fifth
turn. Mr. Ernest McCullough, who gives this method, states that it is
the cheapest way to mix concrete by hand and still secure a good quality
of output.
In work done by Mr. H. P. Boardman the sand is measured in a bottomless
box and over it is spread the cement in an even layer. The cement and
sand are mixed dry with hoes, the water is added in pailfuls and the
whole mixed to a uniform porridge-like consistency. Into this thin
mortar all the stone for a batch is dumped, the measuring box is lifted
and the mixture turned by shovels. A pair of shovelers, one on each
side, is started at one end turning the material back and working toward
the opposite end. A second pair of shovelers takes the turned material
and turns it again. The concrete is then shoveled into the barrows by
the wheelers themselves as fast as it is turned the second time. By this
method a good gang of 20 to 25 men, using two boxes, will, Mr. Boardman
states, mix and place 45 to 60 cu. yds. of concrete in 10 hours,
depending on the wheelbarrow travel necessary. Assuming a gang of 25
men, this is a rate of 1.8 to 2.4 cu. yds. per man per 10-hour day,
concrete mixed and placed.
A method somewhat similar to the one just outlined is given by Mr. O. K.
Morgan. A mixing board made of 7/8-in. matched boards nailed to 2×3-in.
sills is used, with a mixing box about 8 ft. long, 4 ft. wide and 10 to
12 ins. deep. This box is set alongside the mixing board and in it the
cement and sand are mixed first dry and then wet; a fairly wet mortar is
made. Meanwhile the stone is spread in an even layer 6 ins. thick on
the mixing board and thoroughly drenched with water. The mortar from the
mixing box is cast by shovels in a fairly even layer over the stone and
the whole is turned two or three times with shovels, generally two turns
are enough. Six men are employed; two prepare the mortar, while four get
the stone in readiness, then all hands finish the operation.
The following method is given by Mr. E. Sherman Gould: Spread the sand
in a thin layer on the mixing board and over it spread the cement. Mix
dry with shovels, using four men, one at each corner, turning outward
and then working back again. Over the dry sand and cement mixture spread
the broken stone which has been previously wetted and on top of the
stone apply water evenly. The water will thus percolate through the
stone without splashing and evenly wet the sand and cement. Finally turn
the whole, using the same number of men and the same mode of procedure
as were used in dry mixing the sand and cement. Mr. Gould states that by
this method the contractor should average 2 cu. yds. of mixed concrete
per man per 10-hour day.
A novel method of hand mixing and an unusual record of output is
described by Maj. H. M. Chittenden, U. S. A., in connection with the
construction of a concrete arch bridge. The mixing was done by hand on a
single board 25 ft. long and sloping slightly from one end to the other.
The materials were dumped together on the upper end of the board.
Sixteen men were stationed along the board, eight on each side. The
first two men turned the mixture dry. Next to them stood a man who
applied the water after each shovelful. The next mixers kept turning the
material along and another waterman assisted in wetting it further down
the board. The men at the end of the board shoveled the concrete into
the carts which took it to the work. Each batch contained 18 cu. ft., or
0.644 cu. yd., and the rate of mixing was 10 cu. yds. per hour, or 6.25
cu. yds. per man per 10-hour day. The work of getting the materials
properly proportioned to the mixing board is not included in this
figure, but the loading of the mixed concrete is included.
It is plain from the foregoing, that specifications for hand mixing
should always state the method to be followed, and particularly the
number of turns necessary. If these matters are not specified the
contractor has to guess at the probable requirements of the engineer.
The authors have known of inspectors demanding from 6 to 9 turns of the
materials when specifications were ambiguous. It should also be made
clear whether or not the final shoveling into the barrows or carts
constitutes a turn, and whether any subsequent shoveling of the concrete
into place constitutes a turn. Inspectors and foremen have frequent
disputes over these questions.
