The aim of the research project was to develop an alternative bonding technology, which would make the application process much easier and faster. The proposed strengthening system would use thin, flexible FRP laminates as a reinforcement and one-component adhesive that cures in ambient conditions. It was envisaged that removing the mixing of adhesive components from the installation process would allow for an instant application in any conditions. Additionally, the flexible FRP tapes would make the system suitable for strengthening of any shape and size of structural elements, including slabs, ceilings, round columns and rectangular beams. To furtherly improve the flexibility of the strengthening system, the FRP tapes were manufactured with use of thermoplastic polymers, rather than conventional thermoset materials. Contrary to their thermoset counterparts, thermoplastic FRP composites can be re-shaped multiple times with use of locally applied heat.
Rehabilitation of reinforced concrete structures with use of FRP (Fibre-Reinforced Polymer) composites has been a well-recognised solution for many years. FRP composites are used in various situations, including structural repair of damaged structures, external bonding to increase flexural and shear load capacity, or as an internal reinforcement. External reinforcement is usually applied in form of rigid laminates bonded to a concrete surface, or flexible woven mats composed of dry fibres and glued in wet lay-up process. Current strengthening systems use epoxy adhesives for bonding of the FRP composites, which require precise weighing and pre-mixing of two components (resin and hardener) before application on the bonded surfaces. The overall process can be time-consuming, requires trained personnel, and is very susceptible to unfavourable environmental conditions (i.e. humidity, temperature).
The research programme was divided into multiple stages and the ultimate objective to develop a bonding system that can be easily and quickly deployed with minimum on-site work necessary. First stage of the research was focused on the adhesives. A wide range of adhesives were tested in a single lap-shear setup to test their performance in terms of elasticity and bond strength to thermoplastic composites and concrete. In the second stage a selection of adhesives that showed the best performance were used in small-scale flexural strengthening tests in 3-point-bending tests on concrete prisms. The results of the tests showed that the strengthening was effective, even when more elastic one-component adhesives were used.
Further development was focused on development structural one-component adhesive. A strategic partnership was formed with one of the manufacturers of adhesives to investigate custom-made, moisture-curing structural adhesives. The experimental investigation included testing multiple different recipes and admixtures, as well as tests of curing speed and bond strength to various substrates. Various means of packaging the adhesive to prevent its curing were also investigated. As a result it was possible to formulate an adhesive with sufficient bond strength, which was applied on the FRP tape and put in a prototype moisture-proof packaging which allowed for storage and future rapid application.
In the final stage of the research the developed strengthening system was tested in large-scale strengthening tests in 4-point-bending setup. Conventional epoxy adhesive was also tested for comparison. The results have shown that the strengthening with use of the developed system was very effective with respect to the existing systems. 36% increase in flexural load capacity was observed when the one-component adhesive was used, compared to 52% increase obtained with use of conventional two-component epoxy. In an additional trial a concrete beam was strengthened in shear with use of the thermoplastic FRP tapes that were re-shaped on site to match the size of the strengthened element, and bonded with epoxy adhesive. Tests have shown that the shear load capacity was effectively increased thanks to such strengthening.
The results of the above mentioned tests have proven that thermoplastic FRP composites and one-component adhesives can be an effective means of structural strengthening and repair of concrete buildings. It was possible to rapidly deploy a pre-packaged composite reinforcement with adhesive for flexural strengthening, and use the thermoplastic properties of the composite tape for shear strengthening. The results in terms of load capacity gains were also satisfactory. Further development of the adhesive and its properties is however necessary for the strengthening system to be applicable on a commercial scale.
The entire development of the strengthening system was supported with commercial analysis, including market research and price comparison between the proposed and existing, epoxy-based strengthening systems.