Concrete samples provide clues to rebar condition
 uch 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 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.”
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.”
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 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-rays, 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 makes
a unique field-research contribution to the study of reinforced concrete.
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