Comportamento da ancoragem de barras de aço CA70 com cabeça achatada em concreto: estudo experimental com monitoramento por fibra óptica

Abstract

The use of high-strength steel in reinforced concrete structures allows better use of the load-carrying capacity of the reinforcement, but it also places greater demands on the anchorage regions responsible for transferring forces between the steel and the concrete. In this context, this study aimed to experimentally evaluate the anchorage behavior of ribbed CA70 steel bars, with a nominal yield strength of 700 MPa, provided with flattened heads formed from the end of the bar itself, as well as to investigate the transfer of forces to the concrete using fiber Bragg grating (FBG) sensors. The experimental program included 12 pull-out tests on concrete blocks using bars with nominal diameters of 10.0 and 12.5 mm. For each diameter, six bars with flattened heads were tested, with a bonded length equal to five times the bar diameter. The force-slip relationships, anchorage capacity, and failure modes were analyzed. In the blocks with 12.5 mm bars, FBG sensors were embedded in the concrete and placed around the bar to monitor internal strains during loading. In all specimens, the bars exceeded the yield strength and final failure occurred by steel bar fracture, with no failure of the concrete blocks. This shows that the anchorage system was able to maintain force transfer after steel yielding. The confining reinforcement in the blocks also contributed to this behavior by limiting concrete splitting and allowing high force levels to develop before bar fracture. Compared with straight bars without heads previously tested by the research group, the overall anchorage capacity increased by approximately 25% for the 10.0 mm bars and 29% for the 12.5 mm bars. However, for the 12.5 mm bars, the most important effect was the ability of the anchorage system to maintain force transfer after yielding, allowing the bars to reach final fracture. The strains recorded by the FBG sensors increased with the applied load up to levels close to bar fracture and showed different tensile and compressive responses at different positions around the reinforcement, indicating a non-uniform and asymmetric strain field in the concrete. The combined analysis of the mechanical and optical responses indicated that the flattened head was effectively mobilized as an additional force-transfer mechanism, acting together with bond along the bar. It is concluded that, under the experimental conditions evaluated, the combined action of bond, the flattened head, and block confinement was able to mobilize high levels of the load-carrying capacity of the CA70 bars and prevent premature anchorage failure, leading to final fracture of the reinforcement itself. FBG monitoring proved to be a suitable method for monitoring the internal development and non-uniform distribution of concrete strains associated with the anchorage mechanism.