In recent decades, many state departments of transportation have used ultra-high performance concrete (UHPC) in bridge construction because of its advanced mechanical properties. In this study, researchers tested a new mix developed at Washington State University for its structural performance when used in a reinforced spliced connection between adjacent concrete deck bulb tee (DBT) bridge decks.
UHPC is a type of fiber-reinforced concrete that can achieve far superior compression, tension, bond strength, and ductility than conventional concrete, mostly because of the addition of steel fibers as reinforcement. These features allow for very short splice lengths and narrow pour strips. A propriety class of UHPC has been widely used. However, it is quite expensive, so researchers at Washington State University developed a non-proprietary mix design for WSDOT that depends on locally sourced materials to decrease the cost (see WA-RD 869.1).
In deck bulb tee (DBT) girders, a section of driving deck is precast monolithically with the girders so that when they are lifted into place, only the adjacent deck elements need to be connected. This can be done using welded steel inserts and a grouted joint, eliminating the need for fresh concrete on site. Deck bulb tees have been widely used on rural roads that attract less traffic than do major highways. Because DBT bridges can be constructed quickly, WSDOT is interested in using them on major highways; however, their traditional flange connections have exhibited deterioration under truck loading. Therefore, WSDOT seeks a more robust connection and proposes to use one in which rebar projecting laterally from the flanges are spliced within a pour strip of UHPC.
Following a series of material, structural, and bond tests, researchers concluded that UHPC in general, and the mix developed by WSU in particular, is capable of producing a bond to reinforcing bars that has both high strength and high ductility, making it a suitable choice for use in longitudinal bridge joints connected by a non-contact splice. The UHPC mix design showed mechanical properties that closely rivaled those of the proprietary mix, but at a significantly lower cost. Experiments on strength gain over time showed that the UHPC gained its strength relatively quickly, with at least 50 percent of its 14-day strength achieved after two days of curing. Contractors confirmed that this speed would be fast enough for practical purposes.
To characterize the structural performance of WSU’s UHPC, researchers at the University of Washington conducted material and structural tests. The materials tests emphasized the tension strength and bond strength of the epoxy coated #5 reinforcing bars that would be used in DBT flanges. Other ASTM-standard and custom experiments were also conducted to characterize the UHPC’s material properties.
Next, the researchers conducted two types of experiments on bond, one to determine the pullout bond strength where the reinforcing bar would slip through the UHPC, and one to determine the tension strength of a non-contact splice. Finally, the researchers conducted a series of experiments to determine the bond strength of a real UHPC joint connecting two precast concrete deck panels using a non-contact spliced connection.
Analyses were also conducted on bridge decks subjected to wheel loads. They showed that the stresses imposed on the UHPC joint by design-level truck loading would be a small fraction of the joint capacity, and therefore standard wheel loading should not be expected to even cause cracking at the interface between the UHPC and a conventional concrete panel. Furthermore, wheel loading would be expected to cause punching shear failure at a load much lower than that required to cause flexural failure of the UHPC joint. The researchers concluded that the strength of the UHPC joint tested here would be sufficient for anticipated wheel loads.
The benefits of this work to WSDOT will be the ability to construct durable bridge decks with precast, decked members, thereby avoiding the need to form, reinforce, and cast the deck on site. This will result in considerable time and cost savings.
Authors:
Timothy John Peruchini
John Stanton
Paolo Calvi
UW Department of Civil and Environmental Engineering
Sponsor: WSDOT
WSDOT Technical Monitor: Bijan Khaleghi
WSDOT Project Manager: Lu Saechao
Photo credit: Federal Highway Administration