Effects of Shrinkage Reducing Agent and Expansive Admixture on the Volume Deformation of Ultrahigh Performance Concrete

This paper investigated the influences of shrinkage reducing agent and expansive admixture on autogenous and drying shrinkage of ultrahigh performance concrete (UHPC) containing antifoaming admixture. The shrinkage reducing agent was used at dosage of 0.5%, 1%, and 2% and the expansive admixture was used at dosage of 2% to 4% by mass of cementitious material. The results show that the air content of UHPC increases with the higher addition of shrinkage reducing agent and expansive admixtures. However, the fluidity, compressive strength, and shrinkage of UHPC exhibit a declining tendency. The usage of expansive agent at dosage of 4% significantly reduces the shrinkage of UHPC. The 7-day autogenous shrinkage was decreased by 16.0% and 28-day drying shrinkage was decreased by 29.5%, respectively. Shrinkage reducing agent at dosage of 2% reduced the 7-day autogenous shrinkage by 44.3% and 28-day drying shrinkage by 50.2%. Compared with expansive admixture, shrinkage reducing agent exhibits more efficient shrinkage reduction effect on UHPC

Influence of Fiber Alignment and Length on Flexural Properties of UHPC

This paper investigates the influence of fiber alignment induced during casting on flexural properties of ultra-high performance concrete (UHPC). Straight steel fibers with different lengths of 6, 13, and 20 mm were used at 2% volume ratio. Pullout load and energy of fibers with different lengths were evaluated. Flexural strength and toughness were determined for prismatic samples cast using a flow-induced device that can improve fiber orientation during placement. The horizontal outlet height of the device was adjusted to 10, 20, and 30 mm in order to secure different degrees of fiber orientation. Test results indicate that the longest fiber had approximately 5- and 27-time greater pullout load and pullout energy than those of the shortest fiber. Compared to UHPC made with the shortest fiber, UHPC cast with the longest fiber had approximately 75% and 245% higher flexural strength and toughness, respectively. The fiber dispersion and orientation improved when the horizontal outlet height decreased, resulting in greater flexural strength and toughness. Such enhancement was up to 75% and 100%, respectively, compared to similar UHPC cast using the conventional method that can result in random fiber orientation. Synergistic improvement exceeding 130% in flexural strength was obtained for samples cast with fiber length of 13 mm and horizontal outlet height of 10 mm, compared to samples cast with shorter fiber of 6 mm using the conventional casting method. The smoothed particle hydrodynamics simulation indicated that the ratio between fiber length and horizontal outlet height of 1.1 was more efficient to improve flexural properties of UHPC, compared to the ratios of 0.6 and 1.6 where the highest value led to blockage