What is the lateral pressure of fresh concrete on formwork?
Fresh concrete acts like a heavy fluid before it sets, pushing outward on the form face with a near-hydrostatic pressure that is highest at the base of the pour. The full hydrostatic bound is the concrete unit weight (about 24 kN/m³, or roughly 150 lb/ft³) multiplied by the height of fresh concrete. As the lower lifts begin to set they stop transmitting full head upward, which is why a slow pour develops less peak pressure than a fast one.
How do you calculate concrete pressure on formwork?
Start with the hydrostatic bound (unit weight × fresh-concrete height). For conventional mixes placed at a known rate, ACI 347 in North America and CIRIA Report 108 / BS 5975 in the UK and Europe give design formulas that allow a lower pressure based on pour rate, temperature and mix, capped at the hydrostatic value. These are reference methods; the formwork designer or engineer of record runs the final calculation for the actual mix and conditions.
Does pour rate affect concrete pressure on formwork?
Yes. Pour rate is the single biggest lever the site controls. The faster the form fills, the more of the concrete column stays liquid at once, so the peak pressure climbs toward full hydrostatic. Slowing the rate or staging the pour in lifts lets each layer gain a little stiffness before the next, which lowers the maximum pressure the form ever sees. Cold concrete sets slower and also raises pressure.
What plywood thickness handles high formwork pressure?
An 18 mm film-faced panel is the working baseline for typical wall and column pressures on around 600 mm stud spacing. For tall lifts, fast pours, self-compacting concrete, or wider support spacing, 21 mm buys back the stiffness needed to keep deflection within limits and protect the cast face. Match the panel to the calculated design pressure and the deflection limit your finish requires.
