Creep Behavior of Internally Cured UHPC with Calcined Bauxite Aggregate or Lightweight Aggregate

The creep property of ultrahigh-performance concrete (UHPC) is important for long-term performance and safe design of UHPC structures, such as large-span prestressed UHPC bridges. A strong and porous aggregate [calcined bauxite (CB)] has been applied as novel internal curing agent to mitigate the very high autogenous shrinkage of UHPC; however, this shrinkage-reducing component effect on creep performance of UHPC is unknown yet. Thus, UHPCs internally cured with CB aggregate were designed to investigate their mechanical properties, shrinkage, and long-term compressive creep behavior and compared to the UHPCs internally cured by common lightweight aggregate (LWA); meanwhile normal UHPC (with only sand aggregate) or UHPC with basalt aggregate were designated for reference too. Results show that UHPC with CB aggregate exhibits 16% less shrinkage than UHPC with LWA when they were designed at the same theoretical internal curing efficiency, and the UHPC with CB aggregate shows much reduced creep by having 11.2% and 22.6% lower specific creep than the normal UHPC or UHPC with LWA, respectively. Higher dosage of CB aggregate leads to slightly higher (4%) specific creep, but still less (5.7%) than that of the UHPC with basalt aggregate. The X-CT technique and microscopic observation prove that UHPC with CB has fewer air voids than normal UHPC or UHPC with LWA. Using CB aggregate is proved to benefit the time-dependent performance of UHPCs (reduced shrinkage and creep) as well as improve its mechanical properties over normal UHPCs, and it gives significantly better performance than the UHPCs internally cured with LWA. Several creep models were used to predict UHPC creep and verify their applicability, and one model has shown good accuracy in predicting both dry and basic UHPC creep.