Behaviour of Reinforced Concrete Beam-Column Joints under Combined Loads Strengthened with CFRP Schemes Applied to the Beam

This investigation concentrates on the behaviour of beam-column joints subjected to torque interacting with flexural and direct shear forces and in the presence of different Carbon Fibre Polymer (CFRP) strengthening wraps (applied only to the beam). These wrapping schemes have previously been determined by the research community as an effective method of enhancing the torsional capacities of simply supported reinforced concrete beams. Based on the literature and the lack of direct relevant research, an extensive experimental, analytical and numerical investigation has been performed. The experimental work involved a series of exterior reinforced concrete beam-column joints tested under short-term and long-term loads, combinations of which included torque.
The short-term tests performed under monotonically increasing loads illustrated the viability of using CFRP wrapping systems to enhance member capacities under torsional actions. The enhancement levels were affected by the fibre ratio, the confinement degree, and the fibre orientation. The ductility of the members was governed by the strength of the concrete struts. The rebars’ strain readings confirmed the influence of the torsional forces; these forces increased the tensile stresses in the beam rebars, which affected the shear demand and distortion levels of the joints. The long-term tests (under sustained loads) illustrated a significant increase in the beam twist deformations due to the long-term torsional loads. Also, the sustained loads reduced the flexural stiffness; this corresponded to the level of the time-dependent deformations and the induced torsional cracks. The CFRP wraps considerably improved the time-dependent torsional stiffness compared to the unstrengthened specimens.
Formulas are proposed based on the space truss mechanism to evaluate the joint shear demand of beams wrapped in various ways, whilst also considering the interactions of torque with bending and direct shear. Further, an iterative model based on the average stress-strain method has been introduced to predict the joint’s strength. The proposed analytical approaches showed good agreement with the experimental results. In addition, numerical models were developed in order to examine the influence of different torque to bending-shear ratios, multiple CFRP layers, and concrete strengths. These investigations revealed significant increases in the plastic strains, which exceeded those measured in the specimen under concentric loads. The high strength concrete models attained larger load capacities than the normal strength concrete models. The effectiveness of multiple CFRP wraps was limited to the concrete struts capacity. A considerable increase in the beam’s time-dependent deflections was identified when compared with those predicted according to Eurocode-2.