Concrete Foundation Construction Work Procedure at Site
The purpose of this guide is to ensure that all the concrete foundation / plinth are constructed properly and according to standards and requirements.
Before casting concrete the soil sub-grade shall be compacted and leveled, free from standing water and debris. 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.
1. All formwork materials shall be selected and installed to achieve the required concrete surface finishes.
2. All joints between formwork panels shall be adequately sealed, to prevent the leakage of grout during concrete casting and compaction.
3. All formwork shall be measured before concrete casting to confirm the locations, alignment and top of concrete levels.
4. 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.
5. All 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
1. Arrangement of Reinforcing Bars – The minimum distance of individual bars shall be 150mm.
2. After installation of rebar, the following items shall be visually checked and confirmed:
(i) The minimum rebar size is 16mm.
(ii) The tolerance of the position of reinforcement bars shall not exceed ±6mm.
(iii) Cover and spacers for reinforcement. Note that spacers shall consist of either specifically designed plastic spacers, or concrete cubes with fixing wires. Should concrete cubes be utilized, they shall be made from the same grade of concrete (or higher) as the item to be cast.
(iv) Fixing of reinforcement shall be done using 16 SWG soft pliable annealed steel wires. The ends of the fixing wire shall be turned in from the face of the formwork, to avoid rust spots on the surface of the finished concrete.
Removal of formwork
The time for stripping of formwork is 12 hours. The formwork shall not be removed earlier than twelve(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:
(1) Levels of finished concrete.
(2) Alignment of Finished Concrete.
(3) Levels and Alignment of embedded items such as anchor bolts.
(4) Tolerances for levels and alignments
The finished surface of the concrete shall be checked for the following defects:
(2) Spalling and dusting.
(6) Abrupt irregularities
Repair of Foundation Surface Defects with Pre-Packed Grouting
(1) Hack all unsound and cracked concrete up to a depth exceeding 50mm. In areas where rebar is exposed, remove concrete behind rebar.
(2) For corroded rebar, loose rust scales shall be removed by wire brushing. Treat the rebar with an anti corrosion primer.
(3) Feather edges shall be removed along the perimeter of the hacked area by means of a disc grinder.
(4) Clean loose dirt and dust from the prepared surface using clean water.
(5) Pack clean, hand washed aggregates into hacked area and secure with a fine wire mesh.
(6) Install grout tight formwork to prepared surface and secure formwork with suitable ties.
(7) 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).
(8) Mix grout with adequate quantity of water in a mixing drum. Ensure grout achieves a homogeneous consistency while stirring.
(9) 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.
(10) Remove formwork assembly and cure by means of curing compound or wet gunny sacks (burlap).
(11) If necessary, grind the surface to an even finish.