Designing formwork for the pressure of fresh concrete, not hardened concrete
Why formwork is designed for placement rate and temperature rather than the weight of the finished slab, and where the hydrostatic cap applies.
The engineering question
Formwork designed for the weight of hardened concrete still fails on site. What pressure does fresh concrete actually exert, and what limits it?
Worked example
The figures below were produced by running the Concrete Formwork Pressure Calculator with the same inputs — during the build, from its authoritative engine.
| Design lateral pressure | 28.5622 kPa |
|---|---|
| Equivalent fluid head | 1.1901 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 is p = Cw·Cc·(7.2 + 785R/(T+17.8)), scaled by unit weight and chemistry.
- Cold concrete and fast placement both raise the pressure sharply.
- The result is capped by the fresh lift hydrostatic head, which is why lift height controls.
Sanity check
If the calculated pressure exceeds the hydrostatic limit, the lift height governs and no amount of extra reinforcement will change it.
Tools for this problem
These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.
- Concrete Formwork Pressure Calculator Lateral formwork pressure for a concrete pour from placement rate, temperature and unit weight, capped at the hydrostatic head.
- Concrete Volume Calculator Build a multi-item concrete quantity schedule for slabs, footings, beams, walls and columns, with category subtotals and an Excel export.
- Footing & Foundation Volume Calculator Calculate concrete required for spread (pad) footings and strip footings, per-footing and total volumes with ready-mix allowance.
Engineering review required
This topic supports professional engineering work; it does not replace a sealed design. The result depends on site data, the governing standard and engineering judgement, and should be confirmed by a competent engineer before it is built.
References & standards
- ACI 347R — recommended practice for formwork concrete
- ACI 301 — specifications for concrete construction
Related problems
Taking off concrete volumes without mis-ordering the pour
Where concrete volume actually goes — including the parts that are habitually forgotten — and how wastage and batching affect the order.
Also: Civil & Structural
Staircase concrete volume and how it splits up
Waist slab, steps and landing as separate volumes, and why the pour is usually smaller than people expect.