Influence of self-compacting concrete on the lateral pressure on formwork

The paper describes a study carried out to examine the effect of self-compacting concrete (SCC) on the lateral pressure on formwork. This comprised two series: (a) control tests, with SCC and comparative concretes (Portland cement (PC) and superplasticised, high-consistence (SP-HC)) cast in 8.0 m columns and walls, and measurements made of concrete and pore water pressures (ten elements) and (b) site tests, with similar properties considered on actual reinforced SCC members during their construction (seven elements). In the control tests, concrete was poured from above to the rising surface at slow (3.0 m/h) and fast (80.0 m/h) rates of rise (with compaction used in the PC and SPHC concretes only). The results from these indicate that maximum concrete pressures with SCC were slightly lower than those of the comparative concretes at both rates of rise and in columns and walls. Pore water pressures were less than concrete pressures, except with the fast rate of rise for SCC and SP-HC concrete, where the reverse occurred (with the concretes having similar values or SCC slightly higher), which seemed to relate to the concrete properties/pour details and differences in concrete/pore water pressure measurement methods. Maximum pressures for SCC in the control tests were less than hydrostatic and those calculated using the CIRIA report 108 guidance method (Clear and Harrison, 1985). For the site tests, where base-to-top pouring was mainly used (i.e. concrete was passed by way of a hopper at the top of the formwork through a pipe running to its base and kept in place throughout the pour) maximum pressures for SCC were found to exceed hydrostatic in some cases, indicating that this method may give higher pressures. To investigate this further, data from the literature covering SCC pressure measurements on formwork were examined. These showed general agreement with the results of the current study using base-to-top pours and where concrete pumping from below had been adopted, which tended to be at the upper end of the maximum pressure range for SCC. Consideration is given to how SCC may fit within CIRIA 108 for estimating concrete pressure on formwork.