Pouring Concrete

Pouring Concrete

There are several rules to follow when pouring concrete. The first is that concrete should be placed in the forms as soon as possible after it is mixed. Delays result in evaporation of moisture from the mix and a loss of both workability and strength. If the concrete is not placed within 11 ⁄ 2 hours and shows signs of stiffening, it should be discarded. Do not add water to a mix that has begun to stiffen. Even if you succeed in restoring some workability, the concrete will be of poor quality. To avoid delays, it’s important to make sure that all the necessary preparations have been made before the ready mix truck arrives or before you begin mixing. Preparations should include wetting the inside surfaces of plywood and kiln dried lumber forms and the soil sub grade to keep them from absorbing too much water from the concrete mix. Linseed oil or commercial form release oil can be used. Oiled forms will also make form removal easier without damage to the concrete surfaces. Oiling or wetting the forms and soil is especially important on a warm and windy day when moisture evaporation is at its highest.

The second rule in pouring concrete is to place the mix as near to its final location as possible. A ready-mix truck is equipped with metal chutes which can be extended a moderate distance to deliver concrete directly into the forms, and concrete pumps are often used on large commercial projects. On residential projects, it is more common to use wheel barrows or buggies to move the concrete from the mixer to the forms. You can build ramps and runways over the forms to keep from bumping the boards or reinforcing out of place. Deep forms may require pouring in more than one lift or layer.

The third rule in pouring concrete is to effectively compact or consolidate the fresh mix immediately after it is placed and before it begins to stiffen. Concrete must be consolidated to eliminate air pockets and voids and to get the concrete to flow around reinforcement and anchorages. In very small applications, adequate consolidation can be achieved by rodding or puddling by hand with shovels, metal rods, or tampers, but mechanical vibration is preferred on most applications.