Adhesive Bond Performance of CFRP-Patched Concrete

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In civil engineering, modern materials and techniques are crucial for enhancing the durability and strength of concrete structures. Carbon Fiber-Reinforced Polymer (CFRP) patches have emerged as an effective solution for rehabilitating beams, columns, slabs, and walls, offering high tensile strength, lightweight properties, and corrosion resistance. These patches improve load capacity and structural performance, addressing environmental and operational challenges. As the field advances toward sustainable solutions, refining CFRP patching techniques is key. This article discusses interfacial fracture analysis to evaluate the effectiveness of adhesively bonded CFRP patches in reinforcing and rehabilitating concrete structures.

A review of literature highlights extensive research on CFRP for strengthening and refurbishing concrete structures. Studies focus on improving load capacity, ductility, crack resistance, and durability to extend service life.

Key sources include Mays, Biolzi et al., and Frhaan et al., who demonstrate the effectiveness of externally bonded CFRP strips in boosting beam capacity. Research by Golham et al. and Wu et al. validates CFRP use for beams and slabs under varied conditions. Environmental impacts, such as tropical climates or heat, are examined by Hashim et al. and Al-Rousan. Methodologies range from experimental tests to finite element modeling for predicting structural behavior. The ACI Committee 440 provides a valuable design guide for Fiber-Reinforced Polymer (FRP) systems, while Nanni explores CFRP applications in civil engineering, highlighting their versatility. Despite significant advancements, existing studies often focus on epoxy adhesives, limiting insights into CFRP-to-concrete interface failures with varied adhesives. Using a holistic fracture analysis addresses this gap by investigating shear-induced failures at adhesive-bonded CFRP interfaces, offering new insights into reinforcement techniques.

The evaluation of concrete structures falls into two categories: bulk assessment using continuum mechanics which focuses on technical stresses, and surface reinforcement assessment, where traditional methods may be insufficient. Various safety evaluation methods for bonded CFRP patches are documented, primarily using standardized continuum mechanics approaches such as compression, flexure, shear, and pull-off tests. Pull-off tests, standardized by the American Society for Testing and Materials’ ASTM D7234, Standard Test Method for Pull-Off Adhesion Strength of Coatings on Concrete Using Portable Pull-Off Adhesion Testers, 2021, and ASTM C1583, Standard Test Method for Tensile Strength of Concrete Surfaces and the Bond Strength or Tensile Strength of Concrete Repair and Overlay Materials by Direct Tension (Pull-Off Method), 2010, are particularly effective for assessing adhesion in multi-material composites.

Bonded CFRP patches aim to dampen, delay, or stop crack propagation. Figures 1 and 2 show the test specimen design: concrete blocks with U-shaped notches on each front, where CFRP strips were adhered. A cross-section view (Detail X) illustrates how CFRP patches counteract crack formation under mode-I bending-tensile stress, demonstrating their effectiveness in mitigating crack progression. Mode-I bending tensile stress refers to the stress developed when a crack opens perpendicular to its plane under bending loads, such as in a beam or plate. Unlike pure tensile loading (like pull-off tests), bending focuses stress near the crack, better simulating how real structures fail under combined stresses. This method provides more realistic insights into material and joint behavior, especially under complex, service-like conditions.

In the study, three types of CFRP damage patches under mode-I loading were analyzed via interfacial fracture analysis. Figure 3 illustrates the damage types:

The performance of various adhesive systems through fracture analysis was investigated to evaluate their interfacial reliability in CFRP-to-concrete bonding. The three adhesive types—epoxy, polyurethane, and silyl-modified polymers (SMP)—were tested for their effectiveness in bonding CFRP patches under shear loading. While epoxy adhesives dominate practical applications, research on alternative systems using fracture analysis remains limited. Details on the materials are provided in Tables 1 and 2.

Table 1 shows a compilation of the three types of adhesives used for bonding concrete structures of this study. Specifications are taken from product sheets of the manufacturers.

Test Setup

A setup was applied so that specimens were adhesively bonded to concrete plates and cured for seven days at room temperature (Figure 4). Testing was carried out in a laboratory on a universal testing machine. The fracture analytical events were accomplished in quasi-static loading for six samples per series. Several evaluation parameters were identified, with the results presented in Table 2. Further details about the method can be requested from FRACTURE ANALYTICS.

Results and Discussion Basic Considerations

Two key fracture-analytical parameters—flexural notch strength (σfn), and specific fracture energy (GF)—are used to derive an empirical structural safety factor (SF) according to Brandtner-Hafner. The results are summarized in Table 2.

A new method for evaluating the effectiveness of CFRP patching uses three adhesive systems under mode-I loading. Figure 5 illustrates the debonding process for a CFRP-patched concrete specimen. The dashed black line represents the quasi-brittle failure of an unreinforced specimen, where a primary crack forms after maximum load, propagating stably until complete separation. In contrast, the solid red line depicts the patched specimen using a SMP-based adhesive. At approximately half the maximum load (~60 Newtons (N)), Inflection Point 1 marks the onset of CFRP patch load-bearing, allowing the force to increase progressively. This increase continues until reaching 100 N, which corresponds to 83% of the concrete-only strength of 120 N. At the peak stress point of the CFRP patches, a secondary crack initiates within their interface. The debonding process still remains stable and tough, as the flexible adhesive absorbs significant fracture energy, acting as a damping agent and providing a notable safety gain. Once the force drops to below half of the maximum load (Inflection Point 2), secondary cracks propagate until complete separation at rupture. The so-called "Safety Gain Zone," the area between Inflection Points 1 and 2, indicates the absorbed fracture energy, highlighting the desirable safety benefits of this reinforcement.

Finally, to foster a better understanding of how the bonding characteristics of the three adhesive systems influence the failure behavior of patched CFRP, Figure 6 illustrates the fracture patterns observed during the experiments.

The investigation underscores the pivotal role of adhesive selection in the performance of CFRP patches for reinforcing concrete structures under mode-I loading. The analysis of epoxy, polyurethane, and silyl-modified polymers revealed significant differences in their ability to mitigate structural vulnerabilities. While epoxy allows primary cracks to propagate, polyurethane and SMP adhesives demonstrate superior crack damping and delay. Fracture analysis and Modified Compact Tension (MCT) measurements show that SMP adhesives absorb significantly more fracture energy than epoxy and polyurethane. Hybrid adhesives, combining epoxy’s strength with SMP’s fracture energy absorption, emerge as a promising solution for enhancing CFRP patch effectiveness. The findings highlight the importance of balancing strength and fracture energy absorption in adhesive selection to optimize CFRP performance. Future applications should integrate these insights, particularly by exploring hybrid technologies to advance adhesive selection and application practices for concrete structures.

In conclusion, the study suggests reevaluating the industry’s reliance on epoxy adhesives. Emphasis should shift toward alternative systems and hybrid formulations, alongside long-term performance studies, to support the development of sustainable and resilient infrastructure. ■