The Effect of Bonded Length of NSM-CFRP Sheets on The Performance of Shear-Strengthened RC Beams
Ahmad Amin Hakim Bin Azhar 18001039
Dissertation submitted in partial fulfilment of the requirements for the
Bachelor of Engineering (Hons) (Civil Engineering)
Universiti Teknologi PETRONAS Bandar Seri Iskandar
Perak Darul Ridzuan
TABLE OF CONTENT
CERTIFICATION OF APPROVAL………. i.
CERTIFICATION OF ORIGINALITY……… ii.
LIST OF FIGURES………. iv.
LIST OF TABLES………... v.
CHAPTER 1: INTRODUCTION………... 1
1.1 Background of Study………... 1
1.2 Problem Statement………... 3
1.3 Objective………... 4
1.4 Scope of Study……….. 4
CHAPTER 2: LITERATURE REVIEW………... 5
2.1 Shear Strengthening of RC Beam by NSM-FRP…….. 5
2.2 The Effectiveness of CFRP Composites by NSM Technique in RC Beam Strengthening………... 6
2.3 The Effectiveness of 45° Inclination Angle for NSM- FRP……… 8
2.4 The Effect of Groove Size for NSM-FRP……… 9
2.5 The Difference of Epoxy and Cement Based Adhesive for NSM-FRP……… 10
CHAPTER 3: METHODOLOGY………...….. 11
3.1 Flow Chart………. 11
3.2 Properties of RC Beam, CFRP Sheet, and Epoxy Resin……….. 12
3.3 Concrete Mix Design………... 13
3.4 RC Beam Design………... 16
3.5 NSM-FRP Groove Design………... 18
3.6 RC Beam Casting……….. 19
3.7 Retrofitting of Test Beams by NSM-FRP………. 20
3.8 Testing by Four-Point-Loading………... 22
CHAPTER 4: RESULTS AND DISCUSSION……… 24 4.1 Cube Test Result………... 24 4.2 Ultimate Load and Maximum Mid-Span Deflection… 26
4.3 Failure Modes……… 31
CHAPTER 5: CONCLUSION AND RECOMMENDATION…………... 35 REFERENCES………. 36 APPENDICES………... 39
CERTIFICATION OF APPROVAL
The Effect of Bonded Length of NSM-CFRP Sheets on The Performance of Shear-Strengthened RC Beams
Ahmad Amin Hakim Bin Azhar 18001039
A project dissertation submitted to the Civil Engineering Programme Universiti Teknologi PETRONAS in partial fulfilment of the requirement for the
Bachelor of Engineering (Hons) (Civil Engineering)
Ir. Dr. Mohamed Mubarak B. Abdul Wahab
UNIVERSITI TEKNOLOGI PETRONAS TRONOH, PERAK
CERTIFICATION OF ORIGINALITY
This is to certify that I am responsible for the work submitted in this project, that the original work is my own except as specified in the references and acknowledgements, and that the original work contained herein have not been undertaken or done by unspecified sources or persons.
Ahmad Amin Hakim Bin Azhar
Undertaking this Bachelor of Engineering (Hons) in Civil Engineering has been a truly life-changing experience for me and it would not have been possible to do without the support and guidance that I received from many people.
First and foremost, I would like to express my highest gratitude to my supervisor Ir. Dr. Mohamed Mubarak B. Abdul Wahab for all the guidance, support, encouragement, and constant feedback in completing this experimental research work throughout the 7-months journey. Without his vital commitment in supervising this research work, the objectives of this work might not have been achieved. Not just as a supervisor in this research, but as a lecturer and mentor throughout the years of study in UTP, he had taught me more than I could ever give him credit for here.
This work would not have been possible without the facilities, tools and equipment provided as well as grant allocated by Universiti Teknologi Petronas for the whole experimental research project. I am also indebted to Dr. Ehsan Nikbakht for allowing me to utilize his carbon fibre reinforced polymer (CFRP) sheets stored in the laboratory.
I would also like to thank Prof. Nasir Shafiq and Ms. Siti Nooriza for the valuable insights that are full of knowledge in the design of RC beam and concrete mix.
Many thanks to the lead technologists, Mr. Johan Ariff Mohamed, Ms. Raja Intan Shafinaz Raja Mohd Noor, and Ms. Suhaila Meor Hussin for the assistance in utilizing the tools and equipment for the experimental works in the laboratory.
To conclude, I cannot forget to thank my family and friends for all the unconditional support in this very intense academic year. I am grateful to have a very helpful circle of friends among the Civil Engineering department especially Tuan Amirul, Fatih Sinan, Shazriel Hakimi, and Zulfaqar for the assistance that was needed in the laboratory works. I wish to thank my lovely wife, Afika for always being supportive and encouraging in whatever I pursue.
