Role of water film thickness in mix design of high-performance concrete

Abstract

Over the last decade, concrete technology has advanced dramatically, leading to increasing popularity of high-performance concrete (HPC). Meanwhile, the mix design of HPC has remained a tedious trial and error process with no general guidelines whatsoever to follow. One main reason is that apart from the water content and superplasticizer dosage, the particle size distribution of the cementitious materials, inclusive of the supplementary cementitious (SCM), also has significant effects on the rheology of the concrete mix. Particularly, the addition of ultrafine SCM to fill into the voids between the cement grains can release the water trapped inside to lubricate the concrete mix. In this regards, however, researchers have obtained widely different and even contradictory results on the roles of SCM in the rheology of cement paste/mortar/concrete. To resolve this problem and develop a scientific mix design method, a new parameter, water film thickness (WFT), has been proposed by the authors. In this study, a number of mortar samples made with superfine cement and condensed silica fume added as SCM were produced for testing. A wet packing method was used to measure the packing densities of the solid particles, from which the WFT of each mortar sample was determined. The results showed that the addition of the ultrafine SCM can effectively increase the packing density and improve the performance of mortar. Moreover, the WFT has governing effects on the rheology and strength of mortar and thus may be taken as the key factor to be considered in the mix design of HPC.