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. 2023 May 17;8(21):18830–18838. doi: 10.1021/acsomega.3c00988

Employing Carbon Fiber Reinforced Polymer Composites toward the Flexural Strengthening of Reinforced Concrete T-Beams

Mallikarjuna K †,*, P M Ravindra †, Archana D P †, Md Daniyal ‡, Abdullah Naser M Asiri §, Mohammad Amir Khan ∥, Saiful Islam §
PMCID: PMC10233840  PMID: 37273632

Abstract

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Reinforced concrete structures are exposed to various loads under critical environmental conditions, which might lead to the deterioration of the structures prior to their designed service life. Hence, to improve the serviceability of the reinforced concrete (RC) structures and meet the design requirements, the structures are strengthened using carbon fiber reinforced polymer (CFRP) composites. The application of CFRP is done using two major techniques, namely externally bonded (EB) reinforcement and near surface mounted (NSM) techniques. The purpose of this study was to examine and compare the behavior of RC T-beams that had been flexurally reinforced utilizing EB and NSM techniques. Six full-size RC T-beams were evaluated, including a reference beam and four beams that had been flexurally reinforced using laminates made of EB and NSM CFRP. The findings of the experimental tests reveal that, when using the same quantity of CFRP, the beams strengthened with NSM laminates outperformed those strengthened with EB laminates in terms of ultimate load.

1. Introduction

Due to hostile conditions, natural disasters, changes in applied loads, corrosion of reinforcement, and inadequate maintenance, older structures need maintenance and repairs. One of the most promising strategies for extending the life of the structures and providing the necessary reinforcement in recent years has been the use of fiber reinforced polymer (FRP) composites for strengthening and rehabilitation of reinforced concrete (RC) structures.1−3

Flexural and shear strengthening of RC structures via anchorage, externally bonded (EB), and near surface mounted (NSM) approaches using different FRP materials has been investigated by several researchers.4−6 The EBR technique is the simplest and most commonly employed strengthening technique which consists of application of carbon fiber reinforced polymer (CFRP) reinforcements to the exterior surface of RC structures with the help of an appropriate adhesive, whereas in the NSM technique the FRP strips are inserted within precut grooves made into the concrete cover in the tension zone of the RC member.7−9 The FRP strips or laminates are bonded to concrete using an adhesive like epoxy. Panahi et al.10 investigated the flexural strengthening efficiency of RC beams with a hybrid strengthening method such as externally bonded FRP sheets and near-surface mounted FRP rods, numerically. Finally, the performance of these beams was confirmed by comparison with the results of the beams strengthened by the externally bonded and near surface mounted technique, viz. finite element analysis.

The experimental behavior of reinforced and retrofitted RC beams employing NSM CFRP ropes under heat-damaged and unheated circumstances was studied by Murad et al.11 When the beams were reinforced with two layers of continuous CFRP ropes down the length of the beam, anchored at the tensile reinforcement level, the load capacity and stiffness of the unheated beams were increased by 60% and 78%, respectively. It also improved the heat-damaged beams’ load carrying capability by 47% while lowering their peak deflection. Do-Dai et al.12 studied the deformability and the flexural strengthening performance of carbon and basalt FRP sheets reinforced RC beams by varying parameters such as FRP types, reinforcement corrosion levels (0–30%), and numbers of FRP layers (0–4 layers). FRP-reinforced beams showed reduced crack widths by up to 89%/85%, and the flexural resistance was improved by up to 103%/41%. The strengthening effectiveness of the carbon/basalt FRP laminates in the corroded beams was increased by 14%/6% on average compared to the noncorroded beams. Chen et al.13 studied the flexural behavior of RC beams that were externally bonded with flax and jute FRP laminates. The test results showed that natural FRP achieved strengthening effects similar to those of carbon FRP while its cost efficiency was 20–40% higher.

