Strengthening of RC Beams in Shear Using Composites Fabrics An Experimental Study
M.C. Sundarraja, Assistant Professor in Civil Engineering, S. Rajamohan, Associate Professor in Civil Engineering, Thiagarajar College of Engineering, Madurai
Studies on the use of composites materials for the strengthening of shear deficient concrete structures are limited. The research in this area started since 1991. This is because various configurations of composite sheets can be used for shear strengthening and also due to different failure modes that a strengthened beam undergoes at ultimate load. Furthermore, the experimental data bank for shear strengthening of concrete beams using FRP remains relatively sparse due to which the design algorithms for computing the shear contribution of FRP are not yet clear. The objective of this study is to clarify the role of continuous horizontal GFRP fabrics epoxy bonded to the beam web for shear strengthening of RC beams. Included in the study are effects of GFRP area, stirrups spacing, and longitudinal steel rebar section on shear capacity of the RC beam. This study also aims in understanding the behavior of RC beams strengthened in shear by using glass fibre reinforced polymer. By comparing the behavior of strengthened RC beam with normal RC beams attempt has been made to understand the contribution of externally bonded GFRP towards the shear strength of RC beams.
IntroductionReinforced cement concrete is used throughout the world to build infrastructures and buildings. Today, a large number of civil infrastructures around the world are in a state of serious deterioration due to carbonation, chloride attack, etc. Moreover, many civil structures are no longer considered safe due to increased load specifications in the design codes and due to overloading. Changing social needs, upgraded design standards, increased safety requirements, and environmental attacks have made the many existing reinforced concrete structures such as bridges and buildings deficient in strength. Therefore, strengthening techniques of reinforced concrete (RC) beams are used to meet the current design requirements, serious errors made in design calculations, and poor construction practices.
Traditionally, bonded steel plates were used as external reinforcement for existing concrete structures. But there are problems associated with them such as the need for careful surface preparation of the steel prior to bonding, uncertainty regarding adhesive bond durability, corrosion at the steel/adhesive interface, the need for anchor bolts, and maintenance painting. As a result of these problems, alternate materials have been sought by engineers. Compared to the strengthening of RC structures with bonded steel plates, the epoxy-bonded fiber composites sheets have many advantages such as high tensile strength, high fatigue strength, light weight, and especially, corrosion resistance. Other advantages offered by fiber composite sheets are that the sheets can be installed at any location on the RC beam to obtain maximum efficiency. The FRP strengthening technique has found wide attractiveness and acceptance among researchers and engineers in many parts of the world, and is no longer considered as a new technique for strengthening jobs. This technique appears to be a suitable way for increasing the strength and stiffness of an existing structure. The merits of this method can be attributed to the availability of reliable and high quality epoxy resins, simple and inexpensive man power requirements, minimum change in geometric dimensions and structural systems, as well as minimum disruption to the structure. The efficiency of this technique can be measured if composite action (i.e. the transfer of stresses from concrete to the external plate) is maintained at all stages of loading, up to failure.
In recent years, the development of the plate bonding repair technique has shown to be applicable to many existing strengthening problems in the building industry. This technique may be defined as one in which composites sheets or plates of relatively small thickness are bonded with an epoxy resin to concrete structures to improve its structural behavior and strength. The resin that is used to bond the fabric or the laminate to the concrete surface is a two-component epoxy resin. The old structure and the new bonded-on material create a new structural element that has higher strength and stiffness than the original. Researches are being done to understand the strengthening or the repair of RC structures with fiber composite sheets. The shear failure of concrete structures is catastrophic because of their brittle nature and they give no advance warning (without big cracks) prior to failure. Studies on the use of composites materials for the strengthening of shear deficient concrete structures are limited. The research in this area began around 1991. Furthermore, the experimental data bank for shear strengthening of concrete beams using FRP remains relatively sparse due to which the design algorithms for computing the shear contribution of FRP are not yet clear. The bonding of continuous horizontal GFRP fabrics to the beam web is one convenient and effective method of enhancing the shear strength of RC beam. Moreover, due to the continuous shear resisting area provided by the fabrics, peeling is significantly minimised. The plates or sheets provide additional stiffness against bending and contributes to flexural strength too. The objective of this study is to clarify the role of continuous horizontal GFRP fabrics externally bonded to the beam web for shear strengthening of RC beams. Included in this study are the effect of GFRP area, spacing between steel stirrups, and longitudinal steel rebar section on shear capacity of the RC beam. This study also aims in understanding the shear resistance provided by concrete, steel bars, steel stirrups and glass fibre reinforced polymer fabrics. The obtained results of externally bonded GFRP sheets to RC beams are compared with one another and also with control beams to understand the effectiveness of GFRP fabrics in resisting shear.
Experimental InvestigationThe main objective of this investigation is to clarify the role of continuous horizontal GFRP fabrics epoxy bonded to the beam web for shear strengthening of RC beams. A series of 18 beams were tested in this study.
- The RC beams were strengthened by attaching the epoxy bonded GFRP fabrics on the two vertical sides in the shear region of the beam.
- The variables included in this study are GFRP area, spacing between steel stirrups, and longitudinal steel rebar section.
- The Load–deflection behavior, failure modes and the ultimate loads on the strengthened beams were studied.
