Optimization Of Bond Characteristics Between BFRP Bar And Concrete And Evaluation Of Their Long-term Behavior

Fiber-reinforced polymer (FRP) materials have been enjoying wide research and applications in the field of civil engineering, due to their advantages such as light weight, high strength and corrosion resistance. An important and effective way to avoid the problem of corrosion of steel reinforcement in concrete is to use FRP reinforcing bars as an alternative to steel rebars. According to fiber materials,FRP reinforcing bars can be divided into carbon fiber composite reinforcement (CFRP bar), glass fiber composite reinforcement (GFRP bar), aramid fiber composite reinforcement (AFRP bar) and a new type of basalt fiber composite reinforcement (BFRP), etc.The deficiencies of existing FRP bar, like the high price and brittleness of traditional carbon fiber-reinforced polymer (CFRP) bars,the bad alkali-resistance and low creep rupture stress of the glass fiber-reinforced polymer (GFRP) bars as well as the high creep rate, restrict the application of FRP reinforcement. The basalt fiber reinforced polymer (BFRP) has reasonable price, high elongation, high alkali corrosion resistance, creep and fatigue performance, etc. The BFRP bars provide a new space for the development of FRP reinforcement because of the reasonable price, high elongation, good resistance of acidic alkali and salt and creep deformation resistant.Just like the reinforced concrete structure, reliable bond behavior between BFRP and concrete is a fundamental issue for reinforced concrete. However, there is a lack of the researches on the bond behavior especially the long-term bond behavior. Therefore,119 bond specimens were made according to standards ACI440.3R and JSCE-E 531-1995. Researches on bond behavior between BFRP and concrete were carried out by the means of pullout tests under monotonic load, cyclic load and sustained loads.(1)Through the pullout specimens under monotonic load, typical bond-slip curve was obtained. The failure mode, characteristic of each stage in bond-slip curve, process of bond failure and bond failure mechanism was studied. Moreover, a comparison between bond behavior of BFRP and steel bars embedded in concrete was drawn. Based on the results of the pullout tests, the effects of rib geometries (rib height, rib spacing and rib width), rebar surface texture on bond behavior were studied and the optimum analysis of BFRP bar was also conducted. The results of the tests are summarized as follows: ?The ultimate bond strength of the deep integration rib bar was obviously superior to the plain surface BFRP bars.?Through the surface treatment of ribbed BFRP bars,the bonding performance can be changed and the deep integration rib increased most. ?The slip has a essential diference between FRP bars and steel bars in the residual period, the rib of FRP reinforcement has less abrasion wear than steel bars, so there is still a certain bond strength which is helpful for the control of slip.?Based on analysis of the test results, design recommendations involving optimal rib spacing(d), rib height(0.09d) and width(0.25d) were made concerning optimum rib geometries of BFRP ribbed rebars with superior bond-slip performance.(2)The bond behavior between BFRP and concrete under cyclic and sustained load are preliminary discussed in this paper based on the pullout tests. The failure mode, peak slip, bond stiffness and hysteretic energy of the specimens under cyclic loads were inspected in this test. The results of research are summarized as follows:?According to the experiment results, pullout tests under monotonic and cyclic loading had the similar ultimate peak slip.?The peak and residual slip increased with the the cycles of the loading.?The loaded end slip increased with the cycles and tended to be stable until the fatigue damage occured suddenly.?Bond stiffness was almost the same during the process of bond-slip development. Under the sustained loading, the growth of slip along with the time was divided into two stages:the creep rate increased rapidly in the beginning and decreases gradually which called unstable creep stage (0-120h), creep rate reached the minimum and intended to be stable which called stable creep stage. Stress level had a great effect on the creep performance:in the case of smaller load level, the sliding growth were more likely to be stable, the growth rate was smaller;As the loading increased, in the stable creep stage, slip increased and its growth rate increased.(3)Finally based on the test results and worldwide researches, a new equation for bond strength and basic anchorage length of BFRP rebar is proposed in this paper. It provides principles for design code. In addition, a model of bond-slip constitutive relationship, which can describe the process of bond-slip and accord with the results of the tests, is built up after analysis of existing models of bond-slip constitutive relationship.