Stiffness degradation of reinforced concrete beams under repeated low-energy impact loading

Behaviour of reinforced concrete beams under repeated, low-energy impact loading has been studied through a well-designed experimental programme. The impact hammer has been instrumented to obtain the load-time history after any desired number of impact blows. The load-time variation, the deformation-time variation and the natural frequency after a given number of impact blows have been recorded. The time history of deformation pertains essentially to the transverse impulse loading which has a constant duration with varying peak force. With the increase in number of blows, there is penetration of impact hammer head into the beam. This together with the cracking of the beam leads to significant axial impulse loading. The difference between the input energy and the kinetic energy of the beam from the measured response in flexural mode has also been theoretically explained as energy due to axial vibration corroborating existence of axial impulse loading. The flexural and axial impulse loads lead to stiffness degradation. An analytical model has been suggested to evaluate the reduced stiffness of the beam with increase in number of impact blows. Good correlation between the experimental and analytical natural frequencies is seen at all stages of impact loading.