Formwork Striking Time Estimator
Striking times for side and soffit forms by member and temperature.
Quick answer
With the default inputs: Member type Slab span 2-8 m · Daily mean temperature 20 °C · Cement type Ordinary PortlandSuggested striking time 7.0 days
Edit the inputs below to recalculate.Inputs
Result
Suggested striking time
7.0days
| Strength required | 75% of design strength |
| Baseline at 20°C | 7.0days |
| Temperature | 20°C |
| Temperature factor | 1.00 |
| Cement factor | 1.00 |
| Matched cubes (reference) | 3sets |
| Calendar days | 7days |
| Maturity estimate | 5040°C·h |
This is an estimating aid for programming only — actual striking must follow the strength report from matched-cured cubes, which the codes require. Every 10°C drop slows strength gain by about 1.8 times, and below 5°C hydration effectively stops, so insulation is essential. Spans over 8 m and cantilevers must reach 100% before striking.
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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.
How to work it out
- Decide whether it is side form or soffit. Sides only need the concrete to hold its arris, normally 1-2 days at 1.2 MPa.
- Soffit strength follows the member: slabs up to 2 m span 50%, 2-8 m 75%, over 8 m 100%; beams up to 8 m 75%, over 8 m 100%; cantilevers always 100%.
- Estimate the time to reach that strength from temperature and cement type. At 20 °C with ordinary Portland, about 50% at three days, 75% at seven and 100% at 28.
- Strength gain roughly halves for every 10 °C drop: 5-6 days to 50% at 10 °C, and practically nothing at 0 °C.
- Confirm with companion-cured cubes on site. The estimate is for programming only; striking requires the test results.
Where it goes wrong
- Striking on a fixed number of days — the familiar seven days assumes 20 °C, and in winter seven days may be 30% strength, which collapses when struck.
- Striking a cantilever at 75% — hogging moment peaks at the support and an understrength member snaps off at the root; the code requires 100%.
- Substituting standard-cured cubes — those are cured in water at 20 °C and differ greatly from the member, by a factor of two in winter.
- Looking at strength but not span — a 2 m slab and a 9 m beam at the same 75% behave differently, and span is the whole basis of the bands.
Material and cost
Longer striking intervals cut formwork reuse and add to the set — dropping from six cycles to four raises formwork cost 50%. Accelerators to shorten curing add 20-40 CNY/m³ of concrete.
On site
- In winter keep the concrete above freezing by insulating or heating; extending the days does nothing below 0 °C because strength does not develop.
- Long-span slabs and beams need a formal striking permit signed by the technical lead, and props come out from midspan towards supports with temporary props left at midspan.
- If strength looks short or cracks appear after striking, re-prop immediately at closer centres than the original.
Basis
- GB 50204-2015 Code for acceptance of concrete structure construction, Section 4.3
- GB 50666-2011 Code for construction of concrete structures, Section 4.5
- JGJ 162-2008 Technical code for safety of building formwork
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.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Formwork Striking Time Estimator. 2026-10-25. https://jianzhujisuan.com/en/formwork-strip-time
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.