Compounding effects of fly ash and silica fume on basic creep of self-compacting concrete

Double-adding fly ash and silica fume is gaining popularity in self-compacting concrete (SCC) for high-rise and long-span concrete-filled steel tubular (CFST) structures to enhance concrete fresh and hardened performances. Long-term deformation of CFSTs is primarily caused by the basic creep of the concrete core. An accurate prediction of concrete basic creep is critical for the design of high-rise and long-span CFST structures. Yet the fly ash and silica fume effects on basic creep have not been well studied for SCC despite their significant influences on concrete creep. In this paper, 510-day long-term tests were conducted on 45 specimens with different fly ash and silica fume replacement ratios. The mineral admixture effect on the creep development trend was investigated, the different mineral admixture effects at different concrete strengths were illustrated, and the compounding effects between the mineral admixtures were quantified. Using equivalent compressive strength, the EC2 creep model was modified to account for the mineral admixture effects on both the development trend and the final value of SCC creep. Investigation shows that: (1) fly ash and silica fume accelerate SCC basic creep development, and using the traditional EC2 concrete creep model can lead to a 68% overestimation of the SCC creep at the 50-year service time; (2) the fly ash and silica fume effects on basic creep is more significant at higher compressive strength; (3) the fly ash and silica fume have a significant compounding effect on SCC basic creep. The modified EC2 model can well predict both the development trend and the final value of SCC basic creep, with mean value of 0.962, coefficient of determination ( R 2 ) of 0.982, and coefficient of variation ( COV ) of 0.125.