Experimental and analytical study on GFRP reinforced concrete deep beams

Pamuru Mallemkondaiah, Mr. D. Aditya Sairam


Corrosion of steel in reinforced concrete structures is one of the biggest challenges faced by the civil construction industry today. In reinforced concrete structures, corrosion of steel reinforcement due to harsh environmental conditions considerably reduces the durability and life span of these structures. To overcome this corrosion problem, many new techniques have been tried and found to be either expensive or ineffective. Fiber Reinforced Polymer (FRP) materials in the form of solid bars has been successfully tried as a substitute for steel reinforcement in concrete structures.

FRP materials are anti-corrosive, have low weight to strength ratio and are used for various modern engineering applications. Considerable research has been carried out to study the flexural and shear behaviour of FRP reinforced slender concrete beams. However, very little effort has been taken to study the behaviour of Reinforced Concrete (RC) deep beams reinforced with FRP rebars. This work is an attempt to study the shear behaviour of RC deep beams reinforced with Glass Fiber Reinforced Polymer (GFRP) web reinforcement. A concrete deep beam reinforced with FRP is vulnerable to brittle failure under shear load conditions as, individually, both concrete and FRP have the tendency for brittle failure under shear loading conditions.

As     the FRP reinforcements     which were needed for this experimental work with required dimensions were not commercially available, the GFRP reinforcement bars and stirrups used in this work were fabricated by a simple method devised by the researcher called “Manual Fiber-Trusion”. The main advantage of manufacturing GFRP reinforcement by this method is that, the reinforcement can be fabricated to any desired size and shape with a combination of fiber volume content of 75% and resin content of 25% without any filler material being used. This increase in fiber volume fraction in turn has substantially increased the tensile strength of the GFRP reinforcement.

A deep beam is a structure whose depth is comparable to its span. The failure in deep beams is mainly due to shear rather than flexure. In this experimental work, tests were conducted on thirteen concrete deep beams with different configurations of GFRP web reinforcement. The variable parameters considered are the percentage of web reinforcements and “shear span to depth” (a/d) ratio. All the other parameters were kept constant. The thirteen deep beams were cast with and without GFRP web reinforcement and were tested in this work. The testing was done in two stages - in the first stage, i.e. in Series-I, nine deep beams were tested with a “shear span to effective depth” ratio of 0.72 and the results showed a substantial increase in the ultimate shear load carrying capacity for deep beams reinforced with GFRP web reinforcement when compared to those without web reinforcement. Considering this significant increase, four more deep beams were cast in the second stage i.e. Series-II and were tested with a “shear span to effective depth” ratio of 1.08.

The results obtained from the experiments conducted demonstrate that the GFRP web reinforcements were more effective in increasing the ultimate shear capacity of deep beam especially when the “shear span to effective depth” (a/d) ratio had a lower value.

The experimental results were also compared with analytical “Strut-and-tie” models and the results were found to be within the acceptable limits. The Strut-and-Tie method of modelling is widely used in the design of steel reinforced concrete deep beams. This method was adopted due to its flexibility in designing structures which are subjected to a complex state of stress. The results obtained by STM modelling in this work were found to be greater compared to the experimental results. An attempt has been made to propose a suitable modification in the ACI 318-08 code so that it could be adopted for design of GFRP reinforced concrete deep beams with a small a/d ratio to obtain better results.

Finally, after analysis of the experimental results, a design equation was formulated to predict the shear carrying capacity of GFRP web reinforced deep beams. The results obtained by using this equation were found to be acceptable and so, this equation may be adopted for predicting the shear load capacity of deep beams reinforced with GFRP web reinforcement and loaded within a small ‘shear span to depth ‘ratio.

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Copyright (c) 2016 Pamuru Mallemkondaiah, Mr. D. Aditya Sairam

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