Near-surface Mounted Reinforcement Research Articles

Strengthening of the West Gate bridge with carbon fibre composites - a proof engineer’s perspective

The world’s biggest project of structural strengthening by means of bonded, high tensile carbon fibre reinforced polymers (CFRP) has recently been accomplished at the West Gate Bridge in Melbourne. The West Gate Bridge consists of two multi span precast concrete segmental box girder approach viaducts and a central cable stayed steel box girder Bridge. As a relief measure for traffic congestion an extra lane was provided on the western approach viaduct by utilising the former hard shoulder. This together with the increased (current) traffic loadings has resulted in higher traffic loads being applied to the bridge. The most critical areas of overstress that were identified were the torsional and shear capacity of the spine girder and the capacities of the deck slab and the precast cantilever frames. The main spine beam girder was strengthened by draped tendons to increase the global sag and hog capacity. For the other strengthening requirements, VicRoads decided on a solution proposed by the Design and Construct Contractor, of using high-strength CFRP materials due to their advantages as externally bonded reinforcement (EBR) or near surface mounted reinforcement (NSMR). This paper concentrates on this strengthening using CFRP materials. At the time of the design, these strengthening techniques were not very well established in Australia. They require not only knowledge about the mechanical properties of the innovative materials but also thorough understanding of and particular attention to bond and anchorage issues. The detailing and dimensioning by the Designers was carried out with the assistance of a German Engineering sub-consultant. The EBR and NSMR applications also posed a new challenge for the engineering consultants, Hyder Consulting, acting as Proof Engineers for the entire project. The proof engineering was in fact a gradual and stepwise process of assessment and improvement of the various details and was performed in co-operation with a German expert in Carbon Fibre technology (Prof Rostasy and Dr. Neubauer). This paper will cover the entire project, especially focusing on the EBR and NSMR issues. The authors hope by this, to make Australian engineers more familiar with these innovative techniques but also sensitive to their specific requirements.

Adhesive Applications Used During Repair and Strengthening of 30-Year-Old Prestressed Concrete Girders

The use of polymer adhesives during the strengthening of 30-year-old prestressed concrete bridge girders is described. The single-T–shaped specimens strengthened in this study were saw-cut from prestressed concrete double-T girders that had been removed from a bridge originally designed for H-15 highway loading and opened to traffic in 1969. Over time, the bridge had been severely overloaded, resulting in numerous flexural and shear cracks in the stems of each girder and large concrete spalls at the tension faces. In 1998, several damaged members were replaced. To investigate the feasibility of repairing or strengthening similar members in the future, the damaged members were retrofitted with carbon fiber–reinforced polymer (CFRP) sheets and then load tested under service and ultimate load conditions. Polymer adhesives were used to seal all visible cracks in the stems, to inject epoxy into all sealed cracks, to fill spalled areas in the tension face of the stems, to apply longitudinal and transverse CFRP sheets to the stems of the members, and to install near-surface-mounted reinforcement (NSMR) in one of the specimens. Results from load tests indicate that excellent bond was consistently achieved between the CFRP sheets and the 30-year-old concrete surface and between the NSMR and surrounding concrete. The success of epoxy injection of the flexural cracks, however, varied. Specifically, the injected cracks in one specimen reopened at an applied load well below the theoretical cracking load for the members.

A modified pull-out test for bond of near-surface mounted FRP rods in concrete

Among the strengthening techniques based on fiber-reinforced polymer (FRP) composites, the use of near-surface mounted (NSM) FRP rods is emerging as a promising technology for increasing flexural and shear strength of deficient concrete, masonry and timber members. In order for this technique to perform effectively, bond between the NSM reinforcement and the substrate material is a critical issue. Aim of this project was to investigate the mechanics of bond between NSM FRP rods and concrete, and to analyze the influence of the most critical parameters on the bond performance. Following up to previous investigations, a different type of specimen was designed in order to obtain a test procedure as efficient and reliable as possible. Among the investigated variables were: type of FRP rod (material and surface pattern), groove-filling material, bonded length, and groove size. Results of the first phase of the project are presented and discussed in this paper.

Characterization of FRP Rods as Near-Surface Mounted Reinforcement

The use of near surface mounted (NSM) fiber-reinforced polymer (FRP) rods is a promising technology for increasing flexural and shear strength of deficient reinforced concrete (RC) members. As this technology emerges, the structural behavior of RC elements strengthened with NSM FRP rods needs to be fully characterized. Given the variability of material properties and groove geometry, this requires that the tensile properties of the FRP rod and the mechanics of load transfer between NSM FRP rods and concrete be investigated. Tensile and bond tests on commercially available carbon FRP deformed rods for application as NSM reinforcement were carried out using test methods that are expected to become standards in North America. Three full-size beams, one control beam and two beams strengthened in shear with NSM FRP rods, were tested. Test results are presented and compared with the predictions of a simple design approach, showing reasonable agreement.