CONCRETE.1

CONCRETE.¹

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.

[1]

Read July 5, 1887, before the Western Society of Engineers.