Title:
Variations of Lateral and Pore Water Pressure of
Self-Consolidating Concrete at Early Age
Author(s):
Joseph Assaad and Kamal H. Khayat
Publication:
Materials Journal
Volume:
101
Issue:
4
Appears on pages(s):
310-317
Keywords:
aggregate; concrete; pressure
DOI:
10.14359/13365
Date:
7/1/2004
Abstract:
The granular phase in concrete constitutes more than 60% of its total volume and can have significant influence on the development and variation of lateral pressure exerted by the plastic concrete. This study investigates the kinetics of variations in lateral pressure and pore water pressure that can be exerted by self-consolidating concrete (SCC) during the plastic stage of cement hydration and until the early stage of hardening. Nine mixtures of constant slump flow values of 650 ± 15 mm were evaluated by varying the sand-to-total aggregate ratio R from 1.0 to 0.30. The mixture proportioned with an R value of 0.50 was tested with three different coarse aggregate gradations of 10-5, 14-5, and 20-5 mm. Each concrete was tested to determine setting time, adiabatic heat rise and temperature development, and pressure variations with time. Test results show that the kinetics of lateral pressure drop during the plastic stage are significantly affected by the degree of internal friction that can be related to coarse aggregate concentration. For mixtures with R values greater than 0.50, the concrete behaved as an exclusively viscous material with a limited degree of internal friction, which resulted in greater development of lateral pressure. For mixtures with R values lower than 0.50, the concrete appeared to exhibit greater resistance to shear stress given the higher degree of aggregate friction, which led to considerably lower lateral pressure development. The drop in lateral pressure toward zero occurs after the end of the dormant period as the rate of cement hydration is accelerated. Beyond the dormant period, progressive formation of hydration products leads to the creation of a structural network, and the pore water pressure begins to drop abruptly towards negative values. The time for cessation lateral pressure is shown to be influenced by the R value. For R values greater than or equal to 0.40, the concrete can continue to exert some lateral pressure until the onset of final setting. For mixtures made with R values of 0.36 and 0.30, the cessation of lateral pressure occurred before initial setting as the rate of pressure drop was more accentuated than that resulting from SCC with higher R values. The increase in maximum aggregate size from 10 to 14 mm resulted in a sharper rate of pressure drop, which is associated with an increase in the degree of internal friction. A lower rate of pressure drop, however, can be obtained with a further increase in aggregate size to 20 mm, given the relatively decreased packing density of the 20-5 mm coarse aggregate compared to that of the 14-5 mm.