In past decades, many state departments of transportation and the Federal Highway Administration have begun working with ultra-high performance concrete (UHPC), an advanced cementitious material. WSDOT has not used UHPC in highway bridge applications, such as connection joints for precast concrete decks and girders, because of the concrete’s high cost and because of general lack of experience with it.
The goal of this project was to develop a UHPC mixture using materials available locally—fine sand and admixtures—and domestically—steel fibers—as an alternative to commercially available, pre-packaged UHPC products. Selection of these local materials would not only reduce the cost of UHPC but would also improve its sustainability. The expensive materials, such as quartz powder and imported fibers, commonly used in commercial products and other studies, were not used in this study.
Several trial UHPC mixtures were produced and evaluated, from which two viable UHPC mixtures (A4 and C3) were selected for further performance evaluation. The researchers investigated the physical and mechanical properties of the two UHPC mixtures, including workability, compressive and tensile strength, elastic modulus, and shrinkage, as well as their bond properties with epoxy-coated reinforcing bars.
The two considered mix designs exhibited compressive strength of up to 16 ksi, direct tensile strength of 1.1 ksi, elastic modulus of 5,000 ksi, and relatively high bond strength. Shrinkage was also characterized and found to be comparable to that of conventional concrete.
A few of the study\’s many recommendations included the following:
- proper UHPC mixing procedures to achieve adequate workability of mixtures with thorough mixing time
- methods for testing both the fresh and hardened properties of UHPC and for testing the pull-out strength of concrete to characterize the bond strength of reinforcing bars in UHPC
- utilization of high levels of industrial wastes, such as ground-granulated blast-furnace slag and fly ash, to partially replace silica fume and cement and thus produce more economical and eco-friendly UHPC
- further study to investigate the effects of different curing regimens on UHPC properties, such as compressive strength, tensile strength and durability, to identify a practically feasible curing regimen
- further investigation of the freeze-thaw behavior of UHPC exposed to harsh environments and cold climates, such as marine environments, chlorine deicer coupled with freeze-thaw cycles, or magnesium deicer coupled with freeze-thaw cycles.
On the basis of the comparisons of material properties and bond strength, the researchers found the C3 mixture to outperform the A4 mixture and therefore recommended it for use in the second phase of the project, structural-level testing and evaluation in the joint connections of intended bridge deck bulb tees, which will take place at the University of Washington.
Authors:
Pizhong QiaoZhidong Zhou
Srinivas Allena
WSU Department of Civil and Environmental Engineering
Sponsor: WSDOT
WSDOT Technical Monitor: Bijan Khaleghi
WSDOT Project Manager: Lu Saechao