Effect of materials proportion on rheology and mechanical strength and microstructure of ultra-high performance concrete (UHPC)

The present study investigates the effects of constituent materials proportions on mechanical strength and microstructure of UHPC and also attempts to evaluate the rheological properties of UHPC using a squeezing flow test. In this regard, the effects of silica fume replacement value (15%, 20%, 25%), superplasticizer content (1%, 1.2%, 1.4%) and three w/b ratios (0.18, 0.20, 0.22) on fresh properties (setting time, squeeze flow), mechanical strength (compressive or flexural strengths) and microstructure of UHPC were evaluated. The results showed that with the increase of w/b ratios, the compressive strength was reduced up to 11.5%, while high superplasticizer dosage improved it up to 10%. Furthermore, the SEM micrographs showed the very dense cement matrix of UHPC and strong interfacial transition zone without any observable roughness or crack. The squeezing flow test showed that the pastes have three basic characteristics: low plastic deformation, low flowability (high viscosity) and high sensitivity to the squeezing flow rate and the materials proportions. Some rheological parameters, including elongational strain rate, elongational viscosity, yield stress and flow energy, were also determined.