LIST OF FIGURES
FIGURE 2.1: Shear strut angle of beam 9
FIGURE 3.1: Flow chart of the experimental research’s methodology 11
FIGURE 3.2: SikaWrap-300C and Sikadur-330 12
FIGURE 3.3: Flow chart of mix design procedure 13
FIGURE 3.4: Design Mix 1 14
FIGURE 3.5: Design Mix 2 15
FIGURE 3.6: Detailing of RC beam 17
FIGURE 3.7: Minimum dimensions of grooves 18
FIGURE 3.8: Actual casting of RC beams and cubes 19 FIGURE 3.9: Making of manually made CFRP rectangular bars from
CFRP sheets 20
FIGURE 3.10: NSM-FRP configuration with 45° inclination angle 21
FIGURE 3.11: NSM-CFRP strengthening process 21
FIGURE 3.12: Illustrative sketch of the test setup 22 FIGURE 3.13: Actual testing by four-point loading using UTM-200kN
FIGURE 4.1: Load-deflection curves of all test specimens 28
FIGURE 4.2: Typical stress-strain curve 29
FIGURE 4.3: Typical load-deflection curve 29
FIGURE 4.4: Failure mode of CB 30
FIGURE 4.5: Failure mode of TB-1-0 31
FIGURE 4.6: Failure mode of TB-2-10 31
FIGURE 4.7: Failure mode of TB-3-30 32
FIGURE 4.8: Failure mode of TB-4-50 33
FIGURE 4.9: Failure mode of TB-5-70 33
LIST OF TABLES
TABLE 3.1: Details of all test specimens 17
TABLE 3.2: Details of each test specimens with variety of NSM-CFRP
TABLE 4.1: Cube test results for design mix 1 and 2 (wet cured for 7
TABLE 4.2: Cube test result for test specimens (wet cured for 28 days) 25 TABLE 4.3: Ultimate loads and maximum mid-span deflections of
The NSM-FRP approach involves cutting grooves into the surface of the beam and filling them with a suitable adhesive, such as epoxy paste or cement grout. The grooves are then filled with FRP materials having circular, square, or rectangular cross- sections. Variety of parameters towards the contribution of FRP materials’ amount such as the length, thickness, and width play a key role in the amount of resources needed such as material, manpower, cost, time, and tools in order to achieve the best strengthening effect. It is notable that the length of the FRP material bonded to the concrete substrate’s sides has an impact on the shear strengthening of RC beams with NSM-FRP technique. However, these two research studies contradict with each other where one concluded that the shear strengthening effectiveness increases with shorter bonded length, but the other one concluded the opposite. Therefore, the main focus of this experimental study is to test the effect of varied NSM-FRP bonded length for the shear strengthening of RC beams by using manually made CFRP rectangular bar, created in the lab by physically folding CFRP sheets over itself. The load strengthening and ductility improving ratio as well as the failure modes of shear- deficient RC beams that have been strengthened with the adopted NSM-FRP technique were also discussed in this study. The experimental study includes the work of concrete mix design, RC beam design as per Eurocode 2, NSM-FRP groove design as per ACI 440.2 R-08, RC beam casting, retrofitting in shear of RC beam by NSM-FRP, testing by four-point-loading, and results interpretation. From the results, it was found that the ultimate loads and maximum mid-span deflections of all the strengthened specimens increased as the bonded length of NSM-CFRP sheets increased. The failure modes of all the strengthened and un-strengthened shear-deficient beams were discussed in terms of the shear and flexural cracks formation and propagation. The load-deflection curves were also interpreted where failures in elastic or plastic region were experienced by the specimens.
CHAPTER 1 INTRODUCTION 1.1 Background of Study
Concrete is a common building material for reinforced-concrete (RC) structures because of its high durability, fire resistance, and weather resistance. Although it is anticipated that RC structures will deteriorate over time due to exposure to several natural effects like high temperatures, humidity, chemical attacks, and additional loadings, this is in contrast to the massive increase in RC structure construction and the increase in their lifespans. Furthermore, human-made mistakes like poor design, poor maintenance, increases in live loads brought on by the change in facility type, conflicts, or terrorist attacks could cause the RC members in the structures to collapse.
Due to the high costs, protracted construction times, and physical demands, damaged buildings are difficult to tear down and reconstruct. In order to ensure that the necessary strength is achieved and the service life of a structure is subsequently extended, the strengthening and rehabilitation process was introduced using a variety of techniques. Furthermore, one of the most effective ways to adhere to the most recent design rules and meet the strength criteria is to strengthen the damaged RC slabs, beams, columns, and bridges. Therefore, the overarching objectives of strengthening techniques are to improve the flexural or shear capacity, ductility behaviour, and durability under various loading circumstances, thereby enhancing the behaviour of the structure and extending its service life. Experts are already developing cutting-edge solutions for the strengthening and retrofitting of RC structures as they receive more and more attention.
Since the 1990s, fibre reinforced polymer (FRP) materials have been utilised for structural retrofitting. The construction industry has extensively accepted the use of FRP as a strengthening material for reinforced concrete buildings, typically used for beams and columns, despite the fact that they are known to be fairly expensive.
However, due to higher production of FRP materials around the globe for structural strengthening material, the cost of these materials is believed to be continuously reduced. When compared to traditional strengthening materials like steel, wood, and
(2019), the increasing popularity of FRP composites application for RC structure strengthening is due to its’ feature of high strength-to-weight ratio, longitudinal tensile strength, durability (non-corrosiveness) and stiffness; high resistance to chemical attack, insect and fungal; low thermal transmissibility; fast installation due to simple handling, less manpower needed, reduced mechanical fixing; lower cost for the installation and maintenance; and infinite size, shape, and dimension availability.
The most popular method for strengthening reinforced concrete (RC) beams is externally bonded reinforcement (EBR) strengthening. However, due to the low bonding area to cross-sectional area of the FRP element ratio, the approach experiences debonding failure from the concrete substrate. As a result of being open to the surrounding by using EBR technique, the FRP composite is vulnerable to the vandalism, physical damage, and risk of fire. Due to these circumstances, the EBR technique cannot fully enhance the strength of FRP since the entire potential of the FRP reinforcements cannot be utilised. Due to the drawbacks of the EBR-FRP technique, researchers have chosen to use a different method of strengthening to give the retrofitted concrete beams better performance in challenging environments.
Near surface mounted (NSM) technique was determined to be the most suitable solution due to it being embedded and protected by the concrete cover, as well as greater bonded-area to cross-sectional area of the FRP element ratio (Kadhim et. al., 2021). The NSM-FRP technique involves cutting grooves into the surface of the beam and filling them with a suitable adhesive, such as epoxy paste or cement grout. The grooves are then filled with FRP materials having circular, square, or rectangular cross-sections. To this date, the technique of NSM-FRP for shear strengthening of RC beam utilizes different materials of FRP composite. Carbon fibre reinforced polymer (CFRP) in the form of laminates, bars, rods, and plates are the most commonly used type of FRP composite for NSM strengthening technique. CFRP composite in the form of sheet is widely used for the strengthening of RC beam by externally bonded technique.
1.2 Problem Statement
With the embedding of FRP materials into the concrete substrate’s surfaces which is known as the near surface mounted (NSM) technique, the amount of required FRP material to be embedded is crucial in assuring the most effective strengthening effect as possible. Variety of parameters towards the contribution of FRP materials’
amount such as the length, thickness, and width play a key role in the amount of resources needed such as material, manpower, cost, time, and tools in order to achieve the best strengthening effect.
