Concrete Formwork Pressure Calculator (ACI 347R)
Calculate maximum lateral pressure of fresh concrete on formwork. Compliant with ACI 347R standards for safe construction engineering.
About
When fresh concrete is poured into vertical formwork, it acts as a fluid, exerting lateral pressure against the walls. If this pressure exceeds the design capacity of the forms, it can lead to catastrophic "blowouts", endangering workers and causing significant project delays. This calculator uses the standard ACI 347R formulas to determine the maximum lateral pressure (Pmax) based on pour rate, concrete temperature, and height.
Pressure is significantly influenced by the setting time of the concrete. Cold weather slows down the chemical hydration process, keeping the concrete fluid for longer and increasing the pressure load on the forms. Conversely, rapid pouring rates (feet per hour) generate higher pressures before the lower layers have a chance to stiffen.
Formulas
For walls with a pour rate R < 7 ft/hr, the ACI 347R formula is:
For R between 7 and 10 ft/hr:
Subject to the constraint: 600 Cw ≤ Pmax ≤ wh.
Reference Data
| Variable | Symbol | Typical Unit | Description |
|---|---|---|---|
| Pressure | P | psf (lb/ft²) | Lateral force per area |
| Unit Weight | w | 150 pcf | Density of standard concrete |
| Pour Rate | R | ft/hr | Vertical rise speed |
| Temperature | T | °F | Temp of concrete mix |
| Chemistry Coeff | Cc | 1.0 - 1.2 | Retarders increase value |
| Weight Coeff | Cw | 1.0 | 1.0 for roughly 145-155 pcf |