Effects of Elevated Temperatures to High Strength Concrete with Nanosilica

Abstract High strength concrete has risen due to the demand for more complex shapes, sizes, forms, and strength of structures. High strength concrete with nanosilica exhibited improved strength both in tension and compression and improved durability with lower permeability outpacing normal strength concrete. It is falsely assumed that if a material has better mechanical properties at room temperature its performance is also better at higher temperature. The combined effects of calcium silicate hydrate (CSH) disintegration, contraction of cement paste, expansion of aggregates, and vapor pressure resulted to cracking and led to spalling. However, little to no significance is given to high strength concrete despite the irreversible effects of fire. The research subjected cylindrical samples under peak temperature of 25°C, 100°C, 200°C, 450°C and 600°C for two hours with 0%, 2%, 3%, and 4% nanosilica by cement weight. High strength concrete has shown insignificant decrease in compressive strength and tensile strength due to CSH stability when subjected to temperatures less than 200°C. Residual compressive and tensile strength retained lowest under 450°C and 600°C. The reduction in strength is attributed to CSH disintegration, contraction of cement paste, and expansion of aggregate that led to shrinkage and weakening of the interfacial transition zone (ITZ). Significant increase in moisture loss is also observed when temperature reached 450°C. Spalling occurrences are small pop outs to none even after reaching 600°C. Under electron microscope, morphology of 3% nanosilica samples developed block-like medium with long continuous cracks while 0% samples are craterous with short discontinuous cracks. Microcracks are apparent beyond 200°C for all samples. The products of hydrated nanosilica provided fillers between gaps and voids but are weak enough that temperatures above 200°C splits the medium apart.