Strengthening of Reinforced Concrete T-Beams with externally bonded FRP Sheets to improve Shear Strength

The rehabilitation of existing reinforced concrete (RC) bridges and building becomes necessary due to ageing, corrosion of steel reinforcement, defects in construction/design, demand in the increased service loads, and damage in case of seismic events and improvement in the design guidelines. Fiberreinforced polymers (FRP) have emerged as promising material for rehabilitation of existing reinforced concrete structures. The rehabilitation of structures can be in the form of strengthening, repairing or retrofitting for seismic deficiencies. RC T-section is the most common shape of beams and girders in buildings and bridges. Shear failure of RC T-beams is identified as the most disastrous failure mode as it does not give any advance warning before failure. The shear strengthening of RC T-beams using externally bonded (EB) FRP composites has become a popular structural strengthening technique, due to the well-known advantages of FRP composites such as their high strength-toweight ratio and excellent corrosion resistance. A few studies on shear strengthening of RC T-beams using externally bonded FRP sheets have been carried out but still the shear performance of FRP strengthened beams has not been fully understood. The present study therefore explores the prospect of strengthening structurally deficient T-beams by using an externally bonded fiber reinforced polymer (FRP). This study assimilates the experimental works of glass fiber reinforced polymer (FRP) retrofitted RC T-beams under symmetrical four-point static loading system. The eight numbers of beams were of the following configurations, (i) one number of beams was considered as the control beam, (ii) seven number of the beams were strengthened with different configurations and orientations of FRP sheets. The first beam, designated as control beam failed in shear. The failures of strengthened beams are initiated with the debonding failure of FRP sheets followed by brittle shear failure. However, the shear capacity of these beams has increased as compared to the control beam which can be further improved if the debonding failure is prevented. An innovative method of anchorage technique has been used to prevent these premature failures, which as a result ensure full utilization of the strength of FRP. A theoretical study has also been carried out to support few of the experimental findings.


INTRODUCTION
Many old structures which were constructed using old codes and techniques are unable to withstand the latest technology and design methods and hence these old structures are required to be upgraded. Structures like buildings, girders, Bridge decks etc. are susceptible to damage due to age of structure, corrosion, adverse environments. After damages these structures are not capable to carry the load for which they have been designed.
Earthquakes are the most affecting natural disasters in buildings. So having knowledge of earthquakes is an important thing in the current era. This consideration demands revision in seismic loads on structure. The systemic effects have completely changed design methodology that's why older structures need retrofitting because replacing structures may lead to un-economical structure.
Most widely used techniques for retrofitting are steel jacketing and concrete jacketing. In concrete jacketing we Improve load carrying capacity by increasing cross sectional area. This may lead to Increased load due to increase the section, also it requires new formal words therefore it has high cost. Steel jacketing is the most effective technique but it requires difficult welding work on site, also it may have corrosion all this leads to increased maintenance cost.
All these things have led to more and more research work in this field. These researches have created a new milestone in modern Construction Techniques. One of the important outcomes of this research is Fiber reinforced polymer (FRP).
FRP composites comprise fibers of high tensile strength embedded within a thermosetting matrix such as epoxy, polymer or vinyl ester. The most widely used matrix is epoxy.
FRP was developed for using in aero planes spacecrafts Satellites helicopter Space shuttle etc. But later in 1980s it was started for using in civil structures. These where mainly used for rehabilitation of RCC structures. FRP's are having large advantages As follows: 1. FRP materials are not vulnerable to the swift electrochemical corrosion that occurs with steel 2. They can be easily rolled which makes transportation easy 3. High fatigue resistance 4. High strength to weight ratio 5. Fiber composite materials are available in very long lengths while steel plate is generally limited to 6m. The availability of long length and the flexibility of the material simplify the installation process 6. Time required for installation is very less 7. Fiber composite strengthening materials have higher ultimate strength and lower density as compared to those of steel 8. Low energy consumption during fabrication of raw material and structure, and has the potential for real time monitoring 9. Tailor ability and ease of application 10. Excellent durability Although FRP has many advantages it has one disadvantage that it is very sensitive to high growth thermal environment. FRPs are used in forms of bars, plates and sheets for strengthening RC beams. In these forms FRP sheets are commonly used due to their flexibility. As we know beam fails in flexure and shear. So FRP are needed in both zones for strengthening purpose. It may be classified as follows:

A Flexural strengthening
In this type FRPs installed on tension zone using an adhesive like epoxy. Its fibers are placed parallelly to the direction of tensile stress.

