A prediction model for bidirectional fiber patch anchors used to enhance the performance of FRP materials bonded to concrete

Research has demonstrated that the effectiveness of the FRP when applied to concrete members is largely governed by the strength of the bond between the FRP and the concrete. As a result, failure of strengthened members is usually a result of FRP debonding from the concrete substrate. To improve the efficiency of strengthening systems and mitigate the occurrence of end debond, the provision of end anchorage using bidirectional fiber patch anchors have been recently introduced to counteract the peeling and interfacial shear stresses at the FRP ends resulting much higher material utilisations prior to debond. The tests conducted to date have provided promising results, and could be utilised directly provided that the materials matched those used in published experimental programs. However, enough data has been collected to attempt the development of a prediction model which could theoretically relate parameters such as concrete strength, laminate thickness, width and spacing and patch anchor size. Such a model is presented herein and subsequently verified with results from both experimental tests and finite element simulations. The model is proven to provide reasonable predictions in anchorage strength based on the available data

Development of a hybrid anchor to improve the bond performance of multiple plies of FRP laminates bonded to concrete

Fiber reinforced polymer composites (FRPs) are currently one of the leading materials used to strengthen reinforced concrete (RC) structures against various loading actions such as flexure, shear and torsion. Due to the escalating loads being applied to existing structures, significant structural deficiencies have resulted in higher strengthening demands on many projects, resulting in large and often impractical quantities of FRPs being needed to achieve the required level of strengthening. Although FRP possesses significant advantages over traditional materials in terms of strength, the phenomenon of premature debonding currently limits the degree of material utilization achieved in practice. One means by which debonding may be addressed is by the use of anchorage systems that facilitate the utilization of more layers of FRP prior to failure. The paper presents an experimental investigation of a new hybrid anchor, comprising of 3 layers of ±45° bidirectional fiber sheets used to anchor 2 plies of FRP laminate to the concrete. The patch anchor concept is further enhanced by anchoring the ±45° bidirectional fiber sheets to the concrete with FRP spike anchors, with the dowel end of the anchor embedded into the concrete and the fan end attached to the bidirectional fiber sheet. Significant improvements in the bond performance of the FRP laminates were observed as a result of the hybrid anchor

Experimental and finite element analysis of flexural behavior of FRP-strengthened RC beams using cement-based adhesives

The strengthening and rehabilitation of structures are major issues worldwide. In most situations, strengthening is required when there is an increase in the applied load, human error in the initial construction, a legal requirement to comply with updated versions of existing codes, or as a result of the loss of strength due to deterioration over time. Fiber-Reinforced Polymer (FRP) strengthening systems are enjoying a great deal of popularity as a result of the unique properties of FRPs, namely, their light weight, fatigue resistance non-corrosive characteristics and ease of application. The repair and strengthening technique with epoxy-bonded advanced composites has been applied to a large number of bridges around the world. At elevated temperatures, normally beyond the glass transition temperatures of epoxy adhesive, the mechanical properties of the polymer matrix deteriorate rapidly. It will be very beneficial if they can be replaced by cementitious (mineral)-based bonding agents such as modified concrete, in order to produce fire-resistant strengthening systems. Tests conducted for this paper include the investigation of the flexural behavior of FRP-strengthened reinforced concrete beams using cement-based adhesives. It is concluded that the use of cement-based bonding materials is a promising technique in FRP applications for structures located in hot regions or in danger of fire

Finite element and experimental investigation into patch anchor sizes used to enhance the bond performance of FRP-to-concrete joints

The increasing demand to strengthen existing infrastructure has resulted in growing popularity of advanced fiber composite materials (FRPs) applied to reinforced concrete (RC) members as externally bonded reinforcement. Although FRPs contain very high tensile strengths, premature debonding usually prevents the material from reaching its full potential. Research is currently underway to address this shortcoming by the provision of anchorages to the ends of FRP reinforcement. Bi-directional fiber patch anchors have been found to be one of the most effective anchorages available, which are particularly suitable in shear strengthening applications. The ongoing need for verification of the various influencing parameters such as anchor size, spacing and fiber thickness have inspired further numerical and experimental studies resulting in the present work. The paper will investigate the effect of such parameters highlighting key relationships that may be applied for future use in anchorage strength models

Investigation of bond strength and flexural behaviour of FRP-strengthened reinforced concrete beams using cement-based adhesives