Estimates of the cost of hand mixing may usually be figured upon the
number of times that the materials are to be turned by shovels. A
contractor is seldom required to turn the sand and cement more than
three times dry and three times wet, and then turn the mortar and stone
three times. A willing workman, under a good foreman, will turn over
mortar at the rate of 30 cu. yds. in 10 hours, lifting each shovelful
and casting it into a pile. With wages at $1.50 and six turns, this
means a cost of 5 cts. per cubic yard of mortar for each turn; as there
is seldom more than 0.4 cu. yd. of mortar in a cubic yard of concrete,
we have a cost of 2 cts. per cubic yard of concrete for each turn that
is given the mortar. So if the mortar is given six turns before the
stone is added and then the stone and mortar are mixed by three turns we
have: (2 cts. × 6) + (5 cts. × 3) = 12 + 15 = 27 cts. per cubic yard for
mixing concrete. In pavement foundation work two turns of the mortar
followed by two turns of the mortar and stone are considered sufficient.
The cost of mixing per cubic yard of concrete is then (2 cts. × 2) + (5
cts. × 2) = 4 + 10 = 14 cts. per cubic yard of concrete. One
specification known to the authors, requires six turns dry and three
turns wet for the mortar; under such specifications the cost of mixing
the mortar would be 50 per cent. higher than in the first example
assumed. On the other hand, they have seen concrete mixed for street
pavement foundation with only three turns before shoveling it into
place. These costs of mixing apply to work done by diligent men; easy
going men will make the cost 25 to 50 per cent greater.
~LOADING AND HAULING MIXED CONCRETE.~--Wheelbarrows and carts are employed
to haul the mixed concrete to the work. The loading of these with mixed
concrete by shoveling costs less than the loading of the materials
separately before mixing. While the weight is greater because of the
added water the volume of the concrete is much less than that of the
ingredients before mixing. Again the shoveling is done off a smooth
board with the added advantage of having the material lubricated and,
finally, the foreman is usually at this point to crowd the work. A good
worker will load 12½ cu. yds. of concrete per 10-hour day, and with
wages at $1.50 per day this would give a cost of 12 cts. per cu. yd. for
loading.
Practically the same principles govern the transporting of concrete in
barrows as govern the handling of the raw materials in them. The cost of
wheeling concrete is practically the same as for wheeling the dry
ingredients, so that the total cost of loading and wheeling may be
estimated by the following rule:
_To a fixed cost of 16 cts. for loading and lost time add 1 ct. for
every 30 ft. of level haul._
Within a few years wheelbarrows have been supplanted to a considerable
extent by hand carts of the general type shown by Fig. 12, which
illustrates one made by the Ransome Concrete Machinery Co. The bowl of
this cart has a capacity of 6 cu. ft. water measure. It is hung on a
1¼-in. steel axle; the wheels are 42 ins. in diameter with staggered
spokes and 2-in. half oval tires. The top of the bowl is 29½ ins. from
the ground. Owing to the large diameter of the wheels and the fact that
no weight comes on the wheeler, as with a wheelbarrow, this cart is
handled by one man about as rapidly and easily as is a wheelbarrow. It
will be noted that the two ends of the bowl differ in shape; the handle
is removable and can be attached to either end of the bowl. With the
handle attached as shown the bowl can be inverted for discharging onto a
pavement or floor; with the handle transferred to the opposite end the
bowl is fitted for dumping into narrow beam or wall forms. The maximum
load of wet concrete for a wheelbarrow is 2 cu. ft., and this is a heavy
load and one that is seldom averaged--1 to 1½ cu. ft. is more nearly the
general average. A cart of the above type will, therefore, carry from 3
to 5 wheelbarrow loads, and on good runways, which are essential, may be
pushed and dumped about as rapidly as a wheelbarrow. In succeeding pages
are given records of actual work with hand carts which should be studied
in this connection.
Portland cement concrete can be hauled a considerable distance in a dump
cart or wagon before it begins to harden; natural cement sets too
quickly to permit of its being hauled far. Portland cement does not
begin to set in less than 30 minutes. On a good road, with no long,
steep hills a team will haul a loaded wagon at a speed of about 200 ft.
per minute; it, therefore, takes 6½ minutes to travel a quarter of a
mile, 13 minutes to travel half a mile, and 26 minutes to travel a mile.
Portland cement concrete can, therefore, be hauled a mile before it
begins to set. The cost of hauling concrete in carts is about the same
as the cost of hauling the raw materials as given in a preceding
section.
