Experimental studies on strength and ductility of CFRP jacketed reinforced concrete beam-column joints
Introduction
Strengthening of existing reinforced concrete structures is now a major part of the construction activity all over the world. 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, change of codal provisions, improper design, faulty construction, explosions and fire. With the mandate to go vertical, in light of the rising population and space crunch, most of the structures which have come up over the last three or more decades are all framed structures. For all framed structures, the most important component is the beam-column joint, and the structural design of the joint is usually neglected. During the design stage, attention is only restricted to provision of sufficient anchorage for the beam. Unsafe design 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 joint region in reinforced concrete framed structures are recognized as very critical as it transfers the forces and bending moments between the beams and columns. In most cases, during extreme loadings, 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 needs retrofitting and strengthening. Various types of strengthening materials such as steel plates, ferrocement, and fiber reinforced polymers are available in the construction industry to be used for jacketing of the affected components, the most common being steel jackets [1]. These types of jackets increase the weight and dimensions of the structural elements. A few attempts have been made for the use of corrugated or plain steel plates as jacketing material in concrete frames [2]. FRP based strengthening has become attractive as compared to others due to its light weight, high strength and stiffness, corrosion resistance, easier implementation, excellent fatigue etc. and is an attractive alternative to restore the joints to their desired capacity. According to Gergely et al. [3], CFRP composite was able to strengthen the ductility and shear resistance of the beam-column joints. Mukherjee and Joshi [4] found that both GFRP and CFRP can be used for seismic retrofitting as well as rehabilitation of RC joints. Pentelides et al. [5] concluded that the shear capacity of the beam-column joints increased by 35% with use of composites. Ghobarah and El-Amoury [6] observed that rehabilitation techniques using CFRP were effective in avoiding the brittle joint shear and bond slip failure. Toutanj et al. [7] found that CFRP retrofitted beams showed increase in maximum load up to 170% as compared to control beams. Kachlakev and McCurry [8] also found that beams strengthened using CFRP and GFRP laminates did show an increased capacity of more than 150% both in flexure and shear. Antonopoulos and Triantafillou [9] investigated a combination of FRP in various forms plates, sheets, bars in structural enhancement and also discussed their roles in improving the joint shear capacities. Karayannis and Sirkelis [10] used a combination of epoxy resin injection and CFRP sheets and found a significant improvement in the loading capacity, ductility and the energy absorption of the beam-column joints. Ferreira [11] observed that a CFRP strengthened beam showed increase in the stiffness and delay in the tension cracking at higher loads. Significant flexural and shear enhancement has been observed in cases when CFRP bonded laminates have been used for strengthening of beams [12], [13], [14], [15]. D’Ayala et al. [16] conducted tests on different layouts of FRP fabric and sheets bonded to R.C. beam-column joints, and found that the strengthening procedure increases the stiffness and ductility, while increases in shear and flexural strength and in energy dissipation are highly dependent on proper confinement of concrete and anchorage of the wrapping. Karunasena et al. [17] found that externally bonded composites, CFRP or GFRP, improved the moment capacity of damaged concrete beams.
Based upon the published literature, it can be concluded that, CFRP is a viable alternative material for repair and strengthening of reinforced concrete structures. The RCC structures, which require retrofitting, are already stressed to a particular level due to the loads experienced by the structure in its life span, and therefore, it is deemed imperative to study the effect of the initial stress level on the behavior of the structures after retrofitting, specifically of the beam-column joints, for which not much work has been reported. In the present study, the effect of initial stress levels on the various parameters such as ultimate load carrying capacity, stiffness and ductility of reinforced concrete exterior beam-column joints retrofitted with CFRP (carbon fiber reinforced polymer) with two layers, has been presented.
Section snippets
Experimental program
In this study, nine external reinforced concrete beam-column joints were cast using M-20 grade of concrete and Fe-500 steel grade as shown in Fig. 1. Conforming to IS: 10262-1982 [18] the ratio of cement: sand: aggregate was 1:1.43:2.97 with the water-cement ratio 0.48. These beam-column joints were tested based on the available loading arrangement and test facilities as shown in Fig. 2. The column cross-section of 225 mm × 125 mm with an overall length of 1000 mm and the cantilever beam with
Results and discussion
First of all the three specimens of prototype beam-column joints were tested to ultimate load. The average ultimate load of these beam-column joints was observed to be 22.68 kN, with an ultimate deflection of 23.24 mm at the tip of the cantilever beam. Subsequently, out of the six specimens cast, two specimens were stressed up to ultimate load, two were stressed up to 85% of the ultimate load achieved for the control specimen i.e. 19.28 kN, and remaining two specimens were stressed to 50% of
Conclusions
Based on the results of the experimental investigation carried out on the control and retrofitted beam-column joint specimens using CFRP, the following conclusions could be drawn:
- 1.
CFRP jacketed beam-column joints show an increase in ultimate load carrying capacity of 9.47%, 11.99% and 7.76% for stress level-1, stress level-2 and stress level-3, respectively establishing the efficacy of using the material for retrofitting.
- 2.
CFRP jacketed beam-column joints show an increase of nearly 15% in the
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