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Article

Evaluation of Bonding Properties Between CFRP Laminate and Concrete Using Externally Bonded Reinforcement on Transverse Grooves (EBROTG) Method

by
Ahmed H. Al-Abdwais
1,2,* and
Adil K. Al-Tamimi
3
1
Civil Engineering Department, Al-Nahrain University, Baghdad 64040, Iraq
2
Civil Engineering Department, Swinburne University of Technology, Hawthorn, VIC 3122, Australia
3
Civil Engineering Department, American University of Sharjah, Sharjah 26666, United Arab Emirates
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2024, 8(12), 488; https://doi.org/10.3390/jcs8120488
Submission received: 9 September 2024 / Revised: 13 November 2024 / Accepted: 20 November 2024 / Published: 22 November 2024
(This article belongs to the Section Fiber Composites)
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Versions Notes

Abstract

:
The external bonding system using CFRP composite has been extensively utilized for strengthening different structures worldwide. However, premature debonding in this strengthening technique is a critical failure that leads to the fiber not reaching its ultimate capacity. In order to enhance the capacity of the externally bonded (EB) FRP and to slow the premature debonding failure mechanism, numerous anchoring techniques have been applied to improve the bonding capacity. The externally bonded reinforcement on grooves (EBROG) technology is one of the strategies that have been recently developed to delay the debonding issue. Although extensive studies have been conducted in the literature on the EBROG method, most of these studies have been focused on the bonding characteristics of grooves in the longitudinal direction, and few studies on the effect of different designs and configurations (e.g., width, height, and spacing) in the transverse groove direction have been conducted using only CFRP fabric. In the present study, an experimental investigation was carried out to study the bond behavior of the externally bonded reinforcement on transverse grooves (EBROTG) technique on CFRP-to-concrete joints involving different parameters, including groove width, depth, spaces between grooves, and strain evolution with the corresponding bond stress–slip relationships using CFRP laminate. Twenty-four concrete prisms, divided into eight groups of three specimens, were tested using a single-lap shear test set-up. The results of testing proved that the EBROTG method furnished a proper anchor system and highly enhanced the bonding force of the tests. The increasing range of bonding strength in the specimens reinforced with the transverse grooving method ranged from 11 to 86% compared to the externally bonded reinforcement (EBR), reflecting the effect of different widths, depths, and distances between grooves.

