Depositing Concrete

DEPOSITING CONCRETE Concrete should not be allowed to stand un used for any considerable length of time after being mixed, but should be placed or deposited at once. In no case should this be delayed be yond 20 to 30 minutes after the cement is first wet.

Effect of Remixing.

To disturb the concrete after it has begun to set, will not only retard its setting, but may possibly prevent the develop ment of its full, ultimate strength. This is par ticularly true where the cement is to be used in reinforced work, as remixing reduces the ad hesiveness and binding power of the cement. If it is permissible to wait a long time—say several months—for the development of the full strength of the concrete, remixing of both Port land and natural cement mortar and concrete might be allowed, if done shortly after initial set has begun—say within an hour or two; but if, as is usually the case, it is desirable that the mixture shall acquire its full strength within a few days or weeks, then cement mortar or con crete which has begun to set before placing should be rejected.

If the mixing process is kept up for about two hours continuously before placing, it has been found that the strength of the mixture will be considerably increased, this being due, in all probability, to the more thorough blending and interpenetration of the constituents of the mor tar or concrete.

Concrete is usually deposited in layers 4 to 12 inches thick. Unless otherwise specified, it should be placed in layers of about 6 inches.

Method of Placing. In the placing of con crete, the most important thing is to handle it in such a way that its materials will not be sepa rated. The mass should retain its compactness and its even distribution of ingredients; the stone or gravel should never become separated from the mortar. Provided this precaution is taken, concrete may be handled and placed in any manner suited to the nature of the work. It may be shoveled off the mixing board directly into the trench or forms; shoveled into wheel barrows, wheeled to place, and dumped; shov eled into cars or carts, and hauled to the work; shoveled or dumped down an inclined chute; shoveled into buckets or hods and lifted or carried to place, etc.

If dumped into a trench or form from any considerable height, the work should be watched with special care to see that the heavier par ticles , are not forced toward the bottom of the mass, and that the stone or gravel still remains in intimate contact with the mortar. Where de positing from a considerable height is necessary, a chute should invariably be used if the mixture is a "dry" one. A very wet mixture, however, is much easier to handle in such a case than a dry mixture, and the stone or gravel will not so readily be separated from the mortar.

Ramming or Tamping Concrete.

In order to force out air-bubbles and obtain a thoroughly compact job, concrete should be rammed (or tamped) immediately after placing. The amount of ramming needed depends on the con sistency of the mixture. If very wet, only the slightest tamping, if any, is necessary—merely enough to level off across the top. If much tamping is done to a very wet mixture, the par ticles of the coarse aggregate may be forced to gether in one part of the mass, and their uni form distribution be upset. Sometimes a very wet mixture, instead of being tamped, is simply puddled (stirred up), this being done with a piece of reinforcing bar or other convenient material.

A medium wet mixture should be tamped until thoroughly compacted, when it will quake like jelly.

A dry mixture must be very thoroughly tamped—until the water is forced to the surface.

The tamper for medium or dry concrete may consist of a steel shoe or plate with a face about 6 inches square, to the back of which a handle of convenient length is fastened; or, for small work, a satisfactory home-made tamper may be constructed from a cylindrical block of wood into one end of which a handle is fastened. For work in very thin sections or between rein forcing steel members, a suitable rammer can be made from board 1 inch or so in thickness, the blade left at the lower end being about 8 inches high by 3 inches wide.