Influence of steel fibre content on fracture toughness of reinforced concrete

In the present work, the fracture behavior of concrete beams reinforced by steel bars and steel fibers wasinvestigated. The effect of fiber volume fraction, fiber aspect ratio and notch depth on the fracture toughness ofconcrete beam specimens of dimensions 6 x 15 x 65 cm and reinforced with different areas of steel bars were studied.The volume fractions studied were 0.0, 0.5, 0.8 and 1.1%, theaspect ratios were 40, 70 and 100, notch depths were 3, 5, 7.5cm and steel bars reinforcement areas were 1 ~6 mm (0.283 cm2)2and 2 ~6 mm (0.566 cm ). The flexural strength of unnotchedbeams of the same dimensions, splitting tensile strength ofstandard 15 x 30 cm cylinders and compressive strength ofstandard 15 x 15 x 15 cm cubes were measured for the variousfiber volume fractions and fiber aspect ratios mentionedabove, and used in a theoretical fracture mechanics model. Thepredicted values were compared with the obtained experimentalresul ts .It was found that by adding steel fibers to plainconcrete, the corresponding properties and fracture toughnessincreases but with various levels of improvements. Further-more, adding steel fibers to plain concrete increases crackpropagation resistance. It was found also that there is nosingle value for plane strain fracture toughness of concreteand it appears to change with crack size. In addition, thepresence of steel bars reinforcement creates a closing effect.Such closing effect due to the local compressive stress fieldaround steel bars inhibits ability for crack extension. It wasfound that the values of the local compressive stressesincreases with increasing the area of bars reinforcement whichgives higher values for the apparent fracture toughness.Good agreement was found between the estimated fracturetoughness values and the experimentally determined ones. Thisindicates the suitability of fracture mechanics solutions forcrack initiation in concrete members reinforced by steel barsand fibers.