1. Introduction

There are various strengthening techniques applied to different structural members worldwide, such as reinforced concrete (RC), timber, steel, and masonry. The widespread technique is externally bonded fiber-reinforced polymer (FRP) composites with structure substrates [1,2,3,4,5,6,7,8]. However, premature debonding between the CFRP and the concrete substrate at the ends is the major phenomenon that is causing degradation of bonding strength and consequently reducing the ability to efficiently utilize the capacity of FRP materials for this strengthening technique.
Therefore, postponing or preventing debonding between concrete substrates and fibers is a major challenge for researchers to provide more efficient utilization capacity of CFRP reinforcement. For this purpose, anchorage systems are often required to inhibit the delamination of CFRP, prevent peeling-off or separation failure, and increasing the capability to transfer the stresses between the concrete substrate and the CFRP reinforcement [9].
Different types of anchoring systems in the concrete strengthening field have been utilized over the last decade, including mechanical anchorage, the gradient anchorage system, the fiber-based anchoring method, and the near-surface mounted (NSM) anchoring technique [9,10,11,12,13,14,15,16,17].
Despite the ability of the NSM to prevent premature debonding by providing confinement around the FRP, there are some restrictions, including the limits of concrete covers that are prevalent in actual circumstances due to the existence of stirrups, in addition to the difficulty of prestressing the FRP reinforcement within the grooves, which frequently necessitates additional actions.
In 2010, the development of the externally bonded reinforcement on grooves (EBROG) approach aimed to render FRP reinforcing systems more efficient in concrete structures [18]. Such systems were applied in 2010 to ensure that these structures are more efficient compared to the EBR method [18]. The EBROG method, in comparison to the traditional externally bonded reinforcement (EBR) method, can reduce the early-age distress that is caused by debonding by increasing the contact area between the FRP/adhesive and the concrete substrate and ensuring the distribution of bond stresses into deeper layers of the substrate [18,19]. Furthermore, the EBROG approach is stated to have a better degree of FRP bonding than traditional EBR techniques [20,21].
Numerous applications have been used to thoroughly examine the EBROG strengthening method, including beams in flexure [22,23] or shear [24,25], strengthening of RC columns subjected to uniaxial loading [26,27] and subjected to eccentric loads [28], FRP strengthening of concrete beam–column joints [29], examining the bonding behavior between the RC substrate and FRP sheet [30,31] and procured FRP sheets [32,33], evaluating the EBROG method’s durability [34], and, finally, strengthening heat-damaged concrete [35]. In addition, analytical and empirical models were developed for the EBROG method to estimate the bonding strength between FRP and concrete [36,37,38,39,40]. Furthermore, different factors, including groove width and depth, have been investigated to evaluate their effect on the bonding strength [41,42].
As indicated in literature, most studies focused on the bonding properties and the effectiveness of EBROG for longitudinal grooves aligned with the direction of the fiber [43]. However, in some cases, execution of long grooves in a straight line aligned along the specimens is a practically difficult process in the field that requires special cutting machine to ensure straight grooves. Hence, short grooves in the transverse direction are a good alternative and can be easily applied in practical applications. Despite the extensive studies on the EBROG method recorded in literature, there is a lack of significant studies on using the transverse grooves method. Few experimental studies [44,45,46] and numerical approaches [47] have been conducted to assess different depths, widths, and distances of transverse grooves using only CFRP fabric. In this study, the effectiveness of applying externally bonded CFRP laminate strips over transverse grooves was investigated. The investigation evaluated the bonding strength and failure mode with different parameters including groove width, depth, distance between grooves, strain variation, and bond–slip values. The experimental work includes testing 24 concrete prisms divided into eight groups of three specimens, one group as a reference and the other groups with grooves including different widths, depths, and spaces between grooves. The results showed significant improvement in bonding strength compared to the use of CFRP fabric.

2. Experimental Program

2.1. Specimens Layout

The experimental work comprised eight groups of concrete specimens (a total of 24 specimens); each group has three specimens with different parameters. The first group includes two reference specimens without grooves. The other groups were grooved in a transverse direction to study various parameters, including different groove depths (4, 8, and 12 mm), widths (2, 4, and 6), and distances between grooves (20, 40, and 80 mm). The concrete prisms’ dimensions are 75 mm × 75 mm × 250 mm. The CFRP laminate was glued externally on one face of the prisms over the transverse grooves with different dimensions and distances between grooves using epoxy adhesive, as reported in Table 1. A unidirectional CFRP laminate with carbon fiber volume content > 68% and dimensions of 20 mm wide × 1.2 mm thick was used in the experiment. The material was manufactured by Sika in Baar, Switzerland and provided by Sika UAE under commercial name (Sika CarboDur® S). A bonding length of 160 mm was used for all the specimens. Each bond group was tested using three separate identical specimens labeled as “S1”, “S2”, and “S3”. The tests of specimens were performed using a single-lap shear test set-up, and the details of the specimens are provided in Table 1.

2.2. Materials Properties

In order to manufacture the prisms, the concrete mix design was conducted to achieve the target compressive strength of 40 MPa at 28 days of curing. The test was performed for three cylindrical specimens with dimensions of 200 mm heights × 100 mm diameter according to ASTM C39/39M-18 [48]. The CFRP laminate and epoxy adhesive were supplied by commercially named Sika [49]. The tensile strength of the CFRP strips was determined in accordance with ASTM: D3039 [50]. Table 2, Table 3 and Table 4 show the characteristics of the concrete, adhesive, and CFRP correspondingly.

