Keywords: polypropylene fibres, fibre-reinforced concrete, fibrous concrete, high strength concrete, constitutive models, material properties.

The increased use of high strength concrete in recent years has increased the demand to enhance its ductility under compressive loads. The explosive nature of the failure of this type of concrete under compressive stresses leaves little warning if any, prior to failure. In addition, high strength concrete is characterised by having low ductility under compressive loads. Another disadvantage is manifested by its brittle fracture mechanisms in tension and especially when subjected to large diagonal tensile strains under high shearing forces.

It thus became a requirement to try to enhance the behaviour by introducing fibres into this type of concrete. The most common type of fibre used by researchers is the steel fibre. Another type of fibre that is commonly available is the polypropylene fibre. This latter type is characterised by being relatively inexpensive in price and light in weight (low specific weight) when compared to other types of fibres.

For these reasons, an experimental programme has been conducted to investigate the effects of introducing polypropylene fibres into higher strength concrete applications. Four different fibre volume fractions were used in the study namely 0, 0.5, 1.0 and 1.5%. Three types of concrete grades were cast. Typical cylinder compressive strengths for the batches used were 32, 50 and 60 MPa ( = 40, 62.5 and 75 MPa). Uni-axial compressive and splitting cylinder tests were conducted. For the uni-axial compressive tests, longitudinal, transverse and volumetric strains were monitored and reported at different stress levels during testing. The material properties determined from the tests included peak compressive stresses for both cylinders and cubes, splitting cylinder tensile strengths, elastic moduli and Poisson's ratios. In addition, the ratios between the cylinder-to-cube strengths for different concrete grades and fibre volume fractions were also determined.

The study revealed that polypropylene fibres significantly enhanced the tensile properties of higher strength concretes, increased the ductility under compressive loads, reduced transverse deformations however reduced the peak attainable strength under compressive loads. A two parameter model for calculating the tensile strength of concrete as a function of both the concrete grade, as expressed by the cylinder peak strength , and the fibre volume fraction has been developed.

The model given by equation (123.1), shows excellent agreement with the experimental results and represents a solid foundation for the future development of a group of constitutive models that can be used to quantify the behaviour of high strength concretes reinforced with polypropylene fibres.

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