Al-Sulayfani : Flexural Behavior Of R/C Beams With Externally Bonded Gfrp Sheets

Abstract During the latest decades there has been a significant increase in using FRP (Fiber Reinforced Polymers) as a main material for external reinforcement in the construction industry.Externally bonded FRP sheets have been used to increase moment capacity of flexural members and to improve confinement in compression members. This paper summarizes the result of experimental and analytical studies on the flexural strengthening of reinforced concrete beams by external bonding of glass fiber reinforced polymer sheets to the tension face of the beam. Four beams, three with different thickness of GFRP sheets and one without GFRP sheets were tested using third-point loading over a span of 900mm.The tests were carried out under load control. The results indicate that the flexural strength of the beams increased significantly as the thickness of the sheet increased. Analytical study using a computer program based on strain compatibility is presented to predict the ultimate strength and load-deflection behavior of the beams. When the experimental results were compared with theoretical ones, good acceptable agreement was obtained which make it possible to consider and recommended this model in the design.


Introduction :
The use of externally bonded fiber reinforced polymer (FRP) to strengthen reinforced concrete (R/C) structures is increasingly becoming popular retrofit technique. The light weight and formability of FRP reinforcement makes these systems non-corrosive, nonmagnetic, and generally resistant to chemicals, they are an excellent option for external reinforcement (1).Glass fiber reinforced polymer (GFRP), like carbon fiber reinforced polymer(CFRP), are more attractive than steel for use as external reinforcement. Strengthening with externally bonded FRP sheets has been shown to be applicable to many types of R/C structures. Currently, this method has been implemented to strengthen structures such as columns, beams, slabs, walls, chimneys, tunnels, silos (2), girders in structures such as bridges, parking decks, and building. Tests on beams with a bonded GFRP plate (3,4) show that it can enhance the ultimate flexural strength considerably when the steel reinforcement ratio is much lower than the balanced steel ratio, and in most cases the concrete fails long before the GFRP plate reaches its ultimate load capacity. Several research studies have addressed the upgrade of reinforced concrete (RC) element using fiber-reinforced plastic (FRP) plates as a substitute for steel plates (4)(5)(6)(7)(8). The purpose of this paper is thus to enhance the understanding of the flexural behavior of reinforced concrete (R/C) beams strengthened externally by GFRP Sheets, and to develop clearer picture of the role and effectiveness of the GFRP sheets in the structural performance of the strengthened beam.

Experimental program properties of materials:
Concrete: Concrete with compressive strength of 36 MPa is specified for all the concrete beams. Ordinary Portland cement, locally available sand and gravel in Mosul city with maximum aggregate size of (19mm) were used.

GFRP Sheets and epoxy adhesive:
The ultimate tensile stress ( fu ) and Young's modulus (E f ) of GFRP Sheet is determined by conducting tension tests on coupons cut from the sheet. The Young's modulus and the ultimate tensile stress of GFRP Sheet are calculated from load/strain curves and presented in Table ( 1). The properties of epoxy used for bonding the GFRP sheet are also presented in Table (1). Test Specimens: Four R/C beams (1000mm-long) having cross-sectional dimensions of 150×150mm ( Fig.1) were used. The beams were made from the same batch. Control specimens were cast with concrete from the same batch for compressive strength, Modulus of elasticity, tensile splitting, and modulus of rupture. Each concrete beam is reinforced with two 10mm diameter steel bars for tension along with 6mm diameter steel closed stirrups at a spacing of 55mm center-to-center for shear reinforcement. The internal strain was measured with two handmade gage. The hand made gages consisted of a electronic strain gage with 6mm long placed on the middle of rebar. A hand held grinder was used to smoothed out a two inch section were the gage was attached. The devices are shown in Figure (2).The strain was measured by Model 1300 Gauge Installation Tester. The effective span of all the beams is kept as 900mm. The concrete control beam is designated as RB1-0, three beams wrapped with different levels by changing the thickness of GFRP sheet. One beam wrapped with one layer of GFRP Sheet (RB2-1-B), the second beam wrapped with two layers of GFRP Sheet (RB3-2-B) and the third beam wrapped with three layers of GFRP Sheet (RB4-3-B). The details of the beams are presented in Table (2).   Preparation and Curing : Prior to applying adhesive the bonding surface of the concrete beam is made rough by scarifying it with a toothed grinder and cleaning it with an air blower. All beams were completely dried before the epoxy was applied. The epoxy system consists of two parts, resin and hardener, mixed in the ratio of 3:1. The epoxy system was thoroughly hand mixed for at least 5 minutes. at room temperature. A thin layer of epoxy was applied to the concrete surface using paint roller. A GFRP saturated with epoxy sheet was then applied directly on the surface. Special attention was taken to ensure that there were no voids between the GFRP sheet and concrete surface, and the excess epoxy was squeezed out along the edges of the GFRP sheet, assuring complete epoxy coverage. After the application of the first layer of the GFRP sheet, a second layer of epoxy was applied on the surface of the first layer to allow the adhesion of the second layer of the GFRP sheet. Finally a last layer of epoxy was applied on the surface of the wrapped beam. The surfaces were then kept bonded together under pressure using mechanical clamp until the adhesive had cured. The time gap between the GFRP sheets bonding and the beam test was at least 7 days.

