Optimisation and Prediction of Fresh Ultra-High-Performance Concrete Properties Enhanced with Nanosilica

Abstract

The use of high chemical admixture dosages in ultra-high-performance concrete (UHPC) mixtures to achieve adequate water demand can slow down early cement hydration and prolong the setting time. In this study, the effects of nanosilica (Ns) with high chemical admixture dosages on the rheological properties of UHPC was investigated. A factorial design approach was employed to predict and optimise the Ns content, water-binder ratio (W/B), and sand-binder (s/b) ratio to obtain the best flowability, setting time, and compressive strength. This study represents an attempt to modelling and optimise eighteen UHPC mixtures containing various proportions of water, cement, and sand, with the Ns powder as a possible property enhancer to achieve the best rheological properties. Response surface analyses revealed the significant effect of Ns in controlling the prolonged setting time and improving the compressive strength. Based on the applied criterion conditions, the optimisation results indicated two mixtures targeting either the maximum compressive strength or cost effective materials. The use of a 1.12 s/b ratio with a controlling level of 0.8% Ns content was suitable to fulfil the compressive strength, flow, and setting time limit values.