Investigation of “elephant skin” on UHPC surface via a new test method: Influencing factors, formation mechanism and control measures

Shortly after the mixing of ultra-high performance concrete (UHPC), a viscoplastic layer forms on the surface, which is often colloquially called elephant skin. This layer can hinder the venting, through which air bubbles accumulate below the concrete surface, causing poor surface quality and aesthetics after finishing or resulting in an inferior composite in the case of multi-layer structural components. This paper aims to improve the surface performance of UHPC by reducing the effect of elephant skin. In the present work, the influence of various factors on the “elephant skin” formation on UHPC surface is firstly investigated using a newly developed test method that measures the development of the surface resistance of “elephant skin” on UHPC with time, based on the Vicat apparatus for testing the setting time of cement. Then, the formation mechanism of “elephant skin” on the surface of UHPC is discussed. Finally, some measures to minimize the effect of “elephant skin” on the UHPC surface are proposed. The results reveal that next to controlling the ambient relative humidity (RH) and temperature, an optimization of the concrete mix composition and manufacturing process, such as an increase of superplasticizer (SP) dosage, paste content and water-to-binder (W/B) ratio can help to slow down the “elephant skin” formation on UHPC surface, minimizing its negative effect while without significantly compromising the concrete compressive strength.