Elephant skin formation on UHPC surface

Abstract

Ultra-high performance concrete (UHPC), which typically contains extremely high superplasticizer amount, tends to render a special type of dense surface layer, especially under drying conditions. Even if the inner side of fresh UHPC is still in a flowable nature, the top surface of the concrete which is exposed to ambient conditions renders a dense cover called “elephant skin” in a short time after casting. In this study, the effects of climatic conditions and ground granulated blast furnace slag (GGBS) replacement on the elephant skin formation were studied. The influence of elephant skin formation on the macropore profile was assessed by micro-computed tomography scan. Observations revealed that any drying condition that does not lead to water vapor condensation on the fresh surface can trigger the formation of elephant skin within minutes after casting. Both microscopic and micro-computed tomography investigations showed that as a result of elephant skin formation, most of the air bubbles cannot escape from the fresh matrix, and locate under the skin. These air bubbles and elephant skin itself result in a multi-layered structure in UHPC body throughout the cross-section and reduce the aesthetics of the casting surface. Increasing GGBS replacement percentage significantly reduced the superplasticizer demand which is responsible for skin formation, however, did not eliminate the problem.