Wall Form Pressure Calculator

Lateral Pressure is calculated from an ACI 347R formula driven by Pour Rate and Concrete Temperature, but the result is always capped at the LOWEST of three values, not just two: that formula value, Full Hydrostatic (Concrete Unit Weight times Form Height -- the pressure fresh concrete would exert if it behaved as a fully liquid fluid), and a separate ACI ceiling of 2000·Cw psf (for pour rates at or below 7 ft/hr) or 3000·Cw psf (above 7 ft/hr), where Cw is the unit-weight coefficient. That ACI ceiling is easy to overlook because it isn't always the binding one, but it can govern even when hydrostatic pressure would allow more -- for example, at a 7 ft/hr pour rate, 40°F concrete, 20 ft form height, and Cc=1.4, the formula gives 2415 psf and the hydrostatic cap allows 3000 psf, but the displayed Lateral Pressure is 2000 psf because the ACI ceiling is the lowest of the three. At this calculator's default settings, the ACI formula value (about 793 psf) is well below the hydrostatic cap, so Form Height has no effect on Lateral Pressure at all once it's tall enough for hydrostatic pressure to exceed the formula value -- which happens above roughly 5.3 ft at these settings. Below that height, Form Height becomes the ENTIRE story: the hydrostatic ceiling is what's actually limiting the pressure, and Lateral Pressure tracks Form Height directly rather than the pour- rate formula.

Concrete Temperature moves pressure in the opposite direction from what you might expect from a simple "warmer means more pressure" intuition -- because temperature sits in the denominator of the ACI formula (representing faster strength gain at higher temperatures), warmer concrete actually produces LOWER lateral pressure, not higher, since it stiffens and stops behaving like a fluid sooner. Pour Rate raises pressure only up to the point where a cap takes over -- the formula itself switches to a different equation above 7 ft/hr (the threshold between column-type and wall-type placement rates), and both regimes increase the FORMULA value with faster placement, but once that formula value reaches whichever cap (hydrostatic or the ACI ceiling) is binding, further increases in Pour Rate have no effect on the displayed Lateral Pressure. At this calculator's default settings, that happens at a pour rate of about 18.25 ft/hr, where Lateral Pressure hits the 1,500 psf hydrostatic ceiling and then stays flat all the way to the top of Pour Rate's 30 ft/hr range -- covering roughly the top 39% of the input's declared span.