2.3. Specimen Preparation and Reinforcement

After a one-week curing period to ensure the concrete surface was dried before applying the CFRP, the preparation of the specimens started with drawing the necessary lines for the grooves’ placement on the concrete surface. The bonding of CFRP at the ends of the prism could cause cracking in the concrete because of the intense local stresses. In previous researchers’ studies, it is recommended to leave a distance of 50 mm from the edges of the specimens before applying the glue [46,47] to avoid early cracks in the concrete. Two strengthening approaches were applied: the EBR and EBROG methods. In the EBR method, the surface was ground using a fine grinder to remove any exposed particles or mortar, cleaned using a high-pressure water jet, a layer of adhesive was rubbed on the bonding area, and the CFRP was placed. In the EBROG method, grooves were cut using an electric cutting machine equipped with blades of the required depth, and the grooves were then cleaned of any particles and dust, as shown in Figure 1a. Bonding was applied by filling the grooves accurately and removing all bubbles inside the adhesive. Finally, the CFRP was coated with adhesive and applied to the FRP and the bonding place of the prisms. To enable the fixing of the gripping steel plates, the CFRP strips were extended by 150 mm from the prism edge (see Figure 1b). The CFRP strips were bonded externally over the grooves, and specimens underwent curing for 2 weeks at room temperature (23 °C). The CFRP ends were glued between two steel plates for gripping.
In order to record the strain values at the CFRP sheets along the bonding length, three strain gauges were installed along the CFRP strip in the specimen’s load direction. The strain gauge type used in the test was TML, manufactured in Japan, with gauge factor of 2.11 (+/−1%) of the strain values recorded using a strain acquisition system (1/2 bridge) calibrated to synchronize the strain reading with load values. Figure 2 displays the specimens’ dimensions with strain distribution, and Figure 3 shows the instrumented specimens with strain gauges.

2.4. Test Configuration and Set-Up

The test of specimens was implemented using a single shear-lap set-up, and the pull-out loading was applied in the structural laboratory using 100 kN MTS universal testing machine manufactured in Eden Prairie, MN, USA. The application of load was performed on the specimens using displacement load control. The measurements of the load and displacement values were recorded by a load cells and extensometer available in the testing machine. To guarantee equal elastic elongation of this part, the length of the CFRP laminate outside of the bonding zone were employed to measure the displacement at the loading point.
The direction of the CFRP laminate was in alignment with the vertical direction of the load, and its vertical alignment was precisely verified using a laser level to ensure pure axial loading and to avoid any bending moments that may occur on the CFRP. The direct tensile test was performed using 2 mm/min of displacement–control loading applied in accordance with the ASTM D3039/D3039M [50]. The set-up dimensions and test configuration are shown in Figure 4 and Figure 5 respectively.

3. Results and Discussion

3.1. Mode of Failure

In the reference specimen with the EBR method, the failure mode showed detachment of the CFRP from the concrete surface at the concrete/adhesive interface (Figure 6). In contrast with grooved specimens of the EBROTG method, two types of failure modes have been observed. First, the debonding of the CFRP sheet from the concrete surface over the area between the groove occurred at the concrete/adhesive interface, and a thin layer detached from the concrete surface (similar to the EBR method). Second, in the area over the grooves, the delamination of the CFRP occurred at the FRP/adhesive interface (Figure 7, Figure 8, Figure 9, Figure 10 and Figure 11). This mainly indicates that the weak point for debonding is the interface between concrete and adhesive due to the detachment of a thin layer from the concrete’s surface
In the specimen with a 20 mm distance between grooves (nine grooves), it is clearly observed that the change in the type of debonding along the specimen starts with separation in the fiber/adhesive layer over the grooves and proceeds to debonding at the concrete/adhesive interface at the area between grooves followed by debonding in the spaces between the grooves (Figure 6).
For the other EBROTG specimen with a 40 mm distance between grooves and containing five grooves, the change in debonding type along the bonding length followed the spaces between the grooves (Figure 7, Figure 8, Figure 9, Figure 10 and Figure 11), which indicates the effect of existing grooves on improving the bonding properties by affecting the failure mode at the area over the grooves.