When hand mixing is employed in building piers, abutments, walls, etc.,
the concrete often has to be hoisted as well as wheeled. A gallows frame
or a mast with a pulley block at the top and a team of horses can often
be used in such cases as described in Chapter XII for filling cylinder
piers, or in the same chapter for constructing a bridge abutment. It is
also possible often to locate the mixing board on high ground, perhaps
at some little distance from the forms. If this can be done, the use of
derricks may be avoided as above suggested or by building a light pole
trestle from the mixing board to the forms. The concrete can then be
wheeled in barrows and dumped into the forms. If the mixing board can be
located on ground as high as the top of the concrete structure is to be,
obviously a trestle will enable the men to wheel on a level runway. Such
a trestle can be built very cheaply, especially where second-hand
lumber, or lumber that can be used subsequently for forms is available.
A pole trestle whose bents are made entirely of round sticks cut from
the forest is a very cheap structure, if a foreman knows how to throw it
together and up-end the bents after they are made. One of the authors
has put up such trestles for 25 cts. per lineal foot of trestle,
including all labor of cutting the round timber, erecting it, and
placing a plank flooring 4 ft. wide on top. The stringers and flooring
plank were used later for forms, and their cost is not included. A
trestle 100 ft. long can thus be built at less cost than hauling,
erecting and taking down a derrick; and once the trestle is up it saves
the cost of operating a derrick.
In conclusion, it should be remarked that the comparative economy for
concrete work of the different methods of haulage described, does not
depend wholly on the comparative transportation costs; the effect of the
method of haulage on the cost of dumping and spreading costs must be
considered. For example, if carts deliver the material in such form that
the cost of spreading is greatly increased over what it would be were
the concrete delivered in wheelbarrows, the gain made by cart haulage
may be easily wiped out or even turned into loss by the extra spreading
charges. These matters are considered more at length in the succeeding
section.
~DUMPING, SPREADING AND RAMMING.~--The cost of dumping wheelbarrows and
carts is included in the rules of cost already given, excepting that in
some cases it is necessary to add the wages of a man at the dump who
assists the cart drivers or the barrow men. Thus in dumping concrete
from barrows into a deep trench or pit, it is usually advisable to dump
into a galvanized iron hopper provided with an iron pipe chute. One man
can readily dump all the barrows that can be filled from a concrete
mixer in a day, say 150 cu. yds. At this rate of output the cost of
dumping would be only 1 ct. per cu. yd., but if one man were required to
dump the output of a small gang of men, say 25 cu. yds., the cost of
dumping would be 6 cts. per cu. yd.
Concrete dumped through a chute requires very little work to spread it
in 6-in. layers; and, in fact, concrete that can be dumped from
wheelbarrows, which do not all dump in one place, can be spread very
cheaply; for not more than half the pile dumped from the barrow needs to
be moved, and then moved merely by pushing with a shovel. Since the
spreader also rams the concrete, it is difficult to separate these two
items. As nearly as the authors have been able to estimate this item of
spreading "dry" concrete dumped from wheelbarrows in street paving
work, the cost is 5 cts. per cu. yd. If, on the other hand, nearly all
the concrete must be handled by the spreaders, as in spreading concrete
dumped from carts, the cost is fully double, or 10 cts. per cu. yd. And
if the spreader has to walk even 3 or 4 paces to place the concrete
after shoveling it up, the cost of spreading will be 15 cts. per cu. yd.
For this reason it is apparent that carts are not as economical as
wheelbarrows for hauling concrete up to about 200 ft., due to the added
cost of spreading material delivered by carts.
The preceding discussion of spreading is based upon the assumption that
the concrete is not so wet that it will run. Obviously where concrete is
made of small stones and contains an excess of water, it will run so
readily as to require little or no spreading.
The cost of ramming concrete depends almost entirely upon its dryness
and upon the number of cubic yards delivered to the rammers. Concrete
that is mixed with very little water requires long and hard ramming to
flush the water to the surface. The yardage delivered to the rammers is
another factor, because if only a few men are engaged in mixing they
will not be able to deliver enough concrete to keep the rammers properly
busy, yet the rammers by slow though continuous pounding may be keeping
up an appearance of working. Then, again, it has been noticed that the
slower the concrete is delivered the more particular the average
inspector becomes. Concrete made "sloppy" requires no ramming at all,
and very little spading. The authors have had men do very thorough
ramming of moderately dry concrete for 15 cts. per cu. yd., where the
rammers had no spreading to do, the material being delivered in shovels.
