Formwork and Shoring
Formwork and shoring tests whether you understand that fresh concrete behaves like a heavy fluid, and that forms, ties, bracing and shoring must be designed, installed and removed accordingly.
Core rules
- Lateral pressure depends on placing rate, concrete temperature, set time and lift height. Slow placement, cool concrete, retarders and revibration all raise pressure; a fast-set mix or a short lift lowers it.
- Full hydrostatic pressure equals density times gravity times height: with normal concrete near 2,400 kg per cubic metre, about 23.5 kPa per metre of height.
- Ties, walers, strongbacks, kickers and braces work as a system. Tie spacing sets the load per tie; the tie's safe working load must exceed it with a margin.
- Use a compatible release agent on the form face only, never on construction joints or rebar.
- Camber beams and slabs as specified; shore and re-shore so the structure is never unsupported beyond design assumptions.
- Stripping: vertical forms usually after about 24 hours, once concrete can stand; soffits only when strength is verified by cylinders or the specification allows. Cold weather extends every timing.
Worked example
A 3.0 m high wall is placed at a rate that keeps the concrete plastic from bottom to top, so full hydrostatic pressure develops. Pressure equals 2,400 times 9.81 times 3.0, about 70.6 kPa. Ties are set at 600 mm by 800 mm, an area of 0.48 m squared, so each tie carries 70.6 times 0.48, about 33.9 kN. If the tie is rated 26.7 kN, the pattern fails. Required area is 26.7 divided by 70.6, about 0.38 m squared, so a 600 mm by 600 mm pattern at 0.36 m squared works. Alternatives are a slower placing rate or a set-accelerating mix, both of which reduce pressure.
Traps
Assuming pressure is always hydrostatic, or never is. Forgetting that vibration and revibration drive pressure up near the bottom. Stripping soffits by the clock instead of by measured strength. Getting release agent on a construction joint and killing the bond.