Effect of Curing Age on Pull-Out Response of Carbon, Steel, and Synthetic Fiber Embedded in Cementitious Mortar Matrix

Abstract

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.