A new bridge deck in Maryland’s scenic horse country is a sign of big changes coming to the state’s bridges. The Cold Bottom Road overpass, which crosses I-83 some 10 miles north of Baltimore’s I-695 Beltway, is the second of two pilot bridge deck projects conducted by the Maryland State Highway Administration (MDSHA). The projects are providing real-world data as MDSHA prepares to implement a specification for the use of high-performance concrete (HPC) in all state highway bridge decks.
The Cold Bottom Road bridge is just one example of a growing problem across the U.S. Countless overpasses built during the interstate-building boom of the 1950s and 1960s are now requiring rebuilding as they succumb to cracking and corrosion.
Maryland officials would like to see 35-year bridge decks go the way of tailfins and tune-ups. In the years since these bridges were built, advances in materials and engineering now permit the design of concrete bridge decks with a projected life of up to 75 years.
The U.S. Federal Highway Administration is actively promoting development of 75-year roads and bridges—an effort that promises to deliver huge cost savings in the nation’s highway infrastructure. In 1996, MDSHA established a committee to develop a specification to establish a 75-year HPC specification for use in all Maryland bridge decks, parapets and backwalls, components that typically encounter the highest exposure to traffic and weather and which are usually the first part of a bridge to fail.
Mixing with the existing
HPC is an evolutionary, not revolutionary, material. The mixes are composed of essentially the same materials as conventional mixes, but proportions and admixtures are adjusted to provide added strength and durability.
The pilot specification involved materials only. It did not modify existing structural, geometric or construction specs for the project or practices, such as depth of steel-reinforcing cover or epoxy coating of steel. The design mechanical strength of the concrete was not increased, and the specification retained the existing requirements for curing—seven days of wet cure. After evaluating the field performance data from the pilot projects, the committee may consider modifications to design and construction practices to take advantage of the properties of HPC. The first pilot project was a new concrete bridge in Smithsburg, Md., where Rte. 64 crosses railroad tracks. Completed in 2000, the Smithsburg bridge deck remains in excellent condition after several years of traffic, winter weather and salt application.
Based on this success, MDSHA moved forward in 2003 with the Cold Bottom Road project, just outside the town of Hereford. This project was similar in scope to the Smithsburg project; however, it employed a new HPC deck placed over existing steel girders rather than the concrete girders used in the Smithsburg project. During the reconstruction, the steel girders also were refurbished.
Engineers also increased the proportion of slag cement in the mix. The Smithsburg project employed a 60-40 mix of portland and slag cement. The Cold Bottom project used a 50-50 mix of portland cement and Lafarge NewCem slag cement, supplied from Lafarge’s Sparrow’s Point plant in Baltimore. The primary benefit of the higher slag-cement content was to improve placeability of the concrete in warm weather.
Corrosion comes later
MDSHA has used data collected from the pilot projects to analyze the projected performance of the high-performance mix compared with the existing standard specification. Fig. 1 illustrates the estimated chloride content at a depth of 2.5 in. over the life of a bridge deck built under the current specification and the HPC specification. Under the current specification, corrosion is expected to initiate at 13 years, with the epoxy coating on the reinforcing steel inhibiting corrosion for another 25 years. Expected time to first repair is 38 years.
HPC is estimated to increase the age of corrosion initiation to 50 years. With the epoxy coating inhibiting corrosion for another 25 years, expected time to first repair is 75 years, essentially doubling the life of the deck.
Those extra 37 years of bridge life come at a very small cost. Essentially, high-performance concrete will allow Maryland to skip an entire replacement cycle for new bridge decks. The added in-place cost of HPC was approximately $50 per cu yd, or about $15,000 for each project. The cost works out to slightly more than $400 a year over the life of the bridge decks. Construction crews encountered no problems using the new specification. Generally, HPC is easier to work with than standard portland mixes. As noted, placeability was improved on the Cold Bottom Road project because of slag cement’s slower set times. Compliance to the curing specification was crucial for prevention of cracking and was strictly enforced.
Based on its successful completion of the pilot projects, MDSHA now plans to implement the HPC specification for bridges statewide. In fact, HPC has already been specified for the largest bridge project in the state: the construction of the new Woodrow Wilson Bridge, where I-95 crosses the Potomac River near Washington, D.C.
The adoption of the new specification represents a major engineering advance in Maryland’s highways. As bridges are built or replaced throughout the state, future drivers will have fewer detours and delays to contend with, and Maryland’s citizens will be spending fewer tax dollars on highway reconstruction projects. Chances are, these improvements will largely go unnoticed—which is exactly what MDSHA is aiming for.