Experimental tests and numerical simulations on the mechanical response of RC slabs externally strengthened by passive and prestressed FRP strips

Externally Bonded Reinforcement on Groove (EBROG) method has been introduced to enhance the bond resistance of FRP strips to concrete. It has demonstrated that EBROG generally outperforms EBR in terms of loadtransfer capacity between FRP strips and concrete. The present study aims to further demonstrate the potential of EBROG applied for flexural strengthening. A specimen reinforced according to the EBR solution and a nominally equal one reinforced through the EBROG system are first presented. Then, the performance of a newly fully-composite mechanical end anchorage for prestressed FRP strip to be used in conjunction with the EBROG method is investigated. The experimental results show that the premature debonding observed in EBR is avoided by EBROG in the case of "passive" FRP strips. Moreover, the combination of EBROG and end anchorage demonstrates their effectiveness, as the pre-stressed slab exhibits the full exploitation of the FRP up to rupture. Numerical analyses, carried out by means of a model already presented by the authors, show that the structural response of the tested slabs can be simulated in a very accurate manner if consistent assumptions are made in terms of bond-slip laws adopted to describe the interaction between FRP and concrete in EBR and EBROG

Round robin testing initiative for fiber reinforced polymer (FRP) reinforcement

An international Round Robin Testing (RRT) programme on FRP reinforcement was conducted within the framework of the Marie Curie Research Training Network, ENCORE, and with the support of Task Group 9.3 of the International Federation for Structural Concrete (fib). Eleven laboratories and six manufacturers and suppliers participated in this exercise. As part of this extensive experimental endeavour, one or more of the following tests were performed by the participating laboratories: 1) tensile tests on FRP bars and strips; 2) tensile tests on FRP laminates; 3) double bond shear tests on FRP laminates (Externally Bonded Reinforcement, EBR) and FRP bars/strip (Near Surface Mounted reinforcement, NSM). This paper will discusses the results of the RRT initiative, among which the experimental results of bond tests on concrete specimens strengthened with EBR and NSM FRP

Modeling in-plane and out-of-plane displacement fields in pull-off tests on FRP strips

The present paper deals with modeling FRP strips bonded to concrete blocks and tested in pull-off. The investigation starts from the experimental observations obtained by means of an optical image-correlation measurement system which is able to monitor the 3D displacement components of a fine mesh of points on the surface of both the FRP strip and concrete block. Thus, refined measurements of both in-plane and out-of-plane displacements of the FRP strips are available. [...

Effect of partly cured adhesives on bond-slip relation ship of FRP-concrete interface

External prestressing of existing reinforced concrete (RC) beams by means of bonded fiber reinforced polymer (FRP) strips is getting more and more common as a possible strengthening technique for either underdesigned or damaged members. However, the current technical solutions for external prestressing with FRP require a mechanical anchorage system at the end of the strip, in order to react to the high interface stresses arising in that part of the FRP-to-concrete interface. An alternative solution based on a gradual reduction of the prestressing force in the end zone of the FRP strip which avoids any mechanical anchorages has been recently developed at Empa. Generally speaking, this solution, called “gradient method”, works on the accelerated curing of epoxy resins under heating exposure. Therefore, a better understanding of accelerated curing of adhesive by heating is necessary. The present paper describes some experimental results of pull-off tests carried out on FRP strips glued on concrete blocks and cured through heating with variable duration. The maximum force at debonding onset observed in the tests on specimens with different curing durations at elevated emperatures were compared with a reference test carried out on a specimen fully cured at room temperature for three days. Furthermore, the displacements measured by means of an optical image correlation system are presented with the aim of showing the effect of the heating duration. Moreover, a simple calibration procedure has been used for identifying the supposed bi-linear shear-stress-slip relationship describing the effect of partly cured adhesive on the behavior of the FRP-to-concrete adhesive interface

