Experimental Study On Bond-Slip Behavior Between FRP Bar And Concrete Based On Intelligent Bars

Whether it is in the construction phase or the use phase in construction projects at home and abroad, reinforced concrete structural cracks are the common problem. Reinforcement corrosion problem caused by structural cracks is becoming increasingly prominent and it severely reduced the structural durability and safety. To solve this problem, a number of measures have been proposed, which using fiber reinforced polymer (FRP for short) instead of steel is an effective way to solve the problem. The bonding behaviors between FRP bars and concrete are the key issues to determine FRP bars and concrete work together and ensure the two materials playing roles properly. In addition, most of the current study only focused on the average bond stress variation with displacement, and the research of bond stress distribution along the bond length was little, so how to accurately measure the internal bonding mechanism is the future direction of development.In this paper, the 150mm×150mm×150mm concrete axial tension specimens of CFRP bars and GFRP bars were produced and the single-ended axial pullout test was proceeded. Meanwhile by using the single-ended axial pullout test method, the intelligent sensing FBG-GFRP bar specimens and intelligent GFRP bar specimens with bonding strain gauge externally were produced. The bonding properties between GFRP bars and concrete along the bond length were researched and the interface bonding stress transfer rule of intelligent GFRP specimens was analyzed.The test results showed that:The FRP bars and concrete bond failure modes include load-side FRP bars brittle fracture, FRP bars pull failure and FRP bar internal brittle fracture. FRP bars pull failure includes ribs stripping failure and intercostal concrete splitting failure. According to the typical bond-slip curves trend, the bond stress process of ribbed FRP bar specimens can be divided:Micro-slip phase, slip phase, pullout phase, drop down phase and the residual volatile phase; the bond stress process of plain FRP bar specimens can be divided:slip phase, drop down phase and horizontal phase. Generally, the bonding force of FRP bars and concrete consist of chemical glue force, interface friction force and mechanical bite force, the failure mechanism of ribbed FRP bars and plain FRP bars in the process of the experiment were analyzed respectively in this paper. The similarities and differences of bond-slip curves for CFRP and GFRP bar specimens were compared, meanwhile the influence of concrete strength on the bond-slip curves were analyzed. By fitting the experimental data we found that:a cubic curve model is very suitable for the rising bond-slip curve segment in this experiment and the second formula of improved BPE model is very suitable for the decline curve segment of the test. The two models were combined to be one new model which was suitable for the test data. In this paper, four kinds of factors influencing the bond strength of FRP bar and concrete were analyzed and we found that:with the diameter of FRP bars and bond length increase, the bond stress will be reduced; The impact of concrete strength on FRP bars and concrete bond stress is little, generally with the strength of concrete increases, the bond strength increases; the greater loading speed, the greater the bond stress.By analyzing the test results of intelligent FRP bar specimens with bonding strain gauge externally, it was obtained the strain and stress change rules of FRP bars with loading and load side displacement. And it was got that the FRP bar and concrete bond stress along bond length was the right- skewed distribution. By comparing the measured results of strain sensor and strain gauge, we found that, the strain change rules of FRP bars with loading and load side displacement measured by both methods was consistent well, which explained the test data measured by strain gauge has good reliability.Through this research and analysis, it provided some lessons for FRP bars and concrete bond slip test study and laid the foundation for in-depth, comprehensive and systematic study of the mechanical properties of FRP bars.