Using external bonding and near surface mounting techniques, Abdallah et al.14 examined the effects of the CFRP shape, location, number of layers, and weight on the flexural performance of RC beams. The yield and ultimate load capacity of CFRP composites with external bonding might be increased by up to 59.1% and 49.8%, respectively. Salama et al.15 investigated the viability of side-bonded CFRP composite sheets supporting RC beams in flexure discovered load-deflection, failure mechanisms, and ductility. Lamanna et al.16 introduced a novel approach to enhance the flexure strength of RC T-beams. They affixed FRP strips to the RC beams using mechanical fasteners, leading to augmented moment capacity and ductility in the fortified beams. Chellapandian et al.17 scrutinized distinct flexure-strengthening techniques for RC beams via empirical and analytical means. The methods tested comprised NSM, EB with CFRP sheet, and hybrid FRP strengthening, which involved a blend of NSM CFRP laminate and EB CFRP confinement. Hybrid FRP strengthening significantly boosted the strength of the reinforced beams and offered superior energy absorption capability than other fortified beams.

Akbarzadeh et al.18 noted that continuous RC beams are prevalent in construction, but there is a dearth of research on how such beams behave when reinforced externally. Ashour et al.19 carried out an experiment on continuous RC beams with distinct steel and external CFRP reinforcement configurations designed for flexural failure. The failure modes observed in the experiment comprised laminate rupture, laminate separation, and the peeling failure of the concrete cover attached to the composite laminate. Attari et al.20 discovered that the twin-layer GFRP sheet was highly effective in strengthening beams, producing an increase in flexural capacity of up to 114%. Sen et al.21 employed a natural jute fiber textile reinforced composite system to reinforce RC beams in flexure and compared it with carbon and glass FRP systems. The study revealed that the full wrapping technique enhanced the ultimate flexural strength of jute, carbon, and glass FRP reinforced RC beams by 62.5%, 150%, and 125%, respectively. However, the use of Basalt FRP to strengthen beams is minimally documented in the literature.

Serbescu et al.22 investigated the use of BFRP U-jacket strips as external shear reinforcement for RC beams, efficiently delaying debonding failure at the plate end and reducing the brittleness of failure. Sim et al.23 applied externally bonded BFRP strips to the tension side of RC beams to increase the flexural load carrying capacity, with the yielding and ultimate strength of the beam specimen rising by up to 27%, depending on the number of layers used. Experimental comparisons between the flexural performance of eight RC cantilever beams reinforced with CFRP laminates utilizing the BB and side bonding procedures were made by Li et al.24 It was found that the SB and BB laminates have equivalent effects on the flexural stiffness of the examined beam specimens after the SB beams were mechanically attached with plates and anchor bolts. Furthermore, the SB laminates had an impact on the strengthened beams’ crack breadth and pattern, which significantly increased the precrack stage of the specimens of enhanced RC beams. The production and ultimate loads of the SB and BB specimens were also found to be comparable.

Hosen et al.25 conducted experimental and analytical investigations of the flexural behavior of six RC beam specimens reinforced with various side-near-surface-mounted (SNSM) steel and CFRP bar reinforcement ratios. The beams were put through a four-point bending test, and the outcomes were compared to a control beam specimen that hadn’t been strengthened. It was found that the SNSM approach considerably improved the flexural performance of the beams. Grace et al.26 conducted an experiment on five continuous beams and examined four different strengthening systems. Each beam was subjected to at least one loading cycle before failure. The results showed that the use of FRP laminates is effective in reducing deflections and increasing the load carrying capacity of continuous beams. Additionally, the experiment showed that FRP-strengthened beams have smaller and better distributed cracks.

In a subsequent study, Grace et al.27 investigated the experimental effectiveness of CFRP strips used for flexural reinforcement in the negative moment zone of a full-scale reinforced concrete beam. They observed that category I beams exhibited diagonal cracking with local debonding at the top of the beams, while category II beams failed through delamination at the interface of the CFRP strips and the concrete surface.

More recently, Grace et al.28 were also involved in another study that examined three continuous beams. A standard ductile flexural failure occurred in one of those beams because it lacked external reinforcement. The negative and positive moment zones of the other two beams were strengthened as a U-wrap around the top and bottom face on both sides. It was found that reinforced beams failed via tensile fabric rupture above the central support, then fabric rupture at midspan. El-Refaie et al.29 studied 11 RC two-span beams enhanced in flexure with externally bonded CFRP sheets. The beams were divided into two groups depending on the internal steel reinforcement arrangement. They found a maximum number of CFRP layers beyond which there was no further improvement in the beam capacity. They also looked into whether extending the length of the CFRP sheet to include the whole hogging or sagging zones would prevent peeling failure of the CFRP sheets.

