Prediction of service load of concrete structures after cracking using ultrasonic technique

The objective of this study is to investigate the cracking behavior of fractured concrete structures and to introduce a new approach to assess their remaining service lives. A novel ultrasonic technique is utilized to detect crack propagation in notched beams of mortar, plain concrete and reinforced concrete. The study showed that crack growth and microcrack zone in cementitious composites can be quantitatively detected by the ultrasound technique. To predict crack propagation during the fracture process in plain concrete, an effective compliance fracture model is proposed. A large fracture process zone is incorporated in the analytical model by means of an equivalent elastic beam concept, which makes it possible to determine fracture toughness of the concrete member in terms of critical stress intensity factor, KICe. The model was evaluated for size dependency using several sizes of notched beams with different notch lengths. The results showed fracture toughness in terms of KICe , to be size-independent up to a crack over depth ratio of 0.4, making it a material property which may be used for design purpose. By combining the effects of pure bending and an applied axial force on a cracked flexural member, a new fracture model was proposed to predict crack propagations in RC beams. The model assumes elastic-plastic behavior of the reinforcement and a constant fracture angle of the cracked plane, allowing the model to predict crack growth at any instant during the fracture processes. The predicted values of crack growth were validated by comparing with those measured using the ultrasonic technique and found to be in good agreement. Through loading and unloading cycles during testing, the ultrasonic technique was able to detect the size and variation of the fracture process zone during the fracture processes. This is a unique finding that has never been experimentally observed in the past. Finally, a design diagram showing the energy loss ratio versus the crack growth of any fractured concrete beam was developed. The relationship of normalized crack growth and energy loss ratio for non-reinforced concrete, (mortar and plain concrete) and reinforced concrete beams with different reinforcement ratios (under-reinforced, balanced and over-reinforced) were formulated. By using the existing crack growth over uncracked ligament ratio and the reinforcement of the member, the diagram can easily be applied in practice to predict the remaining service life of any cracked concrete beams