Mechanical Properties and Crack Propagation Behavior of Defective Cement Mortar Reinforced with Hybrid Steel–Carbon Fibers

To enhance the crack resistance and toughness of cement mortar, steel fibers (SF), carbon fibers (CF), and their hybrid combinations were incorporated at various volume fractions. Cubic specimens with single or double prefabricated holes were tested using compressive and splitting tensile tests combined with digital image correlation technology (DIC). to evaluate mechanical properties, crack propagation, and failure patterns. Results indicate that steel fibers primarily suppressed macrocracks via bridging and improved strength, while carbon fibers inhibited microcrack initiation and promoted uniform strain distribution. Hybrid fibers achieved combined reinforcement, with the S2C02 mixture exhibiting the best overall performance: compressive strength increased by 39.7–55.1%, tensile strength by 70.45–75.9%, and the toughness and tensile/compressive ratio were enhanced. DIC analysis showed reduced strain concentration, more uniform strain fields, delayed crack propagation, and a transition from brittle to quasi-ductile failure. These findings demonstrate an optimal hybrid fiber dosage and reveal the combined mechanism of steel–carbon fibers in defect-containing cement mortar, providing guidance for material design and performance optimization.