When the United States was coming out of the depths of the Great Depression, one of the solutions for reducing unemployment was to create public work programs. Part of this 1930s effort included the expansion of the highway system. Later, in the late 1950s, construction began on the present interstate highway system with the 1930s construction serving as the backbone for the main transportation routes.
These highway systems were typically designed for a 50-year service life. The bridges typically need rehabilitation in 35 years and replacement in 70 years.
The time has come to replace the 1930s infrastructure and rehabilitate the 1950-60s interstate system. Unfortunately, much of the replacement and rehabilitation has not even taken place.
The consequences could be dismal, according to Richard Weyers, an expert in bridge construction and a professor of civil and environmental engineering at Virginia Tech. He predicts that the bridge system alone in America is facing a trillion dollar investment. “A new structure costs $75 to $100 per square foot to build. A new bridge, on average, is 8,000 square feet. And there are about 500,000 bridges in the federal highway system, not counting any structure under 20 feet or ones on the back roads.”
Complicating the issue is new knowledge about one of the materials that the Federal Highway Administration (FHWA) started using in 1974 in the construction of bridges.
In 1972, after the FHWA noticed a rapid corrosion of the reinforcing steel in concrete bridge decks following the application of deicer salt, it sponsored a research project to assess the feasibility of using organic coatings to protect the steel. After two years of testing, when no sign of corrosion was obvious, the FHWA used ECR in its first bridge. Its use soon became commonplace.
However, Weyers points out, none of the laboratory or field studies concluded that the ECR would not corrode. And only one laboratory study estimated that ECR would provide long-term corrosion protection of 46 years.
In 1986, engineers noticed early failures of ECR in Florida’s bridge substructures where salt water was involved. Due to these failures, a preliminary study was conducted with Virginia’s bridges. Engineers removed drilled cores containing ECR from piles in marine environments and from bridges in deicing salt environments. As a result of their studies, they anticipate that the coatings would be debonded (separated) from the steel bar in about 15 years for bridge decks and in six years from piles in marine environments.
A second, larger study on 18 bridge decks between two and 20 years old concluded that in Virginia the epoxy debonds from the steel in as little as four years. When the chloride arrives at the steel depth, the epoxy coating will debond from the steel surface. The level of corrosion protection provided by ECR is presently uncertain. Projections have been as little as five years of additional service life. A study is being conducted to further develop the estimated service life that ECR will provide.
“Presently, there is no existing method to effectively repair the existing decks with epoxy coated bars. And there is no way to evaluate the corrosion condition of the steel bar due to the coating. They can only be replaced by tearing the bridge decks out,” Weyers explains.
“The product was put into the bridge deck without knowing what reactions might occur in concrete,” Weyers says. “We now need a political solution to a technical problem” because of the dollar amount involved.
Noting the seriousness of the situation, Weyers refers to the failure of the pedestrian walkway bridge at the Lowe’s Motor Speedway in Concord, N.C. In that instance, grout contaminated with calcium chloride corroded the steel cables, weakening the beam and causing its collapse.
“This is absolutely the worst condition you could possibly have,” the materials engineer says about the combination of the salt with the steel and moisture. “For the speedway, the question remains how did the grout become contaminated. Was it a precast problem? Were the engineering specifications wrong? Or was it a materials supply problem?”
Five years ago, Weyers received $2.4 million from the Strategic Highway Research Program (SHRP) to direct the investigation of methods to correct deterioration of concrete bridges. He explored chemical and physical techniques to protect the existing bridges.
Today, industry has recognized Weyers’ research efforts on a model for the deterioration rates as a way to judge different corrosion protection systems. His corrosion service life model is the result of a three-phase study, started 16 years ago. His work identified “average” service lives; today he is working on the variability of service life prediction.
The variability includes the use of corrosion inhibitors, different high performance concretes, and selected materials’ low permeability to chlorides. “We are looking at these systems and selecting the most cost-efficient or minimum life cycle cost treatments,” Weyers explains.
— Written by Lynn Nystrom
