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Composite Structures

Experimental tests of two-span continuous concrete deep beams reinforced with GFRP bars and strut-and-tie method evaluation

Abstract

Test results of nine continuous concrete deep beams reinforced with GFRP bars and one specimen reinforced with steel rebars are reported. The investigated parameters included shear span-to-overall depth ratio ( a / h ), size effect and web reinforcement ratio. Two a / h ratios of 1.0 and 1.7 and three section heights of 300 mm, 600 mm and 800 mm as well as two web reinforcement ratios of 0% and 0.4% were used. The experimental results highlighted that the web reinforcement ratio improved the load capacities by about 10% and 18% for specimens having a / h ratio of 1.0 and 1.7, respectively. Additionally, a considerable reduction in shear strength occurred due to the increase of the section depth from 300 mm to 600 mm. The comparisons between the test results and those calculated using the strut-and-tie methods of the American (ACI 318-2014), European (EC2-2004) and Canadian (S806-2012) codes showed that the American code gave unsafe predictions, while the predictions of the Canadian code underestimated the load capacity of the beams tested. The European code provided the closest predictions to the test results; however, all codes presented do not consider the influence of section depth and/or web reinforcement on the shear strength, the impacts of which were confirmed experimentally.

Introduction

In civil engineering, there are many useful applications of concrete deep beams, such as transfer girders, offshore structures, pile caps, folded plates, bunker walls and tanks. The elastic beam theory, which is used for slender beams is not applicable for deep beams due to the non-linear distribution of strain in concrete deep beams [1]. Deep beams in many structural applications such as, marine structures, parking garages and bridges are exposed to severe environmental conditions, causing steel corrosion which in turn increases the cost of maintenance. Therefore, fibre reinforced polymer (FRP) reinforcement has been introduced as an alternative reinforcement to steel. Many advantages of using FRP bars can be offered over steel. FRP has a high corrosion resistance and high strength to weight ratio which makes it easy to handle and install. The major drawbacks in the use of FRP bars are the low modulus of elasticity, brittle nature and low bond strength compared with steel reinforcement. The strut-and-tie method has been proposed by many code provisions to design concrete deep beams reinforced with steel bars, such as ACI 318-08 [2], Eurocode (EC2-2004) [3] and CSA A23.3-04 [4]. Only the CSA-S806-12 [5] developed a guideline to design concrete deep beams reinforced with FRP using the same equations that were developed for steel-reinforced concrete deep beams of CSA A23.3-04 [4]. The CSA-S806-12 considered the effect of FRP bars through the effectiveness factor which depends on the shear span to depth ratio and strain in the longitudinal FRP bars. A good number of experimental investigations were performed for simply supported deep beams reinforced with FRP bars [6], [7], [8], [9], [10], [11], however, no published investigations are available for continuous once reinforced with FRP bars. Continuous deep beams are more common in practice and behave differently from simply supported ones due to the coexistence of high moment and high shear regions within the interior concrete strut that transfers a considerable part of the applied load to the support. The STMs of American (ACI 318-2014) [12], European (EC2-2004) [3] and Canadian (S806-2012) [5] codes have no total agreement regarding the effect of web reinforcement on the shear strength of the deep beams. The STMs of the Canadian (for steel and FRP reinforcements) and European (for steel reinforcement) codes did not consider the effect of web reinforcement on the shear strength of deep beams. While, the STM of the American code (for steel reinforcement) has considered the effect of web reinforcement on the shear strength in case of the bottle-shaped strut.

Section snippets

Research significance

The current research presents the first experimental investigations on continuous concrete deep beams reinforced with GFRP bars. The only code provision (CSA-S806-12) that considered the effect of FRP reinforcement on the shear resistance of concrete deep beams is based entirely on the simply supported beams as the experimental investigations are not yet available for continuous ones. Additionally, that code, namely S806-2012, did not consider the effects of web reinforcement and section depth

Test specimens

Nine specimens of continuous concrete deep beams reinforced with GFRP bars and one specimen reinforced with steel rebar were designed to be tested experimentally. The effects of three main parameters were investigated in this experimental program, namely the shear span-to-overall depth ratio, a / h , web reinforcement and overall section depth, h . Two shear span-to-overall depth ratios of 1.0 and 1.7 were considered. Six specimens had a / h  = 1.0 and were divided in two series, one series with shear

Cracking propagation and failure modes

The two beam spans and the two faces of all test beams showed approximately the same crack patterns and propagation, confirming the symmetrical arrangement of the test specimens. The flexural cracks at the sagging zone (mid-span) and hogging zone (above the intermediate support) occurred at approximately 22–41% of the failure load as presented in Table 3. Inclined cracks appeared in the shear zones and at least one of them extended diagonally from the edge of the intermediate supporting plate

Test results compared with the strut-and-tie model of the current design codes

Strut-and-tie method (STM) is based on the lower bound theory of plasticity and requires satisfaction of the equilibrium and yield conditions [21]. For the reinforced concrete deep beams the STM is often simplified as a truss model [22]. STMs of the American and European code provisions did not consider the shear strength of concrete deep beams reinforced with FRP bars. Whereas, the STM of the Canadian code (S806-2012) [5] adopted the same equations that were developed for steel-reinforced

Conclusions

Test results of nine continuous concrete deep beams reinforced with GFRP rebars and one specimen reinforced with steel rebars were presented. The parameters investigated were the web reinforcement ratio, the shear span-to-overall depth ratio and the section depth. The difference in behaviour between the deep beams reinforced with GFRP bars and those reinforced with steel rebars was investigated using one specimen reinforced with steel rebars to be equivalent to one of those reinforced with GFRP

Acknowledgment

The authors would like to express their special thanks and gratitude to the Higher Committee of Education Development in Iraq (HCED) for their financial support.

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