Flexural behaviour of RCC beams with externally bonded FRP

The increasing use of carbon and glass fibre reinforced polymer (FRP) sheets for strengthening existing reinforced concrete beams has generated considerable interest in understanding the behavior of the FRP sheets when subjected to bending. The study on flexure includes various parameters like percentage of increase in strength of the member due to the externally bonded Fiber reinforced polymer, examining the crack patterns, reasons of debonding of the fibre from the structure, scaling, convenience of using the fibres, cost effectiveness etc. The present work aims to study experimentally about the reasons behind the failure due to flexure of an EB-FRP concrete beam by studying the various parameters. Deflection control may become as important as flexural strength for the design of FRPreinforced concrete structures. A numerical model is created using FEM software and the results are compared with that of the experiment.


Introduction
Fibre reinforced composites have been widely used to strengthen reinforced concrete (RC) members. Mostly because they have a high strength-to-weight ratio, require relatively limited time to cure, and have mechanical properties that can be engineered to meet the desired structural performance. Fibrereinforced polymer (FRP) composites are made up of continuous fibres and a thermosetting organic resin, are currently the most common type of composite system used for structural strengthening applications. Fibre reinforced cementitious matrix (FRCM) composite was another type of composite that was recently developed which contains continuous fibres with a cementitious (inorganic) matrix. The substantial increase in energy absorption capacity is the most significant improvement imparted by adding fibres to a concrete.
The present work aims to study experimentally about the reasons behind the failure due to flexure of an EB-FRP concrete beam by studying the various parameters stated above. Deflection control may become as important as flexural strength for the design of FRP-reinforced concrete structures.
The strengthening effect of EB-FRP using composite element involving glass fibre was studied [1] Use of glass fibre laminates in composite sections in external bonding is well established. Flexural response was better recorded using the epoxy adhesive. Use of carbon -glass hybrid was studied the stiffness contribution of the composite in bridge decks in rehabilitation [2]. The uniaxial tests under compression was performed based on deflection control condition, stress-strain behaviour for fiber concrete was studied [3]. The numerical model developed was referred the research done on the bending tests on FRP in slabs [4] Parameters varied here were thickness and the effect of changing the thickness is studied under the four point loading test [5]. An increase in the width of the FRP will produce an increase in the load-carrying capacity. Interfacial crack propagation and strain distribution during shear debonding is influenced by the width of the FRP laminate in comparison to that of the beam [6]. also studied [7]. Studies show that Strength increases provided by SRP bonded with cementitious grout were smaller than those obtained using epoxy. [8]. . Although use of cementitious matrix is in practice the transfer of stresses was found to be more efficient by use of epoxy strengthening adhesive [9] 2. Materials and methods

Materials
Ordinary Portland Cement (OPC) of grade 53 confirming to IS 12269 was utilized in the study. The specific gravity of the OPC sample was found to be 3.12 with an initial setting time of 40 minutes and a standard consistence of 28 % with its chemical composition is given in Table 1. FRP plates are noncapillary and non-hygroscopic. Therefore, they provide good moisture resistance. CFRP has a very high temperature resistance and is virtually inert. The grading of the aggregate was categorized using sieve analysis test, and the results are detailed in Table 2 and accordingly the aggregate was categorized as graded under Zone III. Aggregate passed through 16 mm sieve and retained on 12.5 mm sieve, were used as in the concrete mixture. The specific gravity of the coarse aggregate was 2.7. Wire basket method based on ASTM C 127 was used to determine the specific gravity. The steel bars of 12 and 8 mm high yield strength deformed bars were used as reinforcement in the cube specimens.

Methodology
High strength concrete of the grade M50 was designed in order to obtain a characteristic compressive strength of 50 MPa. The design mix ratio arrived was 1:0.8:2.13(Cement: FA: CA). The addition of high range water reducer reduced the w/c ratio to 0.32 and the workability was based on slump cone test according to ASTM C14. Beam specimens were cast and water cured for 28 days to study the flexural behavior.

Flexural Behaviour Test
The specimens cast were in the dimensions of 100 mm x 150 mm x 1200 mm beam prototypes. Curing was done for a total of 28 days. The test was carried out using 1000kN capacity flexural strength testing machine. The test setup includes two point loading using a single point loading system by which the loads are transferred equally to the two points using a spreader beam and two rollers. Deflections were measured in the beam by placing strain gauges at the bottom of the beam. Strains occurring at the points were also measured under using a LVDT strain gauge that was placed at particular intervals. The gauge length between the force points is 333.33 mm in the top and 100 mm from either corner of the beam at the supports. All the specimens were capped for uniform loading prior testing. The control of load over the test was 10 kN/min. All the data like displacement load and strain were recorded using Automatic data acquisition system which in turn connected to the computer.

First crack load
The first crack load and moments for beams cast with different EB-FRP beams as well as for the control beam are given in Table 4

Figure 4. First crack and Ultimate load for beams
The load deflection curves for beams are shown in figure 5. It could be noted that with each application of load, the deflection value changes or in this case increases from the control beam. This implies that the addition of FRP makes the concrete beam more ductile. However, it could also be noted that the deflection for the CC and C-G2 beams are the same. It can be observed that the energy absorption capacity increases significantly from the control beam. However, the value for the beam with CFRP content registers the highest value. The beam C-G2 registered a low value of energy absorption, this was due to the debonding occurred between the concrete and FRP. Also due to the brittle nature of the lesser thickness of carbon in the hybrid FRP laminate.

Load carrying capacity
The theoretical value for load carrying capacity has been arrived at by ACI method. From calculating the depth of the neutral axis, it could be identified that the section was under reinforced. Hence, the theoretical value was arrived at and presented in the Table below:   Table   4.2 Properties of the Elements: The material properties of concrete given as input for it ANSYS model are given in Table 7. The material properties of steel given as input for it ANSYS model are given in Table 8  The material properties of CFRP & GFRP given as input for it ANSYS model are given in Table 9.

Conclusion
External bonding of FRP laminates affects the resistance in terms of durability characteristics. From the results the beam that was fitted with C-G1 beam and GFRP beam has higher load deflection behavior in the experimental analysis and in the numerical analysis. Two different failure modes were observed depending upon the type of FRP laminate used. Rupture and debonding of plates were observed. The ultimate load of the specimen with FRP laminates has also increased in the case of all the beams when compared to the control beam. However, comparing the trends observed in first crack load, the ultimate load, the resilience and the energy absorption capacity, the corresponding values increased till the C-G1 and were very close to the values of the control beam in case of C-G2 specimen. CFRP being more brittle compared to the GFRP tends to rupture first in case of the composite laminates .Hence laminate composite of CFRP-1mm and GFRP -2mm is considered to have a optimum load carrying capacity and energy absorption capacity.