Multiscale Hierarchical Fiber Gradations in Concrete to Reduce Early-Age Shrinkage Cracking Potential

Abstract

Significant efforts have been made to improve concrete mix design for shrinkage reduction and crack control, including reduction of cement content and selection of cement type with less heat of hydration, maximizing aggregate contents, and introduction of discrete fibers in concrete to increase internal restraint. Although high-performance concrete (HPC) mixes including fibers [e.g., steel fibers, polypropylene fibers, or polyvinyl alcohol (PVA) fibers] have reduced the level of shrinkage cracking, they have not eliminated it. Thus, understanding the level of tensile strain the fibers take in the concrete could provide valuable information to aid engineers to make an informed decision on the need for fibers in tension demand in the concrete and determine the appropriate fiber detailing and concrete mix additives. As such, this study aimed to explore a multiscale, hierarchical fiber gradation through a combination of nanoscale nanofabrillated cellulose (NFC) with microscale sisal fibers and macroscale steel fibers in concrete to mitigate the causes of concrete shrinkage cracking at an early age. A total of 22 mixtures were designed and tested. The results show that nanofabrillated cellulose, sisal fiber, and steel fiber can play an excellent synergistic effect to inhibit the early-age shrinkage of concrete. The test data reveal that, when steel fiber is partially replaced by sisal fiber, the early shrinkage of steel fiber concrete is significantly reduced by 26.91% at maximum. Meanwhile, when the NFC content reached 0.15%, the 7-day shrinkage value of concrete reinforced by steel fiber, sisal fiber, and NFC was the lowest, and the shrinkage strain value decreased by 25.03% compared with N0. Compared with the experimental group in which sisal fiber partially replaced steel fiber, the 7-day shrinkage value of the experimental group in which sisal fiber mixed with NFC, sisal fiber mixed with NFC and steel fiber decreased to a certain extent, among which SF1.1FF0.4NFC0.15 showed the best shrinkage inhibition performance, and the shrinkage strain value decreased by 82.56% and 45.39% compared with N0 and SF1.1FF0.4, respectively.