Strengthening of reinforced concrete structures with use of externally applied FRP reinforcement has been a well-recognised solution for many years. Fibre-reinforced polymers are used in various situations, including flexural strengthening, shear strengthening and confinement of columns. External reinforcement is usually applied in form of rigid, unidirectional laminates glued to a concrete surface or bi-directional, flexible mats composed of dry fibres and glued in wet lay-up process.
The aim of the research project is to develop a commercial alternative bonding technology, which would make the application process much easier and faster. Currently used epoxy adhesives 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, a lot of tools and a lot of clean up afterwards.
The proposed strengthening system will use thin, flexible CFRP laminates lined with one-part adhesive that cures in ambient conditions. Removing the adhesive mixing from the installation process will allow for an instant application in any conditions. Additionally, the flexible CFRP tapes will make the system suitable for any shape and size of structural elements, including slabs, ceilings, round columns and rectangular beams.
The CFRP tapes could be used for rapid applications for temporary or permanent strengthening, as well as in emergency situations, where easiness of installation would be crucial. With development of suitable adhesive it might be possible to apply the tapes also on steel, masonry or timber structures. A market analysis will be included in the research activities, to evaluate the market size and identify the opportunities coming with the development of new strengthening system.
The main objective is to develop a new product for structural strengthening, comprising of a CFRP tape that can be easily and quickly applied on a construction. The research program will focus on the fulfilment of the following aims:
Due to the change of the research subject in January 2016, the research work is starting from the very beginning, therefore not many results are ready to be reported at this point. The results of the research on the previous subject are summarised in a separate report.
In the initial phase of the research a market analysis has been conducted. Comparison of existing products, its properties and prices allowed to define certain requirements the researched product will have to fulfill to be successfully introduced to market.
In the next step mechanical properties of different types of adhesives will be investigated, along with its bond to FRP materials and concrete, possibly also to masonry, steel or timber. The most appropriate adhesive will be chosen. The polymer matrix with the best bond properties will be selected for the composition of the FRP tapes.
The selected adhesive will be subjected to material tests, including tensile strength, modulus of elasticity, shear strength and bond strength tests. A trial application on different surfaces will be performed, to assess the feasibility of using the one-part adhesive and thin CFRP tapes.
The efficiency study will be carried out by means of a series of load-capacity tests on non-strengthened and strengthened concrete members. An increase in the load capacity will be the direct measure of the strengthening efficiency. Additional durability study will allow to assess a degree of the strengthening’s degradation under the influence of moisture, freeze-thaw cycles, UV light and other environmental factors.
Finally a market-ready product will be developed. This will include the development of a manufacturing process, packaging and application instructions followed by trial applications in the field.
In the second quarter of 2016 a research on available adhesives and thermoplastic resins has been conducted. The research on adhesives included identification of structural adhesives, suitable for bonding of thermoplastic CFRP tapes to concrete, along with its mechanical properties. Adhesives selected for further investigation comprised of polyurethane (and silane-modified polyurethane), methyl methacrylate and epoxy.
The pilot experimental investigation on adhesives was prepared. Fourteen lap-shear tests on CFRP tapes glued to concrete substrate with seven different adhesives were planned. In the next phase some of the best performing adhesives will be used in small-scale flexural strengthening efficiency investigation. A series of ten bending tests on prism strengthened with externally bonded tapes were planned.
The research on polymer resins for thermoplastic CFRP tapes allowed to select the most suitable range of materials for production of external reinforcement for concrete. Studied parameters comprised of availability, price, mechanical properties, chemical resistance as well as in-house processing capabilities. A range of engineering polymers was selected for further investigation, including polyphenylene sulfide (PPS), polyethersulfone (PES) and polyetherimide (PEI). Tape production process is also being investigated and developed.
The laboratory tests of adhesives for bonding of thermoplastic tapes to concrete have been conducted in the third quarter of 2016. In the first stage 7 different adhesives were tested in single lap-shear setup. The tests included polyurethane, epoxy, methyl methacrylate and silane-modified adhesives (samples A to G). Summary of the results, in form of load-slip curves, are presented in Figure 1.
Fig.1 - Load-slip curves obtained in lap-shear tests
During the tests the epoxy adhesives showed the highest stiffness, but its bond strength to thermoplastic tape was limited. All the other adhesives exhibited significantly lower stiffness which resulted in much higher slip. Nevertheless, the silane-modified adhesives showed similar bond strength to epoxy, while some of the polyurethane adhesives even exceeded epoxy’s bond strength by a factor of 2, which made it an interesting alternative for bonding thermoplastic laminates. Methyl methacrylate adhesive bonded very poorly to concrete. In almost all of the other cases the debonding occurred on the adhesive – tape interface.
Fig.2 - Load-deflection curves obtained in 3-point bending tests
In the second stage 5 selected adhesives were used in small-scale flexural tests. 500mm long concrete prism with 100x100mm square cross-section were strengthened with 2 strips of thermoplastic tape. The samples were subjected to 3-point bending, while its deflection and tensile strain in composite tapes were registered. The results of the tests are summarized in Figures 2 and 3.
Fig.3 - Development of tensile stresses in composite tapes during flexural tests
The efficiency of strengthening with use of silane-modified adhesives, in terms of achieved load capacities, was similar to the results obtained with use of epoxy adhesive. The strongest polyurethane adhesive allowed to achieve significantly higher load capacity and excellent ductility of the strengthened beam. Very elastic adhesive (sample D) led to minimal improvement of beam’s load capacity (Fig. 2), which proves that the strengthening efficiency relies strongly on the modulus of elasticity of the adhesive material.
In terms of maximal strains and stresses generated in composite tapes, silane-modified adhesives proved to be as effective as epoxy, while polyurethane adhesive allowed to achieve extreme stresses, nearly reaching tape’s ultimate tensile strength, again proving its superior performance (Fig. 3).
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