It is rare indeed that spreading and ramming can be made to cost more
than 40 cts. per cu. yd., under the most foolish inspection, yet one
instance is recorded which, because of its rarity, is worth noting: Mr.
Herman Conrow is the authority for the data: 1 foreman, 9 men mixing, 1
ramming, averaged 15 cu. yds. a day, or only 1½ cu. yds. per man per
day, when laying wet concrete. When laying dry concrete the same gang
averaged only 8 cu. yds. a day, there being 4 men ramming. With foreman
at $2 and laborers at $1.50 a day, the cost was $2.12 per cu. yd. for
labor on the dry concrete as against $1.13 per cu. yd. for the wet
concrete. Three turnings of the stone with a wet mortar effected a
better mixture than four turnings with a dry mortar. The ramming of the
wet concrete cost 10 cts. per cu. yd., whereas the ramming of the dry
concrete cost 75 cts. per cu. yd. The authors think this is the highest
cost on record for ramming. It is evident, however, that the men were
under a poor foreman, for an output of only 15 cu. yds. per day with 10
men is very low for ordinary conditions. Moreover, the expensive amount
of ramming indicates either poor management or the most foolish
inspection requirements.
In conclusion it may be noted that if engineers specify a dry concrete
and "thorough ramming," they would do well also to specify what the word
"thorough" is to mean, using language that can be expressed in cents per
cubic yard. It is a common thing, for example, to see a sewer trench
specification in which one tamper is required for each two men shoveling
the back-fill into the trench; and some such specific requirement should
be made in a concrete specification if close estimates from reliable
contractors are desired. Surely no engineer will claim that this is too
unimportant a matter for consideration when it is known that ramming can
easily be made to cost as high as 40 cts. per cu. yd., depending largely
upon the whim of the inspector.
~THE COST OF SUPERINTENDENCE.~--This item is obviously dependent upon the
yardage of concrete handled under one foreman and the daily wages of the
foreman. If a foreman receives $3 a day and is bossing a job where only
12 cu. yds. are placed daily, we have a cost of 25 cts. per cu. yd. for
superintendence. If the same foreman is handling a gang of 20 men whose
output is 50 cu. yds., the superintendence item is only 6 cts. per cu.
yd. If the same foreman is handling a concrete-mixing plant having a
daily output of 150 cu. yds., the cost of superintendence is but 2 cts.
per cu. yd. These elementary examples have been given simply because
figures are more impressive than generalities, and because it is so
common a sight to see money wasted by running too small a gang of men
under one foreman.
Of all classes of contract work, none is more readily estimated day by
day than concrete work, not only because it is usually built in regular
shapes whose volumes are easily ascertained at the end of each day, but
because a record of the bags, or barrels, or batches gives a ready
method of computing the output of each gang. For this reason small gangs
of concrete workers need no foreman at all, provided one of the workers
is given command and required to keep tally of the batches. If the
efficiency of a gang of 6 men were to fall off, say, 15 per cent., by
virtue of having no regular non-working foreman in charge, the loss
would be only $1.35 a day--a loss that would be more than
counterbalanced by the saving of a foreman's wages. Indeed, the
efficiency of a gang of 6 men would have to fall off 25 per cent., or
more, before it would pay to put a foreman in charge. In many cases the
efficiency will not fall off at all, provided the gang knows that its
daily progress is being recorded, and that prompt discharge will follow
laziness. Indeed, one of the authors has more than once had the
efficiency increased by leaving a small gang to themselves in command of
one of the workers who was required to punch a hole in a card for every
batch.
To reduce the cost of superintendence there is no surer method than to
work two gangs of 18 to 20 men, side by side, each gang under a separate
foreman who is striving to make a better showing than his competitor.
This is done with marked advantage in street paving, and could be done
elsewhere oftener than it is.
