Abstract
With massive developments taking place in the construction industry, the demand for high performance concrete (HPC) is steadily increasing. It is possible to use industrial by-products such as silica fume (SF) and fly ash (FA) as supplementary cementitious materials (SCMs), to enhance the attributes of HPC. Although numerous investigations have been carried out to identify the optimum replacement levels of these SCMs, some inconsistences are noticed in the results. In this context, a study was conducted to look into the combined effect of SF and FA on the strength and workability of HPC. Four SF replacement levels: 5%, 10%, 12.5%, and 15%; and five FA replacement levels: 0%, 5%, 10%, 15% and 20% were proposed. A total of nineteen mix proportions were used including a control mix. Water-binder ratio was kept constant at 0.35. The mixes were tested for 7 and 28-day compressive strengths and for their workability. Results obtained revealed that the maximum 7-day compressive strength was in the mix with 10% of SF with no FA and the maximum 28-day compressive strength was in the mix with 12.5% of SF and 5% of FA. The workability increased with the addition of fly ash while the SF content kept below 10%. In terms of economy, the best strength to cost ratio was found in the control mix. The extension of the k-value concept for the water/binder ratio found in EN 206 for SF-FA combinations revealed that the existing parameters kSF = 2.0 and kFA = 0.4 show a good correlation with the experimental results.