Effect Of Fibers And Carbon Black On Bond Behavior Between GFRP Rebar And Concrete

Fiber reinforced polymer/plastic rebar(FRP rebar) shows a promising future as an alternative to steel rebar in improving the safety and durability of conventional concrete structures for its relatively higher corrosion resistance. However, due to the intrinsic difference from steel rebar in manufacture and mechanical property, FRP rebar shows insufficient bond performance in concrete matrix. It is considered to be the prominent trouble for the application of FRP rebar as the good bond behavior is regarded as the basis of cooperation between FRP rebar and concrete matrix. Therefore, the study of improving bond behavior between FRP rebar and concrete is of great importance.Meanwhile, unlike steel rebar, as a linear elastic material, the stress-strain relationship for FRP rebar is almost linear until reaching its ultimate tensile strength, without a distinct yielding point or yielding stage. Therefore, concrete with the FRP reinforcement also acts as a brittle material, and shows no presage before failure. Randomly distributed structural steel fibers in concrete matrix can decrease the brittleness of concrete member and improve its ductility before failure.The safety of structures could be enhanced effectively through using self-diagnosing materials, which can minitor the accumulation of its own strain and damage without any attached sensors, and integrate the structure with function. This can be achieved by adding conductive phase materials, such as carbon black(CB), carbon fiber(CF) and steel fiber(SF), to plain concrete matrix. However, it is still unknown how theses conductive phase materials act on the bond behavior between FRP rebar and concrete matrix, and it really needs further study.On the premise of ensuring the basic mechanical properties of concrete matrix after adding the conductive phase materials, we mainly focus on the effect of CB, CF, SF and their content on the bond behavior between GFRP rebar and concrete matrix by means of the pullout test with centric bar placement in concrete cube. The test results show that the compressive strength of concrete is affected slightly by the admixture. The spilting strength will to some extent be improved when CF and SF is added to the matrix, no matter they are added separately or simultaneously. The same trend is also found when analysing the ratio of spilting strength to compressive strength, which means the brittleness of concrete is improved. Meanwhile, the addition of CB, CF and SF can both enhance the bond strength and improve the bond ductility between GFRP rebar and concrete. And the improvement will increase with the increasing of steel fiber content. On the contrary, comparing to that with a lower content, the increased carbon fiber will produce a negative effect on the bond behavior. This may be attributed to the difficulty of dispersing when the content of carbon fiber increases. The mix of CB and CF will show a positive hybrid effect on the bond behavior.Finally, the bond stress-slip curves achieved in the experiment is compared to the sequent curve model. And the result show that the experimental curve could correspond with the theoretical curve greatly, no matter there is conductive phase material in concrete matrix or not. Then, the computational formulas for bond strength and basic anchorage length of GFRP rebar are suggested, and the calculated basic anchorage length of GFRP rebar is compared to that of steel rebar.