[Footnote 1: Read July 5, 1887, before the Western Society of
Engineers.]
By JOHN LUNDIE.
The subject of cement and concrete has been so well treated of in
engineering literature, that to give an extended paper on the subject
would be but the collection and reiteration of platitudes familiar to
every engineer who has been engaged on foundation works of any
magnitude. It shall therefore be the object of this communication to
place before the society several notes, stated briefly and to the
point, rather as a basis for discussion than as an attempt at an
exhaustive treatment of the subject.
Concrete is simply a low grade of masonry. It is a comparatively
simple matter to trace the line of continuity from heavy squared
ashlar blocks down through coursed and random rubble, to grouted
indiscriminate rubble, and finally to concrete. Improvements in the
manufacture of hydraulic cements have given an impetus to the use of
concrete, but its use is by no means of recent date. It is no uncommon
thing in the taking down of heavy walls several centuries old to find
that the method of building was to carry up face and back with rubble
and stiff mortar, and to fill the interior with bowlders and gravel,
the interstices of which were filled by grouting--the whole mass
becoming virtually a monolith. Modern quick-setting cement
accomplishes this object within a time consistent with the
requirements of modern engineering works; the formation of a
monolithic mass within a reasonable time and with materials requiring
as little handling as possible being the desideratum.
The materials of concrete as used at present are cement, sand, gravel,
broken stone, and, of course, water. It is, perhaps, unnecessary to
say that one of the primary requirements in materials is that they
should be clean. Stone should be angular, gravel well washed, sand
coarse and sharp, cement fine and possessing a fair proportion of the
requirements laid down in the orthodox specification. The addition of
lime water, saccharated or otherwise, has been suggested as an
improvement over water pure and simple, but no satisfactory
experiments are on record justifying the addition of lime water.
Regarding the mixing of cement and lime with saccharated water, the
writer made some experiments several months ago by mixing neat cement
and lime with pure water and with saccharated water, with the result
that the sugar proved positively detrimental to the cement, while it
increased the tenacity of briquettes of lime.
Stone which will pass a 2 inch is usually specified for ordinary
concrete. It will be found that stone broken to this limit of size has
fifty per cent. of its bulk voids. This space must be filled by mortar
or preferably by gravel and mortar. If the mixing of concrete is
perfect, the proportion of stone, by bulk, to other materials should
be two to one. A percentage excess of other materials is, however,
usually allowed to compensate for imperfection in mixing. While an
excess of good mortar is not detrimental to concrete (as it will
harden in course of time to equal the stone), still on the score of
economy it is advisable to use gravel or a finer grade of stone in
addition to the 2 inch ring stone to fill the interstices--gravel is
cheaper than cement. The statement that excess in stone will give body
to concrete is a fallacy hardly worth contradicting. In short, the
proportion of material should be so graded that each particle of sand
should have its jacket of cement, necessitating the cement being finer
than the sand (this forms the mortar); then each pebble and stone
should have its jacket of mortar. The smaller the interstices between
the gravel and stones, the better. The quantity of water necessary to
make good concrete is a sorely debated question. The quantity
necessary depends on various considerations, and will probably be
different for what appears to be the same proportion of materials. It
is a well known fact that brick mortar is made very soft, and bricks
are often wet before being laid, while a very hard stone is usually
set with very stiff mortar. So in concrete the amount of water
necessarily depends, to a great extent, on the porosity or dryness of
the stone and other material used. But as to using a larger or smaller
quantity of water with given materials, as a matter of observation it
will be found that the water should only be limited by its effect in
washing away mortar from the stone. Where can better concrete be found
than that which has set under water? A certain definite amount of
water is necessary and sufficient to hydrate the cement; less than
that amount will be detrimental, while an excess can do no harm,
provided, as before mentioned, that it does not wash the mortar from
the stone. Again, dry concrete is apt to be very porous, which in
certain positions is a very grave objection to it--this, not only from
the fact of its porosity, but from the liability to disintegration
from water freezing in the crevices.
Concrete, when ready to be placed in position, should be of the
consistency of a pulpy mass which will settle into place by its own
weight, every crevice being naturally filled. Pounding dry concrete is
apt to break adjacent work, which will never again set properly. There
should be no other object in pounding concrete than to assist it to
settle into the place it is intended to fill. This is one of the evils
concomitant with imperfection of mixing. The greater perfection of
mixing attained, the nearer we get to the ideal monolith. The less
handling concrete has after being mixed, the better. Immediately after
the mass is mixed setting commences; therefore the sooner it is in
position, the more perfect will be the hardened mass; and, on the
other hand, the more it is handled, the more is the process
interrupted and in like degree is the finished mass deteriorated. A
low drop will be found the best method of placing a batch in position.
Too much of a drop scatters the material and undoes the work of
thorough mixing. Let the mass drop and then let it alone. If of proper
temper, it will find its own place with very little trimming. Care
should be taken to wet adjacent porous material, or the wooden form
into which concrete is being placed; otherwise the water may be
extracted from the concrete, to its detriment.
