Investigations on the workability and microstructure development of UHPC; part 1: Effect of superplasticizer and silica fume on mixing and workability of UHPC

A priority programme (SPP 1182) on sustainable construction with ultra high performance concrete (UHPC) comprising 25 individual projects was initiated by the German Research Foundation in the year 2005 for a period of six years. The present contribution is an intermediate report by the group working on rheology. The effect of superplasticizer and silica fume type on the superplasticizer adsorption and the Theological properties of fresh concrete were investigated. The time needed for liquefaction during mixing is much shorter with rapidly adsorbed allyl ether based polycarboxylate (PCE) superplasticizers (star polymers) than with methacrylate based PCE. A mixture containing three parts by weight of methacrylate PCE and one part allyl ether PCE proved most effective for liquefaction of UHPC. Silica fume addition often leads to low slump flow of fresh UHPC. The interparticle adhesion of silica fume particles enhances the yield stress and thus the degree of slump flow. This varies strongly for different silica fumes. Investigations with silica fume in electrolytic solutions showed that adhesive interparticle forces depend strongly on the ionic strength of the solution correlating with increases in yield stress and viscosity.