3/31/2023 0 Comments Max allowable bottom time calcSetting an absolute minimum formwork pressure helps ensure safety. Regardless of the pmax calculated from the three equations above, ACI 347-04 says not to use pressures less than 600Cw (psf) or greater than wh (psf), or the full fluid pressure. The ACI 347-04 equations are based on concrete having a slump of 7 inches or less and placed with a normal vibration to a depth of 4 feet or less. Cw=unit weight coefficient (for 140 to 150 pcf concrete, Cw=1.0).T=temperature of the concrete during placement (☏).pmax=CwCc for walls with a placement rate less than 7 feet per hour where placement height exceeds 14 feet and for all walls with a placement rate of 7 to 15 feet per hour.pmax=CwCc for walls with a rate of placement of less than 7 feet per hour and placement height not exceeding 14 feet.Even partially hardened or quasi-solid concrete can be returned to a liquid state producing a full fluid pressure when vibrated so be sure to follow guidelines for depth of consolidation.ĪCI 347-04 "Guide to Formwork for Concrete" lists three modified hydrostatic pressure equations: Of course, lateral pressure is strongly influenced by the type, extent and depth of concrete vibration used to consolidate the fresh concrete. This "modified hydrostatic pressure" is influenced by the weight of the fresh concrete, rate of concrete placement, temperature of the concrete mixture, types of admixtures, cement type and amount of supplementary cementitious materials in the concrete. When this occurs, hydrostatic pressure at the bottom of the form drops off to less than full fluid pressure. If placement rates are not too fast and concrete stiffening is not too slow, concrete at the bottom of the form starts to stiffen before the form is full of fresh concrete. This is especially true for columns and walls that can be filled rapidly before stiffening of the concrete occurs. Like diving into and swimming to the bottom of a swimming pool, the pressure increases with depth so the maximum pressure or full fluid pressure occurs at the bottom of the formwork.įull fluid pressure may occur when the concrete placement rate is fast and the rate of stiffening is slow or delayed due to chemical retarders, blended cements and supplementary cementitious materials, or cold weather conditions. To compute the full fluid pressure, multiply the concrete height by the unit weight of the fresh concrete as shown in Example 1.įor a concrete height of 10 feet, the full fluid pressure is equal to 1,500 pounds per square foot (psf). Full fluid pressureĭepending on the placement rate and setting characteristics of the concrete, the lateral pressure can be equal to a full liquid head, also referred to as full fluid pressure. As the fresh concrete changes from a liquid into a quasi-solid, the lateral pressure diminishes. However, due to concrete stiffening, the hydrostatic pressure is only temporary. Freshly placed concrete in wall and column forms behaves like a liquid producing hydrostatic pressure that acts laterally on the vertical faces of the formwork.
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