According to Al-Zu’bi et. al. (2021) and Al-Rjoub et. al. (2019), it is notable that the length of the FRP material bonded to the concrete substrate’s sides has an impact on the shear strengthening of RC beams with NSM-FRP technique. Both the research studies were conducted by using CFRP laminates/strips. However, these two research studies contradict with each other since the study by Al-Zu’bi et. al. (2021) concluded that the shear strengthening effectiveness increases with shorter bonded length, but the study by Al-Rjoub et. al. (2019) concluded the opposite. Therefore, the main focus of this experimental study is to test the effect of varied NSM-FRP bonded length for the shear strengthening of RC beams by using manually made CFRP rectangular bar, created in the lab by physically folding CFRP sheets over itself. The reason behind not using CFRP laminates/strips was due to material availability.
The efficacy of the manually made CFRP rectangular bar was proven through the research study conducted recently by Abdelmohaymen and Salem (2022). The folded CFRP sheets were embedded vertically into the NSM grooves at all four sides of the RC beam test specimens (acting as post-installed stirrups) with different test parameters such as number of NSM-FRP stirrups, FRP cross-sectional area, and development length (overlap length of strips). It was found that the test beams demonstrate improvements in shear load-carrying capacity with the increasing of all three test parameters.
The goal of this experimental research study is to:
i. Investigate the effect of varied NSM-FRP bonded length for the shear strengthening of RC beam by using manually made CFRP rectangular bar, created in the lab by physically folding CFRP sheets over itself.
ii. To look into the load strengthening and ductility improving ratio as well as the failure modes of shear-deficient RC beams that have been strengthened with the adopted NSM-FRP technique
1.4 Scope of Study
The experimental study of the effect of varied NSM-FRP bonded length for the shear strengthening of RC beam by using manually made CFRP rectangular bar, created in the lab by physically folding CFRP sheets over itself includes the work of concrete mix design, RC beam design as per Eurocode 2, NSM-FRP groove design as per ACI 440.2 R-08, RC beam casting, retrofitting in shear of RC beam by NSM-FRP, testing by four-point-loading, and results interpreting.
LITERATURE REVIEW 2.1 Shear Strengthening for RC Beam by NSM-FRP
According to numerous research findings, NSM-FRP strengthened reinforced concrete beams is proven to be improving the shear performance of RC beams. There are variety of NSM-FRP composites where different FRP types like rod, bar, plate, laminate and different FRP materials like carbon, glass and basalt were used for the shear strengthening of RC beams. For example, Ramezanpour et. al. (2018) conducted an experimental research study on shear strengthening of RC beam with NSM-GFRP bars. The beams were designed to ensure dominant failure mechanism of shear failure where the shear span of each beam in one side was left without any shear reinforcement. The rest of the beam specimens was detailed with 8 mm diameter rebars spaced at 120 mm to resist all shear forces induced during the experiment. It was reported that NSM-GFRP bars increases the shear capacity from 31% to 66%. Ibrahim et. al. (2020) conducted an experimental research study shear strengthening of shear deficient RC rectangular deep beam where hybrid glass-carbon NSM-FRP strips were used as the strengthening material. It was reported that the hybrid glass-carbonNSM- FRP increased the deep beam shear strength up to 55.8%. Another experimental research study on the shear strengthening of RC beam was conducted by Al Rjoub et.
al. (2019) using NSM-CFRP strips. The beams were fabricated with varying side concrete cover depths, various strip lengths, and two distinct CFRP strip spacing values at both vertical and inclined (45°) angles. NSM-CFRP strips reportedly enhanced the RC beam's shear capacity by up to 176 % and, in many instances, switched the failure mode from shear to bearing or flexure.
2.2 The Effectiveness of CFRP Composites by NSM Technique in RC Beam Strengthening
Numerous studies have looked into the NSM-CFRP technique for strengthening RC beams. The idea of using carbon fiber-reinforced polymer (CFRP) to increase the shear load-carrying capacity of beams was investigated by Rizzo and Lorenzis (2009).
For comparative purposes, one specimen was reinforced with EB laminates. Utilizing NSM-CFRP bars, NSM-CFRP strips, and EB-CFRP laminates increased the shear strength of the beams by 44%, 41%, and 16%, respectively, as compared to the un- strengthened reference beam. Further investigation by Islam (2009) revealed that the NSM technique used to CFRP bars increased the shear strength of RC thin beams by 17 to 25%.
Recent research by Abdel-Jaber et al. (2021) looked at the impact of utilising NSM-CFRP on the shear behaviour of rectangular RC beams with various compressive strengths through both experimental and analytical means (low, medium, and high). The NSM-CFRP technique was used to strengthen a total of 12 simply- supported RC beams in various configurations, and the reinforced beams were then tested under monotonic three-point loading until failure. The test findings showed that utilising NSM-CFRP strips as a reinforcing method increased the beams' shear capacity by 4% to 66%. Additionally, the analysis found that all beams' shear capacities increase as compressive strengths are also increased. Instead of rupture or de-bonding in the CFRP strips, the failure mode was limited to pure-shear failure in all beams. Furthermore, the experimental findings showed that the results of the finite element analysis and the ACI 440.2R-17 standards were in good conformity.
Khalifa (2016) used experimental research to examine the flexural performance of reinforced concrete beams strengthened with CFRP strips using NSM and EBR procedures. The distribution of two and four CFRP strip numbers in one or two grooves was also investigated. According to the study, when NSM-CFRP strips were distributed in two grooves as opposed to one groove, the ultimate load increased. More notably, the NSM approach demonstrated a better improvement in the flexural capacity than the EBR, ranging from 12% to 18%. Increasing the strip number does not always imply an improvement in flexural strength.
Similar to this, Sharaky et al. (2015) investigated NSM-CFRP-strengthened RC rectangular beams with partially and fully bonded lengths. Different FRP forms were used, and the effects of the FRP properties and reinforcing bond length were examined.
The test findings demonstrated that fully-bonded NSM-FRP strengthened beams had lower deflection, larger bearing capacity and stiffness than partially-bonded NSM- FRP reinforced beams.
RC beams strengthened using NSM CFRP rods experienced a slight loss in ductility, according to Badawi and Soudki (2009). Additionally, they discovered that the ductility of the NSM CFRP strengthened beams reduced as the prestressing level of RC beams increased. The NSM technique may have little impact on the eventual elongation of RC components once a conventional failure mechanism is identified (Al- Mahmoud et. al., 2009). Peeling or pulling out of the CFRP rod, as observed by Al Mahmoud et al. (2012), De Lorenzis and Nanni (2002), and De Lorenzis et al. (2000), can result in a more brittle and less ductile state in non-classical failures, according to reports from Al Mahmoud et al. (2009), De Lorenzis et al. (2002), and Radfar et al.
