Characterization of fresh dry-mix shotcrete and correlation to rebound

Dry-mix shotcrete has all the components of concrete but its particular placement technique generates losses due to rebound. These losses induce a cost increase and a difference between the initial and in-place composition of the concrete. Many parameters influence rebound, but this study focuses on mixture design via supplementary cementitious materials (SCMs). The aim of the paper is to study the relation between fresh properties and rebound of shotcrete, and evaluate how SCMs act on it. This will eventually help to understand what mixture properties are needed to limit rebound and help with the design of mixtures. In this study, 5 mixtures in which cement was partially replaced by metakaolin, ground granulated blast furnace slag or silica fume were shot at different consistencies, in a full scale laboratory facility. Fresh shotcrete was evaluated by penetration tests (static and dynamic). It appeared that the SCMs were efficient in reducing rebound and their efficiency depended on consistency. Water is one of the main parameters in rebound but it is difficult to control because it is adjusted by the operator. Mixtures with metakaolin and silica fume seem to lower this dependence. Static and dynamic penetration stresses seem to be correlated with rebound and, more precisely, the relation between dynamic measurement and rebound seems independent of the mixture tested. Understanding the levels of static and dynamic penetration stress could make mixture design easier.