Development of prestressed unbonded and bonded CFRP strengthening solutions for tensile metallic members

In this study, a novel unbonded mechanical clamping system was developed for the strengthening of tensile metallic members using prestressed carbon fiber reinforced polymer (CFRP) plates. The system clamps a pair of prestressed CFRP reinforcement to a metallic substrate and provides an almost uniform contact pressure over the CFRP plate along the anchorage length. A finite element simulation was used to optimize the design of the mechanical components of the system. Subsequently, a set of static and fatigue tests was performed to evaluate the performance of the optimized design. Experimental results revealed that the proposed mechanical clamping system is capable of transferring the entire tensile capacity of the CFRP plates to the steel substrate, even after experiencing 10 million fatigue cycles. The comparative performance of the developed clamps was further investigated by a set of static tests on steel plate specimens strengthened with the prestressed bonded reinforcement (PBR) and the newly developed prestressed unbonded reinforcement (PUR) systems. Furthermore, simple analytical models are proposed to formulate the stress state in prestressed unbonded and bonded CFRP-strengthened tensile metallic members. The accuracy of the proposed analytical formulations was verified by the experimental results obtained during the current study. Experimental results revealed that the efficacy of having relatively high prestressing forces in the normal modulus (NM) CFRP reinforcements is much higher than the stiffness improvement obtained by using ultra-high modulus (UHM) CFRPs. However, the available capacity of the PBR system before debonding failure is far lower than that of the developed PUR solution.

Flat prestressed unbonded retrofit system for strengthening of existing metallic I-Girders

Elyas Ghafoori, Ardalan Hosseini, Masoud Motavalli, Alain Nussbaumer

In this study, a novel retrofitting system is developed to strengthen existing metallic I-girders using prestressed unbonded carbon fiber reinforced polymer (CFRP) plates. The system relies on a pair of mechanical clamps. Each clamp holds two CFRP plates (each having cross-sectional dimensions of 50 × 1.4 mm) and anchors their prestressing forces to a metallic I-girder via friction. A finite element (FE) model was established to optimize the design of the required mechanical components, and a set of pull-off tests was performed to evaluate the capacity of the optimized system. The proposed flat prestressed unbonded retrofit (FPUR) system was then applied on a 6.4-m-long steel I-beam, and the excellent performance of the system, in terms of stress reduction in the beam bottom flange, was confirmed based on the results of a set of static four-point bending tests. Moreover, a fatigue four-point bending test was conducted on the steel I-beam, strengthened using the proposed FPUR system with a prestressing level of 53% (i.e., approximately 1120 MPa prestress in the CFRPs). Experimental results of the high-cycle fatigue test confirmed the excellent fatigue performance of the proposed FPUR system, as no prestress loss was observed after 20 million fatigue cycles. An analytical model is proposed to accurately predict the stress state in an I-girder strengthened with the proposed FPUR system, while design recommendations are provided concerning the practical use of the system. Relying on the laboratory experimental test results, the proposed FPUR system was used to strengthen the metallic cross-girders of a 121-year-old bridge in Australia.

2018

A novel mechanical clamp for strengthening of steel members using prestressed CFRP plates

Elyas Ghafoori, Ardalan Hosseini, Masoud Motavalli, Alain Nussbaumer

Strengthening of existing steel members using prestressed carbon fiber reinforced polymer (CFRP) plates has certain advantages over non-prestressed reinforcement, especially due to the reduction in permanent tensile stresses acting on the steel members. However, only very low prestressing levels in CFRP plates may be reached when adhesively-bonded reinforcement is used. The reason is that high interfacial shear stresses generated in the adhesive layer during the prestress release process lead to the premature debonding of the prestressed CFRP reinforcements from the steel substrate under transient service loads. In the current study, a novel unbonded mechanical clamp is proposed to anchor high prestressing forces to the steel substrate. A finite element (FE) model was used to optimize the design of the required mechanical parts. Subsequently, a set of static and fatigue tests was performed to evaluate the performance of the optimized system. The experimental results proved that the proposed system is capable of transferring the entire tensile capacity of the normal modulus CFRP reinforcing plates (50 × 1.4 mm each) to the steel substrate without any slippage of the joint. Consequently, based on the static and fatigue tests performed in the current study, it can be concluded that the proposed unbonded mechanical clamping system can be used for strengthening of existing steel structures using prestressed unbonded reinforcements (PURs) to increase the ultimate capacity and/or fatigue life.

2017

Prestressed unbonded reinforcement system with multiple CFRP plates for fatigue strengthening of steel members

Elyas Ghafoori, Ardalan Hosseini, Masoud Motavalli, Alain Nussbaumer

Carbon fiber reinforced polymer (CFRP) composites have exhibited a great potential for strengthening of steel structures. In the current study, an innovative prestressed unbonded reinforcement (PUR) system is introduced for fatigue strengthening of existing steel members. The system relies on a pair of mechanical clamps; each holds multiple CFRP plates and anchors their prestressing forces to the steel substrate via friction. A finite element model was established to optimize the design of the required mechanical components of the system. A set of static and fatigue tests was conducted on the developed mechanical clamps as the key elements of the proposed PUR system. The performance of the PUR system was then evaluated using a set of fatigue tests on two precracked steel plate specimens, one without any strengthening system and the other one strengthened with the proposed PUR system. In the latter specimen, the CFRP plates were prestressed up to about 800 MPa (approximately 30% of the CFRP tensile strength), which resulted in a complete fatigue crack arrest in the precracked steel plate. Furthermore, neither slippage of the mechanical clamps nor any prestress loss in the CFRP plates was observed after 7.5 million fatigue cycles. Based on the promising experimental results, obtained from the sets of fatigue tests performed in the current study, it can be concluded that the proposed PUR system can be considered as an efficient alternative to the conventional bonded reinforcement solutions for fatigue strengthening of damaged steel members.

2018