On small-scale concrete beams and slabs, Mohamed (2002) investigated the performance of several NSM FRP reinforcing bars and strips, as well as externally bonded FRP sheets, and included a cost analysis for each of the FRP strengthening techniques. According to test results, using NSM CFRP reinforcing bars boosted strength by 36%. Employing NSM CFRP strips increased strength by 43% due to strip peeling failure, compared to merely 11% when using the axial stiffness used as externally bonded strips. According to Mohamed, the effectiveness of using FRP reinforcing bars as NSM reinforcement depends on their bonding properties as well as the bond between the epoxy adhesive material and the surrounding concrete in the groove.
2.3 The Effectiveness of 45° Inclination Angle for NSM-FRP
Numerous studies have examined the impact of inclination angle for NSM-FRP configuration and it was determined that NSM-FRP inclined at 45° with the beam axis is more effective than the vertically positioned angle (at 90°) in terms of shear capacity and stiffness of the strengthened beams. Al-Zu’bi et. al. (2021) and Al-Rjoub et. al.
(2019) conducted a similar experimental study on shear strengthening of RC beam with 90° and 45° inclination angle of NSM-CFRP laminate with variation of bonded length. It was found that regardless of the NSM-CFRP laminate bonded length, 45°
inclined laminates are more effective in the shear strengthening compared to vertically positioned laminates.
Abdel-Kareem et. al. (2019) and Ramezanpour et. al. (2018) also conducted a similar experimental study on shear strengthening of RC beam with 90° and 45°
inclination angle of NSM-GFRP composites like rod and strip. It was found that regardless of the GFRP composites, 45° inclined NSM-GFRP composite resulted in a higher effectiveness of shear strengthening than 90° positioned composite. Another experimental study was conducted by Fawzy (2018) on the shear strengthening of RC beam with 90° and 45° inclination angle NSM-CFRP bars. It was found that beam strengthened with 45° inclined NSM-CFRP bars gives the best strengthening results when compared to the control beam which is 35% increment than the 90° positioned NSM-CFRP bars.
The higher effectiveness of RC beam strengthened with 45° inclined NSM-FRP composite compared to the 90° inclined angle is due to it being approximately perpendicular to the shear crack direction which is aligned with the maximum shear stress. Theoretically, it is known that the shear strut angle of a beam is inclined throughout the beam longitudinal axis. Depending on how strong the shear force is being applied, the angle of the concrete strut changes (see Figure 2.1). According to Eurocode 2, the angle is proportional to the stress, with a minimum of 21.8° (where cot = 2.5) proving to be the design variable strut angle.
FIGURE 2.1: Shear strut angle of beam
2.4 The Effect of Groove Size for NSM-FRP
Al-Zu'bi et al. (2021) said that for the strengthening of RC beam with NSM-FRP technique, the groove size for the embedment of FRP material would give significant impact on the efficiency. With a wider groove size, the probability of concrete split failure would decrease, as well as increasing the bond strength of the NSM joints, ultimate load, and fracture energy. Al-Rjoub et al. (2018) claim that the most common cause of RC beam failure is the fracture of FRP strips, which is followed by concrete compression failure. Nevertheless, 70–80% of the strips do not totally fracture. An appropriate groove size can be used to address this splitting issue. By thickening the adhesive layer, which prevents splitting between the strips and the concrete, this increases the bonding resilience of the material. According to Al-Zu’bi et. al. (2021), this is because it lowers the state of stress in the concrete right next to the groove sides and delays the phenomenon of cracking. Al-Rjoub et. al. (2018) also added that using the right length for the bonded strip, which impacts the bond's strength, prevents splitting as well. However, it was discovered by Al-Zu’bi et. al. (2021) that for wider groove size with epoxy adhesive did not give a significant impact on the failure load, but it decreases the failure load for wider grooves with cement based adhesive. For specimens with smaller grooves, the failure loads and the bond strength were certainly higher. Al-Zu’bi et. al. (2021) confirmed that specimens with larger grooves size, failure occurs at the FRP-adhesive interface while for specimens with narrower grooves size, failure occurs at the adhesive-concrete interface.
2.5 The Difference of Epoxy and Cement Based Adhesives for NSM- FRP
Appropriate adhesives like epoxy paste and cement grout are adopted for NSM- FRP technique. Alwash et. al. (2021) reported an experimental investigation of RC beam strengthening in shear using NSM-FRP techniques utilising different types of adhesives which are modified cement-based (CBA) and epoxy resins. It was found that the shear capacity was increased by 101 % and 97 %, respectively, by NSM strengthening with CFRP textile bonded with epoxy and modified CBA. Rahman et al. (2016) carried out an experimental research on the substitution of epoxy resin adhesive with cement mortar for the flexural reinforcement of RC beam with NSM- steel bar technique. The research shows that a cost-effective method for flexural strengthening RC beams is the NSM approach using steel bars and cement mortar.
When 50% of the epoxy adhesive is replaced with cement mortar in the centre of the NSM groove, the flexure performance is nearly identical to that of 100% epoxy adhesive.
The behaviour of RC beams strengthened with rectangular NSM CFRP rods using epoxy adhesive and cement grout as a bonding agent was examined by Taljsten and Carolin (2001). They discovered that, correspondingly, the maximum load- carrying capacity was increased by 77 and 56%. They also found that using high- strength rectangular NSM FRP rods and high-modulus rectangular NSM FRP rods increased the ultimate load capacity by 108 and 93%, respectively. Another experimental research study was conducted by Jadooe et. al. (2017) where they investigated the bond behaviour between NSM-CFRP strips and concrete exposed to high temperatures using cement-based and epoxy adhesives. It was found that the residual bond strength followed by the repair of heat-damaged concrete with NSM- CFRP for temperature exposure of 1 h at 200, 400, and 600 °C using epoxy and cement-based adhesive had given comparable results, where the percentage are 94, 79, and 49% and 91, 79, and 70%, respectively.
CHAPTER 3 METHODOLOGY 3.1 Flow Chart
The flow chart of the experimental research was developed after all the necessary information needed were obtained. Flow chart is important in any experimental research as it provides an overview of the experiment process. The information such as the properties of reinforcement bars, coarse and fine aggregate, cement, as well as the testing machine capacity, as provided in the concrete lab are crucial in developing the flow chart. Figure 3.1 below shows the flow chart of the experimental research’s methodology.
