Shear Strengthening of RC Beam by Externally Bonded Side Gfrp Strips

M.C. Sundarraja, M.E, and S. Rajamohan, M.E., Ph.D Associate Professor in Civil Engineering, Thiagarajar College of Engineering, Madurai

In recent years, several studies have been conducted to investigate the flexural strengthening of reinforced concrete (RC) members with fiber reinforced composite fabrics. Shear failure of RC members is disastrous and occurs with no advance warning of distress. In order to take full advantage of the ductility of an RC member, it is desirable to ensure that flexure rather than shear governs ultimate strength. A large number of structures constructed in the past have been found to be deficient in shear and needs strengthening. Deficiencies can be due to many factors such as insufficient shear reinforcement due to design errors or use of outdated codes, reduction in steel area due to corrosion, increase in demand of service load and construction defects. The main objective of this research work is to gain better understanding about the shear behavior of RC beams strengthened externally with bonded flexible glass fiber reinforced polymer (GFRP) strips on beam web. The present study encompasses some of the important parameters such as spacing of strips on shear span and inclination of GFRP strips both for retrofitted and rehabilitated concrete beams. Also the research work aims to study the failure modes, efficiency, strength gain, and deformability of strengthened beams.

Introduction

The use of reinforced concrete as a structural building material is well established in modern engineering design and construction. From earlier days, many materials have been employed as reinforcing agents for concrete including wood, steel, bamboo, and a number of synthetically manufactured composite materials. Recently, the use of high strength fiber-reinforced polymer (FRP) materials has gained acceptance as structural reinforcement for concrete. The FRP materials were initially developed for aerospace applications in the year 1940. There is an urgent need for a concrete reinforcement material that has high tensile strength and non-corrosive in saline environment. Since FRP has both these properties, research works have been done to use the material as primary reinforcement for concrete.

For each type of degradation there are various possible methods using new materials. However, engineers must exercise caution in applying new technologies to historic structures. There is a long history of misapplication of new materials, which have often caused more damage to historic buildings over long run. For example, in the repair of masonry monuments, preservation architects today view standard interventions of the 20th century, such as the use of Portland cement and reinforced concrete, as outdated and harmful to the historic masonry fabric. Engineers must take a long-term view and must consider the whole life design of an intervention, including the reversibility and the future repair of each intervention.

For historically significant structures, engineers must justify the use of materials that differ from the original fabric. In cases where materials decay regularly, it is generally accepted to replace by the same kind of material for the repair. The Eiffel Tower is a classic example of this kind of replacement. Every iron element of the tower are replaced at least once during its lifetime. In Gothic buildings, masonry pinnacles are replaced approximately every 200 years, using the same source of stone each time, and so is the case of King’s College Chapel. In such cases, there is no question of using new materials to repair elements which have decayed. It is more important to use the same materials and the same technology to maintain the structure, even if new materials have better mechanical properties.

New materials, such as FRP, are more appropriate in cases where there is a lack of strength in a historic structure. For example, FRP may be used to compensate for local decay of a single element in a historic timber roof truss. Similarly, a historic metal bridge may require strengthening to carry greater traffic loads. Because such strengthening methods may be irreversible in historic structures, engineers should carefully consider all the options before choosing the materials for repair.

The seismic resistance of historic masonry buildings is a special scenario to consider. Most earthquake engineers feel that historic buildings do not have sufficient ductility to resist a major seismic event. Many engineers would propose that a structure needs to be strengthened to improve its seismic resistance, but our knowledge of the seismic response of masonry buildings is limited at present. In the repair of historic monuments, FRP may offer some advantages including high strength and improved durability over metallic reinforcing. Such materials may also minimize aesthetic impacts in comparison to other materials. The long term stability of FRP in various moisture and temperature conditions is not tested and there is a need for additional research on the long-term compatibility of FRP with traditional materials.

