Effects of silica fume, ultrafine and mixing sequences on properties of ultra high performance concrete

Abstract

The aim of this study is to investigate the effects of changing silica fume content, amount of crushed quartz powder as ultra fine aggregate and the sequence of mixing procedure on the main properties of Ultra High Performance Concrete (UHPC), with particular emphasis on 28 days compressive strength, density and slump. Several concrete mixes involving different contents of Silica fume and crushed quartz powder are prepared and tested for this purpose. Based on the results of this study, it is concluded that the optimum silica fume content necessary for producing UHPC is about 15% of cement mass. At this percentage, compressive strength is about 60% more than the strength for the zero content of silica fume. Added to this, the results show that adding 40% of the quantity of superplastisizer to the UHPC mixture at the first stage of the mixing with all dry materials has a positive effect on the 28 days compressive strength, while enhancing the workability. Moreover, as the ultrafine content increases, the compressive strength increases with the highest compressive achieved using ultrafine to cement ratio of 0.50.