The study presents experimental investigation on short term mechanical properties and flexural behaviour of conventional Ordinary Portland Cement (OPC) based concrete and slag-fly ash based geopolymer concrete. Conventional and geopolymer concrete mixes were designed to achieve compressive strength equivalent to M40 and M70 grade as per Indian standard code. Mechanical properties of concrete mixes such as compressive strength, split tensile strength, flexural strength, modulus of elasticity and Poisson's ratio were evaluated and compared. The flexural behaviour of reinforced concrete beams for both conventional and geopolymer concrete has been studied using 4-point bend test. The findings suggest that geopolymer concrete shows comparable mechanical properties in terms of split tensile strength, flexural strength and Poisson's ratio. However, modulus of elasticity of geopolymer concrete is lower than the conventional concrete of equivalent strength. Studies on flexure behaviour of reinforced concrete beams shows that both geopolymer and conventional concrete exhibit comparable flexural behavior in terms of load-deflection curves, yield load and yield moment. The amount of energy dissipated in flexure is marginally higher for high strength conventional concrete. Based on the visible cracks developed in flexure, it was concluded that the reinforced conventional concrete and reinforced geopolymer concrete show similar number and type of cracks in flexure.

Flexural design of reinforced concrete members in structures such as buildings, bridges, etc. presented in various standards are based on the equivalent rectangular stress block and stress-block parameters for high strength concrete. However, with design of high strength concrete mixes these existing stress block parameters for normal and high strength concrete have become redundant and does not give a true representation of the obtained stress-strain curves for ultrahigh strength concrete. The primary stress-strain characteristics affecting the stress block parameters are strain at peak stress, the ultimate strain and the shape of the rising limb of the curves. Past studies have confirmed a varying behaviour of all these characteristics in the stress strain curves at higher strength. The present study first evaluated the stress strain curves for high strength concrete from 90 to 140 Mpa. The characteristics of the curves were compared with those of normal and high strength concrete. Based on the observed stress strain characteristics, the modified stress block parameters for the ultra-high strength concrete are presented. The proposed stress block was compared with the Indian standard IS456 and European design standard EC: 02-2004. The proposed stress block will be useful for the required modification and updating of various standards pertaining to the flexural design of ultra-high strength concrete with cylindrical compressive strength in from 90 to 140Mpa.

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The study presents an experimental investigation of the fracture behavior of hardened slag and fly ash-based alkali-activated normal and high-strength geopolymer concrete compared with conventional Ordinary Portland Cement (O.P.C.) based concrete with steel and hybrid fibers. The fracture parameters considered in the experimental investigation include fracture energy, stress intensity factor, energy release rate, and characteristic length. The study concludes that the observed differences in conventional and geopolymer concrete's fracture and mechanical performance agree with the microstructural differences between these concrete systems reported in past literature. The slag-based geopolymer concrete is marginally inferior to the O.P.C.-based concrete, with similar compressive strength in fracture performance. Also, hybrid fiber reinforcement improves the fracture performance of geopolymer concrete more than steel fiber alone. Contrary to geopolymer concrete, steel fiber reinforced conventional concrete is superior to hybrid fiber reinforced conventional concrete in terms of fracture behavior.