Address for Correspondence

Research Paper SHEAR STRENGTHENING OF DIFFERENT BEAMS USING FRP Patel Mitali R 1, Dr.R.K.Gajjar 2 Address for Correspondence 1 Research Scholar, 2 Professor and Head, Applied Mechanics Department, L. D. College of Engineering, Gujarat Technological University, Ahmedabad, Gujarat (India) ABSTRACT The concept of adopting externally bonded FRP laminates or sheets to enhance the shear capacity of Reinforced Concrete flexural members is practiced since many years. Despite of the collective efforts of researchers, the shear behaviour of different types of beams strengthened with web-bonded FRP is not clearly demonstrated at one platform. It is observed that the shear strength of RC flexural members can be enhanced to a good percentage if strengthened with FRP. Although the shear behaviour of different types of flexural members namely, normal beams, box beams, T-beams and deep beams is different inspite of using same FRP laminates for strengthening and same loading pattern. This paper aims with a view to contribute the understanding the shear behaviour of different beams externally strengthened with FRP laminates. It also highlights the type of beam best strengthened with FRP. Different researchers have focused on shear strengthening of different beams using FRP laminates. An effort has been made in this paper to represent the collective contributions of researchers and to focus on the type of beam gaining maximum enhancement in shear strength with use of FRP laminates. The paper also represents the effect of different parameters on the strength of beams strengthened externally with FRP sheets or strips. KEYWORDS Strengthening, FRP, Shear, Box Beams, T-Beams, Normal RC Beams, Deep Beams. I. INTRODUCTION Strengthening of Reinforced Concrete Beam by FRP is a very popular technique. The shear failure in any structural element is catastrophic in nature. So the strengthening of structural element such as beams and columns in shear is required. Three types of Fibre reinforced polymers are mainly used for strengthening of existing structures namely Glass Fibre Reinforced Polymer (GFRP), Carbon fibre Reinforced Polymer (CFRP), and Aramid Reinforced Fibre Polymer (AFRP). Among these three, CFRP is found to be most effective in enhancing the shear capacity of the beam. FRP is effectively used in strengthening of RC beams due to its light weight, non-corrosive non-magnetic nature, and resistance to chemicals. Also the formability of FRP makes its application techniques very easy to install. As CFRP possesses high strength to weight ratio and versatility in coping with different sectional shapes, the application of CFRP is further simplified. The literature shows that to enhance the shear capacity of existing RC beams, the laminates, sheets, or strips of FRP are applied externally to the face of the beam in most of the cases. Shear strengthening of different types of beams such as Normal RC beams, T-Beams, Box Beams and Deep beams with externally bonded FRP strips, sheets or laminates are considered and various parameters pertaining to study are evaluated. II. NEED OF STRENGTHENING The strengthening for any RC structure may be required on account of one or combination of any of the following factors. A. Deterioration due to environmental effects: Concrete is heterogeneous as well as porous material. It allows ingress air and moisture in it. Ingress of air and moisture reaches to the reinforcement of RCC structure and it corrodes the reinforcement. Corrosion of steel reinforcement is one of the main durability problems faced in reinforced concrete infrastructures worldwide. Many structures in adverse environment have experienced unacceptable loss in serviceability or safety far earlier than anticipated period due to the corrosion of reinforced steel and thus need of replacement, rehabilitation or strengthening is required. Corrosion produces problems in reinforced concrete structures for two reasons. First, as steel corrodes there is a corresponding drop in crosssectional area. Secondly, the corrosion products occupy a larger volume than the original steel which exerts substantial tensile forces on the surrounding concrete and causes it to crack and spall off. The expansive forces caused by steel corrosion can cause cracking, spalling and staining of the concrete and hence results in loss of structural bond between the reinforcement and concrete. B. Revision in Loading Standards: From time to time loading standards are being revised. For example, if we see the loading standards of Indian Railway, which the railway companies followed initially when the railway line was started in this country, we find that axel loading was 7.5tonnes. Now-a-days, axel loading has been increased to 25tonnes. Similarly longitudinal force was not considered initially while designing the bridge. It doesn t mean that the engineers were not aware of the longitudinal forces. Actually longitudinal forces at that time were of smaller magnitude, which was not affecting the section of the substructure since vertical load was predominant and the section adopted for vertical load along with practical consideration at that time, was sufficient to cater longitudinal forces. Braking forces which we considered while designing bridge depend on the span length and it may be even more than the tractive effort, in case the span of bridge is more. Due to the revised loading standards, the old bridge which is still in sound condition, needs strengthening on account of its capacity enhancement to meet the increased loading standards. Similarly due to change in occupancy conditions and building service requirements strengthening of building structural elements is required. C. Seismic Retrofitting: With due consideration to our experiences in past, seismic forces are being revised world-wide to ensure safety. Everywhere this revision is on upward side. In the past large number of reinforced concrete structures

