Victor C. Li, University of Michigan
Increasingly, DOTs are expected to maintain bridge inventories in good conditions under a tight budget. Simultaneously, there is a sense of urgency to enhance mobility and sustainability of transportation infrastructure. Given that concrete is the most used material in bridge infrastructure, it would be natural to look to new performance in concrete that could assist in meeting these challenges.
A common cause for repeated maintenance of bridge structures, especially in the northern states with severe winters and coastal states with salt-water environment, is the cracking of concrete cover that often leads to corrosion of steel reinforcement. Although it is common to use crack sealants on concrete and/or epoxy coating of steel reinforcement to slow this process, frequent maintenance of bridge decks remains to be the norm.
A concrete that has an ability to reliably control cracking and limit the diffusion of chloride through the concrete cover in the field would be greatly beneficial to extending the service life of bridge decks, reducing down-time and enhancing driver comfort.
The intent of the Envision rating system is to standardize evaluation of the sustainability of infrastructure projects. It is applicable to projects in sectors such as energy, water, waste, transportation, landscaping, and information. In the transportation sector, project types that can use Envision include airports, roads, highways, railways, public transit facilities, and bridges.
What is ECC?
Engineered Cementitious Composite (ECC) is a special type of high performance concrete with tensile deformability several hundred times that of normal concrete. Cracks in ECC are limited to below 100 micron, often less than 50 micron, even under traffic induced fatigue loading. As a result, the water permeability of ECC in the field can consistently retain that of intact concrete throughout its service life. And it does so without relying on steel reinforcement. Under accelerated chloride tests, the corrosion rate of reinforcing steel inside ECC is significantly below that of reinforcing steel inside normal concrete subjected to the same mechanical loading. Further, it has been demonstrated that ECC is spall resistant. These attributes of ECC – low permeability, low chloride diffusivity, and high corrosion resistant for steel reinforcement and spall resistance in the field – make it a good candidate material for overcoming the challenges faced by those having the responsibility to maintaining bridge deck conditions.
A typical composition of ECC is given in Table 1. Depending on the exact composition, the compressive strength of ECC is in the range of 50-75 MPa, whereas the tensile ductility is 2-4%, about 200-400 times that of normal concrete. The ability of ECC to experience large deformation without fracturing is illustrated in a bending experiment shown in Fig. 2. Because ECC does not include coarse aggregates, shrinkage control should be considered especially for large surface area applications.
| Cement | Fine Aggregates | Fly ash | Water | HRWR* | Fiber** |
| 578 | 462 | 694 | 319 | 7.51 | 26 |
Table 1. A typical mix composition of an ECC (kg/m3)
Michigan’s Experience with Ductile ECC
In 2005, the world’s first ECC link-slab was installed on Grove Street Bridge in Southeast Michigan. The link-slab measuring 225 mm x 5.5 m x 20 m, replaces a conventional expansion joint on a high-skew bridge. The link-slab is connected to the adjacent concrete deck through steel reinforcements, and is partially tied to the supporting steel girders through shear connectors. It is otherwise designed to stretch freely. Movement of the bridge deck induced by thermal expansion and contraction is accommodated by the ductile deformation of the link-slab, thus serving the function of an expansion joint. The ECC link-slab eliminates the typical problems of expansion joints, including joint malfunctioning, water leakage, and rusting or beam-end corrosion of the supporting girders. By this writing, almost ten years have passed since its installation. This ECC link-slab continues to serve its intended functions without any maintenance.
Despite the attentions given to the ECC link-slab, the first use of ECC was actually a small patch repair on Curtis Road Bridge over M-14 in Michigan, in September 2002. The ECC patch was placed side-by-side with a regular patch repair concrete. The deck experienced heavy 11-axle truck loading. This application of ECC demonstrated the durability of ECC under severe Michigan winter weather conditions combined with large mechanical loading. Cracks were monitored on both the ECC and the adjacent repair concrete patches. The maximum crack width in the normal repair concrete grew to about 3.8 mm over the following two years, and had to be re-repaired in late 2005. The maximum crack width in the ECC patch remained tight at approximately 50 micron for the whole monitoring period ending in 2007 when full deck replacement took place.
Another bridge deck patch repair was performed on Ellsworth Road over M-23 in Ann Arbor, Michigan, in late November of 2006. For this repair, a special version of ECC with high early strength of 24 MPa within 4 hours was adopted. This patch remains in good condition to this day.
ECC’s Value Proposition
ECC can extend service life of concrete bridge decks. Although the material is more expensive than normal concrete, it is competitive with repair mortars commonly adopted in small-scale repair projects. Because of the enhancement in bridge deck durability, the life-cost consideration of projects specifying ECC can make the adoption of this newer concrete particularly attractive. Furthermore, by minimizing repair needs, ECC contributes directly to enhancing public mobility by reducing traffic interruptions, while reducing downtime and enhancing environmental sustainability. In the case of the Grove Street Bridge, life-cycle analyses conducted by the Center for Sustainability at the University of Michigan found that the adoption of ECC leads to a reduction of about 40% of carbon and energy footprints over the life-time of the bridge deck. ECC offers values to DOTs, the motorist public, and the natural environment.
References
- Lepech, M.D., and V.C. Li, “Large Scale Processing of Engineered Cementitious Composites,” ACI Materials J., 105(4) 358-366, 2008.
- Lepech, M.D. and V.C. Li, “Application of ECC for Bridge Deck Link Slabs,” RILEM J. of Materials and Structures, 42 (9) 1185-1195, 2009.
- Lepech, M.D. and V.C. Li, “Long Term Durability Performance of Engineered Cementitious Composites,” Int’l J. for Restoration of Buildings and Monuments, 12 (2) 119-132, 2006.
- Li, M., and V.C. Li, “High-Early-Strength ECC for Rapid Durable Repair – Material Properties,” ACI Materials J., 108(1), 3-12, 2011.
- Li, V.C. and M. Li, “Influence of Material Ductility on the Performance of Concrete Repair,” ACI Materials J., 106 (5) 419-428, 2009.