Departments of Transportation (DOTs) across the nation face potential failure of mechanical expansion joints installed between adjacent simple-span bridge decks. These joints are necessary to accommodate the many types of bridge deck movements caused by variations in temperature, vehicle loads, or settlement. In addition, bridge joints need to withstand traffic loads and provide good riding quality while producing minimal noise. Joints that fall into disrepair can lead to the deterioration of the entire structure.
Engineered Cementitious Composite
Working with Michigan Department of Transportation (MDOT) designers, University of Michigan researchers have developed a possible solution for durability and maintenance problems in these expansion joints. To allow designers to maintain simple span designs, and to permit retrofitting of existing bridge structures, University of Michigan researchers proposed the use of engineered cementitious composite (ECC) link slabs, rather than mechanical expansion joints.
Engineered cementitious composite has been shown to limit cracking as well as exhibit high ductility. This ductility has led it to be called “bendable concrete”. Engineered cementitious composite is a proprietary mix design using cement, sand, fly ash, water, admixtures, and fibers. Its most distinctive mechanical property is an ultimate tensile strain capacity of three to five percent. This strain capacity, more than 300 times that of normal concrete, is realized through the formation of a large number of microcracks as the load increases and allows the material to deform similar to ductile metals.
Engineered cementitious composite “link slabs” are created by removing the expansion joint and a portion of each of the two adjoining slabs and replacing it all with a section of ECC material. This creates a continuous deck surface, but the ability of the ECC material to deform allows the link slab to accommodate the deformations imposed by the adjacent decks while protecting the underlying superstructure and substructure from any corrosives present on the deck surface.
Grove Street Bridge Project
A demonstration ECC link slab was completed by MDOT on the Grove Street Bridge over I-94 in Ypsilanti, Michigan, in November, 2005. The Grove Street Bridge, a four-lane bridge constructed in 1971, is a composite steel girder concrete deck structure. The 9- inch thick concrete deck rests upon 10 built-up steel girder sections across the 66 foot width of the bridge. Traffic was carried over I-94 on four pin-and-hangar supported spans. Since construction in the early 1970s, the bridge has experienced significant deterioration. The most recent repair work performed on this structure was a thin overlay of bituminous asphalt placed in 2000 to extend the service life of the bridge another five to seven years.
The goal of this construction work was to return the bridge to fully operational conditions, along with improving the bridge through wider sidewalks and dedicated bike lanes. The Grove Street Bridge project included replacing the entire deck as well as painting the steel girders, but the noteworthy portion of this project was the inclusion of an engineered cementitious composite link slab element at the bridge center.
Hand finishing of ECC link slab on Grove Street Bridge project
Periodic inspections after the first winter have shown no changes in the link slab. The long-term durability and maintenance significance of this project won’t be apparent for many years, but the application of this innovative material is an exciting development in the use of the latest concrete technology. Depending on the results of this project, the use of ECC is expected to increase throughout Michigan in upcoming years.
M. Lepech and V.C. Li, "Bridge Decks in Michigan Go Jointless" in Civil & Environmental Engineering Newsletter for Alumni and Friends, University of Michigan, Ann Arbor, MI 48109-2125, Fall, 2005, pp. 12-16.