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.

Introduction

Reinforced 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 Investigation

The 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.

The materials used in the present investigation and their properties are briefly discussed below.

Ingredients

Ordinary 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 Steel

The 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 Proportion

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/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.

Frp Laminates

Glass Fiber

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 upto failure. The properties of the Glass Fiber supplied by the manufacturer are summarized in the Table 1.

Epoxy

The 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.

Bonding Procedure

Bonding of Composite fabrics to RC beam

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

Beam Description

Table 3 shows the description of beams, ultimate force and contribution of composite fabrics. A total of 18 RC beams of size 100mm x 150mm x 1000mm were tested. In this study, there are two sets of beams; each set consists of three series of beams. The longitudinal bar used on tension side, spacing of stirrups and composite fabric area on shear region of beam were taken as the variables. Based on this concept, the beams were designed, reinforced and strengthened accordingly.

In the first set, the control beams B_25-0, B_15-0and B_9-0 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 B_25-1 and B_25-2 of first series, B_15-1 and B_15-2of second series and B_9-1 and B_9-2 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.

Experimental Set-up

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

Results and Discussions

Failure Mode

Figrue 3, 4 and 5 shows the failure modes of beams. Initially, the vertical cracks occurred in the tensile zone of concrete. As the load increased, more vertical cracks developed followed by a diagonal crack. Different shear failure modes were observed for both control and strengthened RC beams. In case of strengthened RC beams, the major diagonal crack extended up to the loading point from the support. While in case of the control beam, the major diagonal crack developed first along the longitudinal steel bars from the support to the center of the beam and then extended up to the loading point. It was noted that the strengthened RC beam failed in flexure by crushing of concrete in the compression zone.

Shear Strengthening Effect on Ultimate Force

Influence of Strengthened Surface, Stirrup spacing and Longitudinal Steel bar Section.

In the first set, three control beams, namely B_25-0 (stirrup spacing of 250mm), B_15-0 (stirrup spacing of 150mm) and B_9-0 (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.

The effect of GFRP on shear strengthening can be understood by comparing the ultimate load taken by beams B_25-1 and B_25-2. The ultimate load carrying capacity of the beams increased when the GFRP area was increased. The beam B_25-1 had composite fabric height of 75mm but the beam B_25-2 had composite fabric height of 150mm.Both the two beams had same stirrup spacing and also same longitudinal reinforcement. The ultimate load of B_25-1 was 80.5 kn and of B_25-2 was 116.7kn. Since all the other factors like stirrup spacing and longitudinal reinforcement remained same the increase in the load carrying capacity is due to the increase in the GFRP area. This increase in load carrying capacity is evident in all other series of set 1 (B_15-1& B_15-2, B_9-1 & B_9-2). It is to be noted that for the beams with same GFRP area the ultimate load carrying was increased by decreasing the stirrup spacing.

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

Failure Mode

Load Vs deflection curves for the strengthened RC beams B_25-2, B_25-1 and the control RC beam B_25-0 are shown in figure 6. It can be seen that the strength of RC beams increased with an increase in composite fabric area. However, the stiffness of the beams remains same. The deflections after the development of initial cracks in concrete increased linearly upto the failure of the beam. The deflection values corresponding to ultimate force for beams B_25-0, B_25-1, and B_25-2were 2.80mm 4.21mm and 5.32mm respectively. The increase in deflection values for the beams B_25-1and B_25-2 were 150% and 190% respectively when compared to the control beam B_25-0. Figure 7 and 8 shows the load deflection relationship of the control beams B_15-0, B_9-0and the strengthened beams B_15-1, B9-1, B_15-2and B_9-2. The load–deflection slopes of the beams remained unchanged. However, the deflection corresponding to the ultimate force increased with shear strengthening. The measured value increased from 4.86mm for beam B_15-0to 5.72mm for the strengthened beam B_15-1. In comparison with the control beam B_25-0,there is some difference between beam B_15-0and B_25-0.In the case of strengthened RC beam, the deflection value increased slightly from 4.21mm for the beam B_25-1 to 5.72mm for the beam B_15-1. It can be seen from fig 5 that the stiffness of the strengthened beam improved slightly in comparison with that of the control beam. In comparison with the beam B_25-0, the deflection value of the control beam B_9-0increased by 1.77mm. This difference is due to the spacing of the stirrups of the beam. The deflection value of the strengthened beam B9-1is more than that of the control beam B_9-0. When compared to the control beam B_9-0, the stiffness as well as the deflection of beam B_9-1 increased when strengthened. The experimental results showed that the RC beam strengthened by bonded GFRP fabrics exhibited more ductile behavior than the control beam.


Figure 9 to 11 show the load-deflection curves for control beams B'_25-0,B'_15-0, B'_9-0 and the strengthened beams B'_25-1, B'_15-1, B'_9-1, B'_25-2, B'_15-2, B'_9-2. It can be seen that the load deflection behavior of the beams improved considerably. The deflection value at rupture of the beam increased greatly after strengthening in shear by bonded composite fabrics. After strengthening, the deflection of the beam B'_25-0 increased from 4.35mm to 6.61mm, while for the control beam B'_15-0, the deflection increased from 4.93mm to 7.52 mm. In case of control beam B'_9-0, the deflection at the ultimate force after strengthening increased from 6.67mm to 10.18mm. However, in comparison with the control beam B_25-0, the deflection value of beam B'_25-0decreased. The same results were observed in case of the other beams.

Conclusion

The improvement in ultimate strength and load-deflection behavior depends on the composite fabric area, stirrup spacing and the longitudinal steel bar section. Ultimate strength improved with increase in composite surface area. The load corresponding to the diagonal cracks due to shear force increased after the beam was strengthened in shear by the bonded composite fabrics. The bonded GFRP fabrics on the vertical surfaces of the beam delay appearance of diagonal cracks in the concrete and inhibited the development of diagonal cracks. In general, for strengthened RC beam, composite fabrics resist the loading rather than internal steel stirrups. The experimental results indicated that it is possible to increase the ultimate strength of a damaged RC beam or a RC beam insufficient in shear strength or a beam having no stirrups to its maximum shear capacity in shear.

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