Experimental Study of the Crack Widening Mechanisms in Polymer Fiber Reinforced Concrete

Though much has been reported on the creep of conventional concrete and the mechanisms responsible for its time-dependent behaviour, the creep of fibre reinforced concrete (FRC) is just being understood. A number of research articles have reported on the compressive, flexural and direct tensile creep of steel and some types of micro-synthetic FRC. Since fibres are known to be triggered only after the initiation of cracks, the creep response under sustained loading which is usually measured in terms of deflection and/or crack mouth opening is investigated in the cracked state. This enables the simulation of in-service condition. Though the usage of macro-synthetic fibre in concrete is on the rise, not much has been reported on its creep behaviour in concrete and the mechanism causing the time-dependent behaviour. The creep of cracked macro-synthetic FRC has been investigated in uniaxial tension at five stress levels (30%, 40%, 50%, 60% and 70%) of the residual uniaxial tensile strength under controlled temperature and relative humidity of 23±1ºC and 65±5% respectively. Specimens were prepared and cracked to an average crack width of 0.5 mm before they were subjected to the various stress levels. Since all the specimens were tested unsealed, drying shrinkage was measured on two load-free specimens and subtracted from the total creep deformation from the creep specimens to obtain the actual creep deformation. One mix design has been used throughout the investigation and the fibre was used at a volume of 1%. Test results of the investigation has revealed that while the creep response is a function of the applied stress level, significant creep has been recorded after 8 months even at lower stress levels. The mechanism responsible for the time-dependent crack opening of macro-synthetic FRC was investigated through the fibre creep response under a sustained load of 30% of the measured average tensile strength of fibres tested at a rate of 0.5 mm/s. Again, sustained loads at 50%, 60%, 70% and 80% of the average interfacial shear resistance (2.50 MPa) of specimens tested at 25 mm embedment length were applied to the fibres individually to study the time-dependent pullout behaviour. The result of these investigations has shown that macro-synthetic fibre creep and the time-dependent fibre pullout are mechanisms responsible for the creep of cracked macro-synthetic FRC.