B Shear strengthening
Shear failure is one of the important types of failure in beams and it needs special consideration while designing. We can apply FRP to the beam for strengthening in shear in various ways. First by bonding FRP on the side of the beam. Secondly applying FRP in U shape including two side and bottom tension zones and thirdly we can a rap FRP on the whole cross section of beam. In T beams it is not feasible to use a third method. FRP has its highest strength only in the direction of fibres, these directions may be uni-directional, bidirectional or multi directional to achieve more benefit in shear we can use fibre in two directions.
In this project we will be using externally bonded FRP sheets for strengthening T-Beams in shear strength. FRPs will be applied in various shapes and at various locations on beam section. Later these beams will be tested using destructive test. Based on these test results analysis will be done that which will be suitable and most effective way of application of FRP.

C Objectives:
The main objectives of the present work are: 1. III. CASTING OF SPECIMEN Eight number of reinforced concrete T-beams are cast and tested up to failure by applying symmetrical four-point static loading system. Out of eight numbers of beams, one beam was not strengthened by FRP and was considered as a control beam, whereas all other seven beams were strengthened with externally bonded FRP sheets in shear zone of the beam. The variables investigated in this research study included FRP amount and distribution (i.e., continuous wrap versus strips), bonded surface (i.e., lateral sides versus U-wrap), FRP ratio (i.e., no. of layers), and end anchor (i.e., U-wrap with and without end anchor).

A. Fiber Reinforced Polymer (FRP)
Continuous fiber reinforced materials with polymeric matrix (FRP) can be considered as composite, heterogeneous, and anisotropic materials with a prevalent linear elastic behaviour up to failure. Normally, Glass and Carbon fibers are used as reinforcing material for FRP. Epoxy is used as the binding material between fiber layers. For this study, one type of FRP sheet was used during the tests i.e., a bidirectional FRP with the fiber oriented in both longitudinal and transverse directions, due to the flexible nature and ease of handling and application, the FRP sheets are used for shear strengthening. Throughout this study, E-glass was used manufactured by Owens Corning.

B. Epoxy Resin
The success of the strengthening technique primarily depends on the performance of the epoxy resin used for bonding of FRP to concrete surface. Numerous types of epoxy resins with a wide range of mechanical properties are commercially available in the market. These epoxy resins are generally available in two parts, a resin and a hardener. The resin and hardener used in this study are Araldite LY 556 and hardener HY 951 respectively.

C. Fabrication of GFRP Plate for tensile strength
There are two basic processes for moulding, that is, hand layup and spray-up. The hand lay-up process is the oldest, simplest, and most labour intense fabrication method. This process is the most common in FRP marine construction. In hand lay-up method liquid resin is placed along with reinforcement (woven glass fiber) against finished surface of an open mould. Chemical reactions in the resin harden the material to a strong, light weight product. The resin serves as the matrix for the reinforcing glass fibers, much as concrete acts as the matrix for steel reinforcing rods. The percentage of fiber and matrix was 50:50 in weight.
The following constituent materials are used for fabricating the GFRP plate: i. Glass FRP (GFRP) ii. Epoxy as resin iii. Hardener as diamine (catalyst) Fixing of FRP sheets on the beam iv. METHODOLOGY After casting and curing for 28 days all the specimens were tested as simply supported RC T-beams by using four-point static loading frame with shear span of effective depth ratio (a/d) as 2.38. A load cell of 500 kN attached to hydraulic jack was used. The load is transmitted through a load cell and spherical seating on to a spreader beam. This spreader beam is installed on rollers seated on steel plates bedded on the test member with cement in order to provide a smooth levelled surface. The test member is supported on roller bearings acting on similar spreader plates. The loading frame must be capable of carrying the expected test loads without significant distortion.
Ease of access to the middle third for crack observations, deflection readings and possibly strain measurements is an important consideration, as is safety when failure occurs. The specimen is placed over the two steel rollers bearing leaving 150mm from the ends of the beam. The remaining 1000mm is divided into three equal parts of 333mm as shown in the figure. Load is applied by hydraulic jack of capacity 500kN. Lines are marked on the beam to be tested at L/3, L/2, & 2L/3 locations from the left support (L=1300mm), three dial gauges are used for recording the deflection of the beams. One dial gauge is placed just below the centre of the beam, i.e. at L/2 distance and the remaining two dial gauges are placed just below the point loads, i.e. at L/3 and 2L/3 to measure the deflections.

