Some Important Details

In mixing concrete the sand and cement should never be combined very far in advance of adding the required quantity of pebbles or broken stone because moisture in the sand will cause the cement to commence hardening and this will affect the strength of the concrete. The action which takes place when cement and water are combined is what causes the hardening of concrete. This action commences very shortly after the water has been added to the other materials. Therefore when any concrete work is planned forms should be ready so that concrete can at once be placed and all the hardening action take place in the form or mold.

Placing Concrete

There are variations in the maiiner of placing concrete due principally to the nature of the work or the conditions under which it must be carried on. Sometimes in laying the foundation for a building, the earth walls of the trench are firm and no forms are necessary for that portion of the wall below ground level. In such a case, concrete, as soon as mixed, is shoveled into the open trench and compacted either by tamping or spading to its utmost density. Tamping is done only when mixtures containing less water than the quaky or jelly-like consistency are used but even in a mixture containing less water than a quaky one, there should still be enough water so that when the concrete is tamped, a film of water will flush to the surface under vigorous ramming. Care should be taken when dumping concrete into a trench unprotected by forms, not to knock down earth from the side walls of the trench, thus permitting earth to become mixed with the concrete. Planks should be laid alongside and across the trench for convenience of the workers. This will prevent them from causing cave-ins of the trench walls. Concrete work should always be planned so that the quantity of concrete that will be placed during whatever time is to be devoted to the work, can be closely estimated so that when work is temporarily stopped its condition will be such as to make resuming the work easy.

In placing concrete for a foundation or building wall, layers no thicker than 6 or 8 inches should be placed at one time. This will permit thorough tamping or spading to make the concrete bond with or adhere to that previously placed.

Tamping Or Spading Concrete

Quaky concrete, instead of being tamped, is spaded thoroughly between form faces and next to form faces so as to secure the greatest possible density and to release air bubbles that may be in the mixture. Narrow or pointed spading tools are sometimes needed to thoroughly work the concrete around reinforcing metal, such as wire mesh fabric or steel rods, also when placing the concrete in a very thin section, such as the sides of a flower box or vase. In such cases small chisel-pointed sticks or rods will serve as spading tools and only a little concrete should be placed at a time so that it can be thoroughly spaded. Work on such small objects especially should be continuous from start to finish, otherwise there will be a seam disclosed in the work when the form or mold is removed and leakage will take place through such a seam, not to mention the fact that the object will be weakened.

Using an iron rod to compact the concrete in the mold for a flower box being cast bottom side up. Working the rod up and down also helps to secure a smooth surface finish.

Jarring the mold to help release air that might be trapped in the concrete and also to cause the mixture to settle thoroughly to all parts of the form.

It is sometimes necessary to carry the concrete some distance from the mixing- platform as the work progresses, or to frequently move the mixing platform to prevent unnecessary handling or transporting of concrete.