Placing the Concrete - surface, set, stones, wet, layer, dry, inches, water and mixed

PLACING THE CONCRETE. After mixing, the concrete is con veyed in wheelbarrows or in buckets swung from a crane or in cars run ning on a track, and deposited in the structure in layers 6 to 8 inches thick if the concrete is mixed dry, and from 12 to 16 inches thick if it is mixed wet. If the concrete is wet, it can be clumped from almost any height; but if mixed dry, it should not be allowed to fall from any considerable height, as doing so separates the ingredients. If in handling dry concrete the larger fragments become separated, they should be re-turned and be worked into the mass with the edge of a shovel.

If the concrete was mixed either dry or plastic, it should be com pacted by ramming; and if the concrete is wet, it should be stirred to allow the entrained air and surplus water to rise to the surface. The stirring or "puddling" may be done either by plunging a rod up and down in it, or by men wearing rubber boots walking around in it.

The rammer ordinarily used for dry or plastic concrete consists of a block of iron having a face 6 to 8 inches square and weighing anything up to 20 or even 30 pounds. A rammer having a 2- by 4-inch face is convenient for use close to the forms. The face of the rammer is sometimes corrugated, to keep the surface of the layer rough and thus give a better bond with the next, and also to transfer the compacting effect of the blow deeper. The tamping should be vigorous enough to thoroughly compact the mass; but too severe or too long-continued pounding injures the strength of the concrete by forcing the broken stone to the bottom of the layer, or by disturbing the incipient set of the cement.

With wet concrete the so-called rammer may be either a round wooden rod 2 inches in diameter or a piece of scantling having a 2- by 4-inch face at the lower end and rounded at the upper end for a handle.

After the concrete is in place it should be protected from the sun, and should not be disturbed by walking upon it until fully set.

This limit should be at least 12 hours, and is frequently specified as 4 or 5 days.

Bonding New to Old Concrete.

When one layer of concrete is to be deposited upon another partially set, precautions must be taken to secure a good union between the two,—particularly if the joint is likely to be subjected to tensile or shearing stress, or if the concrete is required to be water-tight. If the first layer is only partially set, it is sufficient to simply wet the old concrete, taking care that no puddles of water are left upon its surface. In case the first layer is fully set, it is wise to sweep the surface with a 1 : 1 or 1 : 2 cement mortar to make sure that the two layers adhere firmly. If the sand or gravel contains any appreciable clay and the concrete is mixed wet, clay is liable to be flushed to the surface and prevent the adherence of the next layer; therefore under these conditions the surface should be swept or washed perfectly clean, and then the surface should be thoroughly swept with a rich cement mortar.

An almost invisible film of oil or grease, which often gets on the concrete from the forms, is very effective in preventing a bond, and is very difficult to remove. For this reason it is better to use soap than oil on the inside of the forms, since soap is soluble in cold water and hence is easily washed off. If the joint is likely to be subjected to a considerable shearing stress, it is wise to roughen the surface before it sets or to embed a timber, say 4 or 6 inches square, in such a way that when it is removed a groove will be left which will be filled with the new concrete. Sometimes large stones are partially em bedded in the upper surface for doweling the old to the new work.

In attempting to bond new concrete to that which has been set a long time, it is of appreciable advantage to allow the new concrete to take an initial set and then add water and re-mix it before using it. The advantage is due to two things: First, since concrete shrinks in hardening in air, allowing the concrete to take an initial set and then re-mixing it eliminates part of the ordinary shrinkage. Second, all concrete shrinks through cooling after the elevation of temperature due to the chemical action of setting, and the addition of water the second time cools the concrete and prevents at least part of the shrinkage due to this cause.

In building tanks or other structures which must be water-tight, the surest way is to build the work as a monolith, i.e., without stop ping the work; but with reasonable care a water-tight joint may be made by observing the above precautions. In this connection see I 382.

Laying Rubble Concrete.

Rubble concrete is concrete in which large stones are bedded. The embedded stones decrease the cost of construction, since the cost of crushing the embedded stones is saved, and also since no cement is required to combine these stones into a solid mass. Of course, rubble concrete can be used only in massive work. The important things to be observed in laying rubble concrete are: 1. The large stones should be clean, and should be laid far enough apart to be fully encased in the concrete. 2. The concrete should be wet enough to flow easily around the stones, and should be deposited in layers whose thickness varies with the size of the stone to be embedded. 3. If the large stones do not sink into the concrete by their own weight, they should be driven in with a ram mer.