Reasonable Matching Of Hybrid Fiber Composties And Their Applied Research In Strengthening Concrete Structures

In order to improve the efficiency of strengthening concrete structure with sole fiber reinforced plastic (FRP) and overcome the disadvantages of the primary research of hybrid fiber reinforced plastic (HFRP), a new idea that strengthening concrete structure with reasonable matching of HFRP was proposed in this thesis firstly.In response to some researcher's doubt on the durability and stiffness of HFRP in the world, the durability, stiffness of HFRP and its feasibility to strengthen concrete structure were discussed, based on the summary of properties of HFRP as well as its successful applications in several related fields. In the light of this, we decide to use imported carbon fiber (CF) and local high-strength glass fiber (SGF) to manufacture the HFRP which is adapted to concrete structure strengthening.Based on the discussion above, a primary comparative experiment of concrete columns externally wrapped with HFRP, carbon fiber reinforced polymer (CFRP) and high-strength glass fiber reinforced polymer (SGFRP) was carried out to study the ductility, load carrying capacity, strengthening cost of strengthened columns and the exerting efficiency of CF. Test results showed that the HFRP can fully exploit the advantages and overcome shortages of the materials. To improve the same load carrying capacity and stiffness of the specimens, HFRP can enable the strengthened columns certain ductility and a significant decrease of strengthening cost.Considering the size effect has a noticeable influence on the mechanism of FRPspecimen, after the comparatively experimental study of the small specimens, apractical and proper testing method of tensile properties of FRP to use big specimen was proposed. And a comparative experiment of HFRP with different matching wascarried out to study their failure feature and mechanism. The test results prove that, besides lower cost, the strength and the elongation ratio of HFRP in matching of volume ratios of CF Vc =0.198 and 0.247 are much higher than those of other matching of HFRP and sole CFRP, with a slight influence on stiffness. These two matching ratios are suitable and reasonable for the HFRP.Moreover, we notice that failure of the bonding interface between FRP and concrete is one of the main failure modes. After analyzing the bond mechanism and the strengthening arts, we pointed out that pressure is one of the three factors that ensure the bond strength, and the failure is mainly because of no pressure to the bonding interface. Then a convenient and practical construction technique of hydraulic pad-exerting-pressure was proposed. Through the small specimen tests, we found the failure of the bonding interface between FRP and concrete can be avoided and the efficiency can be highly improved, as long as the pressure is almost 0.2 kg/cm2.According to the results of the above experimental research, the reasonable matching of HFRP and construction technique of hydraulic pad-exerting-pressure were applied to a bending strengthening project of roofing frame beams. In the course of strengthening design, the shortcoming to determine the design strength value of FRP suggested by "Technical Specification for Strengthening Concrete Structure with Carbon Fiber Reinforced Polymer" was pointed out. A method consistent with the one to determine the design strength value of steel fs in "Code for Design of Concrete Structures" was proposed. And reliable design and construction method were employed in the strengthening project.At last some further research proposals of strengthening concrete structures with HFRP were presented: besides the theoretical descriptions of the durability of HFRP and its successful applications, further experimental research should be carried out to study its feasibility in strengthening concrete; in order to inspect and amend the practical effect of the reasonable matching, experimental study of concrete columns and beams (the author have fabricated 30 HFRP strengthening concrete columns with a diameter to height ratio of 1:3 and 15 concret...