3.2. Bond Strength

Debonding and rupture are two possible ways that FRP can fail, as previously indicated. The sheet’s load-bearing capacity, or the sheet’s tensile capacity, is measured at its greatest load at sheet ruptures, whereas its bond strength is the maximum load obtained when the debonding failure occurs. As observed in the test results, the existence of grooves increased the bonding strength by transferring the failure mode from the concrete/adhesive interface to the FRP/adhesive interface over the groove section. The ultimate bond strengths of all the specimens in this study are presented in Table 5.
In the specimens with the EBROTG method, the effect of the groove widths of 2, 4, and 6 mm in specimens G3-(2-4-40), G4-(4,4,40), and G6-(6-4-40) can be clearly observed in the ultimate strength results of 10.3, 12.07, and 14.08 kN, achieving significant improvement in the bonding properties from 37.3 to 60.2% and 86.98%, respectively, compared to the reference specimen. The relationship curves of load versus displacement of the specimens compared to the reference curves are presented in Figure 12. This significant increase is reasonable due to the increased bonding area over the grooves, which indicates a higher tensile strength at the interface between the adhesive and the CFRP than that between the CFRP and the concrete interface.
The distance between grooves significantly affected the bond strength, as indicated in the results of specimens G2 (2-4-20), G3 (2-4-40), and G5 (2-4-80). Increasing the distances to 2, 4, and 8 mm, the bond strength decreased from 13 to 10.34 and 8.36 kN (a decrease from 72.6 to 37.3 and 1102%, respectively, as shown in Figure 13). It has also been shown that the bond strength decreases with the increasing distances between grooves, resulting from the reduced bonding area over the grooves.
The investigation of the effect of groove depth on the bonding strength was performed for three different depths of 4, 8, and 12 mm in groups G4 (4-4-40), G7 (4-8-40), and G8 (4-12-40). The load–displacement curve of the results is illustrated in Figure 14. The results were 12.03, 12.7, and 12.1 kN, respectively, which indicated no significant effect of the groove depth on the bonding strength results. This can be justified since no failure has occurred surrounding the grooves, as observed in the mode of failure in all specimens.

3.3. Strain Values

Through the use of three strain gauges, the strain values were recorded along the 160 mm bond length and along the CFRP itself. The strain evolutions of the EBR and EBROTG specimens are depicted in Figure 15. The highest strain value was recorded at the first strain gages near the loaded end of the CFRP. The EBR specimen had a maximum strain value of 0.955, whereas the EBROTG specimen’s strain increased from 0.965 to 1.377. Since the determination of debonding stress for externally strengthening FRP, as reported in the literature, is governed by the maximum strain in the CFRP, the increase in the strain values for EBROTG specimens prove the effects of existing the grooves for delaying the delamination of CFRP from the concrete surface depending on the percentage of the groove area in the bonding surface.

3.4. Local Bond Stress–Slip Value

To develop a practical and accurate bond–slip model, it is assumed that the bond stress is uniformly distributed between two constitutive strain gages spaced by ∆ x i , and can be calculated using the following formula:
τ x = E f . A f p f   .   ∆ ε i ∆ x i
where:
E f   is the modulus of elasticity of CFRP, A f is the cross-section area, P f is the perimeter of the CFRP, ∆ ε i is the difference in strain, and ∆ x i is the space between two strain gauges.
The corresponding slip of the CFRP can be computed using the following formula, by neglecting the strain in the concrete and assuming that the slip at the unloaded end can be ignored before debonding:
s = ∑ k + 1 n ( ε k + ε k + 1 )   ∆ x k   2  
where:
ε k and ε k + 1 are the strain values of two strain gages k and k + 1 , n represents the total number of strain gages, and ∆ x k is the space between two strain gages.
The predicted curve of bond–slip values for both cases (EBR and EBROTG) is extracted from the mean interpolation of the scattered points of the experimental work and are depicted in Figure 16 and Figure 17, respectively, illustrating a non-linear ascending pattern up to the maximum value in the bond–slip curves. It can be seen that the bonding using the EBROTG method exhibited higher stuffiness compared to the EBR method. The experimental and projected values exhibit a strong correlation.
When compared to the experimentally obtained scattered points and the projected curve using Sena Cruz and Barros’s bond–slip equation (Equation (3)) [51,52], the average bond–slip and bond stress computed at different positions along the bond length provide a clear and compatible interpretation.
τ s = τ m   s s m a ,   i f   s   ≤   s m
where τ m and s m are the bond stress values and their corresponding slip and (a) is a parameter defining the shape of the curve. The calculated value of (a) from the calibration between the predicted curve and the equation of the EBR and EBROTG methods is 0.435. The predicted and calculated bond–slip curves for the two methods show good correlation, as shown in Figure 18 and Figure 19. In order to provide ideal utilization of bond–slip curves in finite element modeling, the EBR curve can be used for the bonding area between grooves, representing the failure at the interface between the adhesive and the concrete substrate, while the EBROTG curve will be applied in the bonding area above the groove, representing the failure at the interface between the CFRP and the adhesive.