In addition to the cost of a foreman in direct charge of the laborers,
there is always a percentage of the cost of general superintendence and
office expenses to be added. In some cases a general superintendent is
put in charge of one or two foremen; and, if he is a high-salaried man,
the cost of superintendence becomes a very appreciable item.
~SUMMARY OF COSTS.~--Having thus analyzed the costs of making and placing
concrete, we can understand why it is that printed records of costs vary
so greatly. Moreover, we are enabled to estimate the labor cost with far
more accuracy than we can guess it; for by studying the requirements of
the specifications, and the local conditions governing the placing of
stock piles, mixing boards, etc., we can estimate each item with
considerable accuracy. The purpose, however, has not been solely to show
how to predict the labor cost, but also to indicate to contractors and
their foremen some of the many possibilities of reducing the cost of
work once the contract has been secured. An analysis of costs, such as
above given, is the most effective way of discovering unnecessary
"leaks," and of opening one's eyes to the possibilities of effecting
economies in any given case.
To indicate the method of summarizing the costs of making concrete by
hand, let us assume that the concrete is to be put into a deep
foundation requiring wheeling a distance of 30 ft.; that the stock piles
are on plank 60 ft. distant from the mixing board; that the
specifications call for 6 turns of gravel concrete thoroughly rammed in
6-in. layers; and that a good sized gang of, say, 16 men (at $1.50 a day
each), is to work under a foreman receiving $2.70 a day. We then have
the following summary by applying the rules already given:
Per cu. yd.
concrete.
Loading sand, stone and cement $ .17
Wheeling 60 ft. in barrows (4 + 2 cts.) .06
Mixing concrete, 6 turns at 5 cts. .30
Wheeling 30 ft. (4 + 1 ct.) .05
Dumping barrows (1 man helping barrowman) .05
Spreading and heavy ramming .15
------
Total cost of labor $.90
Foreman, at $2.70 a day .10
------
Grand total $1.00
To estimate the daily output of this gang of 16 laborers proceed thus:
Divide the daily wages of all the 16 men, expressed in cents, by the
labor cost of the concrete in cents, the quotient will be the cubic
yards output of the gang. Thus, 2,400 ÷ 90 is 27 cu. yds., in this case.
In street paving work where no man is needed to help dump the
wheelbarrows, and where it is usually possible to shovel concrete direct
from the mixing board into place, and where half as much ramming as
above assumed is usually satisfactory, we see that the last four labor
items instead of amounting to 12 + 5 + 5 + 15, or 37 cts., amount only
to one-half of the last item, one-half of 15 cts., or 7½ cts. This
makes the total labor cost only 60 cts. instead of 90 cts. If we divide
2,400 cts. (the total day's wages of 16 men) by 60 cts. (the labor cost
per cu. yd.), we have 40, which is the cubic yards output of the 16 men.
This greater output of the 16 men reduces the cost of superintendence to
7 cts. per cu. yd.
CHAPTER IV.
METHODS AND COST OF MAKING AND PLACING CONCRETE BY MACHINE.
The making and placing of concrete is virtually a manufacturing process.
This process as performed by manual labor is discussed in the preceding
chapter; it will be discussed here as it is performed by machinery. The
objects sought in using machinery for making and placing concrete are:
(1) The securing of a more perfectly mixed and uniform concrete, and (2)
the securing of a cheaper cost of concrete in place. As in every other
manufacturing process both objects cannot be obtained to the highest
degree without co-ordinate and universal efficiency throughout in plant
and methods. For example, the substitution of machine mixing for hand
mixing will not alone ensure cheaper concrete. If all materials are
delivered to the machine in wheelbarrows and if the concrete is conveyed
away in wheelbarrows, the cost of making concrete even with machine
mixers is high. On the other hand, where the materials are fed from bins
by gravity into the mixer and when the mixed concrete is hauled away in
cars, the cost of making the concrete may be very low. Making and
placing concrete by machinery involves not one but several mechanical
operations working in conjunction--in a word, a concrete making plant is
required.
The mechanical equipment of a concrete making plant has four duties to
perform. (1) It has to transport the raw materials from the cars or
boats or pits and place them in the stock piles or storage bins; (2) it
has to take the raw materials from stock and charge them to the mixer;
(3) it has to mix the raw materials into concrete and discharge the
mixture into transportable vehicles; and (4) it has to transport these
vehicles from the mixer to the work and discharge them. As all these
operations are interrelated component parts of one great process, it is
plain why one operation cannot lag without causing all the other
operations to slow up.
