Toughness and tortuosity of polypropylene fibre reinforced concrete

This paper gives the results of an investigation in which the fracture toughness and fracture energy of concrete were determined using notched specimens prepared from standard cubes used for compressive strength tests. The test samples were made from concretes containing several levels of silica fume as partial cement replacements and a range of polypropylene fibre contents. Several notch geometrics were investigated and the tests revealed no significant influence on the fracture toughness by the size of the notches. Significant increases in the fracture toughness were obtained when the cement was replaced by 15% silica fume. The fracture toughness was found to be largely unaffected by the polypropylene fibres. The laboratory tests are augmented by finite element analysis and the investigation enabled an assessment of the tortuosity of the fracture surface to be made. The tortuosity was found to decrease with increase in the silica fume content. This is considered to be due to the enhanced composite action between the aggregates and the hardened cement paste caused by the reduction in the thickness of the interfacial zone effected by the silica fume. The tortuosity also decreased with increasing notch size. This is due to the presence of a greater proportion of coarse aggregates being encountered by the advancing crack in the case of the specimens with shorter notches. The polypropylene fibres were found to be effective in controlling the fracture after first crack. The ductility imparted to the matrix by the fibres was assessed using a toughness index determined from the area under the load-displacement curve obtained from the tests. Significant increases in the toughness indices of all concretes examined were obtained by additions of small amounts of fibre.