Deformation Properties of Nano-Silica Modified Concrete Mixtures under Uniaxial Compression Loading

The enhancement that nano-silica (NS) constitutes in the microstructure of concrete would affect its deformation behavior. In this work, the pre-peak and post-peak stress–strain diagram characteristics of concrete mixtures with NS (1.5% of cement by wt.) under uniaxial compression were investigated. The results were compared with the mixtures containing micro-silica (MS) (7.3% of cement by wt.). Aggregate characteristics in the range of 16 to 22 mm, as another variable, were also changed by utilizing river gravel (RG II) or crushed sandstone (CS II). Compared to reference concrete, the addition of NS did not improve the compressive strength and modulus of elasticity significantly. However, the addition of only 1.5% NS significantly affected the stress–strain diagram characteristics, such as discontinuity and critical stress limits before the peak load. Relative to the mixtures containing MS, the effect of NS on the pre-peak region was more significant. For example, when the mixtures containing RG II as coarse aggregate are considered, with respect to reference mixture, the increase in normalized discontinuity stress limits at 28 days was found as 52% and 26%, respectively, for the mixtures containing NS and MS. However, in the same order, the increase in 28-day compressive strength was just 8.1% and 5.2%. Post-peak behavior of the mixtures also indicated that the fracture of concrete is more brittle when MS or NS is used. Among these, the effect of NS on the post-peak region was found to be more dominant.