Low-Shrinkage Ultra-High-Performance Concrete

Abstract

Ultra-high-performance concrete (UHPC) has been used increasingly in the past decade due to its high strength, rapid strength gain, and enhanced durability. UHPC is a cement-based material that typically has a low w/cm ratio, high paste content, and a 2% volume fraction of high-strength steel fibers. Most commercially available UHPC mixtures are also proprietary, or company-owned, which tends to elevate its cost. To lower the cost of UHPC in Kansas, this research aimed to develop non-proprietary UHPC using primarily Kansas-based materials. It was important that the proposed mixture gain strength quickly for use in accelerated bridge construction; the proposed mixture proportions resulted in 1-, 7-, and 28-day compressive strengths of 13.1, 16.8, and 19.6 ksi. Also, because UHPC typically exhibits high early-age shrinkage relative to conventional concrete, this research explores shrinkage-limiting methods, including a shrinkage reducing admixture (SRA), a shrinkage compensating admixture (SCA), and prewetted lightweight aggregates (LWAs). The SRA effectively reduced UHPC shrinkage by one-third 30 to 60 days after mixing, but not at 90 days. The SCA reduced shrinkage throughout the 90 days of monitoring, and the effect was highly dose dependent. LWA did not reduce UHPC shrinkage in this study, but further research is needed since this finding conflicts with prior research. Results are also reported from tension and bending tests of UHPC with different volume fractions of high-strength straight and hooked steel fibers. Every specimen tested exhibited strain hardening in tension or deflection hardening in bending, suggesting that both fiber types are similarly effective. However, further research is needed to conclusively compare fibers due to the scope of the reported tests.