Behavior of Prestressed Concrete Beams Using GFRP Wrapping

Urmil V. Dave, Associate Professor, and Kinjal H. Trambadia, M. Tech Student Civil Engineering Department, Institute of Technology, Nirma University Ahmedabad

Comparative behavior of Prestressed concrete (PSC) beams subjected to two point loadings in terms of failure load, deflection and failure modes is evaluated. Effect of Glass Fiber Reinforced Polymer (GFRP) strengthening on PSC beams before and after first cracking is measured. Experiment includes testing of twelve simply supported PSC beams having cross-section 150 mm x 200 mm with effective span of 3.0 meter. Four unwrapped PSC beams, four PSC beams wrapped by GFRP after initial loading up to first crack and four un cracked PSC beams strengthened using GFRP are tested up to failure.

Four different wrapping patterns are executed on beams. For (2L/7) & (2L/ 6) span loadings, wrapping of full length at bottom and up to 1/3^rd of depth is provided, forming a U-shape around the beam cross-section. For (2L/4) span loading, wrapping of full length at bottom and up to 1/3^rd of vertical depth is provided and extra wrapping near the supports is provided. For (2L/3) span loading, U shape wrapping is provided near the supports, for full depth.

It is observed that in (2L/7) & (2L/6) span loadings, compared to unwrapped PSC beams, the FRP wrapping along longitudinal direction, reduces deflections and increases the load carrying capacity for wrapped PSC beams. In (2L/4) span loading, combination of vertical and horizontal GFRP sheets, together with a proper epoxy adhesion, lead to increase the ultimate load carrying capacity for wrapped PSC beams. In (2L/3) span loading, presence of vertical GFRP sheets near support reduces the shear effects considerably and increase load carrying capacity.

Introduction

Modern structural engineering tends to progress toward more economical structures through gradually improved methods of design and the use of higher strength materials. Such developments are particularly important in the field of reinforced concrete; the limiting features of ordinary reinforced concrete have been largely over come by the development of prestressed concrete. A Prestressed concrete member can be defined as one in which there have been introduced internal stresses of such magnitude and distribution that the stresses resulting from the given external loading are counteracted to a desired degree. Concrete is basically a compressive material. Prestress applies a precompression to the member that reduces or eliminates undesirable tensile stresses that would otherwise be present. Cracking under service loads can be minimized or even avoided entirely. Deflections may be limited to an acceptable value.

Design of PSC Beams

PSC beams are designed using limit state theory using IS-1343 (1984). All beams have same cross-section of 150 x 200 mm with effective length of 3 m. Requirement of reinforcement is evaluated by applying prestressing force of 85.47 kN in case of all the beams. Six high tensile 4mm Φ prestressing wires and 8mm Φ @ 300 c/c stirrups are used as lateral reinforcement in beams. Two no of 10mm Φ bar at top and bottom respectively is used as non prestressing reinforcement. Specimen detailing is shown in Figure 1.

Development of Prestressing Facilities

Prestressing system exclusively developed for this research work is shown in Figure 2, Screw jack is provided for prestresing of HTS wire and for fixing of HTS wires, Wire locking barrel is sed. The screw jack along with moving anchor block is fixed to the jacking end side (stretching end). The steel plate mould is fabricated and put into place with bolted end plates having holes for pretensioning wires to pass through.

Materials

Selected proportion for M45 grade concrete mix is cement: sand: grit: kapchi: water 1:1.36:2.21:0.55:0.37. 4 mm HTS wires are used for prestressing Tension test for the wire is performed on UTM and its ultimate strength is found 2350 N/mm². Figure 3 shows the load deformation relationship for HTS wires. GFRP laminate is used for wrapping on beams.

Four PSC beams are tested up to first cracking. They are then strengthened using Glass Fiber Reinforced Polymer (GFRP) laminates and after tested upto failure. Four PSC control beams immediately after the required curing and strengthened using GFRP laminates are then tested. Different types of strengthening arrangement are provided based upon different span loadings. Figure 3 shows the different type of strengthening arrangements provided.

