The effect of steel and macro-polypropylene fibers on mechanical properties and flexural toughness of various concretes including microstructure

The synergetic effect of steel fibers, polypropylene fibers and the hybrid effect of steel-polypropylene fibers of various concrete mixes on mechanical properties and flexural toughness was investigated. The mechanical properties include compressive strength, split tensile strength, flexural strength and modulus of elasticity. Total 16 mixes containing various concrete mixes (normal concrete of grade M35, Steel fibers – 0.5%, 1.0%, 1.5%, Polypropylene fibers (PPF) – 0.1%, 0.2%, 0.3%, hybrid fibers – each volume fraction, steel: PPF – 25%:75%, 50%:50%, 75%:25%) were cast and evaluated the mechanical properties and flexural toughness. Flexural strength, flexural toughness and indices were evaluated by using the load-deflection diagram of various concrete mixes by using the guidelines prescribed in various Codes of practices, namely, (i) ASTM C 1018 (ii) ASTM C 1609 (iii) JSCE (iv) ASTM C 1399 (v) Chinese National Standards CECS13. It was primarily observed that a concrete mix containing a fibre combination of 75% steel fibers + 25% Polypropylene fibers yielded superior mechanical properties and flexural toughness. Post peak behaviour of flexural specimens containing combination of 75% steel Fibers + 25% Polypropylene fibers found to be more ductile compared to other combinations. A method has been proposed based on the crack initiation load and percentage of peak load to determine the reserve flexural strength. Further, microstructural analysis has been carried out for the samples of plain concrete, concrete with steel fibers and concrete with hybrid fibers. It was primarily inferred from microstructural studies that due to enhanced bond between the synergy effect of steel and PPF, superior mechanical and flexural toughness were obtained.