Placing Concrete

Place concrete into forms continuously in horizontal lifts not exceeding 1.2 m (4 ft.) in depth. Do not allow concrete to fall into the forms from a height of more than 2.4 m (8 ft.) as this causes the concrete to segregate. For higher drops, use a pipe to deposit the concrete. Buggies, wheelbarrows, chutes or pumping can be used to move the concrete to locations not accessible to ready-mix trucks. The chutes should be metal or metal-lined with round bottoms and sloped with a rise-to-run inclination between 1:2 and 1:3.

Do not deposit the concrete in a pile. Spread and level it by raking or shovelling. Use vibrators to consolidate the concrete but not to move it horizontally. Compact the concrete uniformly by means of tamping hand tools (puddling sticks) or, preferably, by a vibrator.

If it is necessary to interrupt concrete placement, the surface of the concrete placed in the forms should be levelled. If partial setting has started by the time concrete placement is ready to resume, roughen and dampen the surface to provide good bonding between lifts. For a surface that has set, spread a bonding agent comprised of grout (1 part cement to 2 parts sand) about 12 mm (1⁄2 in.) thick over the roughened surface between the two lifts. Place the new lift immediately after the grout has been applied.

When mixing and placing concrete (in all temperature conditions), maintain its temperature between 10°C (50°F) and 25°C (77°F). Do not place concrete against frozen soil and always remove any ice and snow from the formwork.

Source : Canada Mortgage and Housing Corporation (CMHC)