Research on the formation of “elephant skin” and its influence on the workability of UHPC

Ultra-high performance concrete (UHPC) has a very high performance mainly due to its high powder content, low water/cement ratio, high packing density, and the contribution of the high-performance reactive powder. However, these characteristics lead to a very hard and viscoplastic layer on the surface of the fresh UHPC in a span of only several minutes after casting, which is colloquially named “elephant skin”. This layer hinders the air bubbles in the concrete from overflowing, making them accumulate un-der the surface to form a porous layer, which may seriously affect the workability of concrete. This paper mainly investigates the influences of elephant skin on the bounding performance of the multi-layer UHPC structures and the distribution of the air bubbles in the UHPC columns. According to the results, corresponding measures must be considered to avoid the formation of elephant skin as much as possible to ensure the workability of UHPC in large-scale applications.