3.2 Properties of RC Beams, CFRP Sheet and Epoxy Resin
In this experimental study, concrete with 28-days average compressive cylinder strength of 20 MPa was chosen. The reason of choosing this type of concrete grade was due to it being commonly used in the industry as a normal strength concrete. A proper concrete mix design for this concrete grade was done according to the British Standard. Steel reinforcement bars and shear links that were used as the longitudinal and transverse reinforcement have a yield strength of 500 MPa as provided by the manufacturer. CFRP sheet manufactured by Sika with the model type of SikaWrap- 300C was used as the strengthening material. SikaWrap-300C is a unidirectional woven carbon fibre fabric with mid-range strengths, designed for installation using dry or wet application process. According to the manufacturer, they have a 4000 MPa of dry fibre tensile strength and a 230,000 MPa of dry fibre modulus of elasticity in tension. As recommended by the manufacturer, the suitable epoxy resin to be used as the concrete substrate adhesive primer and laminating resin was Sikadur-330. Sikadur- 330 is a 2-Part, thixotropic, epoxy-based impregnating or laminating resin for SikaWrap structural strengthening fabrics. They have a tensile strength and modulus of elasticity in tension of 30 MPa and 4500 MPa, respectively. The mixing ratio by weight as stated by the manufacturer was 4:1 of Part A:Part B. Figure 3.2 below shows the picture of SikaWrap-300C and Sikadur-330 used in this experiment.
FIGURE 3.2: SikaWrap-300C and Sikadur-330
3.3 Concrete Mix Design
The concrete mix design was made according to the report by Building Research Establishment (BRE) that complies with the British Standards. A concrete mix design form was filled by following the procedure, charts, tables, and equations adopted from the BRE report. Figure 3.3 below shows the flow chart of the mix design procedure.
In this experimental study, two (2) design mixes of concrete were made where Design Mix 1 and Design Mix 2 vary in slump of 30-60 s and 60-180 s respectively.
Aggregate size of 10 mm was chosen due to a small size of RC beam to be designed.
The spacing between top and bottom bars were ensured to be sufficient for the aggregate to pass through. Three (3) cubes of size 150m x 150mm x 150mm were cast for each design mixes. This makes a total volume of 0.0035 m3 including 15% extra as a factor of safety for each trial mix. Figure 3.4 below shows Design Mix 1 while Figure 3.5 below shows Design Mix 2. Refer to Appendix 1.1 and 1.2 for the reference of both design mixes from British Standard.
FIGURE 3.5: Design Mix 2
3.4 RC Beam Design
Preliminary design assumption of the RC beam was adopted through the research study by Al-Zubi et. al. (2021) where beam size of 150 mm in width, 250 mm in depth, and 1300 mm in length, was used in the shear strengthening of RC beam by NSM-CFRP laminate technique. The steel reinforcements used in the research study were 2T10 for compression bar, 3T14 for tension bar, and R6@200mm c/c for shear links. For the design of RC beam used in this study, it was done in a smaller scale of 50% reduction. Hence, the beam size was decided to be 75 mm in width, 125 mm in depth, and 650 mm in length. The steel reinforcement bars and concrete cover were determined through the beam design process as per Eurocode 2. Refer to Appendix X for the step-by-step design calculation of the RC beam.
A total of six (6) simply supported RC beams with dimensions of 75 mm x 125 mm x 650 mm were designed. Identical design of longitudinal reinforcements and concrete cover were designed for all beams. The steel reinforcing bars in tension and compression were designed with two (2) 10 mm diameter and two (2) 6 mm diameter rebars, respectively, with a 15 mm concrete cover. A 6 mm diameter of shear links were designed, at a spacing of 500 mm for shear deficient beam design. The reinforcement detail of the RC beam is shown in Figure 3.6 below.
In the design of RC beam as shown in Appendix 1.3, it was found that the optimum shear link to be 210 mm. To ensure an insufficient shear strength of the beam, a spacing of 500 mm was chosen. Spacing of shear links is normally designed with typical increment of 25 mm such as 50 mm, 75 mm, 100 mm, 125 mm, and so on.
Thus, a control beam of 500 mm shear links spacing were designed. All the test beams were designed with 500 mm spacing of shear links. Table 3.1 below shows the details of all six (6) RC beams.
FIGURE 3.6: Detailing of RC beam
TABLE 3.1: Details of all test specimens
No. Type Bottom bars
Spacing of shear links (mm)
Concrete cover (mm)
1 Control beam 2T10 2T8 R6 500 15
2 Test beam 2T10 2T8 R6 500 15
3 Test beam 2T10 2T8 R6 500 15
4 Test beam 2T10 2T8 R6 500 15
5 Test beam 2T10 2T8 R6 500 15
6 Test beam 2T10 2T8 R6 500 15
3.5 NSM-FRP Groove Design
The NSM-FRP technique involves cutting grooves into the surface of the beam and filling them with a suitable adhesive, such as epoxy paste or cement grout. The grooves were then filled with FRP materials having circular, square, or rectangular cross-sections. To ensure adequate adhesive surrounding the FRP composite, the grooves should be dimensioned. As per ACI 440.2 R-08, a minimum groove size of 3.0ab × 1.5bb should be provided, where ab is the smallest strip’s dimension and bb is the biggest strip’s dimension. To prevent overlapping of the tensile stresses around the NSM-FRP composite, the minimum clear groove spacing for NSM-FRP composite should be larger than twice the depth of the NSM groove, where 25 mm spacings were provided. In order to reduce edge effects that could expedite debonding failure, a clear edge spacing of four times the depth of the NSM groove should be allowed, where 50 mm edge spacing was provided. The minimum width and depth of the groove were designed to be 5 mm and 10 mm respectively. Refer to Figure 3.7 below for the NSM- FRP groove design. The effect of groove size and spacing on the beam capacity were not investigated since they were designed identically for all strengthened test beams.
FIGURE 3.7: Minimum dimensions of grooves
3.6 RC Beam Casting
Before the start of RC beams casting, a trial mix of concrete and compressive strength test were done in order to check for the properties of concrete mix such as strength and workability. A total of six (6) 75 mm x 125 mm x 650 mm internal dimension formworks were prepared for the mould of the reinforced concrete beams.
