Author(s): Yemanebirhan Emiru

Abstract: It is well researched that concrete is weak in tension and thus has brittle characteristics resulting in less post-cracking ductility. To alleviate this problem, nano-silica and hook ended steel fibers which were never studied together added to the concrete mix. The main objective of this research is to enhance the durability, bond and mechanical properties of concrete by adding nano-silica and steel fiber to cement paste in high strength concrete. Nano silica of (-0.5%, 1%, and 1.5%-) by weight of cement and steel fiber of 1% volume fraction are added to the concrete mix, and then the compressive, tensile, shear, pull-out, and UPV tests have been conducted on standard specimens as per the ASTM standard for high strength concrete. From the experiment results, the workability of the fresh concrete mixture decreases as the percentage of nano-silica percentage increases. Optimum percentage is found at concrete mix of 1.5% nano-silica and 1% of steel fiber. Significant enhancement in mechanical strength of concrete is observed relative to control specimens, which is explained by the fact that high surface area, nano-filler effect, and pozzolanic reaction of nano silica and post crack stiffening of concrete by steel fiber. By introducing Nano silica and Steel fiber in high strength concrete mix, there was a relative strength gain of 11.4%, 16.2%, 24.61% in compression, indirect tensile strength and shear strength respectively from control specimen at the age of 28 days. Formation of secondary CHS due to addition of nano silica is observed in the study of microstructure tests, i.e., XRD and FTIR. A good correlation between actual compressive strength, UPV and pullout test results has been observed. It is concluded that the concrete mix containing 1.5% of nano silica with 1% steel fiber exhibits better performance the control concrete mix and has a potential to be used as a construction material.

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