A Concrete Placing Guide
This guide will help owner builders to cast foundations. These foundations are for external brick walls around a new house, or for extensions of an existing house.
THE NHBRC (National Homebuilders Association Council) requirements for house foundations:
For single storey buildings with a tiled or sheeted roof in stable soil conditions, the minimum width of strip foundations should be 500mm for external walls and 400mm for internal walls. If a reinforced concrete roof is required on the house then the minimum width is 750mm for external walls and 600mm for internal walls.
Trenches in sloping ground can be stepped with a minimum of 400mm overlap. Sloped trenches must not exceed 1:10 gradient.
The minimum depth of the strip foundation trench must be at least 400mm below the original ground level.
The average minimum thickness of the foundation must be at least 200mm. The strength of the concrete used for unreinforced strip footing must be at least 15MPa or better. For a double storey building reinforced foundations would require 25–30MPa.
Trench sides and must be as near to vertical as possible. Trim the sides of the excavation to avoid holes being filled with concrete on the side faces. This wii require more concrete than estimated and will be a waste.
When you have finished the excavation measure the actual depth, width and length of the trench, and calculate the actual volume of concrete that you will need. Always add an additional 10% to allow for wastage and unforeseen circumstances. See our tables here: Concrete mixes by weight & volume
When placing your concrete there are a few things to consider.
Clear the soil that was excavated well away from the edges of the trench to stop it falling back into the trench and contaminating the concrete. If you are using wheelbarrows to ferry the cement then place planks along the edges where there is soft bedding such as fill material o prevent loose soil falling into trenches due to the weight of the wheelbarrows.
Gently wet the excavation trenches before placing the concrete to avoid moisture being drained out of the concrete mix into the surrounding soil as this could weaken the foundation.
Compact the concrete to remove any air bubbles. This will increase the density and strength.
Fresh concrete needs to be protected from drying out too quickly. Gently spray water over the foundation as soon as the concrete has set. In hot dry areas the concrete can be covered with plastic sheeting to stop the concrete drying too quickly. Remove the plastic and spray water on to the concrete every so often to prevent it drying too quickly. When concrete looses moisture too quickly either by evaporation or absorption by the surrounding soil, it shrinks and if the concrete is not malleable, it will crack. See our article on drying in cold weather.
How soon can bricklaying start on the foundation?
Forty-eight hours after placing the concrete. You can build the first 500mm of brickwork to above the surrounding ground level. Then you must carefully wet the concrete thoroughly and backfill your trenches with soil. Seven days after placing the concrete foundation strips, they should be strong enough and your walls can be built up to roof level. At this stage, the concrete should have reached 60% of the final strength.