Title:
Effect of w /cm and High-Range Water-Reducing
Admixture on Formwork Pressure and Thixotropy
of Self-Consolidating Concrete
Author(s):
Kamal H. Khayat and Joseph J. Assaad
Publication:
Materials Journal
Volume:
103
Issue:
3
Appears on pages(s):
186-193
Keywords:
admixtures; self-consolidating concrete; thixotropy; water-to-cementitious material ratio
DOI:
10.14359/15852
Date:
5/1/2006
Abstract:
An experimental program was undertaken to evaluate the effect of water-cementitious material ratio (w/cm) and type of high-range water-reducing admixture (HRWRA) on the development of formwork pressure that can be exerted when using self-consolidating concrete (SCC). Pressure variation was monitored using an experimental column measuring 2800 mm in height. The tested mixtures were proportioned with a similar initial slump flow consistency of 650 ± 15 mm. Three w/ cm of 0.36, 0.40, and 0.46 and three types of HRWRA (polycarboxylate, polynaphthalene sulphonate, and polymelamine sulphonate) were investigated. Variations in lateral pressure were related to the thixotropy of the concrete. Test results show that the variations in lateral pressure and thixotropy of SCC are significantly affected by the w/cm. Irrespective of the HRWRA type, mixtures proportioned with 0.46 w/cm exhibited greater initial pressure and lower thixotropy compared with mixtures made with a w/cm of 0.40 and 0.36. This is related to the higher water content and lower coarse aggregate volume in concrete proportioned with the higher w/cm, which can lead to a reduction in shear strength properties of the plastic concrete. The rate of pressure drop and increase in thixotropy with time, however, were greater in mixtures made with a higher w/cm. This is attributed to the lower HRWRA demand that can lead to sharper fluidity loss with time. For any given w/cm, the type of HRWRA appears to have a limited effect on initial lateral pressure. Compared with naphthalene- and melamine-based HRWRA, the use of polycarboxylate-based HRWRA in SCC resulted in lower rate of pressure drop with time. This is reflected by the greater fluidity retention of the mixtures containing the polycarboxylate-based HRWRA. The incorporation of a water-reducing agent in mixtures made with polynaphthalene sulphonate-based HRWRA is shown to increase lateral pressure development of the plastic concrete over time.