Strength Modeling of Reinforced Concrete Beam with Externally Bonded Fibre Reinforcement Polymer Reinforcement

This research study presents the evaluation of the structural behaviour of reinforced concrete beams with externally bonded Fibre Reinforced Polymer (FRP) reinforcements. Three different steel ratios with two different Glass Fibre Reinforced Polymer (GFRP) types and two different thicknesses in each type of GFRP were used. Totally fifteen rectangular beams of 3 m length were cast. Three rectangular beams were used as reference beam (Control Beams) and the remaining were fixed with GFRP laminates on the soffit of the rectangular beam. The variables considered for the study includes longitudinal steel ratio, type of GFRP laminates, thickness of GFRP laminates and composite ratios. Flexural test, using simple beam with two-point loading was adopted to study the performance of FRP plated beams interms flexural strength, deflection, ductility and was compared with the unplated beams. The test results show that the beams strengthened with GFRP laminates exhibit better performance. The flexural strength and ductility increase with increase in thickness of GFRP plate. The increase in first crack loads was up to 88.89% for 3 mm thick Woven Rovings GFRP plates and 100.00% for 5 mm WRGFRP plated beams and increase in ductility interms of energy and deflection was found to be 56.01 and 64.69% respectively with 5 mm thick GFRP plated beam. Strength models were developed for predicting the flexural strength (ultimate load, service load) and ductility of FRP beams. The strength model developed give prediction matching the measurements.


INTRODUCTION
Fibre Reinforced Polymer (FRP) composite materials have been successfully used in new construction and for rehabilitation of existing structures. FRP composite materials hold great promise for the future of construction industry. Strengthening of reinforced concrete and prestressed concrete structures may be required as a result of increase in service loads, change in usage pattern, structural degradation and defects in design or construction. Repair with externally bonded FRP reinforcement is a highly practical strengthening system, because of ease and speed of installation, efficiency of the structural repair and corrosion resistance of the materials. The application of FRP poses minimal modification to the geometry, aesthetics and utility of the structure. Several studies on the behavior of reinforced concrete beams strengthened with FRP composite sheets provided valuable information regarding the strength, deformation, ductility and long-term performance of the FRP strengthening systems. Installation of externally bonded upgradation systems using FRP is fast and less labour intensive. Tension delamination of concrete cover in midspan of FRP strengthened beams by combining Carbon Fibre Reinforced Polymer sheets (CFRP) and Glass Fibre Reinforced Polymer sheet (GFRP) sheets at midspan of a beam and wrapped on 3 sides of the beam continuously with unidirectional CFRP on the tension of the beams and bi-directional GFRP sheet [1,2] . The flexural strengthening of reinforced concrete continuous beams with different arrangements of internal steel bars and external CFRP laminates are used for estimating the flexural load capacity and the interface shear stresses between the adhesive and concrete at failure of beams [3] . The effectiveness of the epoxy bonding of CFRP sheets to the tension flange of steel-concrete composite girders was analyzed using an iterative numerical method [4] . The shear strength of GFRP reinforced concrete beams and slabs were verified by the shear design approach and limits were proposed by ACI committee 440H [5,6] . The strengthening of corrosion damaged reinforced concrete bridge girders beams were strengthened by externally  [7] . By increasing the flexural performance of RC beams strengthened with CFRP materials was studied [8] and the results indicated substantial improvements in strength.
Test plan: Experimental investigation was carried out on fifteen beam specimens having three steel ratios, wrap thicknesses and wrap materials. Manual readings were recorded directly into a spreadsheet program for easy processing. The specimens were tested under fourpoint bending. Sufficient data was obtained on the strength, deformation and failure characteristics of GFRP laminated as well as control beams. The details of the fifteen specimens prepared for experimental work are shown in Table 1.

MATERIALS AND METHODS
M20 grade concrete was used for casting the specimens. The designed mix proportion was 1:1.54: 3.01 with water cement ratio of 0.5. Fine aggregates passing through 4.75 mm IS sieve and coarse aggregates passing through 20 mm IS sieves were used for concreting. The compressive strength of cubes tested after 28 days was 23.54 MPa. Glass fibre types such as Chopped Strand Mat and Woven Rovings were used for this investigation. The properties of GFRP laminates and epoxy adhesives used for the investigation are shown in Table 2.

Preparation of specimens:
A tilting type drawn mixer was used for mixing fresh concrete. The cement, sand and coarse aggregate were placed inside the wet drawn and then dry mixed. Concrete was placed in three layers up to the top of rectangular beam and compacted. Curing was carried for a period of 28 days. The soffit of the rectangular beam was well cleaned with a wire brush and roughened with a surface-grinding machine. Two part epoxy adhesive consisting of epoxy resin and silica filler was used to paste the FRP laminates. The glue was spread over the pasting surface with the help of a spreads. A thickness of more than 2 mm was maintained throughout the length of the pasting area. The laminate was pasted by gently pressing it by hand from one end and solely moving toward the other end.
A nominal weight to keep in position was placed over the laminates. The oozing out compound was removed. The final thickness of the glue will be around 2 mm thick. The beam is left undisturbed for one week and then subjected to testing.
Test procedure: The beams were tested under twopoint loading after curing for 28 days at room temperature. Before testing, the beams were painted white and centre line of the beam in the longitudinal direction was marked to study the crack patterns during testing. All the beams were tested in a loading frame of 750 KN capacities and 100 mm bearing was given on both ends, resulting in a effective test span of 2800 mm as shown in Fig. 1. The deflections were measured at midspan and load points using mechanical dial gauges of 0.01 mm accuracy. The crack widths were measured using a crack defection microscope with a least count of 0.02 mm. The deflections and crack width were measured at different load levels until failure. The details of test setup are shown in Fig. 2. Table 3 and Fig. 3-10 show the test results of GFRP beam. The effect of steel ratio on any property was evaluated by comparing the performance levels of beams with steel ratio 0.603% and 0.905% against the beam with steel ratio of 0.419%. The effect of thickness of GFRP plate on performance parameters was measured by comparing the performance of plated beam with that of unplated beam having the same steel ratio. For ascertaining the effect of type of fibre on the performance of beams, comparison was made between CSM and WR plated beams of similar thickness plating. The radius of curvature values were deduced from mid-span deflection using a simple relationship between mid-span deflection and curvature at mid-span.  The data used for the regression analysis is shown in Table 3 and the regression equations are shown in Table 4.

Observations on the regression equations:
where κ is a constant with value of 6/2.8x10 6 ) 9 Ultimate deflection (