El-Refaie et al.30 conducted a study on the flexural strengthening of five reinforced concrete continuous beams using external CFRP laminates. The beams had identical geometrical dimensions and internal steel reinforcement. The study focused on the position and shape of the CFRP laminates. It was observed that the beams strengthened with CFRP laminates in both the hogging and sagging zones exhibited the highest load capacity.

Previous studies demonstrated that the major focus and attention were concentrated on those EBR CFRP laminates on RC T-beams in order to evaluate the flexural strengthening performance. Meager work has been reported on the evaluation of the flexural strengthening of RC T-beams using CFRP laminates with NSM techniques. With reference to the information gathered from the literature, the current work focuses on the use of CFRP laminates on RC T beams for flexural strengthening using the near surface mounted (NSM) technique. The effectiveness of the NSM CFRP laminates for the flexural strengthening of RC T-beams was measured and compared with those RC beams strengthened using EB technique.

2. Experimentation Program

2.1. Materials and Test Specimens

Six RC T-beams were cast, instrumented, and tested as part of this study’s experimental program. All beams had a 350 mm overall height, a 100 mm flange thickness, a 150 mm web width, and a 350 mm flange width for their cross sections. The effective span of each beam was 1700 mm, with a total length of 2000 mm. The identical steel reinforcement was installed in all of the beams, consisting of two steel bars with a diameter of 12 mm for the bottom and six steel bars with a diameter of 8 mm for the top, four of which were installed at the top and two at the bottom of the flange. For shear reinforcement, double-legged 8 mm diameter stirrups with a spacing of 100 mm near the supports and 150 mm at the midspan of the beam were used. The thickness of the concrete cover was 20 mm at the bottom, upper, and lateral faces of the beam. The reinforcement details of the T-beams are shown in Figure 1(a), and Figure 1(b) represents the experimental setup.

Figure 1.

Figure 1

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(a) Cross-section details of T-beam. (b) Experimental test setup.

The employed CFRP laminates were 50 mm wide and 1.4 mm thick (Table 1). According to the manufacturer, the CFRP laminates (supplied by Fosroc Chemicals (India) Pvt. Ltd.) utilized have ultimate tensile strengths and elastic moduli of 2800 MPa and 170 GPa, respectively. According to the manufacturer, the glue was a two-part thixotropic epoxy resin called Nitowrap 40 base and hardener (supplied by Fosroc Chemicals (India) Pvt. Ltd.) with compressive strengths of 60 MPa and 6 GPa, respectively.

Table 1. Material Properties.

Material Dimensions (mm) Compressive Strength (MPa) Ultimate Tensile Strength (GPa) Modulus of Elasticity (GPa)
Concrete   22.66   23
Steel D = 12     200
Reinforcement D = 8     200
CFRP laminates t = 1.4   2800 170
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2.2. Strengthening Configurations

The strengthening configuration for the T-beams is represented in Figure 2. The control beams (CB) were tested without strengthening and therefore considered as control beams. Two beams were strengthened using EB CFRP laminates and the remaining two beams were strengthened with NSM CFRP laminates (Figure 3). The EB process has advantages such as high tensile strength, simplicity of application, light weight, and corrosion resistance. In terms of debonding, the NSM outperforms the EBR; nonetheless, delamination and debonding of FRP sheets from concrete are relatively likely in both systems and continue to be a restriction. The NSM technology may increase the load at the serviceability limit condition as well as the stiffness following concrete cracking significantly. Externally bonded reinforcement on grooves has recently showed significant promise in overcoming the debonding problem. For the EBR-B beams, a laminate 50 mm wide and 600 mm long was attached to the soffit of the T-beam in the pure-bending zone after the concrete surface was ground and cleaned, and a layer of primer was applied as shown in Figure 4. For the NSM-B beams, initially a groove having a dimension of 700 (L) × 60 (W) × 10 (H) mm was cut through the concrete cover of the beams using diamond blade cutter throughout the midspan and then cleaned to remove the dust from the groove. Following that, a primer coat was applied before the groove was filled with epoxy, and then the CFRP laminate 50 mm wide and 600 mm long was inserted into the groove with a slight pressure so that the epoxy fills the gaps between the laminate and the sides of the groove. The surface was then leveled, and the excess epoxy was removed. The application of CFRP laminate using the NSM technique is shown in Figure 5. All the strengthened beams were then allowed to cure at room temperature for a period of 7 days and then tested.

