Formwork and Shuttering for Concrete
This section covers temporary moulds for concrete: materials, joints, release agents, props, bracing, tolerances, camber, and striking. The exam tests safety, concrete quality, and economy, because formwork can fail or leave a poor finish.
Core rules
- Formwork must be strong, rigid, and tight enough to prevent grout loss. Joints should be close-fitting and lined where a fair face is required.
- Apply a suitable release agent before placing reinforcement; do not let it contaminate the reinforcement or construction joints.
- Props must bear on a firm base, be braced in both directions, and be checked for plumb and tightness.
- Provide camber where specified so the soffit does not sag below level.
- Striking time depends on concrete strength gain, member span, and weather. Do not strike soffit formwork until the concrete can carry the load; follow the engineer’s instruction.
- Remove formwork in a sequence that avoids shock loading, and immediately clean and stack panels for reuse.
Worked example
A 6 m × 4 m suspended slab is to be formed with 1.2 m × 2.4 m plywood sheets. Area = 24 m². One sheet covers 2.88 m². Sheets required = 24 ÷ 2.88 = 8.33, so 9 sheets before waste. Add about 10% for cutting and damage: 9 × 1.1 = 9.9, so order 10 sheets. The reasoning: calculate net area, divide by sheet area, then add a realistic waste allowance rather than ordering exactly to the net area.
Traps
- Striking soffit formwork early to speed up the programme.
- Setting props on soft ground or on loose bricks without sole plates.
- Omitting release agent and damaging the concrete face on removal.
- Leaving gaps that let grout escape and create honeycombing.