How is concrete pressure on formwork calculated?
Lateral concrete pressure is calculated from the rate of rise of the pour, the concrete temperature, the mix and its admixtures, and the form geometry. It builds hydrostatically until the concrete starts to stiffen, so the design pressure is the lower of the full hydrostatic head and the pressure at which set begins — and that value sets tie, waler and stud spacing.
Every argument about a blown form ends in the same place: what pour rate was assumed, and what rate was actually run. The two are frequently different, and only one of them is on the drawing.
The three variables that move the number
Rate of rise is the dominant one — pour faster and the concrete below has not begun to stiffen, so pressure keeps building hydrostatically. Temperature is next: cold concrete sets slower and holds pressure higher up the form. Admixtures, especially retarders and superplasticisers, extend the same effect.
A pour rate is a design input, not a preference. If the pump is going faster than the methodology allows, the form is being loaded beyond what its ties were sized for.
From pressure to tie spacing
Once the pressure envelope is known, tie capacity and spacing follow directly, then waler and stud sizing from the load each carries. Doubling the pour rate does not double the ties — it moves the whole envelope, and the layout is redesigned from it.