International Journal of Advanced Engineering Research and Studies E-ISSN2249 8974 have been damaged by severe earthquake and some of these structures have been repaired and strengthened. Systematic studies to determine the behaviour of the repaired or strengthened members under cyclic loading are still very limited. The importance of this information can hardly be underrated. As major earthquakes have affected highly populated regions and industrial centres, basic information on the performance of repaired or strengthened member will be extremely important. The up gradation of the seismic performance of existing reinforced concrete and gravity load designed structures is an important issue that involves economic and social aspects in different areas of the world like Europe, USA and Japan. In fact, the RC frame designed without seismic provisions is often characterized by an unsatisfactory structural behaviour due to the low available ductility and the lack of strength hierarchy inducing global failure mechanism. III. ADVANTAGES AND DISADVANTAGES A. Advantages of FRP FRP laminates have many advantages such as, durability against temperature, moisture and chemical attack, ease of bonding with any type of beam, high stiffness to weight ratio, high strength to weight ratio, low thermal expansion, good fatigue performance, damage tolerance, non-magnetic properties, ease of transportation and handling, low energy consumption during fabrication of raw material, potential for real time monitoring, good tensile strength even at high pressure and most important, FRP laminates are corrosion free. B. Disadvantages of FRP FRP laminates are less ductile, hencee strengthened structure losses its ductility to certain extent. Plastic behaviour of FRP is not constant for uneven surfaces applications of FRP laminates and hence is problematic. FRP laminates are very costly in small quantities and its application may not be favourable from cost point of view. FRP is prone to the attack of ultra violet rays. IV. STRENGTHENING ASPECTS Strengthening for an RC flexural member can be of three types: 1. Flexural Strengthening To enhance the flexural capacity of the element, FRP sheets of designed thickness is applied on the tension face of the element. It acts as an external reinforcement along with internal steel reinforcement. The externally added FRP sheet on the tension face provides desired quantity reinforcement. This addition of FRP sheets shifts the neutral axis on the tension side and enhances the total compressivee force on the compression side. Schematic diagram of the flexural strengthening of beam is as shown below: 2. Shear Strengthening Shear force is tensile in nature. It creates tension in the concrete. As such, to resist shear force, shear reinforcements are provided in the RCC structure. Now, flexural failure is designed for ductile failure while the shear failure is brittle in nature. Since brittle failure is not preferred, it should be ensured that the shear strength capacity should be adequate to avoid shear failure. As such the structure should never fail. But in adverse conditions even if it undergoes any adverse deformations, it should be in flexure only and not in shear. Shear strengthening of the beam can be done by providing FRP strips just as we provide shear reinforcements in the form of ring in RCC beam as shown in fig 2(a). Fig 2(a): FRP strips strengthening of beam in zone prone to shear FRP sheets can also be provided in the form of bands in shear zone as shown in fig 2(b). Provisions for FRP application are either on the vertical face or in the U- shape or wrapped all around. In case of wrapping for shear strengthening, it is advisable to provide overlap on the compression side of the beam. Schematic diagram of shear strengthening is given below: Fig 2(b): FRP band strengthening of beam in zone prone to shear 3. Axial Strengthening Sometimes columns, piers and abutments require strengthening to enhance its capacity. Capacity enhancement might be there either on account of flexure or axial or both. In such case, whether it is axial, flexural or combined FRP needs to be wrapped all around the column. Schematic diagram of such strengthening is given below: Fig 1: Flexural strengthening of beam Fig 3: Strengthening of column by wrapping V. SHEAR STRENGTHENING OF DIFFERENT TYPES OF BEAMS USING FRP There are three types of strengthening done in RC structures. From the three, here we ve considered only one. That is shear strengthening of RC beams

