Quantitative evaluation of the characteristics of air voids and their relationship with the permeability and salt freeze–thaw resistance of hybrid steel-polypropylene fiber–reinforced concrete composites

Abstract

Hybrid fiber affects the structure of air voids of hardened concrete and further affects the performance of concrete. A laboratory test quantitatively analyzes the characteristics of air voids and their relationship with the permeability and salt freeze-thaw (F-T) resistance of hybrid steel fiber (ST)–polypropylene fiber (PP) reinforced concrete. Quantitative parameters such as the fractal dimension, number, tortuosity, and shape factor of the air voids are used to characterize their structure. In addition, the relationship between the structural characteristics of air voids and the permeability and salt F-T resistance of concrete is analyzed using the rough set theory. The hybrid ST–PP fiber can reduce the total air-void content, spacing factor, average chord length, and total chord length of the air voids. However, the number and specific surface area of the air voids increased by 113.9–380.2% and 185.3–422.7%, respectively. In addition, the tortuosity of air voids with a diameter of 180–500 μm is more complex than that of other apertures. Moreover, hybrid ST–PP fibers can change the shape of air voids in concrete and make the shape of air voids in concrete simple and round. The compressive strength and splitting tensile strength of concrete after salt F-T cycles are enhanced by the hybrid ST–PP fiber. Nevertheless, this hybrid fiber has a negative effect on the permeability of the concrete. The total air–void content, number of air voids and tortuosity are the main factors affecting the permeability and F-T resistance of concrete.