Retrofitting of reinforced concrete beam-column joints using bonded laminates

Abstract

Reinforced concrete is one the most abundantly used construction material, not only in the developed world, but also in remotest parts of the developing world. Thousands of reinforced concrete structures are constructed annually and a large number of these deteriorate or become unsafe before the end of their design life. Strengthening of existing reinforced concrete structures is now a major component of construction activity. The RCC structures constructed across the world are often found to exhibit distress and suffer damage, even before service life is over, due to several causes such as earthquakes, corrosion, overloading, improper design, faulty construction, explosions, fire etc. With the mandate to go vertical, due to rising population and space crunch, most of the structures which have come up over the last three or more decades are all framed structures. In such structures, the most important link for transferring loads and stresses are the beam-column joints. The structural design of these joints is usually neglected. Unsafe designs and detailing within the joint region is dangerous for the entire structure, even though the structural members themselves may conform to the design requirements. It is well known that the joint regions in reinforced concrete framed structures are very critical as they transfer the forces and bending moments between the beams and columns. In most cases, during extreme loading, the beam-column joints, if not designed properly are the most vulnerable component. With the advent of revised design and detailing codes and increase in the earthquake vulnerability level of many regions, the existing structures need strengthening to conform to the revised codal provision. The strengthening and enhancement of the performance of deficient structural elements in a structure or the structure as a whole is referred to as retrofitting. Retrofitting of a structure is not the same as repair or rehabilitation. Repair refers to partial improvement of the degraded strength of a structure after an earthquake, in fact, it is only a cosmetic treatment. Rehabilitation is a treatment of the structure aimed at achieveing the original strength of the structure after it has deteriorated and suffered damage. Retrofitting means structural strengthening of a structure to a predefined performance level irrespective of whether the structure is damaged or not.

The repair or strengthening of an existing structure is a greater challenge for a civil engineer compared to designing or constructing a new one. A specfic technology has to be designed and developed to re-establish the strength of damaged structures, and to improve the performance for new functions of old undamaged structures. Thus, the technique to be used should be simple in implementation; offer better performance when handled by less experienced workers and must use materials that are readily available, durable, strong and economical. Retrofitting of individual members is referred to as ‘local retrofitting’. For this, a large number of techniques are being used including replacement technique, removal, injection technique, shotcreting and plate bonding etc. Amongst all the above techniques the plate bonding technique is found to be the most efficient and suitable method for retrofitting purposes. In the plate bonding technique, Ferrocement Plates, Fiber Reinforced Polymer (FRP) Plates, Polymer modified concrete and mortar (PMC/PMM) and Steel plates are most commonly used for retrofitting. Of this techniques the use of Fiber Reinforced Polymer (FRP) Plates has gained significant popularity in the last two to three decades. But this technique is costly and requires skilled labour. Various authors have suggested the use of ferrocement jacketing as a more attractive choice in place of FRP plate bonding technique due to its easy application, lesser weight, higher impermeability, improved tensile strength, economical use, and long life term performance. In the present study, an effort has been made to study the effect of ferrocement jacketing on the strength of retrofitted beam-column joints. The studies have been carried out for various parameters like number of wire mesh layers and their orientation in the ferrocement jackets and initial stress levels of the beam-column joints. The effect of these parameters on the strength of reinforced concrete beam-column joints initially stressed to pre-determined levels, and subsequently retrofitted with ferrocement jackets was investigated. A similar set of beam-column joints was also retrofitted using two layers of CFRP jackets with an orientation of 45° to the longitudinal axis of the joint, to study the behavior of such beam-column joints. Subsequently, a 3D nonlinear finite element (FE) model using software ATENA-3D was used to validate the experimental results. Comparison between the finite element and experimental results confirms a reasonable accuracy of the proposed model. The test results showed that retrofitting beam-column joints with different layers of wire mesh in the ferrocement jackets and two layers of CFRP jacketing significantly increased the ultimate and yield load carrying capacity, stiffness of all the joints stressed to various levels, establishing the efficacy of using the material for retrofitting. The use of ferrocement and CFRP jacketing for retrofitting of initially stressed beam-column joints helped to regain full strength even if stressed to 85% of the ultimate load. Due to the strengthening of beam-column joints of control specimens, the failure of the retrofitted beam-column joint specimens shifted from the joint region to the beam ends in the retrofitted specimens. This would help in preventing progressive collapse of the structure. The retrofitting of the beam-column joints may thus shift the failure from the joint to the beam end to obtain a weak beam- strong joint failure pattern. The comparison between the load-deflection results obtained from ATENA 3D and the experimental study shows that the ATENA 3D results agree reasonably with the experimental results. The variation of experimental and FEM (ATENA 3D) load results for the control as well as retrofitted specimens was within ± 10%. The element modeling (ferrocement and CFRP) showed higher values as compared to the experimentally obtained values.