with FRP laminates, sheets or strips. Four types of beams strengthened with FRP are discussed here. 1. Normal RC beams In 1993, Amir M. Malek & Hamid Saadatmanesh presented an analytical model to calculate the stresses and shear force resisted by composite plate in reinforced concrete beams strengthened with web bonded FRP plates. This paper presented a FEM base method to investigate the effect of the FRP plate on the stress distribution in the concrete beam and also calculated shear force taken by FRP plate. The method for evaluating shear force taken by FRP plate has been developed for uncracked beam as well as for a beam with flexural cracks. And verification of this FEM method is done by modelling of pre-configured beam in ABAQUS software. In this paper effect of most important parameter, fiber orientation angle was investigated through a parametric study for both uncracked and cracked beams. For considered problem the closed form solutions were developed based on the compatibility of the strain in the plate and the beam, assuming that the material behaved linearly elastic and that there was complete composite action between the FRP plate and the beam. In 2003, Barros J. & S. Dias presented an experimental study on shear strengthening of reinforced concrete beams with CFRP strip laminates. This paper represents two series (series A and series B) of beam that were tested. Each series is constituted by a beam without any shear reinforcement and a beam with steel stirrups, strips of CFRP sheet embracing the beam, laminate strips of CFRP at 90 degree & 45 degree to neutral axis of beam. The strips of CFRP sheets were fixed to concrete by resin epoxy while laminate strips of CFRP were bonded to concrete by epoxy adhesive. Series A is composed by beams of cross section of 0.15 0.30m 2, length of 1.6m and span of 1.5m. Series B is composed of beams of cross section of 0.15 0.15m 2, length of 1.0m and span of 0.9m. The shear span of both series of beams was two times the height of the beams. The conventional longitudinal reinforcement at bottom and top surface was composed by 4-10mm Φ and 2-6mm Φ bars respectively. The amount of shear reinforcement applied was evaluated in order to assure that all beams would fail in shear. The beams were tested under the four point loading system. From the experimental results author concluded that the load carrying capacity of RC beams failed in shear can be significantly increased using CFRP shear reinforcing system. In 2011, Ahmed Khalifa, William J. Gold, Antonio Nanni & Abdel Aziz M.I represented the concept of shear strengthening with FRP and proposed design algorithms to compute the contribution of FRP to the shear capacity of reinforced concrete flexural members. The two design approaches for computing shear capacity of the FRP sheets were discussed in this paper. One approach was based on the stress level that caused the facture of the FRP sheets and other approach was based on delamination of the sheet from the concrete surface. The parametric study was carried out for different parameters such as stiffness of CFRP sheets, quality of thermosetting resin, compressive strength of concrete and number of plies of CFRP sheets, wrapping scheme and fibre orientation angle. One good example to show how to increase the shear capacity of RC flexural member was given in this paper. The first approach was based on the effective stress and it is only suitable if the failure is controlled by FRP sheet rupture. The second approach was based on bond mechanism and was suitable only if the failure is controlled by the FRP sheet delamination. 2. RC T-Beams In 2003, Deniaud C. & J. J. Roger Cheng represented research studies on interaction of concrete, steel stirrups and external FRP sheets in carrying shear load in reinforced concrete beams. The tests series was carried out in laboratory in which total 8 tests were carried out on 4 controlled beams with four point loading system. In this experiment total three types of FRP were used to externally strengthen the web of the T-beams: (1) uniaxial carbon fiber, (2) uniaxial glass fiber and (3) triaxial glass fiber. The glass fibers were applied at right angle to the longitudinal direction along full length of the shear span and carbon fiber sheets were applied at a 45 0 angle to the longitudinal axis. The spacing of the steel stirrups 200mm, 400mm and no stirrups beams were considered in this test set up. This experimental study was carried out to investigate the effect of concrete strength, stirrups spacing, height of the beam web and type of FRP on the behaviour of FRP strengthened concrete beams. In 2006, Abdelhak Bousselham & Omar Chaallal carried out an experimental study to investigate the behaviour of reinforced concrete T-beams strengthened in shear with CFRP by varying the parameters. The main objective of this paper was to understanding of the resistance mechanisms involved for RC T-beams strengthened in shear with externally bonded FRP. This understanding is of paramount importance because it leads to a more rigorous approach toward safer and rational design guidelines. This experimental program involved 22 tests on 11 full scale T-beams under three point loading condition. In this study two types of beam specimen were used namely Deep Specimen and Slender Specimen which depends upon the ratio of shear span to depth. The parametric study was carried out including parameters such as CFRP ratio, the transverse steel ratio and shear span to beam s depth ratio. In 2008, Abdelhak Bousselham & Omar Chaallal studied on the mechanism of shear resistance of concrete beams strengthened in shear with externally bonded FRP. In this experimental research study, author performed 17 tests on full size T section beam under the three point loading condition. The parameters considered in this work were CFRP ratio, transverse steel ratio and size of beam. The resistance mechanism was studied by observing the behaviour under the increasing loads of the beam strengthened with CFRP, from the first formation of flexural crack to failure. The CFRP sheets were applied continuously over the test zone in a U-shape around the web. In 2010, Lee H.K, Cheong S.H, Ha S.K & Lee C.G represented the behaviour and performance of RC T- section deep beams externally strengthened with CFRP sheets. In this test series total of 14 RC T- section deep beams were designed to be deficient in shear with a shear span to effective depth ratio of 1.22.

