i\lushy concrete may he deposited in somewhat thicker layers, being lightly tamped or worked with of small section, usually about 2x3 inches, for the purpose of eliminating air bubbles and snaking sure that there are no open spaces unfilled with mortar. A flat spade is commonly- run down next to the form to bring the to the surface and prevent voids Nvhich often occur when. the stones of the concrete are iu contact with lhc• form.
Laitance.—In the use of very soft concrete, when an excess of water is used, there is a tendency for certain parts of the cement to be taken up by the surplus water and deposited on the upper surface of the concrete as a sort of light-colored slime, which is known as laitance. Its formation involves a loss of cement in the concrete and, if left in the body of the concrete, it forms a plane of weakness in the mass of concrete. Laitance is often found to an objectionable extent when very wet concrete is ehuted to place, and deposited in masses of considerable vertical thickness. A column of wet concrete poured through chutes may have a cap of laitanee 2 or 3 inches thick which must be removed.
Bonding to Old Work.—Joints must frequently be made with work previously placed. In massive work, subjected only to com pressive stresses normal to the joints, the surface of the old concrete must be clean and should be wet before the placing of the new con crete. In work where the strength of the bond of the new to the old work is of special importance, the old work should be cleaned, all laitance removed, the skin on smooth surfaces broken by scarifying, and the surface thoroughly wet. A coating of cement paste will then aid the bond with the new work.
Joints between different days' work should be carefully located where they will be least injurious to the strength of the structure. When feasible it is desirable to divide a structure into integral parts, each of which may he constructed without stopping the work.
Depositing under Water.—Conerete work for under-water con struction is sometimes clone by passing the mixed concrete through the water to the desired position. It is common to use a tremie for this purpose. This consists of a tube or closed chute, which is kept full of concrete so that the water has no chance to wash the concrete as it passes downward. The bottom of the tremie is moved about over the surface upon which the concrete is being deposited, so that the concrete does not fall through the water. Concrete for this
purpose must be quite wet, in order to flow readily to place without being washed by the water through which it is passing.
Buckets, which arc filled with concrete, lowered through the water, and dumped by opening the bottom when in contact with the surface upon which the concrete is to be placed, are also some times used for under-water work. The method of enclosing the concrete in bags and placing these in contact with each other has also been used for this purpose.
84. Placing Concrete in Freezing Weather.—The setting and hardening of concrete are greatly retarded at low temperatures; in cold weather much longer time is needed to gain strength, and forms must be left longer before removal. Accidents have sometimes occurred to concrete structures through premature removal of forms, on account of failure to consider the influence of temperature upon the rate of hardening. At 40° F., the time required to gain a given strength is two or three times as long as at 70° F.; below 40° F., the required time rapidly increases as the temperature is lowered.
Cement mortar or concrete made and frozen before it has time to set is uninjured by freezing and sets and hardens properly after it thaws out. If the mortar is frozen when partially set or soon after it has set and before any considerable strength has been gained, the expansion caused by freezing breaks the bond and destroys the cohesion of the mass, causing it to crumble upon thawing out.
The use of concrete in freezing weather, except in large masses, should be avoided in so far as possible. When it is necessary to place concrete at freezing temperature, or when it is likely to be frozen soon after placing, extreme care should be taken to minimize the probable effect of freezing upon the concrete. The methods employed may be intended to hasten the setting and hardening of the cement, to prevent the concrete from freezing soon after placing, or both, and for this purpose, materials may be selected that will act quickly when made into mortar. Quick-setting cements, how ever, are sometimes more retarded by low temperatures than others setting more slowly at normal temperatures. In selecting materials, it is more important to get those acquiring strength quickly than those setting quickly.