Analytical Study:
The purpose of this theoretical study is to develop a model that accurately predicts the flexural behavior of reinforced concrete beams (RB2-1-B), (RB3-2-B) and (RB4-3-B) strengthened with GFRP sheets. An iterative method is used in which increases the strain in the top compression fiber of the concrete is increased until the concrete crushes, tensile steel ruptures, or the FRP sheet ruptures. The following model is based on derivation presented by Saadatmanesh and Ehsani (9). It employs strain compatibility, force equilibrium, using the following assumptions: 1. Plane sections before deflection remain plane after deflection.

No Shear deformation. 3. Perfect bond between different materials.
The Stress-Strain curve for concrete is approximated using Al-Sulayfani (10)curve based on unique function shown schematically in Figure (3).
where, c=compressive stress of concrete. For this model, a maximum concrete strain of (0.003) was used in the model developed for this research. The steel is assumed to exhibit a bilinear stress-strain behavior. A post yield modulus equal to 1.5 percent of the initial elastic modulus was used (10). The stress-strain curve for the reinforcing steel was simplified as shown in Figure(5).The stressstrain curve for the GFRP was determined experimentally as discussed in paragraph(2-1-2), it exhibit a bilinear stress-strain behavior as shown in figure (6).The following equations for the stresses of the GFRP and reinforcing steel, are obtained from their stress-strain behavior:   4) where, y = the yield strain of the reinforcing steel St = the stress in the tension reinforcing steel E S = the modulus of elasticity of the tension reinforcing steel fy = the yield stress of the reinforcing steel Esh=Post yield modulus of steel reinforcement st= Strain in tensile steel reinforcement. The linear strain distribution through the cross section is shown in Figure(7) .The concrete forces in tension, which are only used before the concrete cracks in tension, are not shown in Figure(7 Before the concrete has cracked or the tensile steel has yielded, using Equations (6),(7), (8), and (9), Equation (22)

Test Results and Discussions
The experimental load/midspan deflection curves for beams RB1-0, RB2-1-B, RB3-2-B and RB4-3-B are shown in Fig (10). Although the initial stiffness of the beam remains unchanged, the stiffness has changed considerably after cracking. The increase in stiffness is proportional to the sheet thickness. As shown in Table (3) glass wrapping increased the ultimate strength of reinforced concrete beams, The percentage increase in (Load-Carrying Capacity) through wrapping is a function of the number of longitudinal Glass layer up to certain thickness of the wrap. Crack Pattern and Failure Modes: The crack patterns at collapse for the tested beams are shown in Fig. (11) and Fig. (12). The control beam exhibited widely spaced and lesser number of cracks compared to strengthened beam. The strengthened beams have also shown cracks at relatively closer spacing. This shows the enhanced concrete confinement due to the

Comparison of Analytical Calculations with Experimental Results:
Steel Stress: It is seen that at a given load level, rebar stresses and deflections were found to be less in Glass-wrapped beams compared to rebar stresses of unreinforced beams. Yielding of steel bars occurred at about (48.3%) higher load in beam wrapped with three layers as compared to beams without wrapping as seen in table (3).  Fig.(14-17) respectively. It can be seen from these figures that the model has predicted the load deformation behavior with reasonable accuracy. The analytical procedure underestimates the failure loads. The model under-predicted the failure load by about (20%) as seen in table (4).   3-Two of the beams (RB3-2-B) and (RB4-3-B) failed by debonding of GFRP sheets due to high stress concentration at beam ends, to preventing that mode of failure a U-Strips maybe provided to gain better anchoring mechanism. 4-The cracks at ultimate load of strengthened beam were more in number compared with that of the control beam indicating clearly the composite action due to GFRP sheets. 5-The adopted analytical procedure can be used for the design of concrete beams Strengthened/Retrofitted with GFRP sheets.