Strengthening of concrete structures by the use of mineral based composites

Abstract: Strengthening of concrete structures with epoxy bonded carbon fiber reinforced polymers (CFRP) has been proved to be an excellent strengthening technique. However, using epoxy adhesives for bonding do contain some disadvantages such as diffusion closeness, thermal incompatibility to the base concrete, regulations regarding the working environment and minimum application temperature. Some of these drawbacks can be lessened by substituting the epoxy to a polymer reinforced mortar as the bonding agent. A new acronym for strengthening concrete structures with CFRP and polymer reinforced mortars is introduced, mineral based composites (MBC). This thesis presents experimental tests for both flexural strengthening and shear strengthening techniques using CFRP grids and polymer modified mortar as the bonding agent are presented. Flexural strengthening using the MBC system was designed as a pilot study to evaluate suitability of different mortars with different mechanical properties. The outcome of the pilot study onflexural strengthened small scale concrete beams gave indications on the choice of mortar used in the MBC system. A total amount of 21 concrete beams with and without shear strengthening subjected to four-point bending is evaluated in the thesis. The concrete beams were 4.5 m long and had a rectangular cross section of 180 x 500 mm. A number of parameters were varied for these beam specimens namely; concrete strength, shear reinforcement design, mortar properties, grid design and the addition of flexural strengthening using Near Surface Mounted Reinforcement (NSMR). Considering the steel shear reinforcement, three different variations were utilized; no shear reinforcement, a stirrup distance of 250 mm and 350 mm respectively. The results from the experimental study of the shear capacity using MBC on beams with no shear reinforcement indicates that strengthening concrete structures with the MBC system has competitive properties compared to epoxy bonded strengthening techniques. The MBC system reached 97% ofthe ultimate load achieved by a strengthening system with vertically applied epoxy bonded carbon fibre sheets. The ultimate failure load was increased with the increase of carbon fibre amount in the grid. Using a grid with small distance between the CFRP tows generated a higher first visible shear crack load. A simplified analytical design proposal to estimate the shear resistance contribution of the MBC strengthening system is proposed in the thesis. Reasonable results were obtained compared to experimental ultimate failure loads. Strains were monitored in the longitudinal steel reinforcement and the steel stirrups for the beams with internal steel shear reinforcement. Here it as clear that the use of the MBC strengthening system reduces the strains in the steel stirrups. Further, photmetric strain measurement on the surface of the strengthening system was also assessed. The use of the MBC strengthening system also reduces the prinicpal strain at the surface of the shear strengthening system. It can be concluded that the MBC strengthening system has great potentials to strengthen concrete structures. Further research are however needed in both development of the materials included