Implication of calcium sulfoaluminate-based expansive agent on tensile behavior of ultra-high-performance fiber-reinforced concrete

This study aims to investigate the effects of a calcium sulfoaluminate (CSA) based expansive agent (EA) on the tensile behaviors of ultra-high-performance concrete (UHPC) reinforced with straight and twisted steel fibers. For this, five different CSA EA amounts ranging from 0% to 8% were considered. Because the major purpose of adding a CSA EA into a cementitious matrix is to reduce the amount of shrinkage, its implication on the free-shrinkage response of UHPC was also evaluated. The test results indicate that approximately 7% and 10% smaller shrinkage strains were obtained in UHPC matrices with 6% and 8% CSA EAs than that of plain UHPC, respectively. Most of the tensile parameters, including the tensile strength, strain capacity, and g-value, of UHPCs with straight and twisted steel fibers were deteriorated when including a CSA EA and increasing its amount, except for the tensile strength of straight fiber specimens. Thus, the CSA EA needs to be carefully applied for UHPC mixture, despite its positive effect on reducing the amount of shrinkage strains by greater than 6%. There was no clear trend regarding the effects of the amount of CSA EA on the number of fibers located near the localized crack of ultra-high-performance fiber-reinforced concrete (UHPFRC), and the unit number of fibers in UHPFRC specimens with 2% (by volume) of 0.3-mm diameter steel fibers ranged from 25 to 35 ea/cm². The tensile strengths of UHPC reinforced with straight and twisted steel fibers were both improved when increasing the number of fibers, although the former showed a much higher rate of increase and degree of correlation than the latter. The strain capacity and g-value of UHPC with straight and twisted fibers showed only marginal correlations with the number of fibers regardless of the amount of CSA EA applied.