Formwork Striking Time Estimator
Striking times for side and soffit forms by member and temperature.
Inputs
Result
Formula
| Side forms | strength ≥ 1.2 MPa |
| Slab ≤ 2 m span | ≥ 50% |
| Slab 2-8 m / beam ≤ 8 m | ≥ 75% |
| Span > 8 m / cantilever | ≥ 100% |
| Temperature | t = t₂₀ × 1.8^((20−T)/10) |
| Cement | rapid ×0.75, slag ×1.3 |
A slab spanning 2-8 m at 20°C with ordinary Portland cement: 75% strength needed, a 7.0-day baseline and a factor of 1.00, so strike the soffit at 7.0 days — the same slab at 10°C needs 12.6 days.
FAQ
Why go by cubes rather than days?
Because seven days at 20°C and seven at 5°C differ by more than a factor of two in strength, before cement type, admixture and curing moisture are considered. Codes require matched-cured cube results, with days used only for programming. A good share of collapses come from “striking on the calendar” — the date arrives, strength is still under 60%, and the member cracks on release.
Why can side forms come off early?
Because they resist fresh concrete pressure only, not the member's own weight. At 1.2 MPa — about a day at 20°C — the edges will not spall or stick. But curing must start the moment they come off: wrap columns to hold moisture, or surface drying shrinks and cracks the concrete, hurting impermeability and durability.
How do early striking systems work?
By leaving independent props behind. The drop head transfers the slab load onto props that stay, so the soffit panels and bearers can be recovered and reused while the member remains supported. One set of forms then serves far more cycles, cutting formwork cost by 30-40%. The props themselves stay until 100% strength, and enough must remain — recovering them to save time is how failures happen.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.