ACI 447.1R -18 Report on the Modeling Techniques Used in Finite Element Simulations of Concrete Structures Strengthened Using Fiber-Reinforced Polymer (FRP) Materials.
In contrast to longitudinal steel reinforcement, researchers have found that the predicted load displacement response for RC beams was less sensitive to the assumed bond condilion between the steel stirrups and concrete. Although many existing studies conducted in the area of flexural strengthening of RC beams adopted an accurate bond-slip model between the internal steel stirrup reinforcement and the concrete; the practice was less common for shear strengthened beams. Only a single study by Chen et al. (2012) was found to investigate alternative bond-slip models between internal steel stirrup reinforcement and surrounding concrete. Both weakly bonded, strongly bonded, and perfectly bonded scenarios were investigated to represent the bond behavior between the internal steel stirrups and the concrete. When examining the effects of the assumed bond condition with respect to the peak load predictions, the weakly bonded stirrups provided an underestimation of the failure load while a strongly bonded scenario provided good prediclions that matched the experimental data in terms of peak load and ductility. The assumption of perfect bond resulted in a similarly good prediction of peak load, hut produced greater post-peak ductility behavior, which was absent from the experimental results. Furthermore, it was found that the bond condition affected the crack pattern, and the process of crack propagation in a complex manner and further investigation was required to achieve a better understanding.
3.6—Modeling of FRP-to-concrete interface
Debonding ofFR P-to-concretejoints is generally governed by bond characteristics of the FRP-to-concrete interface. Three general approaches used by researchers to numerically represent the interfacial bond properties between FRP and concrete are summarized by Kalfat (2014): 1) perfect bond at the interface; 2) the use of one-dimensional nonlinear spring elements between the adjacent concrete and adhesive layers (L.uo at al. 2012); and 3)a layer of interface elements between the FRP and the concrete (Wu et al. 2009).
Lastly, the fnal approach used to represent the FRP-to- concrete interface and the occurrence FRP debonding is to assign interface elements between the FRP and concrete (Fig. 3.6a and Fig. 3.6b). The interface elements are typi- cally assigned with a constitutive bond-slip model repre- senting the strength and stifness of the interface, which can be derived from the experimental data or available theoret- ical models based on linear or nonlinear fracture mechanics (Ferracuti et al. 2007). A typical FRP-to-concrete bond-slip relationship consists of two stages: 1) an initially elastic stage where the interfacial stresses increase with the slip until the strength of the interface is reached; and 2) a soft- ening stage in which interfacial stress decreases with the slip resulting in debonding. To obtain the bond-slip information from experimental tests, axial strains should be measured along FRP laminate length in the direction of the fbers at diferent loading levels and used to calculate bond-slip relation. ACI 447.1R pdf download.