FRP is a composite of two material groups: (1) reinforcing fibers; and (2) polymer resin matrix. Reinforcing fibers are generally made from glass, carbon, aramid, or its combinations. This study emphasizes the use of glass fiber reinforced polymer (GFRP) reinforcement exclusively. The material properties and failure response of FRP differ significantly from those of steel. A design methodology is required that reflects the material specific characteristics of the FRP reinforcement. To prevent a sudden brittle tensile failure of the FRP reinforcement, beams must be designed to be over reinforced relative to the balanced strain condition at ultimate (Nanni 1993). FRP has much potential as longitudinal reinforcement in concrete structures susceptible to reinforcement corrosion and stressed primarily in bending. Examples of such structural components include bridge decks, footings, floor slabs, and wall type structures (abutments, stems, and wing walls). In these members, flexural strength is essentially provided by the longitudinal reinforcement, and shear strength is provided by the concrete alone because of the lack of transverse (shear) reinforcement.

Several research studies have been done in upgrading rein forced concrete (RC) elements using fiber reinforced plastic (FRP) plates instead of steel-plates. Most of them deals with RC beams and slabs strengthened for flexure by bonding FRP plates or strips to their soffits. Recently, several researchers studied external shear strengthening of RC beams using FRP sheets. Shear strengthening of RC Beams by externally bonded side CFRP sheets (O.Chaallal, M.J.Nollet and D.Perraton) gave results showing the feasibility of using epoxy bonded strips to restore or increase the load carrying capacity in shear of RC beams. This method also reduce substantially shear cracking.

The overall objective this R&D work is to study failure mode, shear strengthening effect on ultimate force and load deflection behaviour of RC beams bonded externally with GFRP strips. Results of experimental studies on 1000mm long beam models strengthened with externally bonded GFRP strips are presented and discussed.

Scope of the Investigation

The purpose of this paper is to provide experimental data on the response of RC beams strengthened in shear using bidirectional GFRP fabrics. In this study, the RC beams were strengthened in shear with epoxy bonded GFRP sheets attached on the vertical sides in the shear region of the beam. Three sets of 1000mm long RC beams (five in total) having cross sectional dimensions of 100mm x 150mm were considered. The beam’s height was selected on the basis of shear strengthening with special regards given to anchorage of side strips; it therefore resulted in relatively stiff beam. The first set of beam was designed at full strength in shear and considered as control beam. The second and third set of beams were under-designed in shear, but were strengthened in shear to achieve the same shear strength as control beam using externally bonded side GFRP strips. The side strips were placed perpendicular to the beam’s longitudinal axis for two beams and at 1350 with respect to the beam’s longitudinal axis for the two beams of third set. All beams of these three sets had the same flexural reinforcing steel ratio.

The effect of shear strengthening is discussed with the help of experimental results. In this, the failure modes, efficiency, strength gain and deformability of strengthened beams were studied and analyzed.

Experimental Program Materials

Ordinary Portland cement (OPC)– 53 grade (Birla-super) was used for the investigation. It was tested for its physical properties in accordance with Indian Standard specifications. 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 was zone II as per Indian Standard specifications. Machine crushed Blue Granite broken stone angular in shape was used as coarse aggregate. The maximum size of coarse aggregate was 20mm and specific gravity of 2.78. Ordinary clean portable water free from suspended particles and chemical substances was used for both mixing and curing of concrete.

Reinforcement

The longitudinal reinforcements used were deformed, hotrolled,high-yield strength bars of 10mm and 8mm diameter. The stirrups were made from mild steel bars with 6mm diameter. The yield strength of steel reinforcements used in this experimental program was determined by performing the standard tensile test on the three specimens of each bar. The average yield stresses of steel bars obtained were 390 N/mm², 375 N/ mm² and 240 N/mm² for 10mm, 8mm and 6mm diameter respectively.

Concrete

For concrete, the maximum aggregate size used was 20 mm. The concrete mix proportion designed by IS method to achieve the strength of 20 N/mm² and was 1:1.68:3.46 by weight. The designed water cement ratio was 0.55. Three cube specimens were cast 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.

