Pressions latérales développées par les bétons autoplaçants contre les coffrages : influence de la thixotropie

Recently, a new generation of concrete referred to as self-consolidating concrete (SCC) appeared on the market. This kind of concrete is characterized by its high fluidity and resistance to segregation. It enables the elimination of the vibration during the casting process and reduces the need for manpower on the construction site. The use of SCC has gained increased acceptance and it became essential to better understand the lateral pressure that can be developed on the vertical formwork systems. Presently, formworks are designed prudently by assuming that the plastic concrete exerts full hydrostatic pressure until the setting time. Such pressure is expressed as: P[subscript max] = p × g × H where p, g, and H correspond to the concrete unit weight, gravity, and head of concrete, respectively. Such approach can result in increased construction costs and can limit the rate of rise of the concrete in the formwork. An experimental program was undertaken in the University of Sherbrooke to evaluate the lateral pressure developed by SCC mixtures. A special emphasis was placed on the influence of concrete thixotropy on the pressure development. Various mixture parameters, including the coarse aggregate characteristics, binder type and content, as well as the water-to-binder ratio were investigated. The effect of incorporating chemical admixtures such as the set-modifying and water-reducing agents, high-range water-reducing and viscosity-modifying admixtures (HRWRA and VMA, respectively) were evaluated. The effect of consistency level, concrete temperature, and casting rate on formwork pressure were also determined. Test results showed that the variations in lateral pressure of SCC are highly affected by the development of shear strength properties, namely the internal friction and cohesion. The higher the degree of internal friction, the lower is the maximum initial pressure measured right after the end of casting. Increased degree of internal friction can be ensured by incorporating a greater volume of coarse aggregate. This can also be achieved indirectly by proportioning mixtures with lower binder contents and/or water-to-binder ratios."--Résumé abrégé par UMI