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abstract

The formwork pressure exerted by freshly mixed concrete is one of the most important considerations in ensuring the safety of construction in the field. The formwork pressure also affects the quality of concrete structures, determining the final shape of concrete members. Previous studies on the form pressure mostly investigated the intrinsic material effect. For example, the use of self-consolidating concrete increases the maximum pressure on the formwork. The extrinsic factor of a formwork system itself has rarely been considered. In the mockup tests and theoretical evaluations, it is identified that the bleed water-out controlled by the formwork tightness is critical. A pore water pressure diffusion model consistently addresses the high form pressure exerted by self-consolidating concrete, showing little bleeding, or by normally vibrated concrete placed in a waterproofed formwork.

4. Conclusions

The form pressure exerted by freshly mixed concrete is critical to ensure safety in the construction field. The mechanism of the decay of form pressure is still unclear, even though its design load is presented in a guideline or specification. This paper described a proposal that the diffusion of pore water pressure governs the change of the form pressure. The diffusivity and Poisson's ratio of the unset concrete mix are the material properties in the situation considered. Drainage of bleed water releases the excess pore water pressure, which results in a reduction of the form pressure. A lowdiffusivity mix, such as SCC, does not allow bleeding; thus, its form pressure remains almost hydrostatic. The mockup test results, where the form pressure exerted in a waterproofed form is compared with a general case, support the proposed hypothesis. The bleed water of the NVC generally leaks through the joints of the form plates. The pore water pressure diffusion model, considering the capillary pressure proposed in the paper, predicts the lateral pressure decay on the form allowing for bleed water-out. In contrast, NVC in a waterproofed form exerts the hydrostatic pressure that is the same as for SCC.