Mechanical and Microstructural Properties of Polypropylene Fiber-Reinforced Concretes Exposed to Low-Temperature Curing

Many major engineering projects have been building in cold regions, and concretes would be inevitably cured under low-temperature conditions. In this study, the influences of curing media (air, water, and 5% NaCl solution), curing temperatures (− 6, − 2, 2, and 6 ℃), and dosages of polypropylene fibers (0, 1.0, 2.0, and 4.0%) on concrete performances were investigated. Results are illustrated that the positive-curing temperature and curing age have beneficial effects on the mechanical properties of concretes, the compressive strength of concretes increases with the curing temperature raised. Negative-curing temperature inhibits hydration reaction, reduction of liquid water and frost heave seriously deteriorate the mechanical properties of concretes cured in the water and air. Besides, the compressive strengths of PPFRCs first increase and then decrease with the polypropylene fiber dosages raised, and the optimal polypropylene fiber dosage for PPFRCs to resist external loads is 2.0% mass fraction (PP2). Additionally, the types of hydration products are not changed with the curing conditions, and the main characteristic peaks of hydration products are basically the same. The deterioration of concrete samples cured in 5% NaCl solution is the most serious, following by the air and water. Furthermore, adding a reasonable dosage of polypropylene fibers into concretes could form effective fiber-matrix combination and three-dimensional network structures, and a remarkable effect on the crack propagations and frost-resistance of PPFRCs could be exhibited. However, high dosages of polypropylene fibers might cause agglomeration and increase the harmful pores in concretes, which is not contributed to the strength enhancement.