Formwork Pressure

Fresh concrete behaves as a fluid

While it remains fluid, concrete exerts hydrostatic pressure on the form: density times gravity times depth. At roughly 2,400 kilograms per cubic metre that is about 23.5 kilopascals per metre of fluid depth, and it is a large number by the time a column is three metres tall.

The crucial word is fluid. Concrete does not stay fluid — it stiffens, and once it does, it carries its own weight rather than pressing outward. The design pressure is therefore governed not by the total height but by how much of that height is still fluid when the pour finishes.

Why columns and walls differ

A column has a small plan area and is filled in minutes. The concrete at the bottom is still fluid when the top arrives, so the full height acts hydrostatically and the pressure envelope is the plain hydrostatic line.

A wall of the same height has a large plan area and is filled over a much longer period. Its lower concrete stiffens and begins carrying itself well before the pour finishes, so the pressure rises with depth to a point and then caps. Same geometry, same mix, entirely different design pressure — and the only difference is the rate of rise.

Temperature and admixtures move the cap

Anything that keeps concrete fluid for longer raises the pressure. Cold weather, retarding admixtures, high slump and high cement replacement with GGBS or fly ash all extend the working time, which is usually the intention — and all raise the formwork pressure as a side effect.

This is why formwork pressure formulas contain temperature explicitly, and why a mix changed for placeability without telling the formwork designer is a recognised route to a blowout. Vibration does the same thing more abruptly: it re-liquefies concrete that had begun to stiffen, undoing the relief that time was providing.

Calculators that use this method

• ACI 347 Column Formwork Rate-of-Placement Pressure CalculatorCalculate the maximum lateral pressure on column formwork from ordinary (non-SCC) concrete, using the ACI 347R rate-of-placement formula.
• ACI 347 Wall Formwork Rate-of-Placement Pressure CalculatorCalculate the maximum lateral pressure on wall formwork from ordinary (non-SCC) concrete, using the ACI 347R rate-of-placement formula.