Formwork for a Suspended Slab After Rain
A suspended slab 6.0 m by 4.5 m, 150 mm thick, is to be cast at 3.0 m above ground beside a newly filled area. The formwork uses 12 mm plywood that has been used three times; edges are delaminating and one sheet is soft underfoot. Props are at 1.2 m centres, but six props sit on bare fill without sole plates, and two props are spliced with wire. There is no diagonal bracing. The mason plans to pour the 4 m³ slab from one end in about 45 minutes. Rain fell last night and the fill is soft. The concrete pump is booked for 7 a.m. and the client is pressing for progress. You are the carpenter responsible for the formwork.
1. Assess the collapse risk and identify which defects must stop the pour.
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The formwork is not ready and the pour must stop. Load path: the wet concrete, reinforcement, workers and equipment pass through the plywood to the joists, bearers and props, then to the ground. The soft fill after rain may not carry the prop loads, especially with no sole plates; six props on bare fill can settle unevenly and load the slab. The two wire-spliced props cannot carry compression safely and can buckle. The delaminated 12 mm plywood has lost stiffness and may puncture or deflect, creating a local failure that spreads. No diagonal bracing means the whole assembly is laterally unstable and can sway or rack during pouring. Pouring 4 m³ from one end creates an unbalanced load and can tip or push the formwork before the concrete spreads.
Stop-work defects: soft fill without sole plates, spliced props, no bracing, damaged plywood, and a pour sequence that loads one end. The pump booking and client pressure are not engineering reasons to proceed.
2. Prepare a corrective action list and a safe pour sequence.
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Corrective actions: strip the affected area; compact and level the fill and provide timber sole plates under every prop; replace spliced props with continuous props and set them plumb with adjustable heads and wedges; add diagonal bracing in both directions and tie the formwork to stable structure; replace soft or delaminated plywood and seal the edges; check joist and bearer spacing against the design; oil the form face; provide camber and edge formwork; inspect and sign off before the pump arrives.
Pour sequence: place concrete in layers across the whole slab, not in one heap at one end; keep the depth roughly even; avoid dropping concrete from height; have a carpenter watching props and wedges throughout; stop immediately if any prop settles, any plywood deflects excessively or any bracing loosens. Keep workers off the formwork except those placing concrete, and keep everyone out from under the slab. If the pump cannot start until the checks are complete, delay the pour; a day's delay is cheaper than a collapse.
3. When can the formwork be struck, and what records are needed?
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Striking is based on strength, not the clock. The soffit formwork and props should remain until the concrete has gained enough strength to carry its own weight plus construction loads. For a suspended slab, that usually means waiting for cube test results or a maturity assessment, and following the specification or engineer's instruction. Strike edge formwork earlier only if the concrete can resist damage. Leave some props under long spans or cantilevers as specified; do not remove all props at once.
Records: concrete pour date and time, cube samples and test results, ambient and concrete temperatures, any delays or additions, the engineer's approval to strike, the sequence of prop removal, and a post-strike inspection for deflection or cracking. If cubes fail, do not strike; inform the engineer and keep the props in place. The common error is to strike after a fixed number of days because the programme says so. On this slab, with the defects found, no striking decision should be made until the formwork has been rebuilt and the concrete tested.