High- and ultra-high-performance concrete produced with sulfate-resisting cement and steel microfiber: Autogenous shrinkage, fresh-state, mechanical properties and microstructure characterization

Abstract

High-performance concrete (HPC) and ultra-high-performance concrete (UHPC) are cement-based composites that present improved mechanical performance and durability. However, their low water/binder ratio and high binder content lead to significant autogenous shrinkage. This work investigated the autogenous shrinkage of HPC and UHPC produced with sulfate-resisting cement and steel microfiber in volumetric fractions (Vf) of 0–1.5%, from the first minutes of hydration. In addition, fresh performance (mini slump flow), hardened properties (dynamic modulus of elasticity – Ed, compressive and flexural strength at 28 days) and microstructure (SEM-EDS) were evaluated. Results showed that steel microfiber incorporation progressively reduced the autogenous shrinkage of both HPC and UHPC (by up to 11% and 22%, respectively) because of the increased resistance to volumetric reduction caused by the capillary pressure. The fresh-state performance of UHPC was not affected by microfiber incorporation (mini slump flow of 305–310 mm), while increasing Vf progressively reduced the mini slump flow of HPC from 305 mm (Vf 0%) to 262 mm (Vf 1.5%); this behavior was associated with the viscosity of the mixes. Furthermore, increasing Vf progressively increased the compressive strength (by up to 8% for HPC and 23% for UHPC) and the flexural strength (by up to 23% for HPC and 47% for UHPC) due the inhibition in propagation and opening of microcracks, while no significant differences were observed in the Ed. In turn, 0.75% microfiber incorporation in UHPC increased the air content by 80%, thus reducing its flexural strength. SEM-EDS analysis indicated the absence of chemical bond between the fiber and the cementitious matrix, suggesting that this interaction was essentially mechanical. Overall, steel microfibers incorporation was effective in reducing the autogenous shrinkage of HPC and UHPC produced with sulfate-resisting cement, in addition to keeping or even improving the mechanical performance of the composites.