رفتار سازه تیرهای RC با خمش خارجی و تحکیم خمشی-برشی با ورق های FRP

This paper presents experimental research on reinforced concrete (RC) beams with external flexural and flexural–shear strengthening by fibre reinforced polymer (FRP) sheets consisting of carbon FRP (CFRP) and glass FRP (GFRP). The work carried out has examined both the flexural and flexural–shear strengthening capacities of retrofitted RC beams and has indicated how different strengthening arrangements of CFRP and GFRP sheets affect behaviour of the RC beams strengthened. Research output shows that the flexural–shear strengthening arrangement is much more effective than the flexural one in enhancing the stiffness, the ultimate strength and hardening behaviour of the RC beam. In addition theoretical calculations are developed to estimate the bending and shear capacities of the beams tested, which are compared with the corresponding experimental results.

Strengthening techniques for building structures have been developed for many years to lengthen the service period and retrofit the damaged structures. There are many structures, due to original design limits and construction errors or an aggressive environment conditions [1] and [2] such as disastrous earthquakes in China and Japan in recent years, that need to be retrofitted to meet the demand usage in a more economic and effective way [3] and [4]. The techniques based on the externally bonded fibre reinforced polymer (FRP) materials are the most widely used ones for retrofitting damaged existing structures [5]. The reason for that is due to their high strength-to-weight ratio, high stiffness, easy installation and stable geometry during the service life [6], [7] and [8]. Many studies [1], [2], [3], [4], [7], [8], [9] and [10] have been undertaken on the RC beams retrofitted in flexural by FRP sheets through experimental, finite element and analytical approaches. The studies have shown that the beams strengthened with FRP in flexural strengthening would avoid the debonding failure mode when a carefully designed anchorage is applied [1], [7], [10], [11], [12] and [13], which gives a good flexural performance in terms of strength and ductility. Although a lot of research has been undertaken on the flexural strengthening by using of FRP materials bonded onto the tension face of the beam, the main focus was on the influence of FRP type, thickness and width on the failure modes of RC beams strengthened [2], [8], [9], [11], [12], [13], [14], [15], [16], [17] and [18]. There were few of these studies covering the effect of beam size [19] and concrete cover thickness [20] on the flexural strength of beams strengthened. Recently, there is increasingly widespread interesting on strengthening RC beams using externally bonded FRP materials to enhance their shear capacities [21], [22], [23], [24], [25] and [26]. Shear failure has different characters as compared to bending failure, in which the former is more brittle and often occurs without any forewarning [27]. Research was also carried out to investigate the effects of longitudinal tensile reinforcement ratio [15], [21] and [28], shear span to effective depth ratio [21], [28] and [29], spacing of CFRP strips, and amount and orientation of CFRP strips on the shear capacity of the precracked and non-precracked beams [28]. Further experimental tests were undertaken to investigate the effect of cross section depth and concrete strength on the shear performance of FRP strengthened RC beams [15], [30], [31], [32], [33], [34], [35] and [36]. However, a lot of research focused on either the flexural or shear failure, with few studies investigating the combined failures [37]. The existing experimental [5] and [6] and analytical [7] research demonstrated that FRP sheets and strips could enhance strength and improve ductility of a beam more effectively if a combined flexural and shear strengthening configuration was applied. Costa and Barros [6] undertook research on the shear capacity of RC beams and found that the load carrying capacity increased 50% when the flexural strengthening was combined with the U or O (full wrapping) shear strengthening. The research work presented here focuses on the strengthening efficiency of RC beams with different layers of CFRP sheets, and under different pre-crack width, concrete cover thickness and the flexural reinforcement ratio and shear reinforcement configurations. A detailed experimental programme provides evaluations of structural behaviour of the combined flexural–shear retrofitting of RC beams by using CFRP sheets prebonded on the tension face of the beam for flexural strengthening, and then reinforced in shear by GFRP or CFRP sheets in U or L configurations. Other experimental parameters have been covered, which include the cross section depth, the stirrups reinforcement ratio and the concrete strength in the flexural–shear retrofitting tests. Research output shows that the flexural–shear strengthening arrangement is much more effective than the flexural one in enhancing the stiffness, the ultimate strength and hardening behaviour of the beam. In addition theoretical calculations on estimating the bending and shear capacities of the beams tested are presented and compared with the corresponding experimental results in a reasonably good agreement.

The experimental results have demonstrated that both the flexural and the flexural–shear strengthening capacity of the RC beams using externally bonded of CFRP or GFRP sheets on bottom and/or lateral faces can significantly enhance the flexural and shear capacity of the beams strengthened. The increase on the overall flexural capacity of the CFRP strengthened beams varies between 41% and 125% over the control beam, and on the shear capacity of the GFRP or CFRP strengthened beams between 31% and 74%. From the investigation, it has shown that the FRP sheets could not only increase strength and stiffness of the beams strengthened but also control development of cracks and increase ductility of the beams. In comparison the flexural–shear strengthening to the flexural strengthening, the former has demonstrated much more significant enhancement on load carrying capacity, initial stiffness and hardening behaviour for the beams strengthened. For the beams with a pre-crack and strengthened with externally bonded of CFRP sheets in the flexural strengthening, the reinforcement arrangements applied in this work are effective in enhancing the flexural strength and ductility. Both of the strength and ductility for the beams strengthened can be enhanced significantly by increasing the internal longitudinal reinforcement, but the preloading may result in a small decrease in flexural strength, stiffness and ductility. The flexural load carrying capacity and deformability of beams with the same strengthening arrangements are significantly increased with an additional layer of CFRP sheets. In addition for the same reinforcing arrangement, the cross-section depth and the reinforcement ratio seem having more influence on the load carrying capacity and initial stiffness on the beam. For the beams with the same flexural–shear strengthening, their load carrying capacity and stiffness can be increased with the higher concrete strength. However, such the increases may not be realised by enhancing the internal stirrups reinforcement ratio or by only one layer of the FRP reinforcement. Theoretical predictions of the flexural strength and the ultimate shear carrying capacity have been developed based on the existing theories, which show reasonably good correlation to the corresponding experimental results. However, in order to develop more rational and accurate predictive models further experimental work and theoretical research are required to cover various concrete strengths, internal longitudinal steel reinforcements and FRP types.