Retrofitting of structures has become a major issue worldwide due to increases in the applied loads, human error in initial construction, legal requirement to comply with updated versions of existing codes, or the loss of strength due to deterioration over time. In this regard, fibre-reinforced polymer (FRP) retrofitting systems are enjoying a great deal of popularity as a result of the unique properties of FRPs. Retrofitting with epoxy-bonded FRP composites is suitable for environments where the temperature is well below the glass transition temperature (T g) of the epoxy adhesive. [T.sub.g] is normally in the range of 55-60 [degrees]C (fib, 2001; Saafi, 2002). It would be very beneficial if it is replaced with cementitious (mineral) based bonding agents in order to produce fire-resistant strengthening systems. Pilot testing conducted by the authors has shown that excellent bonding properties can be achieved using the cement-based adhesives (Hashemi & Al-Mahaidi, 2009). Test results were applied for the next stage of the project presented in the current paper. Tests include the investigation of bond strength of FRP fabrics to the concrete substrate by single-lap shear test and flexural behaviour of FRP-strengthened reinforced concrete beams using cement-based adhesives. The bond-slip response has been developed for the strengthening system. It is concluded that using cement-based bonding materials is a promising technique in FRP applications for structures located in hot regions or in danger of fire

Flexural performance of CFRP textile-retrofitted RC beams using cement-based adhesives at high temperature

Strengthening of concrete structures with epoxy-bonded FRP composites is suitable for environments where the temperature is well below the glass transition temperature of the epoxy adhesive, Tg, which is normally in the range of 55-60°C. It is very beneficial if cementitious mineral-based bonding agents replace epoxy adhesives in order to produce a fire-resistant strengthening system.Tests conducted by the authors have already shown that excellent reinforcement action can be achieved using cement-based adhesives. Tests included in the current paper include the heat endurance of CFRP textile-retrofitted RC beams under constant service load. The strengthened RC beam with cement-based adhesive showed a considerable improvement in flexural performance at high temperature compared to the specimens with epoxy. The specimens with cement-based adhesive failed at temperatures nearly double of that with epoxy-based adhesive

The Influence of FRP spike and patch anchors on the bond performance of FRP-to-concrete joints

It has been demonstrated that the governing failure mode of concrete structures strengthened with fiber reinforced polymer composites (FRP) is by premature debonding of the FRP material from the concrete substrate. Research has shown that one means by which the FRP-to-concrete bond performance may be improved is to provide anchorage measures that resist the interfacial shear and peeling stresses that are generated along the FRP bond line. FRP spike anchors and bidirectional fiber patch anchors are a proven means to enhance the bond performance of FRP materials when bonded to concrete. Although the above mentioned anchorage systems have shown significant promise when investigated independently, the present research aims to combine their unique properties into a hybrid anchorage system. In this study, FRP spike anchors were used to anchor bidirectional fiber patches and used to restrain FRP laminates tested in direct shear resulting in a superior anchorage strength which was demonstrated through experimental testing

Detection of cracks in concrete strengthened with CFRP systems using infra-red thermography

Bond defects due to the development of cracks in concrete strengthened externally with CFRP can degrade the integrity of the composite system. Previous studies have addressed this issue by using different non-destructive testing (NDT) methods, and most have attempted to determine a reliable method to detect cracks and recognize their properties. Infrared thermography (IRT) has emerged as an effective method to detect the propagation of cracks and determine their width in the substrate structure of the composite system. This paper presents the findings of an investigation of CFRP-concrete samples containing various kinds of artificial and loading cracks at the concrete surface. Different types of FRP fabrics and laminate combinations were used in the design. Active IRT was adopted for the thermal observation. Thermal pulses were applied with different angles to enhance crack measurement. The results show that the technique is capable of detecting the location and width of cracks quite adequately. Moreover, the location of the external heating and interval pulse has a considerable effect on crack detection. However, the results show that it is not possible to determine crack depth by using pulsed IRT

Prediction models for debonding failure loads of CFRP retrofitted RC beams

This study focuses on debonding failure in RC beams with CFRP bonded on the soffit using wet lay-up method. An experimental study, which involved 26 tests, was carried out. The experiments showed two failure modes: midspan debond and end debond. The first failure is the result of high bond stress near the tip of flexure-shear cracks; whereas the second type of failure is due to high shear stress developed in the weakest concrete layer at tension reinforcement level. The experiment indicates that U-straps can be effective in preventing midspan debond by limiting opening of flexure-shear cracks. When a U-strap is used as end anchorage, end debond can also be prevented since concrete shear resistance at tension reinforcement level is increased significantly. Based on observation from the experiments, two theoretical models are developed and erified with the experimental data together with a large database of other existing tests