4. Conclusions

In this study, 24 concrete specimens were tested (3 specimens for EBR and 21 for EBROTG) to examine the impact of groove width, depth, and distance between grooves on bonding behavior in addition to the strain variation and bond stress–slip relationships. The objective was to find the optimal transverse groove dimensions for the EBROTG method in practical applications of concrete beam strengthening. The outcomes are as follows:
  • The results showed significant improvement in bond strength using the EBROTG method by delaying the debonding of the CFRP, when compared to specimens strengthened using the EBR method. The specimens strengthened using the EBROTG approach showed an improvement of 11 to 86% in bond strength according to the groove dimensions.
  • The effect of groove width has noteworthy consequences on bond strength. Increasing the groove width to 2, 4, and 6 mm improved the bond strength by 37%, 60%, and 86%, respectively, compared with EBR specimens.
  • The distances between grooves with identical groove section dimensions have a significant effect when increasing the distances by 2, 4, and 8 mm. The bond strength decreased from 13 to 10.34 and 8.36 kN (from 72% to 37% and 11%), respectively. By combining the effects of the width and the distances between grooves, it can be concluded that the total cross-section area of the grooves at a certain bonding length and their distribution along this bonding length are crucial factors in determining the increase in the bonding strength.
  • The findings showed that there was no significant increase in bond strength as groove depth increased since there was no failure observed in the concrete surrounding the grooves.
  • Bond stress–slip diagrams were extracted from the different strain gages along the bonding length, and the predicted curves can be utilized in the finite element modeling of concrete members strengthened using the EBROTG method.

Author Contributions

Conceptualization, A.H.A.-A.; Methodology, A.H.A.-A.; Validation, A.H.A.-A. and A.K.A.-T.; Formal analysis, A.H.A.-A.; Resources, A.H.A.-A.; Writing—original draft, A.H.A.-A.; Writing—review and editing, A.K.A.-T.; Funding acquisition, A.K.A.-T. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding authors.