Longitudinal reinforcement of 8 mm and 10 mm diameter tensile steel reinforcing bars were cut and bent into the desired length, as per the design. A total of ten (10) numbers each for respective diameter steel bars were prepared. Shear links of 6 mm diameter round reinforcing bar were cut and bent into the desired length as per the design. The shear links were bent into shape, before the erection of steel reinforcements began.
The tying of all steel reinforcements was done by using steel wire. Concrete spacers of 15 mm x 15 mm were cast and tied to the outer sides of the erected longitudinal steel reinforcing bars for the purpose of concrete cover. The erected steel reinforcing bars were placed inside the formwork and the beam were ready to be casted. Concrete mixing of grade 20 MPa for the beam casting took place and concrete cubes with a standard size of 150 mm width, length and height were prepared, before the concrete were poured into the formwork. Table vibrator was used to ensure a proper compaction of the reinforced concrete beams and concrete cubes. Twenty-four hours after casting, the beams and concrete cubes were de-moulded and wet cured for 28 days. Figure 3.8 below shows the actual casting of RC beams and cubes.
FIGURE 3.8: Actual casting of RC beams and cubes
3.7 Retrofitting of Test Beams by NSM-FRP
Groove lines of minimum 5 mm in width and 10 mm in depth, as per the design were constructed on the sides of the five (5) test beams by using a diamond wheel cutter. Inclination angle of 45° were applied to the groove lines. The manually made CFRP rectangular bars, as shown in Figure 3.9, were created in the lab by physically folding CFRP sheets over itself were cut and folded to the desired length of 177, 157, 117, 77, and 37 mm, with ten (10) numbers each of the respective length for both sides of the beam. Epoxy resin Sikadur-330 were used as the adhesive pastes. The grooves were cleaned by compressed air and filled halfway with the epoxy resin. Before the CFRP were placed into the grooves, a layer of epoxy resin was applied on the faces of the CFRP. Any excessive epoxy resin applied were removed. Table 3.2 below shows the details of each test specimens with variety of NSM-CFRP length. Figure 3.10 and 3.11 shows the configuration of the NSM-CFRP and strengthening process.
FIGURE 3.9: Making of manually made CFRP rectangular bars from CFRP sheets
TABLE 3.2: Details of each test specimens with variety of NSM-CFRP length Specimen Trimming of NSM-CFRP
Length of NSM-CFRP sheet (mm)
TB-1-0 0 177
TB-2-10 10 157
TB-3-30 30 117
TB-4-50 50 77
TB-5-70 70 37
FIGURE 3.10: NSM-FRP configuration with 45° inclination angle
FIGURE 3.11: NSM-CFRP strengthening process
3.8 Testing by Four-Point-Loading
All beams were tested under four-point loading using Universal Testing Machine (UTM) with a capacity of 200 kN and a load rate of 0.3 kN/s which was manually changed appropriately for testing. Beams were positioned on a support at the beam ends, which makes the beam to be simply supported. The vertical displacement at the point of the load application were measured automatically by the UTM-200kN machine. The load was gradually increased automatically with a rate of 0.3 kN/s while visually monitoring the concrete's surface for shear and flexural cracks up until failure.
With a clear span of 550 mm between the supports, all beams were 4-point monotonically loaded. As illustrated in Figure 3.12, the loading points were placed 225 mm from each support and 200 mm from the other loading point. The load at first crack, ultimate load at failure, maximum mid-span displacement values were recorded.
Failure modes of the beams were visually observed and recorded. Load strengthening and ductility improving ratio were calculated by comparing the ultimate load and maximum mid-span deflection of respective test beam with the control beam. Figure 3.13 below shows the actual testing by four-point loading using UTM-200kN machine.
FIGURE 3.12: Illustrative sketch of the test setup
FIGURE 3.13: Actual testing by four-point loading using UTM-200kN machine
RESULT AND DISCUSSION 4.1 Cube Test Result
A concrete trial mix by using three (3) cubes of size 150 x 150 x 150 mm was done with two (2) different mix designs where they vary in slump of 30-60s and 60- 180s. As shown in Table 4.1 below, the results of Design Mix 1 and Design Mix 2 indicate an average of cube compressive strength of 20.68 MPa and 18.41 MPa, respectively. The average cube compressive strength values were converted to a cylinder compressive strength, by a ratio of 0.8. This conversion is necessary due to the beam design process that utilizes a 20 MPa of concrete characteristic strength, fck
of cylinder, according to Eurocode 2. A 7-days of wet cured concrete is typically around 80% of the full characteristic strength at 28-days. 80% of 20 MPa is around 16 MPa. Therefore, the most suited design mix was Design Mix 1, with slump value of 30-60s due to the average cylinder compressive strength of 16.54 MPa.
TABLE 4.1: Cube test results for design mix 1 and 2 (wet cured for 7 days) Design mix 1 (wet cured for 7 days)
Cube Ultimate load (kN) Cube compressive strength, fck (MPa)
Cylinder compressive strength, fck (MPa)
Cube 1 211.30 21.13 16.91
Cube 2 201.00 20.10 16.08
Cube 3 208.10 20.81 16.65
Average 206.80 20.68 16.54
Design mix 2 (wet cured for 7 days) Cube Ultimate load (kN) Cube compressive
strength, fck (MPa)
Cylinder compressive strength, fck (MPa)
Cube 1 185.70 18.57 14.86
Cube 2 198.10 19.81 15.85
Cube 3 168.50 16.85 13.48
For the process of RC beam casting, it is necessary to also cast concrete cubes for the compressive strength test. This measure is done to check the concrete characteristic strength at the day of beam testing. Thus, three (3) concrete cubes of 150 x 150 x 150 mm in size were cast with the same concrete mix used to cast the six (6) numbers of RC beam specimens. As shown in Table 4.2 below, an average compressive strength of 26.81 MPa and 21.45 MPa for cube and cylinder, respectively were obtained at 28-days of wet curing. With the beam design process that utilizes 20 MPa grade of concrete characteristic strength by cylinder, therefore the concrete mix was proven to be acceptable.