The mechanical devices which may be used for each of these operations
are various, and they may be combined in various ways to make the
complete train of machinery necessary to the complete process. In this
chapter we shall describe the character and qualities of each type of
devices separately. The practicable ways of combining them to form a
complete concrete making plant are best illustrated by descriptions and
records of work of actual plants, and such descriptions and records for
each class of structure considered in this book are given in the
following chapters and may be found by consulting the index. In
describing the various machines and devices we have made one
classification for those used in handling raw materials and mixed
concrete, for the reason that nearly all of them are suitable for either
purpose.
~UNLOADING WITH GRAB BUCKETS.~--The orange-peel or clam-shell bucket is an
excellent device for unloading sand or stone from cars or barges. The
cost of unloading, including cleaning up the portions not reached by the
bucket, is not more than from 2 to 5 cts. per cu. yd. A grab bucket of
either of these types can be applied to any derrick. In unloading broken
stone from barges at Ossining, N. Y., a Hayward clam-shell on a
stiff-leg derrick unloaded 100 cu. yds. of broken stone per day from
barge into wagons, with one engineman and one helper. In addition to the
bucket work there was 24 hours' labor cleaning on each 500-cu. yd. barge
load. The labor cost of unloading a 500-cu. yd. barge was as follows:
Per Cu. Yd.
One engineman, at $2.50 2.5 cts.
One helper, at $1.50 1.5 cts.
Labor cleaning, at $1.50 0.7 cts.
--------
Total cost per cubic yard 4.7 cts.
~INCLINES.~--Inclines to reach the tops of mixer and storage bins and the
level of concrete work can be operated on about the following grades:
For teams hauling wagons or cars, 2 per cent. maximum grade. A single
heavy team will haul a 5-cu. yd. car, with ordinary bearings, weighing
2½ tons empty and 12 tons loaded, with ease on a 1½ per cent. grade,
and with some difficulty on a 2 per cent. grade. A locomotive will
handle cars on a grade of from 4 to 5 per cent. For team haulage 20-lb.
rails may be used, and for locomotives 30-lb. rails. Grades steeper than
about 5 per cent. require cable haulage.
~TRESTLE AND CAR PLANTS.~--Trestle and car plants for handling both
concrete materials and mixed concrete have a wide range of application
and numerous examples of such plants are described in succeeding
chapters, and are noted in the index at the end of the book. The
following estimates of the cost of a trestle and car plant are given by
Mr. Wm. G. Fargo. The work is assumed to cover an area of 100×200 ft.
and to have three-fourths of its bulk below the economical elevation of
the mixer, which stands within 50 ft. of the near side of the work. If
the work is under 3,000 cu. yds. in bulk and there is a reasonable time
limit for completion one mixer of 200 cu. yds. capacity per 10-hour day
is assumed to be sufficient. The items of car plant cost will be about
as follows:
150 ft. trestle, at $1.50 $225
5 split switches with spring bridles, at $18 90
2 iron turntables, at $30 60
3-2/3 cu. yd. steel cars with roller bearings 190
------
Total $565
The trestle assumed is double 24-in. gage track, 6 ft. on centers;
stringers 6×8 ins.×22 to 24 ft.; ties 2×6 ins., 2½ ft. on centers;
running boards 2×12 ins. for each track, and 12-lb. rails; trestle legs,
average length 30 ft., of green poles at 5 cts. per foot. This outfit
with repairs and renewals amounting to 10 per cent., is considered good
for five season's work and the timber work for several jobs if not too
far apart. The yearly rental on the basis of five seasons' work would be
$124.30, or $1 per working day for a season of five months. Three cars
delivering ½ cu. yd. batches can deliver 200 cu. yds. of concrete, an
average of 100 ft. from the mixer in 10 hours. Five men, including a man
tending switches and turntable and one man to help dump, can operate the
plant. With wages at $1.75 per day the labor cost of handling 200 cu.
yds. of concrete would be 4-1/8 cts. per cu. yd.