Acknowledgments

We would like to express our great appreciation and acknowledgement to the civil engineering department at the American University of Sharjah and Baghdad University for their technical support during the research period.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Specimen preparation: (a) cutting grooves; (b) application of adhesive.
Figure 1. Specimen preparation: (a) cutting grooves; (b) application of adhesive.
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Figure 2. Dimensions and strain gauge plan of the specimens.
Figure 2. Dimensions and strain gauge plan of the specimens.
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Figure 3. Cured specimens with “strain gauges” and gripping plates.
Figure 3. Cured specimens with “strain gauges” and gripping plates.
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Figure 4. Dimensions of single-lap test set-up.
Figure 4. Dimensions of single-lap test set-up.
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Figure 5. Test set-up configurations.
Figure 5. Test set-up configurations.
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Figure 6. Failure mode of specimen G1-R.
Figure 6. Failure mode of specimen G1-R.
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Figure 7. Failure mode of specimen G2-(2-4-20).
Figure 7. Failure mode of specimen G2-(2-4-20).
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Figure 8. Failure mode of specimen G3-(2-4-40).
Figure 8. Failure mode of specimen G3-(2-4-40).
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Figure 9. Failure mode of specimen G4-(4-4-40).
Figure 9. Failure mode of specimen G4-(4-4-40).
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Figure 10. Failure mode of specimen G5-(2-4-80).
Figure 10. Failure mode of specimen G5-(2-4-80).
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Figure 11. Failure mode of specimen G6-(6-4-40).
Figure 11. Failure mode of specimen G6-(6-4-40).
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Figure 12. Load–displacement curve of specimens G1, G3, G4, and G5.
Figure 12. Load–displacement curve of specimens G1, G3, G4, and G5.
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Figure 13. Load–displacement curve of specimens G2, G3, and G5.
Figure 13. Load–displacement curve of specimens G2, G3, and G5.
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Figure 14. Load–displacement curve of specimens G4, G7, and G8.
Figure 14. Load–displacement curve of specimens G4, G7, and G8.
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Figure 15. Strain evolution of the EBR and EBROTG specimens.
Figure 15. Strain evolution of the EBR and EBROTG specimens.
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Figure 16. Bond stress–slip relationships for EBR.
Figure 16. Bond stress–slip relationships for EBR.
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Figure 17. Bond stress–slip relationships for EBROTG.
Figure 17. Bond stress–slip relationships for EBROTG.
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Figure 18. Correlation of the predicted and calculated bond–slip curves for EBR.
Figure 18. Correlation of the predicted and calculated bond–slip curves for EBR.
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Figure 19. Correlation of the predicted and the calculated bond–slip curves for EBROG.
Figure 19. Correlation of the predicted and the calculated bond–slip curves for EBROG.
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Table 1. Specimen details.
Table 1. Specimen details.
Groups DesignationNo. of SpecimensGroove Width (mm)Groove Depth (mm)Distance Between Grooves (mm)
G1-R3Reference--
G2-(2-4-20)32420
G3-(2-4-40)32440
G4-(4,4,40)34440
G5-(4-4-80)34480
G6-(6-4-40)36440
G7-(4-8-40)34840
G8-(4-12-40)341240
G(w,d,s): Group of Specimens (width, depth, and spaces between grooves).
Table 2. Properties of concrete.
Table 2. Properties of concrete.
MaterialDimensions
(mm)		Compressive Strength (MPa)Tensile Strength (MPa)Modulus of Elasticity (MPa)
Concrete200 heights × 100 mm diameter413.8625,600
Table 3. Specifications for reinforcing epoxy as stated in the product data sheet [45].
Table 3. Specifications for reinforcing epoxy as stated in the product data sheet [45].
AdhesiveCompressive Strength (MPa)Tensile Strength (MPa)Modulus of Elasticity (MPa)
Sikadur-3160304500
Table 4. Specifications for reinforcing carbon fiber plate as stated in the product data sheet [42].
Table 4. Specifications for reinforcing carbon fiber plate as stated in the product data sheet [42].
MaterialDimensions
(mm)		Cross-Section Area (mm2)Tensile Strength (MPa)Modulus of Elasticity (MPa)
CFRP Laminate1.2 × 20 mm242900165,000
Table 5. Specimen designation, specifications, and ultimate bond strength results.
Table 5. Specimen designation, specifications, and ultimate bond strength results.
Groups Designationbg (mm)dg (mm)Sg (mm)Pmax, Avg. (kN)% Increase
G1-R------7.53
G2-(2-4-20)242013.072.6%
G3-(2-4-40)244010.3437.3%
G4-(4-4-40)444012.0760.2%
G5-(2-4-80)24808.3611.02%
G6-(6-4-40)644014.0886.98%
G7-(4-8-40)484012.363.3%
G8-(4-12-40)4124012.160.06%
G-(bg, dg, Sg): Group of Specimens (width, depth, and spaces between grooves).
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MDPI and ACS Style

Al-Abdwais, A.H.; Al-Tamimi, A.K. Evaluation of Bonding Properties Between CFRP Laminate and Concrete Using Externally Bonded Reinforcement on Transverse Grooves (EBROTG) Method. J. Compos. Sci. 2024, 8, 488. https://doi.org/10.3390/jcs8120488

AMA Style

Al-Abdwais AH, Al-Tamimi AK. Evaluation of Bonding Properties Between CFRP Laminate and Concrete Using Externally Bonded Reinforcement on Transverse Grooves (EBROTG) Method. Journal of Composites Science. 2024; 8(12):488. https://doi.org/10.3390/jcs8120488

Chicago/Turabian Style

Al-Abdwais, Ahmed H., and Adil K. Al-Tamimi. 2024. "Evaluation of Bonding Properties Between CFRP Laminate and Concrete Using Externally Bonded Reinforcement on Transverse Grooves (EBROTG) Method" Journal of Composites Science 8, no. 12: 488. https://doi.org/10.3390/jcs8120488

APA Style

Al-Abdwais, A. H., & Al-Tamimi, A. K. (2024). Evaluation of Bonding Properties Between CFRP Laminate and Concrete Using Externally Bonded Reinforcement on Transverse Grooves (EBROTG) Method. Journal of Composites Science, 8(12), 488. https://doi.org/10.3390/jcs8120488

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