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Fiber-matrix interface bonding is an important area of concern in fiber-reinforced composites because it is directly related to the mechanical behavior of composites. In this study, interfacial bond properties of four types of fiber at different curing ages were investigated by analyzing fiber pull-out responses. Special attention was given to the fiber–matrix bonding behavior, including fiber tensile strength, average bond strength, equivalent bond strength, and average pull-out energy. Pull-out tests were conducted at 7, 14, 28, and 56 days of curing. Different failure modes in steel fiber, synthetic macrofiber, polyacrylonitrile (PAN)-based carbon fiber, and pitch-based carbon fiber were investigated. According to the pull-out force versus slip curves, different failure patterns were recorded based on the fiber type. Carbon fibers experienced a sudden drop after reaching the peak load, whereas the load decrease in steel and synthetic fiber was not as abrupt. Results also confirmed that steel fibers exhibited the highest pull-out load and energy absorption capacity followed by lower values for synthetic and carbon fibers. While monofilament of steel fiber was able to absorb 1,050 N·mm, monofilament of synthetic fiber and twisted bundles of carbon fibers could absorb 277 and 55 N·mm, respectively. However, the bond strength of straight carbon fibers was comparable to that of synthetic fiber and still lower than steel fiber. It was also derived from the experimental data that an increase in cement matrix age correlates to an improvement in fiber maximum pull-out load, bond strength, and tensile strength. These parameters were identified and compared in all fiber types. [ABSTRACT FROM AUTHOR]

Concrete is the most used construction material in the world; however, its deficiency in shrinkage and low tensile resistance is undeniable. Used as secondary reinforcement, fibers can modify concrete properties in various ways. Carbon-fiber-reinforced concrete is highly suitable to maintain longevity of infrastructure where corrosion of steel can shorten the useful service life of the structure while polypropylene fibers can mostly improve the shrinkage of concrete. However, the biggest challenge with fiber-reinforced concrete is the appearance of the poorly structured interfacial transition zone around the fibers. In this study, environmentally friendly and low-cost attempts were made to coat fibers with fly ash to enhance the structure of mortar around the fibers. Coated carbon and polypropylene fibers were used in mortar in single and hybrid forms to investigate the efficiency of fiber coating methods on mechanical and durability properties of fiber-reinforced cement mortar. A minimal dosage of 0.25% and 0.5% (by volume) PAN-based carbon fiber and polypropylene fiber was added to mortar to make low-cost mixes. Compressive, tensile and three-point bending tests were done after 14 and 28 days of curing, and the results were analyzed. The results showed higher compressive strength in coated fiber-reinforced samples and comparable results in tensile strength, flexural strength, and toughness parameters. Scanning Electron Microscopy (SEM) photos and Energy-Dispersive X-ray (EDX) analysis approved the efficacy of the coating methods. [ABSTRACT FROM AUTHOR]