Fatigue of reinforced concrete beams retrofitted with ultra-high performance fibre- reinforced concrete

Concrete structures deteriorate over time due to different reasons and thus may not perform their function satisfactorily. Repair and rehabilitation of deteriorated concrete structures is often preferred over demolition and rebuilding for economic reasons. Various metallic and nonmetallic materials have been used in the past for repair and rehabilitation. These materials have advantages and disadvantages. The latter are connected with the mismatch in the properties of these materials with the material of the structure being repaired which often resulted in unwanted failure modes, e.g. delamination. For this reason, new cement-based ultra-high performance reinforced with steel fibres repair materials have been developed in the last two decades, which restore (and even enhance) the structural response and improve the durability of repaired concrete structures. One such ultra-high-performance fibre-reinforced concrete material is CARDIFRC. It is characterized by very high compressive strength, high tensile /flexural strength, and high energy-absorption capacity. However, it is very expensive and thus industrially uncompetitive due to the very high cost of thin brass-coated steel fibres used in it. It is therefore important to develop a version of CARDIFRC that is industrially competitive. This is one of the objectives of this research. An ultra-high-performance fibre-reinforced concrete (UHPFRC) has been developed that is far less expensive than CARDIFRC and at the same time self-compacting. The steps necessary to achieve this have been described in this work. In addition, a full mechanical and fracture characterisation (i.e. size-independent fracture energy and the corresponding bi-linear stress-crack opening relationship) of this UHPFRC is presented. A nonlinear cracked hinge model has been used to back calculate the stress-crack opening relation of this material in an inverse manner from the test data. The second objective of this research concerns the flexural fatigue behaviour of this new UHPFRC. Tests have been conducted under several stress amplitude ranges. It has been found that the distribution of fibres plays a vital role in its fatigue resistance. Regions with few or no fibres can drastically reduce its fatigue life. As expected, non-zero mean stress leads to a significant reduction in the fatigue life of a material compared to cyclic loading with zero mean. The variation in compliance during cyclic loading has been used to estimate the expected fatigue life under a given cyclic load range, since the tests were terminated at one million cycles. The third objective of this research concerns the flexural fatigue behaviour of RC beams retrofitted with precast strips of this self-compacting UHPFRC on the tension face. Fatigue tests under several stress amplitude ranges have shown that this UHPFRC is an excellent retrofit material under fatigue loading. Again, the variation in compliance during the fatigue loading has been used to estimate the expected fatigue life for retrofitted RC beams