FRP Laminates

Glass Fiber composites are among the oldest and least expensive of all composites. E-glass is the most common type of glass fiber used in resin matrix composite structures and was used in this investigation. The principal advantages of E-glass are low cost, high tensile and impact strengths and high chemical resistance. The disadvantages of E-glass, compared to other structural fibers are lower modulus, lower fatigue resistance and higher fiber self-abrasion characteristics. In general, fiber composites behave linearly elastic to failure. The properties of the Glass Fiber supplied by the manufacturer are summarized in the Table 1.

Epoxy Resin

The success of the strengthening technique critically depends on the performance of the epoxy resin used. Numerous types of epoxy resins with a wide range of mechanical properties are commercially available. These epoxy resins are generally two part systems, a resin and a hardener. The resin and hardener are used in this study is Araldite GY 257 and Hardener HY 840 respectively. The properties of epoxy resin and hardener supplied by the manufacturer are summarized in Table 2.

Bonding Procedure

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 continued until the mixture was in uniform colour. When this was completed and the fabrics had been cut to size, 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. Air bubbles entrapped at the epoxy/concrete or epoxy/fabric interface were to be 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.

Experimental Setup

A Two-point loading system was adopted for the tests. At the end of each load increment, deflection, ultimate load, type of failure etc., were carefully observed and recorded.

Results and Discussions

The test results were based on the value of load-deflection behavior. The load versus deflection curves are shown in Figure 1 to 5. A summary of the experimental results is presented in the Table 3 & 4. This includes loads and deflection at ultimate stages as well as modes of failure. Here ultimate stage is defined as the stage of loading beyond which the beam would not sustain additional deformation at the same load intensity.

In the following sections, results are discussed for each set of the study in terms of load deflection, cracking behavior and model failure.

In the case of retrofitted beams by EB-GFRP strips, initial cracking occurred at 37.5 kN for control beam, at 45 kN for the retrofitted beam with inclined strips and at 55 kN for retrofitted beam with vertical strips. After cracking, the stiffness of all models dropped but the reduction in stiffness was more pronounced in the control beams than in the retrofitted beams. Also, from the stiffness enhancement point of view, vertical strips were slightly more efficient than inclined strips.

Cracking in control beams started at mid span with vertical flexural cracks. Later, diagonal cracks developed and widened around 60 kN, introducing shear failure. In the retrofitted beam with vertical strips, failure was due to shear cracking, which produced a severe delamination along the middle two strips. The diagonal strips had forced the diagonal crack to bend, whereas the vertical strips inhibited the propagation of diagonal shear cracks.


In the case of rehabilitated beams by EB-GFRP strips, initial cracking occurred at 37.5 kN for control beam, at55 kN for the rehabilitated beam with inclined strips and at 55 kN for rehabilitated beam with vertical strips. From the stiffness enhancement point of view, vertical strips were slightly more efficient than diagonal strips. In the rehabilitated beam with vertical strips, failure was due toshear cracking. In the case of rehabilitated beams with inclined strips, the failure was due to shear diagonal cracking & flexural cracks. The diagonal strips had forced the diagonal crack to bend, whereas the vertical strips inhibited the propagation of shear diagonal cracks.

Conclusion

Based on the investigation the following conclusions were made:

• For the retrofitted beam with vertical strips, mode of failure was shear cracking, which produces a severe delamination along the middle two strips
• For the rehabilitated beam with vertical strips mode of failure was shear cracking
• In the case of rehabilitated beams with inclined strips, the failure was due to shear diagonal cracking and flexural cracks
• The diagonal strips had forced the diagonal crack to bend, whereas the vertical strips inhibited the propagation of shear diagonal cracks
• From the stiffness enhancement point of view, vertical strips were slightly more efficient than diagonal strips
• It is easier to maintain a relatively uniform thickness of epoxy resin through out the bonding length.
• Restoring or upgrading beam shear strength using FRP side strips can result in increased shear strength and stiffness with substantial reduction in the shear cracking. Restoring beam shear strength using GFRP is a highly effective technique
• Vertical side strips outperformed diagonal side strips for shear strengthening in terms of crack propagation, stiffness and shear strength.

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