Installation of ECC bridge link slab: (a) location of ECC slab, (b) placement of reinforcing steel within link slab segment, (c) pouring of ECC material and (d) finishing of exterior surface. Credit: Li, J. of Material Structures
I’ve posted before about Victor Li’s work at the University of Michigan using engineered cementitious composites.
A new paper by Li recently became available regarding a demonstration project in which ECC is being tested on a bridge deck within Michigan to replace a conventional joint within the deck.
ECC is in the family of materials known as high performance fiber reinforced cementitious composites. According to Li, ECC has the ability to strain harden under uniaxial tension while forming large numbers of microcracks up to an ultimate strain capacity typically over 4 percent, a level that he says is 400 times that of normal concrete. Under high levels of tensile strain, ECC does not form cracks with large crack width openings (e.g., between 50 µm and 70 µm). Designers can tailor the material on three phases within the composite: the fiber, matrix and fiber/matrix interface.
This particular application for ECC is aimed at coming up with solutions for a growing problem facing highway engineers. In the U.S. bridge deterioration is a big concern, and one problematic area is the
mechanical expansion joints in bridges. These joints are placed between sections of bridge decking, and are necessary because of the changing dimensions of these sections of deck that occur because of thermal expansion or other forms of thermal deformation.
Unfortunately, these joints deteriorate fairly quickly, begin to leak and then bigger problems begin. For example, in colder climates, water containing de-icing salts corrode the ends of the steel girders or penetrate into the rebar in precast concrete deck slabs. At best, costly repairs have to be made. At worst, a catastrophic failure of the bridge can result.
In response, engineers have essentially figured out a way to eliminate these expansion links in new bridges. But, the dilemma is what to do about existing structures.
Li’s idea is to retrofit bridges with ECC ‘‘link slabs’’ by removing the expansion joint and replacing a portion of the two adjacent decks with a section of ECC material overtop the joint. From the exterior, it would appear as a continuous deck surface.
ECC link slab schematic: Credit: Li, J. of Materials and Structures
In a demonstration project sponsored by the Michigan Department of Transportation, completed in 2005, a 225 mm thick, 5.5 m x 20.25 ECC link was added to a demonstration bridge. Thirty cubic meters of the ECC as delivered by standard ready-mix concrete trucks from a batching plant in a mix supervised by Li’s team.
Full scale load tests showed that the ECC link slab did not alter the supported nature of the bridge spans, and that ample strain capacity of the ECC is reserved for temperature-induced straining as designed.
The good news is that, so far, the performance of this link slab remains has stayed constant. More long-term performance monitoring and other demonstrations will be needed, but Li is optimistic that an ECC link slab will provide an excellent expansion joint replacement option for highway engineers.