Inspectors normally use the amount of rust on the steel reinforcing bars in concrete as one indication of a structure's remaining lifetime, but Penn State engineers say this could lead to miscalculations about a structure's safety.
Dr. Kalliopi Aligizaki, who received her doctorate in civil engineering in May, and her adviser, Dr. Digby Macdonald, director of the Center for Advanced Materials and professor of materials science and engineering, say the iron oxides that accumulate in concrete and form colored areas up to an inch or more away from the bars, also have to be taken into account.
Aligizaki and Macdonald, a faculty member in the College of Earth and Mineral Sciences, reported their findings today (Nov. 30) at the annual meeting of the Materials Research Society in a paper, "Formation of Iron Oxides at Locations Distant from a Corroding Reinforcing Steel Bar."
"Corrosion of steel reinforcing bars constitutes one of the major causes of damage and early failure of concrete structures," Aligizaki says. "During corrosion, the iron in the steel converts to iron oxides, the volume of which can be two to four times greater than the volume of the steel they replace. The resulting increase in volume induces stress in the concrete around the bar, which ultimately causes cracking in the concrete," she explains.
As part of her doctoral thesis, Aligizaki prepared 120 specimens in which a steel bar was imbedded in concrete. Corrosion of the steel bars was accelerated by applying direct electric current with the positive pole connected to the steel bar while the negative pole was connected to the counter electrodes on the sides of the concrete specimens.
When the steel rods were removed after the concrete specimens cracked, Aligizaki noticed that the corrosion products on the steel bars were black, dark brown-black and dark brown. Different colored corrosion products were also found in the concrete about an inch away from the rod and were clearly visible to the naked eye.
"Research carried out in the past aimed at analyzing oxides surrounding the steel bars. Ours was the first study to examine the iron oxides formed away from the corroding steel reinforcing bar," she says.
While these corrosion products represent metal loss from the bars, they do not contribute to the expansive forces at the surface of the reinforcing steel that cause concrete cracking.
The Penn State researcher says, "These corrosion products, which have migrated away from the steel bar, could cause an inspector to assume that a structure is safer than it actually is because, even though the amount of rust on the bar has not yet reached a critical level to result in cracking, metal has, in fact, been lost from the steel bar. Loss of sufficient metal could compromise the reinforcing action of the steel bar."
It is also possible that the hardened concrete has entrapped air, which created voids where iron oxides can accumulate. The accumulation of corrosion products in the entrapped voids requires higher amounts of steel corrosion to produce the same extent of cracking compared to that for steel bars corroding in concrete without entrapped voids. The process is complex, however, because a higher void fraction in the concrete usually means greater access to the steel bar by oxygen and chloride, both of which accelerate corrosion.
Aligizaki's participation in the study was supported, in part, by the Ferondelis Foundation.
EDITORS: Dr. Aligizaki is at kxa104@psu.edu
by email.
Dr. Macdonald can be reached at ddm2@psu.edu
or at (814) 863-7772.