Prestressed FRP systems

This chapter provides an overview on the state-of-the-art in prestressing systems for the structural retrofitting of reinforced concrete (RC) structures using Externally Bonded (EB) Fibre Reinforced Polymers (FRP). Focus is put on flexural strengthening, which currently is the most common application field for composite materials in structural engineering. The manuscript provides information regarding commercially available prestressing systems and their anchorage procedures. In addition to conventional mechanical anchorages, the innovative ‘gradient anchorage’ that lacks any remaining plates or bolts is also presented. Additionally, the authors mention various current prototypes at the laboratory-scale level. Performed experimental investigations, results, and conclusions represent the core content of this chapter. Several studies from various universities and research institutes worldwide are presented and explained. In these research projects, the previously mentioned systems are applied to specific reinforced or prestressed reinforced concrete members for strengthening purposes. Static and/or dynamic loading indicate the efficiency of the retrofitting concept compared to the reference structure. Generally, prestressed FRP will be demonstrated to follow the principle of conventional prestressed concrete by resulting in higher cracking, yielding, and bearing loads. Especially under service loads, the structural behaviour is improved

Round robin test on the bond behaviour of externally bonded FRP systems to concrete

The reliable characterisation of the bond behaviour of an FRP system bonded to concrete is critical for the optimal design and detailing of a strengthening solution. Although various test methods to examine the bond behaviour of externally bonded FRP systems have been proposed thus far, their implementation can lead to a wide range of results and a standard methodology has yet to be generally accepted. With these issues in mind, a Round Robin Testing programme was carried out to assess the performance and reliability of small scale testing on six externally bonded FRP strengthening systems. Seven laboratories and four manufacturers and suppliers participated in this extensive international exercise, which was initiated within the framework of the European funded Marie Curie Research Training Network, EN-CORE, with the support of Task Group 9.3 of the International Federation for Structural Concrete (fib). This paper describes the proposed testing programme and summarizes the results obtained by the participating laboratories

ROUND ROBIN TEST ON THE BOND BEHAVIOUR OF EXTERNALLY BONDED FRP SYSTEMS TO CONCRETE

The performance of an externally bonded FRP strengthening system is often limited by its ability to effectively transfer stresses between the FRP reinforcement and the concrete substrate. As evidenced in the literature, debonding of the FRP reinforcement can occur at a level of strain well below its ultimate strain limit and generally results in a brittle type of failure. An accurate characterisation of the bond behaviour between an FRP system and concrete is therefore critical for the optimal design and detailing of a strengthening solution. Although various test methods to examine the local bond behaviour of externally bonded FRP systems have been proposed thus far, their implementation can lead to a wide range of results and a standard methodology has yet to be generally accepted. With these issues in mind, a Round Robin Testing (RRT) programme was carried out to assess the performance and reliability of small scale testing on various externally bonded FRP strengthening systems. Seven laboratories and four manufacturers and suppliers participated in this extensive international exercise, which was initiated within the framework of the European funded Marie Curie Research Training Network, EN-CORE, with the support of Task Group 9.3 of the International Federation for Structural Concrete (fib). This paper describes the proposed testing programme and summarizes the results obtained by the participating laboratories. The influence of various parameters on the bond behaviour of the examined strengthening systems is discussed and recommendations on testing procedures are given

Bond of FRP strengthening systems for concrete structures: a round Robin test

Although various test methods to examine the local bond behaviour of FRP strengthening systems to concrete have been proposed thus far, their implementation can lead to a wide range of results and a standard methodology has yet to be generally accepted. With these issues in mind, a Round Robin Testing (RRT) programme was carried out to assess the performance and reliability of small scale testing on various FRP strengthening systems, including both externally bonded laminates and near surface mounted reinforcement. Ten laboratories and eight manufacturers and suppliers participated in this extensive international exercise, which was initiated within the framework of the European funded Marie Curie Research Training Network, EN-CORE, with the support of Task Group 9.3 of the International Federation for Structural Concrete (fib). This paper describes the proposed testing programme and summarized some of the results obtained by the participating laboratories