TABLE 4.2: Cube test result for test specimens (wet cured for 28 days) Cube test for test specimens (wet cured for 28 days)
Cube Ultimate load (kN) Cube compressive strength, fck (MPa)
Cylinder compressive strength, fck (MPa)
Cube 1 261.30 26.13 20.90
Cube 2 273.60 27.36 21.89
Cube 3 269.50 26.95 21.56
Average 268.13 26.81 21.45
4.2 Ultimate Load and Maximum Mid-Span Deflection
The ultimate load of the tested beams and their maximum mid-span deflections are provided in Table 4.3. All the strengthened beams experienced significant increases in load-carrying capacity and deflection compared with the control beams.
The results indicate that a higher bonded length of NSM-CFRP sheets can sustain more load than the control beams. Thus, it is clear that NSM-CFRP sheets are effective in increasing the load carrying capacity of beams strengthened thereby.From Table 4.3, the increase in ultimate load is reported in terms of load strengthening ratio, while the increase in maximum mid-span deflection is reported in terms of ductility improving ratio, where both are denoted by a percentage (%). Refer to mathematical equations below for the calculation of load strengthening and ductility improving ratio.
𝐿𝑜𝑎𝑑 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ𝑒𝑛𝑖𝑛𝑔 𝑟𝑎𝑡𝑖𝑜 =𝑈𝑙𝑡𝑖𝑚𝑎𝑡𝑒 𝑙𝑜𝑎𝑑 𝑇𝐵 − 𝑈𝑙𝑡𝑖𝑚𝑎𝑡𝑒 𝑙𝑜𝑎𝑑 𝐶𝐵
𝑈𝑙𝑡𝑖𝑚𝑎𝑡𝑒 𝑙𝑜𝑎𝑑 𝐶𝐵 𝑥 100 (1) 𝐷𝑢𝑐𝑡𝑖𝑙𝑖𝑡𝑦 𝑖𝑚𝑝𝑟𝑜𝑣𝑖𝑛𝑔 𝑟𝑎𝑡𝑖𝑜 =𝑀𝑎𝑥. 𝑑𝑒𝑓𝑙𝑒𝑐𝑡𝑖𝑜𝑛 𝑇𝐵 − 𝑀𝑎𝑥. 𝑑𝑒𝑓𝑙𝑒𝑐𝑡𝑖𝑜𝑛 𝐶𝐵
𝑀𝑎𝑥. 𝑑𝑒𝑓𝑙𝑒𝑐𝑡𝑖𝑜𝑛 𝐶𝐵 𝑥 100 (2) The load strengthening and ductility improving ratio of the test beams is compared with the control specimen CB due to the identical beam design. For different bonded length of NSM-CFRP sheet for TB-1-0, TB-2-10, TB-3-30, TB-4-50, and TB- 5-70, the load strengthening ratio is found to be 48.93%, 47.00%, 40.36%, 21.54%, and 8.95% respectively while the ductility improving ratio is found to be 83.47%, 66.53%, 43.39%, 9.30%, and 2.48% respectively. This shows that regardless of NSM- CFRP bonded length, the strengthened beam is capable of sustaining a higher load and deflection than that of the control beam. Furthermore, it is clear that the strengthening effectiveness by NSM-CFRP sheets increases when the bonded length of NSM-CFRP increases through the depth of beams.
From the results, it is clear that the best NSM bonded length for strengthening of RC beam was with a full-depth coverage due to it having the highest load strengthening and ductility improving ratio. It can be seen that the ultimate load after strengthening for beams with a full bonded length of NSM-CFRP (TB-1-0) is higher than the ultimate load observed for the control beam and other strengthened beams.
The specimen TB-1-0 failed with an ultimate load of 64.65 kN and maximum mid-
improving ratio increased by 48.93% and 83.47% respectively over the control specimen CB. This can be attributed to the fact that the bonded length of the NSM- CFRP sheets increased the beam’s shear capacity and delayed the delamination of the sheet by increasing the bonded area hence reducing the stress concentrations.
TABLE 4.3: Ultimate loads and maximum mid-span deflections of specimens
Specimen Load at first crack (kN)
load (kN) Failure mode Maximum deflection (mm)
Load strengthening ratio (%)
Ductility improving ratio (%)
CB 9 43.409 Shear 4.84 - -
TB-1-0 22 64.650 Flexure and concrete
crushing 8.88 48.93 83.47
TB-2-10 18 63.813 Flexure and concrete
crushing 8.06 47.00 66.53
TB-3-30 14 60.930 Flexure, shear, concrete
crushing and peeling 6.94 40.36 43.39
TB-4-50 12 52.760 Shear and peeling 5.29 21.54 9.30
TB-5-70 12 47.296 Shear and peeling 4.96 8.95 2.48
The load-deflection curve is a crucial curve that must be created in order to assess structural performance. For all tested specimens, the relationship between ultimate load and maximum mid-span deflection response is shown in Figure 4.1.
According to the bonded length of NSM employed in strengthening various beams, load-deflection behaviour appears to vary. The following can be used to summarise the load-deflection curve: prior to the cracking stage, the relationship was linear;
however, when cracks developed, the slope of the curves deteriorated with increasing stress.
FIGURE 4.1: Load-deflection curves of all test specimens
The load-deflection curve indicated that test specimens TB-1-0, TB-2-10, and TB-3-30 had better ductility and strength, where they failed with flexural mode of failure at a higher deflection, compared to specimens TB-4-50, TB-5-70 and CB where they fail with shear mode of failure with lower ductility and strength. Specimens that fail with better ductility were indicated by a higher deflection, while specimens that fail with better strength were indicated by a higher ultimate load. Specimens TB-1-0, TB-2-10, and TB-3-30 had better ductility and strength were due to the steel reinforcements that were able to yield first and take up the loads at plastic region before they failed. While specimens TB-4-50, TB-5-70 and CB had lower ductility and strength were due to the failure happened in the elastic region where the steel
0 10 20 30 40 50 60 70
-2 0 2 4 6 8 10
Load vs. Deflection
CB TB-1-0 TB-2-10 TB-3-30 TB-4-50 TB-5-70
interpretation of typical stress-strain and load-deflection curves where elastic and plastic region were indicated, as well as the points of yielding of steel, ultimate load and fracture.
FIGURE 4.2: Typical stress-strain curve
FIGURE 4.3: Typical load-deflection curve
In summary, the findings demonstrated that as the bonded length of NSM-CFRP sheets increased, so did the ultimate loads and maximum mid-span deflections of the strengthened specimens. This could validate the idea that employing higher NSM- CFRP bonded length might lead to a stronger strengthening process.
