Depositing Concrete

In choosing materials for the mixture, re member that the proper size of stone or gravel will vary with the type of construction.

Remember that the binding value of Port land cement is lowered by exposing the concrete to a hot sun during the first four or five days after it has been placed.

Use anything up to inches in diameter for foundations or large thick structures. Use inches and under (generally about inch), graded, for thin walls and reinforced concrete. Use Y2 inch and under for a finish or very hard coating. The best results are obtained from a mixture of sizes graded from the largest to the smallest.

If the aggregate is porous, it should be well soaked in water before coming in contact with the cement; otherwise it will absorb the water necessary for mixing and for developing the full activity and strength of the cement.

It occasionally happens that concrete must be deposited under water. In such cases, special precaution must be taken to prevent separation of the materials. The governing principle is to see that the concrete shall be disturbed as little as possible in being placed.

There are three methods used: First, the concrete may be lowered under water to the place of deposit, in a closed bucket, which should be provided with an automatically opening hinged bottom, allowing the concrete to slide out gently when its destination is reached.

Second, the concrete may be lowered to place in tubes. If only moderate quantities are to be deposited, a small tube—say 4 to 6 inches in diameter—with a removable cap at the lower end, may be used. This is first filled with the mixture, then lowered, the cap removed, and con crete fed through the tube in a continuous stream. Where large quantities are to be placed, requiring a tube of inconvenient weight for handling, this large tube may be lowered empty, the water rising therein. Into this chute the con crete may be dumped until the water has been displaced. The tube is then slowly raised, al lowing the concrete to run out. Under this method, there is always more or less danger of badly washed concrete.

Third, concrete may be placed under water by means of paper bags or of cloth bags of open mesh weave. Paper bags will retain their shape

long enough to permit of proper placing in courses like masonry, and the paper soon loses all its strength through soaking, and allows the bonding or blending of the concrete. A similar union is effected through the open meshes of the cloth bags.

One of the troubles which for a time mystified concrete workers, was the appearance of fine cracks—known as crazing or hair cracks on con crete surfaces, frequently long after the con crete had set hard. Investigation, however, showed that this disfigurement was entirely superficial. Hair cracks are confined to the sur face, being generally of no greater depth or width than a coarse hair; they are no indication of weakness in the concrete. It has long been known that very wet concrete is more apt to craze than dry concrete is. Dry concrete is objectionable to some for the reason that it is lacking in strength and density; and on that ac count its use is not advocated even though hair cracks are to some extent avoided. Experiments have demonstrated the fact that in wet con crete a portion of the very finest particles of ce ment is carried to the surface by the action of the excess water which is being absorbed by the atmosphere. This excess water is to a large ex tent drawn from the interior of the concrete to the exterior, carrying with it the finer particles, which, being deposited on the surface, form a richer mortar than is contained in the interior. Under certain conditions these finer particles on the surface practically form a coating of neat cement. Neat cements and rich mortars are found to be much more liable to crazing than mortars containing a larger proportion of sand or finely crushed stone. The cracks are due entirely to a contraction of the surface, and not to any con traction of the interior.

The trouble may be almost entirely avoided by seasoning the concrete in a very moist atmos phere or under a cover of very wet sand, or, in the case of blocks, by immersing entirely for a considerable time in water.