Crack patterns and behaviour of deep beams were observed during four point loading test. The key variables evaluated in this study were strengthening length, fiber direction combination of CFRP sheets and an anchorage using U-wrapped CFRP sheetsin addition, a series of comparative studies in accordance with the common design codes were carried out to evaluate the shear strength of non-strengthened and strengthened CFRP beams. The theoretical results, where the shear strength of the non-strengthened beam was derived using the equations for deep beam in accordance with the ACI 318, CIRIA model code and Eurocode. 3. RC Box Beams In 2005, Grace N. F., Singh S. B., Shinouda M. N. & Mathew S. S. carried an experimental study to investigate the shear performance of box beams strengthened with CFRP. This experimental study was carried out to investigate shear cracking load and ultimate load carrying capacity of the box beams. In this study total of 6 box beams were evaluated for shear behaviour. The parameters considered in this study were, arrangement and type of the stirrups. Each box beam was 4.9m long, 965mm wide 305mm deep and reinforced with pairs of steel stirrups at a uniform spacing of 75mm in one shear span and with single steel stirrups at a uniform spacing 230mm through its mid span. The remaining shear span was the test zone. One beam was a control beam, with no stirrups in the test zone and the another beam was reinforced with 9.5mm diameter steel stirrups at a uniform spacing of 125mm in the test zone. All beams were reinforced longitudinally with a total of eleven non-prestressing steel. And seven bonded pre-tensioning tendons were installed in the bottom flange and six unbounded posttensioning tendons were installed in the hollow portion of the box beam. To measure the strain distribution in the box beam total 15 strain gauges were installed in which five gauge on the top surface and five gauges on each side surface. In this all beams were simply supported with roller at one end and hinged support at the other end. Jack and hydraulic pump were used to apply four point loading system. 4. RC Deep Beams In 2004, Islam M.R., Mansur M.A. & Maalej M. represented an experimental research work on shear strengthening of RC deep beams using externally bonded FRP systems. The main objective of this work was to explore the prospects of strengthening structurally deficient deep beams by using externally bonded FRP system and finding out the effectiveness of different types of FRP systems. Six identical beams were fabricated and tested in two symmetrical point loadings. The strengthening was done by using CFRP fiber wrap, strips or grids. Deep beams usually fail in shear so when strengthening of it using externally bonded FRP systems is considered, the system may be regarded as additional web reinforcement, but fixed externally. The parameters considered in this research work were size of grid bars, orientation of grid system and method of bonding. VI. CONCLUSIONS 1. Normal RC Beams For the uncracked beam, shear force resisted by the composite plate was negligible. However, for beams with flexural cracks, shear force resisted by FRP plate was considerably higher, and depended on the thickness and fibre orientation of plate. For the strengthening of beam in shear, use of strips of CFRP was best mechanism. This mechanism has largest residual strength and less brittle failure. With increase in height of beam, the 45 0 laminate strip of CFRP became more effective rather than 90 0 laminate strip of CFRP. A CFRP laminates strip provides higher protection against fire and vandalism acts. 51% increase in shear capacity was achieved with addition of CFRP reinforcement externally. 