Bond Behavior of NSM Steel Bars for Structural Strengthening: A Numerical Study

This study presents a three-dimensional numerical model developed to simulate the pull-out behavior of helically shaped stainless steel bars embedded in cylindrical mortar specimens. These bars are commonly used in the repointing technique–also known as the Near Surface Mounted (NSM) method, which is particularly suitable for strengthening masonry structures while preserving the visual integrity of fair-faced historic facades. The technique involves cutting shallow grooves along horizontal mortar joints by partially removing the existing mortar. High-tensile strength reinforcement elements, such as steel or carbon bars, are then inserted and bonded using either organic or inorganic materials. The effectiveness of this method primarily depends on the stress transfer between the reinforcement and the surrounding material, a mechanism that is still not fully understood. The numerical model is validated by comparing results with experimental data from previous pull-out tests conducted on the same type of twisted steel bars. To accurately capture the interaction at the bar-mortar interface, cohesive elements are implemented within the simulation. The findings indicate that, during extraction, the bars undergo a complex stress and deformation state, which includes rotational movement driven by their spiral geometry