Fresh concrete exerts pressure on vertical form surfaces, and an assessment of that pressure is needed for designing forms. In the simplest theory, fresh concrete acts as a fluid exerting pressure equally in all directions at whatever point the measurement is made essentially assuming a hydrostatic pressure effect. This is reasonable because the fresh concrete behaves much like a fluid at least briefly during vibration, or for a longer time if flowability of the mixture has been enhanced through use of admixtures or special proportioning and materials selection.
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| Heavy-duty steel formwork resists concrete pressure in a 16 ft (5 m) high retaining wall. Custom form assembly permitted the contractor to place the wall and projecting corbels monolithically (Photo courtesy of EFCO Corp.) |
But concrete is not a true fluid, and some method of evaluating the concrete’s actual pressure is needed. Evaluating pressure has been a significant part of the work of ACI Committee 347, Formwork for Concrete. As early as 1958, Committee 347 (then Committee 622) studied available field measurements of lateral pressure on formwork and used the data to develop pressure formulas that could be safely used for form design. A report was published in 1958 and the formulas, with some modifications, were included in ACI’s first formwork standard, ACI 347-63.2 In the days before the advent of the personal computer, the committee considered it important to keep the equations simple, reasoning that this would encourage their use and minimize mathematical errors.