Microstructural investigations on the skinning of ultra-high performance concrete

A skinning phenomenon occurring in the first seconds to minutes on ultra-high performance concrete (UHPC) was analysed by microstructural investigations. The low water content and the resulting high content of superplasticizers, i.e. polycarboxylate ethers, lead to differences in rheological behaviour of fine grained UHPC containing silica fume. A visco-elastic surface layer impedes the deaeration and constrains smoothing of the concrete surface. A detailed multi-method characterization of the microstructure of the layered cross-section revealed enrichments of polycarboxylate ether superplasticizers (PCE) on the surface of the UHPC. FITC-staining of the PCE made a localisation and relative quantification possible. A hypothesis for the formation of the skin comprising PCE enrichment driven by evaporation is derived from this microstructural study.