Mitigating early-age cracking in thin UHPFRC precast concrete products using shrinkage-reducing admixtures

This study investigates the effects of shrinkage-reducing admixture (SRA) and cover depth on the shrinkage and surface-cracking behaviors of ultra-high-performance fiber-reinforced concrete (UHPFRC) slabs. Three different SRA'to'cement weight ratios of 0%, 1%, and 2%, and three different cover depths, 5, 10, and 20 mm (0.2, 0.4, and 0.8 in.), were considered using nine UHPFRC slabs with steel reinforcing bars. Three more unreinforced UHPFRC slabs with three different SRA ratios and four different depths were also fabricated for the comparisons. The use of SRA exhibited better behavior with regard to the decrease of shrinkage, maximum evaporation rate, and maximum surface-crack width. In addition, the application of a larger cover depth was favorable because it caused a lower degree of restraint, leading to a lower cracking potential and a higher surface-cracking resistance. Finally, the minimum required cover depth of the UHPFRC slab is suggested to be 1.3 times larger than the diameter of the upper steel reinforcing bar in order to prevent surface cracking.