A Concrete Foundations Placing Guide
Cast Your foundations
This guide will help owner-builders to cast the concrete 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-story buildings with tiled or sheeted roofs in stable soil conditions, the minimum width of strip concrete 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 on sloping ground can be stepped with a minimum of 400mm overlap. Sloped trenches must not exceed the 1:10 gradient.
The minimum depth of the strip concrete foundations trenches must be at least 400mm below the original ground level.
The average minimum thickness of the concrete foundations 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-story building reinforced foundations would require 25–30MPa.
Trench sides and walls 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 will 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 foundations there are a few things to consider.
Clear the soil that was excavated well away from the edges of the trench to stop it from 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 to prevent loose soil from 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 concrete foundations as soon as the concrete has set. In hot dry areas, the concrete can be covered with plastic sheeting to stop the concrete from drying too quickly. Remove the plastic and spray water onto the concrete every so often to prevent it from drying too quickly. When concrete loses 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.
Here is an extract about water and concrete from a Wiki article:
The most destructive agent of concrete structures and components is probably water. Indeed, water often directly participates in chemical reactions as a reagent and is always necessary as a solvent, or a reacting medium, making the transport of solutes and reactions possible. Without water, many harmful reactions cannot progress or are so slow that their harmful consequences become negligible during the planned service life of the construction. Dry concrete has a much longer lifetime than water-saturated concrete in contact with circulating water. So, when possible concrete must first be protected from water infiltrations.
How soon can bricklaying start on the foundation?
Forty-eight hours after placing the concrete. You can build the first 500mm of brickwork 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 foundations 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.