Figure 2.

Figure 2

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Strengthening configurations of T-beams.

Figure 3.

Figure 3

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(a, b) Reinforcement cage of T-beam in different views. (c) Compaction using vibrator. (d) Casted T-beam. (e) Demolded T-beam. (f) Front view of T-beam.

Figure 4.

Figure 4

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(a) Concrete surface preparation. (b) Mixing of base and hardener. (c) Application of primer coat followed by epoxy. (d) Placement of CFRP laminate.

Figure 5.

Figure 5

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(a) Cutting of groove through the cover. (b) Finished groove. (c) Filling of epoxy adhesive to the groove. (d) Placing the laminate above the epoxy layer. (e) Remaining portion of the groove filled with epoxy.

2.3. Instrumentation and Test Configuration

As illustrated in Figure 6, all of the T-beams were simply supported and tested using a two-point loading configuration until they failed. A hydraulic jack was used to apply the load through a steel spreader beam positioned on the rollers placed on the flange of T-beam. The test specimens were supported on the rollers excluding 150 mm from each end of beam. The effective span of 1700 mm is equally split into three sections of 567 mm. The midspan deflections of the T-beams were measured using three dial gauges. Two dial gauges were placed directly below the points of load application and one at the center of the beam. The load was applied gradually until the failure of beams. The corresponding deflections for each load increment were noted, and the spread of cracks was noted.

Figure 6.

Figure 6

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Laboratory setup.

3. Experimental Results and Discussion

In the laboratory of the Department of Civil Engineering, Bangalore Institute of Technology, RC T-beams were tested in a loading frame with a 50-ton capacity. The surfaces of the beams were left to dry after casting and curing for 28 days. At 1/3, 1/2, and 2/3 of the effective span from one end of the support, the beams were marked. All simply supported T-beams were put to the test with two point loads. The center region of this type of loading arrangement, also known as the pure bending region, has a relatively uniform moment distribution and negligible shearing. The loading configuration is depicted in Figure 6. The hydraulic jack is used to apply the load. The steel beam, which is positioned above the rollers on the flange of the beam, receives it after being transferred through the load cell. The test specimens were supported on the rollers, 150 mm from the beam’s ends being the furthest away. Three parts of 567 mm each make up the 1700 mm effective span. Three dial gauges were used to measure the deflections of the T-beams. One dial gauge was situated below the center of the beam, at a distance of 1/2 of the effective length from the support, and two dial gauges were positioned right below the point of load application, at distances of 1/3 and 2/3 of the effective length.

3.1. Load Carrying Capacity of Control and Strengthened T-Beams

The NSM approach’s performance and efficacy in enhancing the flexural strength of RC T-beams were compared with those of the EBR technique. The summary of the experimental test results of all the T-beams is represented in Table 2. The control beams CB-1 and CB-2 failed at loads of 158 and 156 kN by concrete crushing at the supports after the steel reinforcement. The first visible hairline crack was observed in the web of the T-beams at the midspan at loads of about 56 and 52 kN, respectively. On further increasing the load, the stiffness of the beam starts to reduce leading to the formation of new flexural cracks in the web of the T-beam. Beyond this stage, under a load of 104 kN cracks started appearing at the bottom of the flange, which on further increment of load extended to the sides of the flange reaching to the top of the flange. At the failure load the cracks existing in the web of the T-beam widened to a greater extent, which further extended to the flange of the beam indicating the failure of the web prior to the failure of flange. The midspan deflection at the failure load for CBs was 6.631 and 5.454 mm, respectively. The typical crack pattern of the control beams CB is as shown in Figure 7.

Table 2. Summary of Experimental Test Results.

