Concrete structures are prone to deterioration due to ageing, e.g., exposure to severe environments causing corrosion of steel reinforcement, natural and human-made extreme events such as earthquakes, vehicular impacts, fire exposure etc., or simply due to changes in design codes or changes in use [1]. Rehabilitation of damaged concrete structures to meet the current codes' requirements and strengthening existing concrete structures to carry higher permissible loads appear to be a more cost-effective and appealing option than demolishing and rebuilding[2][3]. Several methods have traditionally been used to repair and strengthen reinforced concrete structures, such as reinforced concrete jacketing, epoxy bonding steel plates, external post-tensioning, and externally bonding carbon fibre-reinforced polymer [4][5]. Despite their effectiveness in achieving strengthening goals and improving strength and durability, these methods have some drawbacks. For example, using concrete jacketing increases the weight and size of the structural element, while using steel plate technique increases the risk of corrosion, fire resistance and debonding. On the other hand, ageing adhesion materials at the interface and fire resistance represent a challenge when using FRP strengthening technique [6][5].
Ultra-high performance concrete (UHPC) is a relatively new building material with great compressive and tensile strength and excellent durability, making it an ideal material for strengthening/rehabilitation [7]. UHPC has superior mechanical properties, including a compressive strength at least 120 MPa [8], sustainable post-cracking tensile strength exceeding 5 MPa[9], high energy absorption capacity, and low permeability, shrinkage and creep [10]. The first attempt that used UHPC material to strengthen reinforced concrete structures was in Switzerland and North America [11]. In recent years, numerous experimental and simulation studies have been conducted to explore the flexural behaviour of reinforced concrete beams strengthened by the UHPC layer taking into consideration many variables, such as the overlay thickness, the reinforcement ratio in the overlay, and other aspects.
Paschalis et al. [12] indicated that the UHPC layer used in strengthening reinforced concrete beams is a promising technique based on experimental results. A large increase in stiffness was noted with a slight increase in ultimate load due to strengthening the RC beam by the UHPC layer. In comparison, the ultimate load capacity increased significantly (90%) when the steel rebar reinforced the UHPC layer. Al-Osta et al. [13] investigated different techniques to glue the UHPC layer on the reinforced concrete beam. The first one was by sandblasting RC beams surfaces and casting UHPC in situ. The other was by bonding prefabricated UHPC strips to the RC using epoxy adhesive. Both techniques showed an overall better performance. The results showed that no significant difference in the results for flexural testing of strengthened beams was observed based on variations in interface preparation technique. The addition of a steel reinforcing bar in the UHPC layer was found to be significant to improve the ultimate load and stiffness of the strengthened member [4][14]. The cracking and flexural resistance of the strengthened beams with the reinforced UHPC layer toughened by the placement of steel wire mesh was further improved [15]. Tanarslan [16] investigated the reinforced concrete beams' behaviour that were strengthened with prefabricated UHPC laminates. Different applied UHPC laminates were tested to determine which method is more effective for the flexural strengthening of RC beams. It was found that UHPC laminate usage is an effective technique to enhance RC beams' behaviour and load carrying capacity and can be preferred to strengthen deteriorated structures.
According to the author's knowledge, most experimental and numerical studies in this field extend the UHPC layer beyond the support. This is not the same as what would be done at a practical construction site. The present study aims to investigate the flexural behaviour of reinforced concrete beam strengthening by the UHPC layer extending to the face of the beam supports representing the face of the column. On the other hand, the study investigates the effect of construction joints in the UHPC layer.