Micro- and Nanoscale Characterization of Effect of Interfacial Transition Zone on Tensile Creep of Ultra-High-Performance Concrete

Ultra-high-performance concretes (UHPCs) are nano- to microstructurally optimized construction materials whose use presents significant opportunities for improving the performance of prestressed bridge girders. In UHPC girders, transverse shear reinforcement may be eliminated because of the high tensile strength of the material achieved through the use of short dispersed steel fibers as part of the mix. Use of the concrete's tensile strength requires that the long-term tensile performance be understood to avoid brittle shear failure in service. The scope of the present study was characterization of the tensile creep of UHPCs under different thermal treatment regimens, with complementary assessment of the underlying mechanisms by characterization by nanoindentation and scanning electron microscopy. In this study, tensile-creep tests were conducted for a period of 1 year with UHPCs subjected to three different moist thermal curing regimes (i.e., early curing at 90 degrees C, early curing at 60 degrees C, and curing at 23 degrees C). The effects of the curing conditions were further examined by nanoindentation and scanning electron microscopy, with particular emphasis being placed on the influence of thermal curing on the fiber-matrix interface. On the basis of the findings of this multiscale study, it is proposed that an enhanced fiber-cementitious matrix interfacial region, created by thermal curing, contributes significantly to the observed reductions in tensile-creep deformation.