Improvement of the surface quality and aesthetics of ultra-high-performance concrete Yang Chen MSc MS student, Department of Civil and Environmental Engineering, Michigan State University, East Lansing, MI, USA Faris Matalkah MSc PhD student, Department of Civil and Environmental Engineering, Michigan State University, East Lansing, MI, USA (corresponding author: Matalkah@msu.edu) (Orcid:0000-0003-3899-2036) Weerirsiri Rankothge MSc Chemist, Metna Co., Lansing, MI, USA Anagi Balachandra PhD Senior Scientist, Metna Co., Lansing, MI, USA Parviz Soroushian PhD, PE, FACI, FASCE President, Metna Co., Lansing, MI, USA The low moisture content of ultra-high-performance concrete (UHPC) combined with its minimal bleeding leads to quick drying of its exposed surfaces by way of moisture evaporation and self-desiccation. The result is the formation of a rough texture and occasional cracks on the exposed surfaces of UHPC within hours after its placement and finishing. This phenomenon is referred to as ‘elephant skin’ formation. Different strategies were devised and experimentally evaluated for the control of elephant skin formation on exposed UHPC surfaces. These strategies emphasised reduction of the moisture loss from the surface, enhancement of early-age dimensional stability and increase in plastic shrinkage crack resistance. The potential for elephant skin formation was assessed through monitoring of the surface aesthetics, crack area, penetration resistance and temperature under exposure to air flow and radiation. The results indicated that measures that effectively reduce moisture loss from the surface are most effective in controlling elephant skin formation on the exposed surfaces of UHPC. 1. Introduction Ultra-high-performance concrete (UHPC), with compressive strengths exceeding 150 MPa (Richard, 1996; Richard and Cheyrezy, 1995; Russell and Graybeal, 2013; Schmidt et al., 2004; Shi et al., 2015), is produced with a high content of cementitious materials, low ratios of water to cementitious materials (0·15–0·20) and aggregates of relatively small particle size (Richard and Cheyrezy, 1995). UHPC exhibits relatively high rates of shrinkage (Holt, 2005). The low moisture content of UHPC combined with its high barrier qualities leads to quick drying of its surfaces due to moisture loss from evaporation and self-desiccation (Bentur et al., 2001), and minimal bleeding. The result is a surface that dries rapidly and experiences relatively large plastic shrinkage movements. Rapid drying of the surface combined with restraining of plastic shrinkage movements by the interior of UHPC leads to the formation of a rough surface texture and occasional cracking (Uno, 1998). Figure 1(a) shows an example of a UHPC surface upon finish- ing. The appearance of the same surface after 24 h is shown in Figure 1(b), where a rough surface texture with some cracks can be observed. This surface texture is occasionally referred to as ‘elephant skin’. It should be noted that elephant skin formation is marked mostly by the formation of a rough texture rather than by pronounced cracking. Its adverse effects on the aesthetics of UHPC products and structures are thus more pronounced than its negative durability implications. The surface appearance of the UHPC slab-on-grade shown in Figure 1 did not change after 3 years of exposure to the mid-Michigan (USA) weather. The formation of ‘elephant skin’ is not pronounced on the surface of small laboratory specimens, although it can be detected on the surfaces of medium-scale slabs prepared in the laboratory. As field applications of UHPC grow, the tendency towards elephant skin formation cannot be neglected. Different strategies were devised and evaluated in this work for preventing elephant skin formation on UHPC surfaces (Acker and Behloul, 2004; Shi et al., 2015; Soroushian et al., 1993; Yang et al., 2005). These strategies were reduction of moisture loss from the surface (Goldman and Bentur, 1994; Graybeal, 2006), enhancement of surface ductility and shrinkage control (Banthia and Gupta, 2006; Cohen et al., 1990; Soroushian and Ravanbakhsh, 1998) and the introduction of discrete reinforce- ment systems for enhancement of shrinkage cracking resistance (Altoubat and Lange, 2001). While elephant skin formation on UHPC surfaces is highly pronounced and can occur under different climatic conditions, it may be compared with the plastic shrinkage cracking (Holt, 2005) of normal concrete that is occasionally observed under weather conditions that promote rapid surface drying (e.g. windy and dry conditions) (Ravina and Shalon, 1968; Torrenti et al., 1999; Wang et al., 2001). 1.1 Research significance UHPC offers significant advantages in terms of mechanical performance, durability, sustainability, structural efficiency and life-cycle economy. Field applications of UHPC, however, encounter problems associated with the formation of exposed 1 Construction Materials Improvement of the surface quality and aesthetics of ultra-high-performance concrete Chen, Matalkah, Rankothge, Balachandra and Soroushian Proceedings of the Institution of Civil Engineers http://dx.doi.org/10.1680/jcoma.17.00009 Paper 1700009 Received 22/02/2017 Accepted 23/06/2017 Keywords: concrete technology & manufacture/ field testing & monitoring/sustainability ICE Publishing: All rights reserved