Behrouz Shafeishafei@iastate.edu email >
Structural Engineer, BEC
Peter Taylorptaylor@iastate.edu email >
Director, CP Tech Center
About the research
Ultra-high performance concrete (UHPC) provides superior properties in strength and durability for the long-term performance of bridges. Despite these desirable properties and the potential to be applicable in the majority of projects, UHPC is still not widely used, mainly because of the cost associated with it.
This report details a study performed on the design of non-proprietary UHPC mixes that provide comparable strength properties to that of commercially available mixtures. A set of base mixtures were explored by varying the ratios for various constituents and investigating their durability, strength, and transport properties, including volume stability and freeze-thaw resistance.
In the later stage of the project, the selected non-proprietary mixes were evaluated for their flexural strength. The flexural strength in UHPC comes mainly from the fibers used in the mix. Bearing in mind the role of fibers, the effects of various types of steel fibers (i.e., variation in shape, size, and dosage) were evaluated. The role of fibers on strength and post-cracking behavior was carefully examined using laboratory testing and image analysis utilizing digital image correlation techniques. The efforts found that an optimal combination of micro- and macrofibers can enhance the flexural strength of UHPC mixtures.
Steel fibers contribute to more than a third of the cost of UHPC mixtures, so the possibility of utilizing less expensive and more environmentally friendly synthetic fibers—polypropylene, polyvinyl alcohol, nylon, alkali resistant glass, or carbon—to partially replace the steel fibers could reduce the cost of UHPC. The steel fibers were partially replaced by the different synthetic fibers to see their effect on the UHPC’s fresh properties and flexural strength. Utilizing digital image correlation, the synthetic fiber contribution to post-cracking behavior was evaluated, especially from the crack width control and crack propagation aspects. The replacement of steel fibers with synthetic fibers showed promise for flexural strength and post-cracking behavior.
This report provides recommendations for the preparation of cost-effective, non-proprietary UHPC mixtures that could be used for various transportation infrastructure applications. Further recommendations are also made for the optimal combination of different types of steel micro- and macrofibers to get the best flexural response. Recommendations are then extended for the use of different types of synthetic fibers and the optimum percentage of dosage replacement for steel fibers.