An experimental study on structural performance of reinforced concrete beams strengthened for combined flexure and shear by using NSM-GFRP strips

Near surface mounted fiber reinforced polymer (NSM-FRP) technique is the latest & most effective techniques used in strengthening the existing structural components, which requires strengthening against increased service loads, faulty design, environmental effects, etc. In the past, many researches on NSM technique have been carried out by using different cross-sectional shapes & dimensions of FRP materials to strengthen for shear & flexure. In the present experimental study, the behaviour of R.C. beams strengthened for combined flexure & shear studied. The beams of 150x150x700 mm size with M30 grade of concrete & Fe500 steel were considered. The control beams (unstrengthened) & strengthened beams using GFRP (Glass fibre reinforced polymer) strips were subjected to three point loading to study the behaviour of beams in combined shear & flexure. The main objectives were to assess the behaviour of strengthened beams with control beams & to study the influence of the orientation of shear strips. The beams were studied for the failure characteristics, yield & ultimate load carrying capacity, stress-strain behaviour, deflection, & crack patterns. From the test results, it’s been observed that, the load carrying capacity of the strengthened beams were increased, the improved in deflection, when compared to the control beams. The ultimate load carrying capacity of beams with shear strip orientation of 45°, 60° & 90° were increased by 50.3%, 39.3% & 25% respectively, compared to the control beams. This NSM strengthening technique can be advantageous & suitably adopted to enhance the load carrying capacity of the beams.

In the experimental study by Ummar Shariff et.al., [23], an enhancement of 30-50 % in the ultimate load carrying capacity was observed when GFRP strips were used for flexural strengthening.Also, there was a delay in the appearance of 1 st crack in NSM strengthened beam compared to unstrengthened beam.To increase the flexural capacity of R.C. beams using GFRP strips as a strengthening FRP material, Kusuma et al. [24] experimentally compared the B-NSM (Bottom NSM) & S-NSM techniques (Side NSM).Beam Load carrying capacity was increased by 87.5 % in beams with B-NSM, 62.5 % in beams with S-NSM where the strips are oriented horizontally & 81.25 % in beams with S-NSM where the strips are placed vertically.As per the results the B-NSM is effective when compared to S-NSM.The load carrying capability of R.C. beams strengthened for only shear using NSM-GFRP strips was experimentally planned and studied by M Rashmi et al. [25] by orienting the shear strips at 45°& 90°with varied strip spacing's.The load carrying capability of the beams strengthened with GFRP strips increased significantly from 48 to 93.5 % as compared to the control beam.T Rashmi et, al [26] compared the strength between the control beam & the beam strengthened for shear using NSM-GFRP strips by orienting the strips at an angle of 60°& 75°with different spacing.According to the investigations done by M Rashmi et al. [25] & T Rashmi et, al [26] , the load carrying capacity of the strengthened beams has significantly increased as the angle of strips orientation with respect to the horizontal has decreased & as the strips spacing reduced.The R.C. beams when strengthened for shear or flexure, there is an enhancement in the ultimate load carrying capacity.In the current work, an attempt is made to strengthen the R.C. beams against both flexure & shear, which is a feasible scenario where three of the beams' faces are accessible for strengthening.To 3 determine the influence of the NSM strips orientation angle for the shear strengthening, the orientation of the shear strips is varied.

2
Present study involves the use of GFRP strips for improving the structural performance of reinforced concrete beams by adopting NSM technique.The NSM approach entails cutting a groove in the concrete cover of the beams, followed by filling the groove using an adhesive & GFRP.Twelve beams of size 150x150x700 mm, with M30 grade concrete & Fe500 steel were grouped into two groups shown in Table 1.Group A comprises of three beams that were not reinforced using the NSM approach & were known as the control beams (CB) or the reference beams (Figure 1

Material Propertie 2.1 s
The R. respectively.As per the literature review, the GFRP spacing incorporated in the experimental investigation was set to ensure the spacing between grooves was at least twice the width of the strips & that the spacing between the GFRP shear reinforcement is not exceeding L net [28].The spacing followed in the present study is shown in Table 1.
Where, L net is net length of GFRP strips, L b is actual length of GFRP strips, C is the clear cover of beam & α is orientation of NSM-GFRP.

Spacing between groove & the beam edg 2.3 e
A minimum of 20mm distance should be maintained between the edge of the support & the NSM-GFRP in order to prevent splitting failure of the concrete cover.Cracks were not observed at the edges of the members strengthened using NSM technique when a minimum edge distance of 30 mm was maintained [25].Taking this into account, an edge distance of 30 mm was followed in the present study.Strengthened beam(B45) .

3.1.3
The 1 st crack appeared & was initiated at a load of 50kN in all the strengthened beam samples in the tension zone.The cracks started to propagate towards the neutral (towards centre) axis of the beam a

3.2.2
The first crack in CB was observed at 40 kN load, whereas in the beams strengthened for combined flexure & shear with shear strips oriented at an angle of 60°the first crack started to propagate at a load 1.3 times higher than that of CB, i.e., at 50 kN.Also, the service & ultimate loads of the strengthened beams were increased by 42.

