Concrete samples provide clues to coated rebar condition
Much like the skeleton offers physical support within the human body, a steel reinforcing bar, or rebar, adds strength and stiffness to such concrete structures as bridge decks and building foundations.
As is the case with the body, when that rebar skeleton begins to corrode, the concrete around it breaks down, too. As a result, cracks and potholes form, increasing maintenance time and cost and aging the concrete prematurely. “When the bars corrode, you get a layer of rust around them,” says Civil and Environmental Engineering Professor José Pincheira. “As the bars continue to corrode, that layer increases in volume and eventually it will split the concrete around it and you will have cracks. When the problem gets really bad, you eventually can break the ‘cover’ and the concrete will spall off.”
Expectations for epoxy-coated rebar
Epoxy-coated rebar has been around since the mid-1970s. An alternative to conventional, corrosion-prone rebar, it debuted amidst high expectations that the coating would eliminate corrosion of the bars. However, bridge owners found that this more expensive material didn’t completely solve their corrosion problems.
Back then, some researchers believed the coating didn’t work, others thought nicks in the epoxy coating caused during construction created highly concentrated corrosion in those areas, and still others pointed to manufacturer variances in coating quality, application process, thickness and adherence. “It was hard to be able to establish a relationship between the performance of the bars in the field, when they were not doing very well, when you had all these other variables,” says Pincheira.
Since the ’70s, accelerated laboratory studies have shown that a properly applied epoxy coating on rebar can extend the life of reinforced concrete structures like bridge decks, he says.
But what does a two-year laboratory study mean for a bridge that’s supposed to last 50 years? “The only way we can answer that question is to actually measure the performance of the bars in the field,” says Pincheira. “And for that, we have to wait a lot of years.”
Lessons from the past
Enter four Minneapolis-area bridges, built in the late 1970s and early 1980s with reinforced concrete that contains epoxy-coated rebar. Researchers conducted a 1996 study to assess the rebar condition; recently, the Minnesota Department of Transportation asked Pincheira and Civil and Environmental Engineering Assistant Professor Dante Fratta to perform a follow-up.
The duo used several minimally invasive measurement techniques to identify areas in which the rebar likely was corroded. In one, called half-cell potential, Pincheira and Fratta used an instrument similar to a stud-finder to locate a piece of rebar. Then they drilled a small hole through the concrete to the bar, connected a wire to it and applied a current. “If there is corrosion in the bar, you should detect, relatively speaking, high voltages,” says Pincheira.
They mapped their voltage readings and created color-coded X-ray, so to speak, of the rebar in that portion of the bridge deck. Red indicated areas of high corrosion; green or yellow showed little or no corrosion activity.
Once they identified areas their measurements indicated were corroded, they worked with a Minnesota DOT crew to extract “cores,” or cylinder-shaped samples about the size of a gallon paint can, from those areas. In many cases, cores in areas with corroded bars also included vertical cracks running from the roadway surface straight down to the rebar. “And that, essentially, is a direct path for the chlorides to get to the bars,” says Pincheira.
In northern climates, road salt is the main factor that contributes to rebar corrosion. Pincheira and Fratta also measured the diffusion coefficient, or how quickly humidity and road salt penetrate the concrete to the bar level. From the cores, they drilled out and pulverized small concrete samples at varying depths, and analyzed their chemical makeup to learn how salt penetration affects rebar health, and vice versa.
Considering such variables as weather, traffic frequency and maintenance practices, Pincheira and Fratta compared their results with the 1996 data. Their final report gives the state of Minnesota a better idea about the service life of its bridges—and simultaneously