What this calculator does
Fresh concrete is heavy and it flows, so formwork is designed for the pressure it exerts while it is being placed — not for the weight of the hardened slab. This calculator applies the ACI expression for the placement rate, the concrete temperature and the unit weight, then caps the result at the hydrostatic head that the pour depth allows.
How to use it
- Enter the placement rate in metres per hour; 1.5 m/h is fast, 0.5 m/h is controlled.
- Enter the concrete temperature — cold weather delays set and raises the pressure.
- Enter the unit weight; the calculation normalises against 23.5 kN/m³.
- Set the chemistry factor: 1.0 for normal mixes, up to 1.4 for retarding or high-early-strength admixtures.
Governing formula
p = C_w · C_c · (7.2 + 785·R/(T + 17.8)) Where:
-
R= Placement rate [m/h] -
T= Concrete temperature [°C] -
C_w= Unit weight factor = γ/23.5 [–] -
C_c= Chemistry factor [–]
Derived Equations:
p_max = min(p, γ·H_lift) Worked example — 1 m/h placement of 24 kN/m³ concrete at 20 °C
The figures below were produced by running this page's own engine with the example inputs (Placement rate (m/h), Concrete temperature (°C), Concrete unit weight (kN/m³), Chemistry factor (Cc) (–)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Design lateral pressure | 28.56 kPa |
|---|---|
| Equivalent fluid head | 1.19 m |
| Hydrostatic limit at 2 m | 48 kPa |
| Assessment | ACI 347R column pressure ≈ 29 kPa at 1 m/h placement and 20 °C. |
- The ACI expression gives 7.2 + 785/(20 + 17.8) ≈ 31 kPa, scaled by the unit-weight factor of about 1.02.
- The hydrostatic limit at a 2 m lift is 48 kPa, so the calculated pressure governs.
- Design the formwork and ties for that pressure and limit the lift height accordingly.
Assumptions & limitations
Engineering calculation. ACI 347R lateral pressure for columns with slump ≤ 100 mm, capped by the fresh-concrete hydrostatic head. Walls and mass concrete use different rules.
- The expression applies to columns placed with slump ≤ 100 mm; walls have their own rules and self-compacting concrete needs a different approach.
- Formwork tie design, deflection and vibration of the formwork are separate checks.
- Temperature, not rate, is usually the variable that changes the answer most — measure it at the point of placement.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- ACI 347R — recommended practice for formwork concrete.
- ACI 301 — specifications for concrete construction.