Temperature stability and durability of externally bonded CFRP strips in bridge construction
The use of Externally Bonded (EB) Carbon Fiber Reinforced Polymer (CFRP) strips is a popular strengthening technique for structurally deficient Reinforced Concrete (RC) structures. One possible application of this strengthening method are box-girder bridges with their cantilevers in transverse direction. Such an installation raises the following two questions: 1) how does the CFRP-epoxy-concrete system behave during and after the warm asphalt application, and 2) how does the system behave on the long-term?.
The typical road build-up for bridges in the canton of Zurich is comprised of three main steps: a) appliying a sealing layer (epoxy mixture) directly on the concrete substrate, b) PBD (German acronym for polymer-bitumen sealing layer), and c) warm mastic asphalt (approximately 240°C in the transportation truck).
The PBD and warm mastic asphalt application steps induce elevated temperatures in the epoxy adhesive layer below the CFRP strips. Whilst being almost negligible for the PBD installation, the epoxy temperature climbs to almost 80°C followed by a plateau and a slow cooling process for the mastic asphalt application. The ambient temperature is attained only after several hours. If the CFRP strips are non-prestressed and unloaded, no problems regarding a softer epoxy due to exceeding the glass transition temperature are expected. Epoxy post-curing might even be beneficial for the bonding between the different components. In the framework of this project the residual bond strength of the CFRP-epoxy-concrete system after the warm mastic asphalt exposure and the related temperature rising and cooling process has been investigated. Lap-shear as well as large-scale tests were performed before and after the temperature exposure to assess any possible degradation and hence any reduction in the load-carrying capacity. It was found that the mastic asphalt application does not reduce the residual load carrying capacity of the system; however, the prestressed CFRP strips cannot resist this temperature scenario.
The lateral cantilever of highway concrete box-girder bridges can also suffer damage owing to fatigue loading. In this project, the fatigue behaviour of bridge deck slabs belonging to lateral cantilever of highway concrete box-girder bridges that have been strengthened on the upper side with EB CFRP strips has been investigated. Several experimental studies have demonstrated that at room temperature, the typical fatigue failure is due to fracturing of the longitudinal steel reinforcement followed by delamination of the EB CFRP strip. This delamination usually happens a few cycles after the fatigue failure of the steel reinforcement. However, concerning the long-term behaviour, the strips and the epoxy adhesive can reach elevated temperatures up to 50°C during service when they are directly exposed to the sun, changing the usual fatigue failure mode of these elements at room temperature. Therefore, several large-scale experiments on RC slabs strengthened with EB CFRP strips and tested under fatigue load at an elevated temperature of 50°C were performed. It was found that the two million load cycles under 50°C did not negatively influence the residual load carrying capacity of the strengthened slabs. It was even beneficial because of the post-curing of the epoxy adhesive.
The long-term behaviour of such strengthening systems is another concern. Measurements on strengthened RC cantilever slabs with an asphalt layer on the top and exposed to external weather conditions in the Zurich area showed temperatures at the asphalt surface of approximately 60°C under direct sun exposure. Owing to the heating of the mastic asphalt layer, these elevated temperatures cause an increase in the epoxy temperature that could affect the long-term properties of the adhesive. In this project, the long-term behaviour of RC slabs strengthened with EB CFRP strips and subjected to sustained loading was studied. For this purpose, several large-scale RC slabs strengthened with non-prestressed and prestressed EB CFRP strips were fabricated, fixed as cantilever slabs and exposed to outside weather conditions in the Zurich region for several months. Strain gauges were installed on the strips, and the changes in the strip strains were recorded with a long-term monitoring system. So far, a very good behavior of the unstressed and prestressed EB CFRP strips was observed.