2. RC T-Beams Effectiveness of FRP in shear resistance doesn t depend on the FRP thickness but it directly depends on the amount of transverse steel reinforcement present in the beam. FRP sheets delayed the loss of plane section behaviour and shear forces carried by arching action were also delayed. Triaxial glass fiber reinforced provided the beam with more ductile failure than unidirectional glass fiber or unidirectional carbon fiber. Shear capacity gain due to the CFRP was greater for deep specimen and small size specimen rather than slender specimen. CFRP ratio does not lead to an additional shear capacity gain in proportion to the added CFRP. Maximum strain in CFRP was inversely proportional to the CFRP thickness. Contribution of concrete under the increasing loading is effective from the application of loading and is increasing until first diagonal crack. The contribution of CFRP and transverse steel was active after the formation of first diagonal crack which does not linearly vary with the increased loading. Bonded FRP system led to much slower growth of critical diagonal crack and enhanced the load carrying capacity of the beams to satisfy most of the practical upgrading requirement. Shear strengthening performance of CFRP sheets increases as the strengthening length increases with respect to ultimate load, initial stiffness and ductility. Fiber direction combination CFRP sheets had significant influence on the shear performance of deep beams under ultimate load and ductility, and had no significant change with increase in initial stiffness. The anchorage using in U-wrapped CFRP sheets was shown to be more effective in increasing load carrying capacity, initial stiffness and ductility of the beam. The shear strength contribution by CFRP calculated by ACI 440 is overestimated compared to the experimental results. 3. RC Box Beams Energy ratio was apparently not affected by the stirrups spacing. Stirrups with grater center to center spacing experienced higher strain at beam failure.

Shear cracking force for the beam with steel stirrups was significantly higher than the cracking force for the beam with CFRP stirrups at the same spacing. Shear cracking occurred at higher loads with the steel stirrups. 4. RC Deep Beams Strengthening of deep beam in shear is practically possible. FRP grid placed in normal orientation is the most effective system as far as the amount of material used in strengthening is concerned. 40% enhanced shear strength was achieved. REFERENCES 1. Amir M. Malek, Hamid Saadatmanesh, Analytical Study of Reinforced Concrete Beams Strengthened with Web-Bonded fiber Reinforced Plastic Plates or fabrics, ACI Structural Journal, Vol-95(3), May-June 1993. 2. Ahmed Khalifa, William J. Gold, Antonio Nanni & Abdel Aziz M.I, Contribution of Externally Bonded FRP To shear Capacity of RC Flexural Members, Journal of Composites for Constuction, Vol-2(4), Nov- 1998, page-195 to 202. 3. Barros J., S. Dias, Shear strengthening of reinforced concrete beams with laminates strips of CFRP, 2003. 4. Deniaud C., J. J. Roger Cheng, Reinforced Concrete T- Beams Strengthened in Shear with Fiber Reinforced Polymer Sheets, Journal of Composite for Construction, Vol-7(4), Nov-2003, Page-302 to 310. 5. Abdelhak Bousselham, Omar Chaallal, Behaviour of reinforced Concrete T-Beams Strengthened in Shear with Carbon Reinforced Polymer-An Experimental Study, ACI Structural Journal, Vol-103(3), May-June 2006, Page-339 to 347. 6. Abdelhak Bousselham, Omar Chaallal, Mechanism of Shear Resistance of Concrete Beams Strengthened in Shear with Externally Bonded FRP, Journal of Composite for Construction, Vol-12(5), Sep-Oct 2008, Page-499 to 512. 7. Lee H.K, Cheong S.H, Ha S.K, Lee C.G, Behaviour and performance of Rc T-section deep beams externally strengthened in shear with CFRP sheets, Journal of Composite Structures, Vol-93, 2010, Page-911 to 922. 8. Grace N. F., Singh S. B., Shinouda M. N., Mathew S. S, Investigation the shear performanace of Box Beams strengthened with carbon fiber reinforced polymer, Concrete International, Feb-2005, Page-1 to 5. 9. Islam M.R., Mansur M.A. & Maalej M., shear Strengthening of RC Deep Beams using extenally Bonded FRP Systems, Cement & Concrete Composites, Vol-27, 2005, Page- 413-420. 10. Mitali R. Patel, R.K.Gajjar, Shear Strengthening of RC Beams using CFRP, International Journal of Advanced Engineering Technology, Vol-3(1), Jan-Mar 2012, Page- 338 to 342.