IngredientsOrdinary Portland cement (OPC)–53 grade (Birla-super) was used in this investigation. It was tested for its physical properties in accordance with IS code. The fine aggregate used in this investigation was clean river sand passing through 4.75mm sieve with specific gravity of 2.63. The grading zone of fine aggregate is II as per IS 383–1970. Machine crushed Blue Granite broken stones, angular in shape were used as coarse aggregate. The maximum size of coarse aggregate was 20mm and had specific gravity of 2.78. Ordinary clean potable water free from suspended particles, chemical substances etc., was used both for mixing of concrete and curing.
Reinforcing SteelThe yield strength of steel reinforcements used in this experimental program was determined by performing the standard tensile test on three specimens of each bar diameter. The average yield stresses of steel bars of 10 mm diameter and 8 mm diameter were 390 N/mm2and 375 N/mm2respectively.
Mix ProportionFor concrete, the maximum aggregate size used was 20 mm. The concrete mix proportion designed by IS method to achieve the strength of 20 N/mm2was 1:1.68:3.46 by weight. The designed water cement ratio was 0.55. Three cube specimens were casted and tested at the time of beam test (at the age of 28 days) to determine the compressive strength of concrete. The average compressive strength of the concrete was 29.11 N/mm2.
EpoxyThe success of the strengthening technique critically depends on the performance of the epoxy resin used. Numerous types of epoxies with a wide range of mechanical properties are commercially available. These epoxies are generally two part systems, a resin and a hardener. The resin and hardener are used in this study was Araldite GY 257 and Hardener HY 840. The properties of epoxy resin and hardener supplied by the manufacturer are summarized in Table 2.
Before bonding the composite fabric onto the concrete surface, the shear region of concrete surface was made rough using a coarse sand paper texture and cleaned with an air blower to remove all dirt and debris. Once the surface was prepared to the required standard, the epoxy resin was mixed in accordance with manufacturer's instructions. Mixing was carried out in a metal container (Araldite GY 257–100 parts by weight and Hardener HY 840 - 50 parts by weight) and was stirred until the mixture was uniform in color. Then fabrics were cut to size and the epoxy resin was applied to the concrete surface. The composite fabric was then placed on top of epoxy resin coating and the resin was squeezed through the roving of the fabric with plastic laminating roller. Large entrapped air bubbles at the epoxy/concrete or epoxy/fabric interface were eliminated. During hardening of the epoxy, a constant uniform pressure was applied on the composite fabric surface in order to extrude the excess epoxy resin and to ensure good contact between the epoxy, the concrete and the fabric. This operation was carried out at room temperature. Concrete beams strengthened with glass fiber fabric were cured for 24 hours at room temperature before testing.
Testing of RC Beam
In the first set, the control beams B25-0, B15-0and B9-0 were not strengthened externally with GFRP sheets, but were reinforced with 8 mm diameter bar on tension side and the stirrups were spaced at 250mm, 150mm and 90mm respectively. Similarly, the control beams B'25-0, B'15-0 and B'9-0 were reinforced with 10 mm diameter bar in tension side and the stirrups were spaced at 250mm, 150mm and 90mm respectively. In set 1, the beams B25-1 and B25-2 of first series, B15-1 and B15-2of second series and B9-1 and B9-2 of third series were reinforced with 8mm diameter bar on tension side and stirrups were spaced at 250,150 and 90mm respectively. Also in both the two sets, the second and third beams of each series were strengthened externally with GFRP sheets of areas 0.33mx0.075m and 0.33mx0.15m respectively on shear region of beams. In set 2 the beams B'25-1 and B'25-2 of first series, B'15-1 and B'15-2 of second series and B'9-1and B'9-2of third series were reinforced with 10mm diameter bar on tension side and stirrups were spaced at 250,150 and 90mm respectively. Also the first and second beams of each series were strengthened externally with GFRP sheets on shear region of beams and had areas 0.33x0.075m2 and 0.33x0.15m2 respectively.
Results and Discussions
Shear Strengthening Effect on Ultimate ForceInfluence of Strengthened Surface, Stirrup spacing and Longitudinal Steel bar Section.
In the first set, three control beams, namely B25-0 (stirrup spacing of 250mm), B15-0 (stirrup spacing of 150mm) and B9-0 (stirrup spacing of 90mm) were taken. The ultimate forces of the control beams were 66.7 kN, 95.9 kN and 100.8 kN respectively. These beams were reinforced with 8mm diameter bar on tension zone. It was observed that the ultimate force of control beams increased when the stirrup spacing were decreased. In the second set, the control beams were reinforced with 10mm diameter bars on the tension zone (spacing remained the same). It was observed that the ultimate force of these control beams B'25-0, B'15-0 and B'9-0 increased to 100.2 kN, 119.4 kN and 131.2 kN respectively. This indicated that the ultimate force can be increased not only by decreasing the spacing between stirrups but also by increasing the diameter of longitudinal bar.
In the second set of beams, 10mm diameter bars were used as main reinforcement. The ultimate strength of control beams and strengthened beam increased with the variation in stirrup spacing and GFRP area. The ultimate strength of second set of beams is more than the first set of beams due to increase in the diameter of main reinforcement.
Load Vs Deflection Behavior
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