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Guidelines for the construction of concrete foundations ensure strength, durability, and longevity of foundation structures. Consequently, the concrete foundations are constructed properly and according to standards and requirements.
Construction guidelines for concrete foundations can be obtained from Building Codes such as American Concrete Institute and Indian Standards, and engineering experience and judgement to a certain extent.
Guidelines are substantially crucial for the construction of concrete foundation (base, reinforcement placement, formworks, concreting, stripping works, defects detection and their repairs) that adequately serves its purpose.
Base Preparation For Foundation
Concrete foundation should be constructed on firm undisturbed soil, engineered fill, or coarse gravel. Standing water, mud, frozen ground, and other debris should be removed. If the sub-grade soil is of poor quality, it shall be excavated, and a layer of gravel 100mm thick shall be provided. The level tolerances for the prepared sub-grade shall not exceed +5mm, -15mm.
Up to 25 mm of standing water may be displaced by the concrete if it does not mix with the water. Removal of inappropriate materials below the designed bottom of the footing is required to develop a sound subgrade base for the footing. The over excavation can be filled with engineered fill or concrete.
- All formwork materials shall be selected and installed to achieve the required concrete surface finishes.
- Joints between formwork panels shall be adequately sealed, to prevent the leakage of grout during concrete casting and compaction.
- Formwork shall be measured before concrete casting to confirm the locations, alignment and top of concrete levels.
- Where the formwork extends above the top of concrete level, the top of concrete level shall be clearly marked on the formwork with nails and / or a chalk line.
- Foundation formwork shall be selected and arranged such that it has adequate strength, stiffness and stability to retain the weight of wet concrete during placement.
- The formwork shall be stiffened, as necessary, to ensure no significant deformation of the same during concrete casting.
Arrangement of Reinforcing Bars
- Reinforcement in the foundation is needed to ensure the continuity of the structure. This is particularly important in cases of bad ground or where the building may be subjected to earthquake forces.
- Steel reinforcement in foundation consists of reinforcing bars placed longitudinally, transversely, or both, to the direction of the footing.
- Longitudinal reinforcing is sometimes used to bridge soft spots and minor trenches or to increase strength over narrow excavations.
- Transverse footing reinforcement is typically not required unless the soil-bearing capacity is poor or wall loads are high.
- Transverse or longitudinal reinforcing bars should be located near the center of the footing thickness with a minimum cover of 75 mm to the bottom and sides.
- Foundation reinforcements can be set on chairs, construction bricks, or other support devices.
- The minimum distance of individual bars shall be 150mm.
- After installation of rebar, the following items shall be visually checked and confirmed.
- The minimum rebar size is 16mm.
- The tolerance of the position of reinforcement bars shall not exceed ±6mm.
- Where reinforcement cannot be installed in one length to meet reinforcement requirements, reinforcement bars must be lapped to develop the bars’ full tensile capacity across the splice.
- In accordance with ACI-318, a minimum lap length of 40 times the diameter of the reinforcement bar is required for splices in the reinforcement.
- The separation between spliced or lapped bars is not to exceed eight times the diameter of the reinforcement bar, or 6 inches, whichever is less.
Concrete for Foundations
- The strength of concrete should not be less than 17 MPa. This need to be increased if the foundation is exposed to aggressive environmental conditions and earthquake forces.
- Concrete cover shall be 75mm for foundation in direct contact with soil.
- Placement of concrete for footings can be by any conventional method, including direct chute, wheelbarrows, crane, pump, or conveyor.
Removal of Formwork
The time for stripping of formwork is 12 hours. The formwork shall not be removed earlier than 12 hours after completion of concrete finishing works.
Surface Defects Correction
After formwork has been removed, and curing completed, each concrete structure shall be visually inspected for surface defects. The points which shall be inspected shall be as follows:
- Levels of finished concrete.
- Alignment of Finished Concrete.
- Levels and Alignment of embedded items such as anchor bolts.
- Tolerances for levels and alignments
The finished surface of the concrete shall be checked for the following defects:
- Spalling and dusting.
- Abrupt irregularities
Repair of Foundation Surface Defects
- Hack all unsound and cracked concrete up to a depth exceeding 50mm. In areas where rebar is exposed, remove concrete behind rebar.
- For corroded rebar, loose rust scales shall be removed by wire brushing. Treat the rebar with an anti corrosion primer.
- Feather edges shall be removed along the perimeter of the hacked area by means of a disc grinder.
- Clean loose dirt and dust from the prepared surface using clean water.
- Pack clean, hand washed aggregates into hacked area and secure with a fine wire mesh.
- Install grout tight formwork to prepared surface and secure formwork with suitable ties.
- Dispense clean water into formwork assembly through inlet ports using a hand-operated grout pump. Ensure prepared surface and aggregates achieve Saturated Surface Dry condition (SSD).
- Mix grout with adequate quantity of water in a mixing drum. Ensure grout achieves a homogeneous consistency while stirring.
- Dispense grout mix into the formwork assembly through the inlet ports. Grouting shall proceed from the lower-most inlet port. As soon as grout is observed appearing from the adjacent port, lock off the first port and grout through the adjacent inlet port. Continue grouting in sequence until the entire formwork assembly is filled with grout.
- Remove formwork assembly and cure by means of curing compound or wet gunny sacks (burlap).
- If necessary, grind the surface to an even finish.