ABSTRACT
Ultra-high performance concrete (UHPC) is an emerging material that provides advanced mechanical and durability properties The high compressive strength and increased environmental resistance provides the potential for longer structural lifespans and a corresponding reduction in lifecycle environmental and economic impact (Ahlborn et al. 2008). Increased tensile and flexural capacity is achieved with the addition of high strength steel fibers. To improve the understanding of the behavior of the UHPC and the contribution of the high strength steel fibers to UHPC, two scaled beams were designed and tested under flexural loading. Digital Image Correlation (DIC) was used during the flexural tests to enhance the study and provided additional information on the performance of the UHPC in the shear regions. Following the completion of the flexural tests, the “undamaged” portions of the beams were then re-tested to further investigate the shear behavior of the beams.
The UHPC investigated in this research was developed primarily using materials local to New Mexico, USA. Different from typical commercially available products, the fiber content for the UHPC was reduced from 2.0% by volume (typical of commercial products) to 1.5% by volume. Full details on the development and optimization of the UPHC can be found in Weldon et al. (2012).
The two scaled fully prestressed rectangular specimens were designed, cast, and cured using the locally developed UHPC. The beams were designed in accordance with the American Association of State and Highway Transportation Officials (AASHTO) Load Resistance Factor Design (LRFD) 2012 Bridge Design Specifications using a design compressive strength of 20,000 psi (152 MPa). Properties including the modulus of elasticity, modulus of rupture, and concrete post cracking strength are significantly different than conventional or high strength concretes. Thus, modifications to the Specifications for material properties were made based on previous studies on UHPC for the scaled girders.
The results demonstrated that the prestressed rectangular UHPC specimens with 1.5% fiber reinforcement by volume, in the absence of shear reinforcement, resisted loads greater than the AASHTO (2012) estimated capacity indicating that the steel fibers provided significantly to the shear strength of the specimen.
The use of DIC identified key differences in the behavior in the shear regions of the two specimens under flexural load. Additionally, DIC was able to identify cracking in the specimen with no shear reinforcement; however, the fibers were able to bridge the cracks and provided the necessary strength to continue to carry load past cracking.