A. SPECIMEN DETAILS
Eight specimens were considered as simply supported RC Tbeams. Reinforcement used for all specimens was two numbers of 20mm φ and one number of 10mm φ.

B. CONTROL BEAM (CB)
This beam is not strengthened with FRP.

D. STRENGTHENED BEAM 2
Two layers of FRP on web portions on shear span at distance L/3 from both supports was used in this beam.

E. STRENGTHENED BEAM 3
Strengthened by applying two layers of GFRP U-strips on web portions and bottom on shear span at distance L/3 from both supports was used in this beam with three equal strips on both sides of the beam, each strip of size thickness as 50mm and the spacing between the strips is 50mm.

F. STRENGTHENED BEAM 4
The beam (SB4) was strengthened by applying two layers of FRP strips only on web portions on shear span at distance L/3 from both supports with three equal strips on both sides of the beam.
Model of T-beam with FRP -SB4

G. STRENGTHENED BEAM 5
The beam (SB5) was strengthened by applying two layers of FRP strips only onsides of web portions on shear span region (distance L/3 from both supports) with two equal strips on both sides of the beam which is inclined to 450 as shown in figure, each strip of size thickness as 50mm and the spacing between the strips is 50mm.

H. STRENGTHENED BEAM 6
The beam is modeled with two layers of FRP having U-wrap on bottom and web portions on the shear span (distance L/3 from both supports) of the beam. It is found that in most cases debonding happens between the glass-fiber and the concrete.
To reduce the debonding effect steel plates are used and tightened with bolts i.e., the anchorage system

I. STRENGTHENED BEAM 7
The beam is modelled with four layers of FRP having U-wrap on bottom and web portions on the shear span (at distance L/3 from both supports) of the beam. Here, also to reduce the debonding effect steel plates are used and tightened with bolts. • Wrapping schemes (U-wrap or fiber attached on the two web sides of the beam).

Model of T-beam with GFRP -SB7
• Presence of FRP end anchor.
• Concrete surface preparation and surface roughness. Vu is the total shear force applied at a given section due to the factored loads. The nominal shear strength of an FRP-strengthened concrete member can be determined by adding the contribution of the FRP reinforcing to the contributions from the reinforcing steel (stirrups, ties, or spirals) and the concrete. An additional reduction factor ψf is applied to the contribution of the FRP system.

ϕVn = ϕ (Vc + Vs + ψfVf)
It is suggested that an additional reduction factor ψf be applied to the shear contribution of the FRP reinforcement. For bondcritical shear reinforcement, an additional reduction factor of 0.85 (Completely wrapped members) is recommended. For contact-critical shear reinforcement, an additional reduction factor of 0.95 (Three-sided U-wraps or bonded face piles) is recommended in code ACI 440.2R-02. E. FRP system contribution to shear strength strengthening calculations for repair, retrofit, or strengthening using FRP laminates.
The contribution of the FRP system to shear strength of a member is based on the fiber orientation and an assumed crack pattern [Khalifa et al. 1998]. The shear strength provided by the FRP reinforcement can be determined by calculating the force resulting from the tensile stress in the FRP across the assumed crack.

F. Effective strain in FRP laminates
All possible failure modes should be considered and the effective strain should be used which is the representative of the critical failure mode. Maximum strain that can be achieved in the FRP system at the ultimate load stage is called as effective strain and it is governed by the failure mode of the FRP system and the strengthened reinforced concrete member. 1. FRP can improve the shear strength of RC T-beams.
2. As per test, beams strengthened with FRP shows initial cracks at higher load. 3. Strengthening of on the webs with FRP is most vulnerable to debonding with premature failure. 4. The beam strengthened with a U-wrap configuration is more effective than the side-wrap configuration. 5. The use of anchorage system eliminates the debonding of the FRP sheet, and consequently results in a better utilization of the full capacity of the FRP sheet. 6. Applying FRP to the beam with end anchorage is better than strengthening without end anchorage. 7. Strengthened beam has high load carrying capacity than the control beam. 8. T-beam strengthened with U-wrap has more shear strength than that of the beam without openings. 9. From this study we can recommend the use of FRP sheets as an external reinforcement in order to increase the shear strength of T-beams with anchorage system.