Additivity effects in the rheology of fresh concrete containing water-reducing admixtures

Freshly mixed concrete must be capable of being transported and placed, flowing into moulds and around reinforcement, compacted and finished, all without segregating. Modern concrete admixtures can reduce the water content needed for such performance by up to 30% with a resulting considerable increase in strength. It is well established that fresh concrete conforms to the Bingham model and since 1970s considerable data on the effects of constituent materials and their relative proportions on the yield stress and plastic viscosity has been collected. In mix selection it has been assumed, but never explicitly demonstrated, that these effects are additive. This paper reports a series of experimental water-reduced concretes, where the addition of superplasticiser reduces the yield stress, which is offset by reduced water content to return the yield stress to the original value. The reduced water content increases the plastic viscosity, leading to mixes which are sticky and difficult to handle. The results provide evidence in support of the concept of the additivity of constituent effects in the rheology of fresh concrete, which can be visualised as the vector sum of the effects when plotted on a graph of yield stress against plastic viscosity.