Cover, formwork and curing
Concrete pressure on formwork (CIRIA Report 108)
- where
- temperature coefficient
- concrete temperature at placing (°C)
- full hydrostatic pressure (kN/m²)
- weight density of concrete (kN/m³)
- vertical pour height (m)
- design maximum pressure (kN/m²)
- shape coefficient (1.0 walls, 1.5 columns)
- rate of rise of concrete (m/h)
- concrete materials coefficient
- vertical form height (m)
Explain this equation
Estimates the maximum pressure wet concrete puts on wall or column formwork, by the CIRIA Report 108 method. Use it to check that formwork and ties can take the pour.
Fresh concrete behaves like a heavy liquid at first, so the most it could push is its weight density times the pour height. But it starts to stiffen as it is placed, so a fast pour builds more pressure than a slow one. Cold concrete stiffens more slowly, which the factor K raises. C₁ reflects the form shape and C₂ the cement type and admixtures. The design pressure is whichever is smaller: full liquid pressure or the stiffening-limited value.
An empirical design method based on site measurements, capped by hydrostatic pressure.
Sense check: a 3.3 m wall poured at 3 m/h at 15 °C gives about 62 kN/m², less than the 82.5 kN/m² full liquid head.
Watch out: self-compacting concrete, retarders and very fast pours can approach full hydrostatic pressure. Get formwork designed for large pours.
Estimates the maximum sideways pressure of fresh concrete on wall or column formwork, using the CIRIA Report 108 method, never more than full fluid (hydrostatic) pressure.
Enter your values to see the result.
Good to know
Fresh concrete starts to stiffen as it is placed, so in a tall form poured slowly the pressure stops rising before it reaches full fluid head. Faster rate of rise, colder concrete and retarding cements or admixtures all push the pressure up. C1 is 1.0 for walls and 1.5 for columns (plan dimensions up to about 2 m). C2 depends on the cement and admixtures: 0.3 for plain Portland cements, 0.45 for blends or with retarders, 0.6 for blended cements with retarders; check R108 for your mix. 1 kN/m² = 1 kPa.
Common mistake
Pouring a column or short wall in one quick lift. Above a certain rate the formula gives full hydrostatic pressure (D × h), which can be two or three times what the formwork was designed for.
Limits
For conventional vibrated concrete in vertical forms. Self-compacting concrete, external vibration, re-vibration and very slow pours are outside the simplified method: design for full hydrostatic pressure or follow CIRIA R108 / the formwork supplier. Formwork design is temporary works and needs a competent designer.
Sources
- CIRIA Report 108 Concrete pressure on formwork (Clear and Harrison, 1985): Pmax = D[C1√R + C2K√(H − C1√R)] or Dh, whichever is less; K = (36/(T+16))²
- No-Strip Stop End technical data sheet: CIRIA 108 design check (C1 1.0, C2 0.60, T 32 °C, D 25 kN/m³, R 3 m/h, H = h = 3.3 m → 53.87 kN/m²; Dh 82.5 kN/m²)
- Timber formwork members design, concrete pressure estimation worked example (C1 1.0, C2 0.3, D 24, H 4 m, R 0.5 m/h, T 30 °C → 24.97 kPa)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS 8500, BS EN 206, BS EN 752, Building Regulations Part H, manufacturers’ data and your engineer’s design. Structural concrete and foundations need a competent designer.