Beam designation Pcra (kN) Pwa (kN) Pua (kN) Δua (mm) CFRPb strengthening effectiveness ratio (%) Failure Modec
CB-1 56 105.33 158 6.631   CC + SY
CB-2 52 104 156 5.454   CC + SY
EBR-B-1 70 120 180 8.063   ECS + EID
EBR-B-2 76 126.67 190 10.310 17.83 ECS + EID
NSM-B-1 54 126.67 190 13.017   EID
NSM-B-2 60 138.67 208 14.933 26.75 EID + ECS
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a

Pcr = cracking load, Pw = working load, Pu = ultimate load, Δu = ultimate deflection.

b

Strengthening effectiveness ratio = (failure load of beam – failure load of CB)/failure load of CB.

c

CC = concrete crushing, SY = steel yielding, ECS = end cover separation, EID = end interfacial debonding.

Figure 7.

Figure 7

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Crack pattern at failure for CB.

For beams EBR-B-1 and EBR-B-2 strengthened with EB CFRP laminates, the typical crack patterns are shown in Figures 8 and 9. The first vertical crack was observed in the web of the T-beams at the midspan at loads of about 70 and 76 kN, respectively. On further increase in the applied load, the existing web cracks continued to widen and propagate vertically upward toward the bottom of the flange, but the presence the CFRP laminate controlled the crack width and the crack propagation toward the flange in comparison with the control beams. The failure occurred at loads corresponding to 180 and 190 kN, respectively, by peeling of the concrete cover along the CFRP laminate at one or both ends of the laminate. The cover separation was initiated by flexural-shear cracks which were initiated at one or both ends of the laminate. It should be noted that EBR-B beams resulted in a 17.83% increase in the ultimate load in comparison with that of control beams. The midspan deflections at the failure load for EBR-B beams were 8.063 and 10.310 mm, respectively.

Figure 8.

Figure 8

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Crack pattern at failure for EBR-B-1.

Figure 9.

Figure 9

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Crack pattern at failure for EBR-B-2.

For beams NSM-B-1 and NSM-B-2 with NSM CFRP laminates, the typical crack patterns are shown in Figures 10 and 11. The first crack was observed in the web of the T-beams at the midspan at loads of about 54 and 60 kN, respectively. On further increasing the load their behavior was almost similar to that of EBR-B beams, where the NSM CFRP reinforcements arrested the existing cracks and the prevented them from propagating toward the flange. The NSM beams failed at loads 190 and 208 kN, respectively, and the corresponding midspan deflections were 13.017 and 14.933 mm, respectively. After severe concrete cracking at the midspan, debonding started at one end of the CFRP laminate, leading to ripping off the epoxy cover along with the laminate. The maximum ultimate load of NSM-B beams was 26.75% and 7.6% higher than that of the control beams CB and that of EBR-B beams. It should be noted the NSM-B beams were strengthened with the same amount of CFRP reinforcements as that of EBR-B beams.

Figure 10.

Figure 10

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Crack pattern at failure for NSM-B-1.

Figure 11.

Figure 11

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Crack pattern at failure for NSM-B-2.

3.2. Experimental Load-Deflection Behavior

The load-deflection curves for the control and EBR beams are displayed in Figure 12, and the load-deflection curves for the control and NSM beams are plotted in Figure 13. Up to a cracking stress, all reinforced beams behaved similarly to control beams. The load deflection relationship of beams reinforced with NSM CFRP laminates after cracking was nearly identical to that of beams reinforced with EBR CFRP laminates.

Figure 12.

Figure 12

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Load versus deflection curves of control and NSM-B beams.

Figure 13.

Figure 13

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Load versus deflection curves of control and EBR-B beams

The experimental load–displacement curves of the T-beams featured three crucial phases: up to concrete breaking, between concrete cracking and longitudinal steel reinforcement yielding, and between steel reinforcement yielding and ultimate load. All of the examined beams displayed linear elastic behavior in the first stage, prior to concrete cracking, and the EB and NSM laminates had only a little impact on the cracking load. Both EB and NSM laminates enhanced the beam’s stiffness and the yielding load during the second stage, which spanned the period from the concrete splitting up to initiation of the steel reinforcements. The second phase’s essentially linear slope suggests that this phase of cracking propagation was driven by the linear behavior of both steel bars and CFRP laminates. The control beams performed as predicted throughout the third phase, which was the interval between the beginning of the steel yielding and the breakdown of the beam. Due to the contribution of the CFRP, the third phase of strengthened beams behaved practically linearly, whereas the steel reinforcement was in a flexible state and concrete had severely fractured.

