Concrete foundations

KEYWORDS:

SHORT DESCRIPTION:

· Concrete foundations on hard, uniform ground can be made without steel reinforcement, if not in an earthquake or hurricane prone area.

· All non-uniform andproblem soils require reinforced concrete foundations, especially in areas of medium to high rainfall and natural hazard regions.

· Depending on the strengths required, concrete mixes can vary from 1: 3: 4 (cement: sand : gravel) to 1: 4: 7, the higher proportion of cement being required for reinforced concrete.

· Water contents of fresh mixes should make them just easily workable. Excessive water leaves pores in the concrete, making it weak and water absorbent. Foundation trenches should also be properly wetted to avoid excessive absorption of the water from the mix.

· The concrete should be wet-cured for 3 to 7 days before building the walls. A damp-proof course should be laid between foundation and wall.

Further information: Bibl. 20.03, 20.04, 20.05.

Cast Concrete Foundations (Bibl. 20.05)

Reinforced Foundations

Placing concrete footing without shuttering: the reinforcement is laid after the lowest course of lean concrete is hardened. The richer second layer holds the reinforcement.

Foundation strip poured into shuttering of wood or plywood. These should be oiled before pouring concrete, to facilitate removal after hardening.

The finished foundation, with the trench filled up with the previously excavated soil and well compacted.

Foundations on Expansive Clay (Bibl. 20.03)

· Certain clayey soils respond to moisture movements (in rainy and dry seasons, moisture extraction by trees, etc.) with excessive swelling and shrinkage, which can severely damage foundations and consequently entire buildings.

· Damage can be avoided by either installing foundations which penetrate through the zone of ground movement, or by constructing foundations and superstructures which are tolerant of ground movement.

· Pile-and-beam-foundation: Small diameter piles are installed below the zone of clay movement; RC ground beams, which span between pile heads are constructed on compressible material (eg expanded polystyrene), which absorbs ground movement without affecting the beams and superstructure.

· Pad-and-beam-foundation: Pads are installed on stable ground below the movement zone; RC columns support ground beams, which are constructed in the same way as in the pile-and-beam-foundation.

Pile-and-beam-foundation; Pad-and-beam-foundation