Bond Behavior Between Steel-Polypropylene Hybrid Fiber-Reinforced High Strength Concrete And Deformed Bar

The bond performance between steel bar and concrete is not only very important in engineering practice, but also critical in theory of reinforced concrete. However, due to the complexity of load-transferring mechanism in the interface between reinforcement and concrete, numerous influence factors, non-linearity and randomness of concrete itself, discrepancies of test methods and measurement techniques, the experimental results are scattering. Therefore, this kind of research is difficult and develops slowly, and becomes a difficult problem in basic theory of reinforced concrete structures.High strength concrete has a broad application prospect for the sake of long-term and comprehensive economy. As we known, the higher of concrete strength, the greater of material brittleness. Researchers added steel fiber, polypropylene fiber and other fibers in concrete, in order to improve the mechanical properties and durability of concrete structure. However, concrete is a kind of non-homogeneous, heterogeneous materials, adding a certain kind of fiber can only significantly improve or enhance the performance of a certain phase, in order to get better effect, two or more complementary fibers are mixed into concrete to improve strength, toughness and durability of concrete matrix at different levels and stages.In order to make Steel-Polypropylene Hybrid Fiber-Reinforced High Strength Concrete (referred to as SPHFRC) and steel bar play their respective advantages to work together, good bonding between these two kinds of materials should be ensured. At present, researches on bond behavior between SPHFRC and steel bar are rarely reported, while bond stress-slip constitutive relation is one of the common concerns in fields of engineering and mechanics, which plays an important and core role in numerical simulation of engineering structure. As a result, based on experimental study and theoretical analysis, bond properties between SPHFRC and deformed bar were carried out systematically in this paper, and the main work and achievements are as follows:(1) Influences of volume fraction and aspect ratio of steel fiber and polypropylene fiber, and interaction of various factors on bond strength between SPHFRC and deformed steel bar were analyzed quantitatively by pullout test; according to test results, calculation formula of bond strength between SPHFRC and deformed steel bar based on strength of SPHFRC was established, as well as conversion formula of SPHFRC bond strength and normal concrete bond strength. At the same time, a conclusion could be drawn that designed anchorage length of tensile reinforcement in SPHFRC could be shortened accordingly under the condition of anchoring reliability consistent guaranteed in reinforced concrete structure, and application of SPHFRC was favorable to overlap joint of steel bar and anchoring, etc.(2) Using single ended central pullout test, researches on average bond stress-slip constitutive relation between SPHFRC and deformed reinforcement were carried out, actual observation of load-slip overall process curve was recorded, and overall process of destruction of bond between SPHFRC and deformed bar was revealed. According to test results, strengthen mechanism of hybrid fiber on concrete matrix in overall process of bond failure resistance was analyzed, and mathematical expression of relation curve of average bond stress and free end slip was proposed by nonlinear fitting method in DATAFIT software. At the same time, the results were compared with those of bond stress-slip relation between normal concrete and deformed bar, and the differences between them were analyzed in detail.(3) Based on pullout test results and fiber strengthen theory, bond mechanism between SPHFRC and deformed bar was analyzed. Namely, hybrid fiber not only increased concrete strength, but also enhanced constraints to steel bar on the one hand; on the other hand, crack resistance composite effect of hybrid fiber was more conducive to impose restriction on generation and development of micro cracks in concrete, so as to reduce defects of concrete, and to enhance bonding strength accordingly. In hybrid fiber effect, steel fiber played a major role, and steel fiber with longer aspect ratio had better strengthening effect.(4) According to results of local bond stress-slip test in Slot-Patch-Method, strain and free end slip of steel bar along the bond anchorage length in different positions and stages were actually measured. Therefore, position function was proposed to reflect bond stress changing with bonding position in different stages; combined with product of average bond stress-slip constitutive equation and position function of hybrid fiber reinforced concrete, a more accurate description of bond stress-slip constitutive relation between SPHFRC and deformed bar was presented.Finally, on the basis of summarizing the dissertation work, some related research prospects are put forward.