It has been found on removing boxing that the portion adjacent to the
wood was frequently friable and of poor quality, owing to the fact
just stated. It is usual to face or plaster concrete work after
removing the boxing. On breakwater work, where the writer was engaged,
the wall was faced with cement and flint grit, and this was found to
form a particularly hard and lasting protection to the face of the
work.
Batches of concrete should be placed in position as if they were
stones in block masonry, as the union of one day's work with a
previous is not by any means so perfect as where one batch is placed
in contact with another which has not yet set. A slope cannot be added
to with the same degree of perfection that one horizontal layer can be
placed on another; consequently, where work must necessarily be
interrupted, it should be stepped, and not sloped off.
Experience in concrete work has shown that its true place is in heavy
foundations, retaining walls, and such like, and then perfectly
independent of other material. Arches, thin walls, and such like are
very questionable structures in continuous concrete, and are on record
rather as failures than otherwise. This may to a certain degree be due
to the high coefficient of expansion Portland cement concrete has by
heat. This was found by Cunningham to be 0.000005 of its bulk for one
degree Fahrenheit. It is a matter which any intelligent observer may
remark, the invariable breakage of continuous concrete sidewalks,
while those made in small sections remain good. This may be traced to
expansion and contraction by heat, together with friction on the lower
side.
In foundations, according to the same authority above quoted, properly
made Portland cement concrete may be trusted with a safe load of 25
tons per square foot.
In large masses concrete should be worked continuously, while in small
masses it should be moulded in small sections, which should be
independent of each other and simply form artificial stones.
The facility with which concrete can be used in founding under water
renders it particularly suitable for subaqueous structures. The method
of dropping it from hopper barges in masses of 100 tons at a time,
inclosed in a bag of coarse stuff, has been successfully employed by
Dyce Cay and others. This can be carried on till the concrete appears
above water, when the ordinary method of boxing can be employed to
complete the work. This method was employed in the north pier
breakwater at Aberdeen, the breakwater being founded on the sand, with
a very broad base. The advantage of bags is apparent in the leveling
off of an uneven foundation. In breakwater works on the Tay, in
Scotland, where the writer was engaged, large blocks perforated
vertically were employed. These were constructed below high water
mark, and an air tight cover placed over them. They were lifted by
pontoons as the tide rose, and conveyed to and deposited in place, the
hollows being filled with air, serving to give buoyancy to the mass.
After placing in position the vertical hollows were filled with
concrete, so binding the whole together--they being placed vertically
over each other.
As mentioned before, continuous stretches of concrete in small
sections should be guarded against, owing to expansion by heat; but
the fact of a few cracks appearing in heavy masses of concrete should
not cause apprehension. These occur from unequal settlement and other
causes. They should continue to be carefully grouted and faced until
settlement is complete.
The use of concrete is becoming more and more general for foundation
works. The desideratum hitherto has been a perfect and at the same
time an economical mixer. Concrete can be mixed by hand and the
materials well incorporated, but this is an expensive and man-killing
method, as the handling of the wet mass by the shovel is extremely
hard work, besides which the slowness of the method allows part of a
large batch to set before the other is mixed, so that small batches,
with attendant extra handling, are necessary to make a good job.
Mixers with a multiplicity of knives to toss the material have been
used, but with little economical success. Of simple conveyers, such as
a worm screw, little need be said; they are not mixers, and it seems a
positive waste of time to pass material through a machine when it
comes out in little better shape than it is put in. A box of the shape
of a barrel has been used, it being trunnioned at the sides. The
objection to this is that the material is thrown from side to side as
a mass, there being a waste of energy in throwing about the material
in mass without accomplishing an equivalent amount of mixing. Then a
rectangular box has been used, trunnioned at opposite corners; but
here the grave objection is that the concrete collects in the corners,
and after a few turns it requires cleaning out, the material so
sticking in the corners that it gets clogged up and ceases to mix.
The writer has just protected by letters patent a machine, in devising
which the following objects were borne in mind:
1st. That every motion of the machine should do some useful work.
Hitherto box or barrel mixers have gone on the principle of
throwing the material about indiscriminately, expecting that
somehow or other it would get mixed.
2d. That the sticking of the material anywhere within the mixer
should be obviated.
3d. That an easy discharge should be obtained.
4th. That the water should be introduced while the mixer
revolves.
With these desiderata in view, a box was designed which in half a turn
gathers the material, then spreads it, and throws it from one side to
the other at the same time that water is being introduced through a
hollow trunnion.
It is also so constructed that all the sides slope steeply toward the
discharge, and there is not a rectangular or acute angle within the
box. A machine has now been worked steadily for several weeks, putting
in the concrete in the foundations of the new Jackson Street bridge in
this city, by General Fitz-Simons. The result exceeds expectations.
The concrete is perfectly mixed, the discharge is simple, complete and
effective, and at the same time the cost of labor in mixing and
placing in position is lessened by 50 per cent. as compared with any
known to have been put in under similar circumstances.--_Jour.
Association of Engineering Societies._
* * * * *