Testing of Beams

Total 12 PSC beams are cast and tested by applying two point loading. Four PSC beams are tested with four loading positions mentioned as below. Four PSC beams are tested up to first cracking, after that provided with wrapping and are tested up to failures which are denoted by PSCWFC1, PSCWFC2, PSCWFC3 & PSCWFC4 respectively. Four un cracked PSC beams are wrapped and tested, who are denoted by PSCW1, PSCW2, PSCW3 & PSCW4 respectively. Four different types of loading positions i.e. 2L/7, 2L/6, 2L/4 & 2L/3 span loadings respectively are employed for testing of beams and are denoted with notations 1, 2, 3 & 4 respectively. As shown above 1, 2, 3 & 4 generally written after category of specimen exhibit the loading span employed for that category of specimen.

Results and Discussions

The overall comparison is obtained between the experimental observations and theoretical calculations and results for Failure Load and Load-Displacement for different span loadings in case of all the beams are given in Table 1. Basic beam theory is applied for calculating deflection and failure load in case of all the beams.

Failure Load

Table1 as well as Figure 6 give the comparison of experimental and theoretical values of failure loads for all the beams. Load carrying capacity of unwrapped PSC beams is observed significantly less compared to the wrapped PSC beams. Higher load carrying capacity is observed in case of un cracked wrapped PSC beams compared to PSC beams wrapped after loading up to first visible crack. The experimentally observed failure loads are higher than the capacity of the beam evaluated theoretically. Table 1 gives the percentage increase in failure load in case of all the wrapped PSC beams compared to unwrapped PSC beams tested at different span loadings obtained experimentally. The percentage increased in failure load for PSCWFC and PSCW beams are 17.24%, 16.67%, 15.78% and 25%, and 20.68%, 30%, 39.47% and 40.91% respectively as compared to unwrapped PSC beams for different span loadings as shown in table 1.

Load-deflection

Maximum displacements at the time of failure load in case of all the specimens are given in Table 1. These are observed at the centre of the beams during the testing by using LVDT (Linear variable displacement transducer). Figures 7 a), 7 b), 7 c) & 7 d) represent the load– displacement relationship for all the beams tested at 2L/7, 2L/6, 2L/4 and 2L/ 3 loading spans respectively. From Figures. 7 a) to 7 d), it can be seen that as the span loading increases, the tendency of the beams to deflect under lesser loading decreases. This is primarily due to the reduction in the moment caused by the point loads. It is apparently shown by Figures that the load carrying capacity of the beam increases as the loading span increases. But actually, it is the reduction in the moment that causes the beam to deflect lesser as the span increases. Because of the increase in loading span, the load required inducing same amount of moment increases, and hence for the same loading the deflection observed is lesser than the load for the smaller loading span.

Failure Mode

Catastrophic failure i.e. failure with large sound is observed in wrapped beams during the experiments due to the sudden failure of the fibers in tension. Specimens also fail due to the de-bonding of the fibers from the concrete surface. Figures . 8 a), 8 b), 8 c), & 8 d) show mechanism behind failure of the PSC beam wrapped after first crack. Here load is given in the form of push, due to which bottom fibers are in tension. Due to this, bottom fibers of the beam get de-bonded in tension as shown in Figures. 8 a) & 8 b) respectively, as the tension carrying capacity of the wrapping system is more than its bonding capacity.

Concluding Remarks

• In PSC beams, due to the high tensile strength tendons present in the tension zone and are bonded well with the high strength concrete of grade M45, the pretension in the tendons prevents the widening of the cracks at the earlier stages.
• The highest failure load is observed in wrapped uncracked PSC beams compared to other beams. This is due to the transfer of stresses from concrete to the fibres through bonding, thus preventing the beam to fail even after the strain in concrete crosses the allowable limit. The wrapping acts as an external reinforcement and takes complete tensile load after the concrete fails to transfer effectively the stresses to the tendons/ reinforcement.
• Theoretical failure load is higher compared to experimentally observed failure load due to assumption of linear stress distribution in the side reinforcement. But in reality very few amount of side fibers at extreme bottom faces take part during the loading. Parabolic stress distribution is generally observed in case of the fibers.
• The wrapping also helps the PSC beams to take up more shear load compared to unwrapped PSC beams. This is clear from the fact that the beams loaded with higher span loadings in anticipation of shear failure have actually failed in flexure.
• Failure modes observed in all unwrapped PSC specimens are closely matching with expected flexural & shear failure. De-bonding of bottom fibers and tension failure of side fibers are observed as failure mode in case of all wrapped beams.
• Also in case of wrapped beams, the failure takes place primarily due to bond failure i.e. de-bonding between concrete surface and FRP and/or due to tension failure of the FRP sheets. This avoids the local failure or failure of the concrete in compression.

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