Investigation of key parameters affecting the use of Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) as strengthening material

The use of Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) for strengthening applications is gaining increasing attention due to its advantageous mechanical and durability characteristics. However, the lack of design guidelines and code recommendations hinders the extensive use of UHPFRC in strengthening applications. The aim of the present study is to offer valuable insights regarding the effect of critical parameters on the performance of UHPFRC as a strengthening material. In this current research, experimental and numerical results have been used to evaluate the effectiveness of UHPFRC layers for the structural upgrade of existing Reinforced Concrete (RC) beams. Critical parameters such as the thickness of the additional layers, the connection at the old-to-new concrete interface, the mechanical performance of UHPFRC, and the amount of additional longitudinal reinforcement have been studied, and recommendations for the effective design of the strengthened elements, considering both performance and cost, are presented. The results of the present study indicate that the addition of steel bars with different diameters in the layer increased the load-carrying capacity by up to 183%. On the contrary, an increase of 5.8% in the load-carrying capacity was achieved when the RC beams had 30 mm layers, and an 18.1% increment was achieved for layers with a higher thickness of 70 mm. The assumption of perfect connection at the interface, which can be achieved with dowels at the interface, resulted in further enhancement of the load carrying capacity, which was found to be equal to 11.8% for the 30 mm layer and 35.3% for the 70 mm layer. An increment of the UHPFRC tensile strength of the strengthening layer from 11.5 MPa to 14 MPa resulted in a further increment of the load-carrying capacity by 8%.