Impact of Substitution of Silica Nanoparticles on Ultrasound Pulse Velocity in Concrete Specimens

In present work, we studied the effect of substitution of silica nanoparticles (SNPs) by replacement of cement on ultrasound velocity in concrete specimens (cube shape 150 mm × 150 mm × 150 mm). Higher ultrasound pulse velocity indicates better strength and quality of concrete structure. The quality of concrete specimens was assessed by measuring ultrasound pulse velocity (UPV) in m/s and compressive strength (N/ mm²). The average value of UPV on 7^th day of curing turned out to become 3200 ± 36, 3215 ± 42, 3290 ± 41, 3349 ± 24, 3450 ± 17 and 3456 ± 12 for 0%, 0.5%, 1.0%, 1.5%, 2.0% and 2.5% content of SNPs in the specimens respectively. Similarly, the average value of UPV on 28th day was 3540 ± 36, 3580 ± 38, 3696 ± 42, 3820 ± 39, 4160 ± 40, 4163 ± 41 for same amount of SNPs substitution respectively. Overall, the trend for rise of UPV could be modeled as linear in both cases. When compared the result of UPV on 7^th and 28^th days, the UPV was higher on 28^th day in comparison to 7^th day data, obviously, because the samples were getting stronger with time and also due to addition of silica nanoparticles the UPV in the specimens. The UPV was observed to have achieved maximum at nearly 2% (2.0-2.5%) nano-silica content. As UPV increased so did the compressive strength. Higher velocity would suggest better quality and continuity inside the material, while decreased velocity imply poor quality, cracks, bubbles or voids within the structure of concrete. Thus, in defective domain of concrete, the compression wave velocity becomes slower than the normal (un-defective) region and the signal amplitude is also attenuated. Compressive strength and USV increased due to hydration reaction leading to C-S-H gel formation which filled the pores in the concrete matrix, making it stronger and the pores filling nature of inactive silica-particles made the specimen compact and strong.