Comprehensive study of GFRP bar-concrete bond behavior using pull-out and three-point bending tests

The GFRP bar-concrete bond behavior is known to be influenced by factors such as bar surface treatment, bar diameter, and concrete strength. This study aims to provide a comprehensive understanding of the bond behavior of GFRP bars embedded in concrete. Pull-out specimens are cast from two concrete batches with different compressive strengths. Two surface coating conditions are considered, i.e., one with a lacquer finish and one without. Specimens with three bonded lengths are used to determine the role of the bonded length in the bond mechanism (i.e., 5db, 10db, and 20db, where db = nominal bar diameter). Two free end conditions of the pull-out specimens are examined: one with the bar protruding at the free end and another with the bar cut at the free end (i.e., the bar is cut flush with the concrete surface). The pull-out test responses are analyzed in terms of applied load versus loaded end slip and free end slip. Three-point bending (TPB) tests are conducted on concrete notched beams with two different widths (75 mm and 150 mm). Beams are either reinforced with a single GFRP bar or they are not reinforced and used as benchmark. Notched beams are only cast from one of the concrete batches used for the pull-out test specimens. This study shows how the protrusion of the bar can affect the bond strength in pull-out tests. In addition, the different bond behavior of the GFRP bars observed in pull-out tests and TPB tests of notched beams is discussed considering the length of the stress transfer zone for bars with different surface conditions.