The Interfacial Bond Stresses in Concrete Filled FRP Tubes

Composite structures have gained more attention these days due to their advantages, such as high strength (because of complementary performance of core concrete and FRP tube), excellent durability, light weight, and fast erection. One of the composite structures is concrete filled fiber-reinforced polymer (FRP) tubes (CFFTs). The technology has been investigated in the past, but more attention should be paid to some specific problems, such as quantifying an adequate bond between the tube and the concrete core to act as a full-composite structure, which is an important issue, especially in flexural member. This study proposes a new and simple analytical method calculating the bond stress in flexural members. The equilibrium between the tension and the compression forces is used to develop a MATLAB code to calculate the bond stress. The section is divided into some fibers. The force in each fiber is calculated according to the stress distribution. The total tension and compression forces are calculated by the sum of fibers’ forces. The bond stress is the total tension or compression force divided by the interface between the concrete core and the FRP tube. However, the ultimate moment capacities given from tests are used in the simplified method to calculate the bond stress. The tension and the compression forces are calculated based on the arm between them. Finally, the bond stress is determined. Furthermore, a comparison between the bond stress calculated according to two methods and the bond strength data derived from push-off tests is made. The results show that although the bond stresses are a bit more than the bond strength at the ultimate condition, there is an adequate bond between the concrete and the FRP tube before reaching the ultimate condition as the differences are not too much.