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"Bendable Concrete" Replaces Bridge
Expansion Joints
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 necessary
movements of the bridge decks 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.
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, the use of “Engineered
Cementitious Composites (ECC) Link Slabs,” rather than mechanical
expansion joints, was proposed by the University of Michigan researchers.
 ECC
is an ultra-ductile, high-performance fiber-reinforced cementitious
composite (HPFRCC) developed by the Advanced Civil Engineering Materials
Research Laboratory (ACE-MRL) under the direction of Professor Victor
C. Li at the University of Michigan. ECC has been shown to limit
cracking as well as exhibit high ductility. This ductility has led
to ECC being called “bendable concrete”. ECC has been
developed as a proprietary mix design using cement, sand, fly ash,
water, admixtures, and fibers. It’s most distinctive mechanical
property is an ultimate tensile strain capacity of 3%-5%. This strain
capacity, over 300 times that of normal concrete, is realized through
the formation of a large number of microcracks as the load increases.
This allows the material to deform similar to ductile metals.
ECC “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 ECC 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 over I-94, a four-lane bridge constructed
in 1971, is a composite steel girder concrete deck structure. The
230 mm (9 in.) thick concrete deck rests upon 10 built-up steel
girder sections across the 20 m (66 ft) width of the bridge. Traffic
was carried over I-94 on four pin-and-hangar supported spans. Since
construction in the early 1970’s, 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 5 to 7
years.
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| Hand finishing of ECC link slab on Grove Street
Bridge project |
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 ECC link slab element at the bridge center.
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
the State of Michigan in upcoming years.
Reference
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.
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