Load carrying capacities and failure modes

Failure of FRP reinforced concrete beams in flexure can be caused by rupture of FRP bars in the tension zone when the reinforcement ratio is smaller than the balance

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ratio, which is called under reinforcement, or by crushing the concrete in the compression zone when the reinforcement ratio is greater than the balance ratio which is also called over reinforcement. For each case, the ultimate flexural strength can be calculated by using the same equations when the reinforcement is steel or FRP bars57. This phenomenon has been addressed by the different authors by studying and testing concrete beams reinforced with different types of FRP bars and different reinforcement ratios and different concrete strengths.

Benmokrane et al.12 investigated eight concrete beams under monotonic four-point loading. The cross-section dimensions of the beams were 200 mm in width and 300 and 550 mm in height and the clear span was 3000 mm. The specimens were reinforced with the same amount of GFRP or steel bars in tension and steel bars in compression. They observed that the maximum experimental moment was similar for GFRP and steel reinforced beams for 300 mm height, while it was approximately 8% greater in GFRP-RC beams than steel-RC beams for 550 mm height. Also, the specimens failed either in tension for under reinforced beams or in compression (concrete crushing) for over reinforced beams, as they had been designed.

Masmoudi et al.58 studied ten concrete beams under cyclic four-point loading with 200 mm × 300 mm × 3300 mm in width, height and length, respectively. The specimens were reinforced with either GFRP or steel bars and had different reinforcement ratios according to their predicted failure modes which were balance, tension and compression failure. The compressive strength of the concrete was varied between 45 MPa to 52 MPa. They noticed that with the increasing of the

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reinforcement ratio, the ultimate moment capacity also increased. However, this increase in the GFRP reinforcement ratio was limited by the concrete compressive failure strain. The failure of over reinforced specimens consisted of two stages. The first stage was the crushing of the concrete which was followed instantly by the decrease and then the increase of the moment resistance strength. The second stage was the initiation of failure of the GFRP bars. In addition, it was observed that, if the reinforcement was more than one level, a partial failure can occur. When a partial failure happens, the specimens can regain the elastic deflection component with a large deformation.

The effect of CFRP reinforcement ratio on the flexural behaviour of the concrete beams has been studied by El-Salakawy et al.59_{. They constructed 14 full-scale} concrete beams with 200 mm × 300 mm × 3300 mm in width, depth and length, respectively. The specimens were cast with 39.3 to 44.8 MPa concrete and tested under four-point loading and were reinforced with different reinforcement ratios of CFRP bars or steel bars. They concluded that increasing the reinforcement ratio by 50% to 100% led to increase the load carrying capacity of the beams by 4% to 11%. Also, the failure of the specimens was caused by concrete crushing, as they were designed with an over reinforcement ratio, except the specimen that was reinforced with a 1.2 balanced reinforcement ratio. For this beam a simultaneous failure by concrete crushing and CFRP rupture occurred.

Kassem et al.60 conducted a test on 24 concrete beams to investigate the flexural behaviour under four-point loading. The beams were 200 mm × 300 mm × 3300 mm

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in width, depth and length, respectively and they were cast with 40 MPa concrete. The beams were reinforced with carbon, glass and aramid FRP bars which had different surface textures and different reinforcement ratios. They noticed that the failures of specimens were controlled by the concrete compressive strength, while the increase of reinforcement ratio did not significantly increase the flexural capacities. The flexural capacity increased by 4% and 16% as a result of increasing the reinforcement ratio by 50% and 100%, respectively.

The effect of concrete strength and FRP reinforcement ratio on the flexural capacity of concrete beams were studied by Kalpana and Subramanian61. They conducted an experimental investigation on nine concrete beams with 200 mm × 250 mm × 1800 mm in width, depth and length, respectively and reinforced with GFRP bars under four-point monotonic loading. The specimens were reinforced with three different reinforcement ratios and they were cast with three different concrete strengths (20, 40 and 60 MPa). They observed that increasing the reinforcement ratio by 0.52%, it resulted in increases of 13% and 17% of the ultimate load capacity in the normal and high strength concrete specimens, respectively. Also, increasing the reinforcement ratio by 1.3% resulted in a better increase of 28% and 36% in the ultimate load carrying capacity for the normal- and high-strength concrete, respectively. However, a comparable improvement in the ultimate load carrying capacity of the specimens cast with moderate concrete strength could not be observed due to change in the reinforcement ratio.

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In a study by El-Nemr et al.62 about the impact of the concrete strength and FRP reinforcement ratio on the flexural behaviour of concrete beams, they revealed that for normal strength concrete beams, increasing reinforcement ratio from 0.36 to 1.47% increased the load carrying capacity of the specimens by 143% and increasing the reinforcement ratio from 0.55 to 1.78% increased the load carrying capacity by 224%. In addition, for high strength concrete beams, increasing the reinforcement ratio from 0.36 to 1.47% increased the load carrying capacity of the specimens by 28% and increasing the reinforcement ratio from 0.55 to 1.78% increased the ultimate capacity by 116%. These results were deduced through testing 14 concrete beams which were 200 mm × 400 mm × 4250 mm in width, depth and length, respectively under four-point monotonic loading. The specimens were reinforced with different type and ratio of GFRP bars and they were cast with 30 and 65 MPa concrete strength.