4.3 Failure Modes
The failure modes of all tested specimens are depicted in Figures 4.4 to 4.9. The load at first crack was recorded visually, as shown in Table 4.3. The outcomes provide a generalised understanding of how NSM affects the behaviour of strengthened beams.
It was discovered that the NSM-CFRP was able to prevent shear cracking in the majority of beams strengthened with various bonded lengths of NSM-CFRP.
Regardless of the bonded lengths, it was discovered that the majority of the NSM- CFRP-enhanced beams were stronger than the control beams. Reduced NSM-CFRP length from whole beam depth had resulted in shear failure, whereas full depth length results in flexural failure. This improvement can be attributed to the NSM-effect CFRP's on the concrete's increased tensile cracking strength. As an extra interpretation, using 45º inclined NSM-CFRP had raised the shear capacity and at the same time enhanced the flexural capacity, according to the failure modes of all test specimens. A component of the inclined NSM-CFRP force that is horizontal and perpendicular to the flexural cracks may be the cause of the increased flexural capacity by avoiding the formation and spread of flexural cracks within the NSM-CFRP.
At an ultimate load of 43.409 kN, the control specimen had pure diagonal shear cracking as the damage mode (see Figure 4.4). It is evident that specimen CB failed as a result of shear cracks that caused the concrete cover to delaminate and peel off. This failure mode was developed due to the beam's inadequate shear reinforcement while having adequate flexural reinforcement. At 9 kN, shear cracks first appeared, and they kept growing until failure.
FIGURE 4.4: Failure mode of CB
Both specimens TB-1-0 and TB-2-10 demonstrated a similar failure mode, which is flexural cracks and concrete crushing, as shown in Figures 4.5 and 4.6. With an ultimate load of 63.813–64.65 kN, which is higher than that of all other test specimens, the first crack first occurred at 18–22 kN. It was observed that there were no shear cracks formed and propagated through the NSM-CFRP areas. This might be understood as the shear crack coming into contact with a high-scale NSM-CFRP strength that was able to prevent crack formation, giving the specimen additional resistance to shear crack propagation and so increasing the specimen's shear capacity.
In contrast to all other test specimens, a delayed failure at a higher stress was brought on by the avoidance of shear crack growth. This could be due to the transition from shear to flexural failure mode that took longer time and resulted in a higher load- carrying capacity. Flexural cracks started to form and spread towards the specimen's mid-span, in which the largest moment occurs.
FIGURE 4.5: Failure mode of TB-1-0
FIGURE 4.6: Failure mode of TB-2-10
In the case of specimen TB-3-30, as depicted in Figure 4.7, it was seen that both flexure and shear cracks were developed, followed by concrete crushing and concrete cover peeling off, which had caused a failure in both flexure and shear. The specimen may be considered in the mid-range of strengthening effectiveness among all strengthened test specimens due to its’ ability in slightly delaying crack formation compared to the control specimen CB where the first crack appeared at 14 kN, with an ultimate load of 60.93 kN. The sides' concrete cover peeled off as a result of shear cracks that developed and spread at the ends of NSM-CFRP. While flexural cracks formed at the mid-span of the specimen was smaller than those that had appeared in specimen TB-1-0 and TB-2-10. With the formation and propagation of both shear and flexural cracks up until failure, it may be interpreted that the specimen was under the transition of flexure-to-shear strength capacity deficient.
FIGURE 4.7: Failure mode of TB-3-30
Specimen TB-4-50 showed a similar failure mode to specimen TB-5-70, as shown in Figure 4.8 and 4.9, with the same load at the first crack of 12 kN but with different ultimate load, that is 52.76 kN and 47.296 kN. The specimens demonstrated a failure mode of shear and peeling-off of concrete cover. The damage from shear cracks that were formed and propagated at the ends of NSM-CFRP causing the peeling-off of the sides concrete cover were much bigger than those that had appeared in specimen TB-3-30. This was due to the length of NSM-CFRP that were in a further distance from the top and bottom of the specimen, which led the crack to face a non- strengthened area. As a result, the specimens had lower resistance to shear crack propagation because they were unable to stop the formation of cracks. It may be interpreted that the failure mode of shear instead of flexure in these two specimens
were due to insufficient bonded length of NSM-CFRP, resulting in insufficient shear strengthening effectiveness.
FIGURE 4.8: Failure mode of TB-4-50
FIGURE 4.9: Failure mode of TB-5-70
CONCLUSION AND RECOMMENDATION
The experimental investigation on the effect of varied NSM-FRP bonded length for the shear strengthening of RC beam by using manually made CFRP rectangular bar created in the lab by physically folding CFRP sheets over itself was able to address the research gap of the experimental study by Al-Zubi et. al. (2021) and Al-Rjoub et.
al. (2019) where:
• The strengthening effectiveness in terms of ultimate loads and maximum mid- span deflections of the strengthened specimens increased as the bonded length of NSM-CFRP increased.
• NSM-CFRP bonded with different lengths for TB-1-0, TB-2-10, TB-3-30, TB- 4-50, and TB-5-70, the load strengthening ratio increased by 48.93%, 47.00%, 40.36%, 21.54%, and 8.95% respectively, while the ductility improving ratio increased by 83.47%, 66.53%, 43.39%, 9.30%, and 2.48% respectively.
• The failure modes of all the strengthened and un-strengthened shear-deficient beams were discussed in terms of the shear and flexural cracks formation and propagation.
For further studies and to produce more accurate result on the effect of varied NSM-FRP bonded length for the shear strengthening of RC beam by using manually made CFRP rectangular bar created in the lab by physically folding CFRP sheets over itself, several recommendations are suggested:
• To design a shear-deficient RC beam only at the critical shear span (CSS) by using software including justification by manual design calculation as per Eurocode 2 to obtain a more accurate representation of a real-life situation of RC beams with low shear strength.
• To utilize an advanced testing machine that is able to automatically determine the load at first crack of tested specimens.
• To validate the experimental results with analytical investigation using formulas that can be found in literatures.
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1.1 Design Mix 1
1.2 Design Mix 2
1.3 Beam Design Calculation
1.4 Cube Test Results for Design Mix 1 (7 days Wet Cured)
1.5 Cube Test Results for Design Mix 2 (7 days Wet Cured)