Effect of Binder Content and Sand Type on Mechanical Characteristics of Ultra-High Performance Concrete

Ultra-high performance concrete (UHPC) mixtures are typically proportioned using high binder content and low water-to-binder ratio (w/b). Yet, in most cases, such recipes denote the presence of unhydrated cement and other pozzolanic materials, thus playing the role of costly fillers in the matrix. Two UHPC categories proportioned with two cement types and various binder contents ranging from 505 to 1050 kg/m3 are investigated in this study. Test results showed that the compressive and flexural strengths followed an increasing trend with binder additions, but then tended to stabilize or decrease at higher binder concentration. The highest strengths were achieved at binder contents hovering about 575 or 690 kg/m3 for 0.26 w/b mixtures prepared with river or quartz sands, respectively. The thermogravimetric analysis showed that UHPC made with higher binder contents exhibited increased mass loss at 500 °C, which corroborates the presence of increased amounts of unreacted cement compounds (i.e., CH) in matrices containing high binder content. The effect of reducing w/b and/or incorporating silica fume or metakaolin helped improving the strength of UHPC prepared with given binder content. The use of CEM I (52.5 MPa strength cement class) yielded consistently superior mechanical properties than equivalent mixtures containing CEM II (42.5 MPa strength class). The resistance against water permeability improved for UHPC containing higher binder content, suggesting that the increased cementitious phase plays a beneficial role to block the capillary pores and refine the matrix porosity.