Title: Influence of Internal Friction and Cohesion on the Variations of Formwork Pressure of Self-Consolidating Concrete
Author(s): J. Assaad and K. H. Khayat
Publication: Symposium Paper
Appears on pages(s): 19-32
Keywords: aggregate; binder; cohesion; formwork lateral pressure; internal friction; self-consolidating concrete; set-modifying admixture; thixotropy
A comprehensive research program was undertaken to determine the influence of coarse aggregate concentration, binder type and content, and the use of set-modifying admixtures on lateral pressure exerted by self-consolidating concrete (SCC). Experimental columns measuring 200 mm in diameter and either 2100 or 2800 mm in height were used to determine the distribution of lateral pressure during the plastic stage of cement hydration. The effect of thixotropy of the concrete on pressure variations was investigated. Test results show that lateral pressure exerted by SCC is significantly affected by the development of shear strength properties of the plastic concrete, namely internal friction and cohesion. Mixtures incorporating greater coarse aggregate volumes and/or lower binder contents were found to exhibit higher degree of internal friction. This can reduce the mobility of the concrete and result in lower initial pressure. However, given that internal friction is an inherent property of the material which remains constant with time, the rate of drop in pressure was shown to depend mainly on the increase in cohesion. Therefore, mixtures containing higher binder contents and/or a set-accelerating admixture can exhibit sharper rate of pressure drop with time. Concrete with higher degree of thixotropy was found to develop lower initial lateral pressure and higher rate of pressure drop with time. This is attributed to the stiffening effect which enables the material to re-gain its shear strength when left at rest with-out any shearing action.