3.2.3
The first crack in CB was observed at 40 kN load, whereas in the beams strengthened for combined flexure & shear with shear strips oriented at an angle of 90°showed the 1 st crack at a load 1.1 times higher than that of CB, i.e., at 45 kN.Also, the service & the ultimate loads of the strengthened beams were increased by 40.

Effect of different angle of orientations of shear strip 3.3 s
The appearance of the 1 st crack was delayed & the load carrying capacities of the beams were enhanced when the angle of orientation of the shear strips with respect to vertical was decreased.The first crack in the beams with 60°& 45°shear strip orientation appeared at a load 1.1 times more than the load on beams with 90°shear strip orientation.The service load was increased from 80.68 kN to

Stress strain relationshi 3.4 p
The first crack in the CB was observed at 40 kN load which is taken as the reference.The shear stress & flexural stress values corresponding to 40 kN loads were 1.33 N/mm 2 & 10.67 N/mm 2 .The strain values are tabulated in Table 3.It can be observed that as the angle of orientation of strip is varied from 90 to 45 degree, the strain in the member is reduced, along with reduction in the lateral deformation.Compared to the group A with group B beams has exhibited a better result in terms of lateral deformations.• When the strengthened beams were compared to the control beams, it was found that the 1 st crack load, service load, ultimate load, & deflection characteristics were enhanced.
• Comparing all of the strengthened beams to the control beams, a delay in the onset of the 1 st crack was observed.
• Compared to the control beams, all of the strengthened beams exhibited lower deflection values.
Particularly, the observed deflection values of B45 beams were lower than those of B60 beams & the observed deflection values of B60 beams were lower than those of B90 beams.
• When compared to 90°NSM strip orientation of shear strips, the beams with 60°NSM orientation has shown the better results, similarly the beams with 45°NSM orientation has shown better results compared to 60°.It can be inferred that as the orientation angle is reduced to 45°from 90°t he load carrying capacities of the beams is enhanced.
• The service load carrying of the beams was increased by 44.7% in B45, 42.4% in B60 & 40.3% in B90 beams when compared to reference beams (CB).Similarly, the ultimate load carrying of the beams was increased by 50.3% in B45, 39.3% in B60 & 25% in B90 beams compared to control beam.
• The ultimate load carrying capacity of the beams was increased by 20.2 & 11.4 % when the NSM orientation angle of shear strips was changed from 90° to 45° & 60° with respect to the horizontal.
• The strain values of the strengthened beams at the tension zone, zone near the neutral axis & compression zone were less compared to the control beams.
). Group B composed of nine beams that were reinforced against combined flexure & shear by utilising the NSM-GFRP technology.For all the group B beams, same amount of shear strips & flexure strips were employed as reinforcement.The size of GFRP strips used for flexural & shear strengthening is 10 mm wide & 2 mm thick.At the soffit of the beams, one flexural strip of 700x10x2mm (long, x wide, x thick) is provided.Twelve shear strips, six each on one side face of the beams were provided to account for shear strengthening as shown Figure 2. The influences of three different angles of orientations of shear strips were studied Experimental programme .

Figure 5 : 7 Control 2
Figure 5: Three-point loading test set up

Figure 9 :
(a) crack pattern on the side face of B90, (b) Debonding failure at the concrete and epoxy interface, (c) Crack pattern of B90 at the soffit, (d) Failure of GFRP strips at the soffit 3.2 Comparison of the control beam & strengthened beams 3.2.1The 1 st crack in CB was observed at 40 kN load, whereas in the beams strengthened for combined flexure & shear with 45°orientation of shear strip, the first crack started to propagate at a load 1.3 times more than that of CB, i.e., at 50 kN.Also, the service & ultimate loads carrying of th Control beam & B45 e 1282 (2023) 012006 IOP Publishing doi:10.1088/1757-899X/1282/1/01200610 strengthened beams were increased by 44.7 % & 50.3 % respectively.Since the dial gauges were removed at a load of 75kN in the CB, the deflection at 75 kN is taken as the reference.The deflection of 3.13mm & 1.68mm were observed at the mid span of the CB & B45 respectively.Similarly, the deflection of 1.61mm & 0.97mm were observed at the quarter span of the CB & B45 respectively.Compared to CB, the deflection values of the B45 beams were lower at the mid & quarter spans because of the introduction of GFRP strips as the strengthening material increased the stiffness of the beam.
4 % & 39.3 % respectively.The deflection values at 75 kN load observed at the mid span of the CB & B60 were 3.13mm & 1.9mm respectively & the deflection of 1.61mm & 0.98mm were observed at the quarter span of the CB & B60 respectively.The deflection values of the B60 beams were less at mid & quarter spans due to the increase in stiffness of the beam due to the introduction of GFRP strips as the strengthening material Control beam & B60 .
3 % & 25 % respectively.The deflection of 3.13mm & 2.04mm were observed at mid span of the CB & B90 respectively & the deflection of 1.61mm & 1.18 mm were observed at the quarter span of the CB & B90 respectively for 75 kN load.The deflection values of the B90 beams were less at mid & quarter spans due to the increase in stiffness of the beam Control beam & B90 .