Title:
Effect of Viscosity-Enhancing Admixtures on Formwork Pressure and Thixotropy of Self-Consolidating Concrete
Author(s):
Joseph J. Assaad and Kamal H. Khayat
Publication:
Materials Journal
Volume:
103
Issue:
4
Appears on pages(s):
280-287
Keywords:
formwork; high-range water-reducing admixture; self-consolidating concrete; thixotropy
DOI:
10.14359/16612
Date:
7/1/2006
Abstract:
An experimental program was undertaken to determine the influence of the type and concentration of viscosity-enhancing admixture (VEA) on formwork pressure that can be exerted by self-consolidating concrete (SCC). A liquid polysaccharide, powder polysaccharide, and a cellulose-based VEA were used in this study. Both naphthalene- and polycarboxylate-based high-range water-reducing admixtures (HRWRAs) were employed. Thixotropy of the concrete was evaluated and correlated to the initial lateral pressure and its variations in time. Test results show that the type, combination, and dosage rate of VEA-HRWRA have a marked effect on thixotropy and formwork pressure. SCC made with cellulose-based VEA and polycarboxylate-based HRWRA are shown to yield a greater degree of thixotropy compared to similar concrete prepared with polysaccharide-based VEA and naphthalene-based HRWRA. SCC made with cellulose-based VEA, however, exhibited a lower rate of increase in thixotropy with time. This is associated with the greater degree of fluidity retention when using polycarboxylate-based HRWRA. Irrespective of the VEA type and combination with HRWRA, results indicate that the incorporation of VEA at relatively low concentrations could result in lower formwork pressure compared to reference mixtures made without any VEA and those containing medium or high concentrations of VEA. SCC made with a low VEA concentration necessitated lower HRWRA demand to secure the targeted slump flow. This results in a sharper loss in fluidity, enabling the concrete to develop enough cohesiveness to resist some of the applied vertical stress. A good relationship was established between formwork pressure and thixotropy determined for SCC mixtures containing low concentrations of VEA with mixtures of greater degree of thixotropy exerting lower lateral pressure on the formwork. The increase of VEA concentration, however, necessitated a greater HRWRA dosage, thus leading to higher lateral pressure measured initially and over time, despite the increase in thixotropy.