Influence of Superplasticizers on Workability of Concrete

This paper outlines the theoretical foundations governing the rheological properties of concrete. The rheological equation for fresh concrete is given as a modification of Bingham's equation. The shear resistance of the concrete mix is explained in terms of two constants—yield value and plastic viscosity. The complex nature of the yield value is described. It is pointed out that there are three different zones of rheological behavior of fresh concrete, which arise as a result of the relation between shear stress and yield value of the mix. This paper also describes an experimental study that measures the effects of superplasticizer, water-to-cement (w/c) ratio, condensed silica fume (CSF) and elapsed time on the rheological parameters for fresh concrete. The rheological tests were carried out on a ROD1E rotational rheometer. In total, 102 flow curves were obtained from which the yield values g and the plastic viscosities h for a wide range of concrete mixes were determined. The results showed that superplasticizers became less effective with increase in w/c ratio and that different superplasticizers have widely different effects on changes in g and h. It was also established that CSF additions generally reduce workability, although above a certain CSF content a sharp reduction in plastic viscosity occurs. In addition it was observed that superplasticizers generally extend the period over which a mix remains workable. The results are discussed in terms of the theoretical foundations.