3.3. Failure Modes

The beams strengthened with EB CFRP laminates failed by end cover separation which is shown in Figure 14. In this failure mode, the debonding of the CFRP laminate starts from one end of the CFRP laminate and propagates to the midspan of the beam. Hence, high interfacial shear stress and high normal stress act simultaneously at the end of the laminate due to the abrupt termination of the CFRP laminate, which influences the cover separation failure. Due to these stresses, a flexure-shear crack initiated at one or both ends of the laminate propagates horizontally at the level of longitudinal reinforcement, leading to the peeling off of the cover along with the laminate. This type of failure may occur because of two factors: (1) the EB CFRP laminates’ strong bond with the surrounding concrete, which reduces the likelihood of interfacial debonding failure, and (2) the substantial radial stresses that the steel bars place on the concrete during their tensioning process. It should be noted that along with cover separation deboning also occurred at the adhesive–concrete interface.

Figure 14.

Figure 14

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End cover separation failure in EBR-B (a) at both ends and (b) at one end of laminate.

The failure of the NSM CFRP laminate-strengthened beams was caused by end-interfacial debonding at the laminate–adhesive interface, as illustrated in Figure 15. Due to the abrupt termination of the laminate, substantial interfacial shear and normal stresses emerge close to the end of the NSM CFRP laminate when the NSM T-beam is loaded. A flexural shear fracture and an inclined crack both developed in the bonded zone of the NSM CFRP as a result of the high interfacial stresses, as seen in Figure 16. These interfacial stresses increase with the applied load; hence, these cracks propagated horizontally at the level of the laminate–adhesive interface, finally leading to the debonding of laminate along with the epoxy cover. Also, in one of the NSM beams cover separation was initiated along with interfacial debonding at one end of the CFRP laminate as shown in Figure 16(b). This type of failure may arise from two possibilities, i.e., improper bond between the epoxy–concrete or epoxy–laminate and improper surface preparation for strengthening.

Figure 15.

Figure 15

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End interfacial debonding failure in NSM-B.

Figure 16.

Figure 16

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Cover separation: (a) EBR-B, (b) NSM-B.

4. Conclusions

The performance and efficacy of the NSM and EBR approaches for the flexural strengthening of RC T-beams were compared and studied in an experimental examination. Six T-beams were cast and put to the test as part of the experiment. The following findings were obtained from the study that was conducted:

  • (1)

    The ultimate load bearing capacity of NSM beams was found to be greater than that of EBR beams with the same amount of CFRP reinforcement, of around 7.6%. This is because, in the case of the NSM process, the CFRP laminates have a stronger bond.

  • (2)

    For the T-beams strengthened with EB CFRP and NSM CFRP laminates, the working and ultimate load increased by up to 17.83% and 26.75%, respectively, in reference to the control beams.

  • (3)

    Up to a cracking load, all reinforced T-beams behaved similarly to that of control beams. After cracking, the load deflection relationship between reinforced beams using NSM CFRP laminates and those using EB CFRP laminates was practically identical. The ductile behavior of NSM beams, however, was shown by the fact that their deflection at the failure load was larger than that of the EB and control beams, showing the ductile behavior before failure takes place.

  • (4)

    The beams strengthened with EB CFRP laminates failed by end cover separation along with the laminate, which occurred at one or both the ends of laminate, while the NSM beams failed by end interfacial debonding between the laminate and adhesive which occurred at one end of the laminate.

  • (5)

    Hence, both the EBR and NSM techniques using CFRP laminates are found to be effective in enhancing flexural.

Acknowledgments

The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University, Kingdom of Saudi Arabia, for funding this work through the Large Groups Research Project under Grant Number RGP2/165/44.

Data Availability Statement

The data used to support the findings of this study are included within the article.

The authors declare no competing financial interest.

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Data Availability Statement

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