Effect of macro polyoxymethylene fiber on the flexural behavior of concrete beam under cyclic loading and the potential for self-monitoring of crack development

This work investigates the mechanical properties and crack self-sensing behavior of polyoxymethylene (POM) fiber reinforced concrete (POMFRC) with nano-carbon powder (NCP) under monotonic and cyclic bending. Cyclic bending tests are conducted on notched beams, and the fractional change in resistance (FCR) is measured to enable real-time crack monitoring. The effects of varying macro POM fiber and NCP dosages on the mechanical properties and self-sensing sensitivity of POMFRC are analyzed. The relationships between deflection, crack mouth opening displacement (CMOD), and FCR are examined under both monotonic and cyclic loading conditions. Results reveal that POM fibers greatly improve post-cracking performance, while NCP enhances self-sensing sensitivity; on average, 1 kg/m³ NCP enhances the FCR value at a 1 mm deflection by 61.1 %. For the first time, the self-sensing of concrete cracks subjected to cyclic loading is explored. From the first loading-unloading cycle, the variation in FCR shows a similar trend to that of CMOD. After concrete cracking, both FCR and CMOD increase or decrease simultaneously during loading and unloading, which reflects a positive correlation in the post-cracking stage. A linear model derived from monotonic test data offers a simple and practical approach for preliminary prediction of the FCR-CMOD relationships under cyclic bending, providing conservative and reliable estimates of crack opening.