Effect of fibre reinforcements on shear capacity of geopolymer concrete beams subjected to impact load

This study investigates the shear capacity of fibre-reinforced geopolymer concrete (GPC) beams subjected to impact loads. For easy examination of the shear capacities, GPC beams, as well as two reference beams made of ordinary Portland cement (OPC) based concrete, without stirrups were prepared and subjected to the dropweight impact tests with different contact conditions (direct contact and rubber pad contact). In the case of the beams under direct contact, the failure mode was observed to be a purely diagonal shear failure. The change in concrete material from OPC concrete to GPC showed a marginal effect on the impact response of the beams. Adding fibres into the GPC matrix improved considerably the post-failure behaviour of the beams. The beams reinforced with fibres exhibited not only less concrete spalling and fragmentation but also much higher reaction forces and the second impulse of the impact force. However, the fibre reinforcement seemed to have only a minor effect on the local and contact stiffness of the beams and thus the first impulse of impact force of all the beams was quite similar. The fast Fourier transform (FFT) analysis showed that the adoption of rubber pad contact reduced the highest dominated frequency of impact force from 2.5 kHz to 0.5 kHz. Using rubber pad contact led to the change in the failure pattern of the beams from the purely diagonal shear to the flexure-shear combined failure. The methods used for estimating the imparted and absorbed energy were compared and evaluated. The analysis results demonstrated that the method based on the impact force vs displacement yielded inaccurate results when the inclination angle between the drop hammer and the beams increased. Therefore, this study suggests that the variation in the kinetic energy of the drop hammer should be used to calculate the imparted energy to the impacted beam.