capacity of conc section with correded reinf 2

[doejohn]

I may shortly be asked to compute the axial compression and flexural bending moment capacities of a reinforced concrete section with assumed amounts of the reinforcement "lost" through corrosion. That is, I know what the section looks like today, before any corrosion, and I'll be given the "amount" of corrosion in terms of steel weight lost. One approach would be to simply use the amount of remaining, non-correded steel in a standard analysis of RC section capacity (i.e. the bending capacity of a beam with 2 bars with 50% weight lost is equal to that of a non-corroded beam with one bar). But since the corrosion starts at the outside face of the bars, at the steel-conc interface, I wonder if the effects on bar bond are such that this "linear" approach isn't appropriate? Perhaps loosing (say) 20% of bar weight renders a bar completely ineffective. (Important detail: I'm told that the nature of the corrosion is such that the buildup of expanded, corroded material, which commonly spalls the concrete, isn't expected.) What approaches have the forum members taken? Thanks for any insights.

 

[Dinosaur]

Most structural engineers think of section loss when discussing the capacity of R/C sections and corrosion; however, an examination of photos after a collapse of a parking garage will show the bars never broke. The true failure mode is most often a loss of development and the concrete simply slides past the bar on its way down. When attempting to get me to understand this, my old concrete professor (from Turkey) said to me while holding a photo up, "Look at the spaggettis, look at the spaggettis, what do you think cause this?" Therefore, I would take a hard look at development of the reinforcement. Also, I believe the major contributer to this loss of development is the swelling of the bar as it corrodes. Good Luck - Dinosaur

 

[JAE]

Sometimes it depends on where along the length of the member the corrosion occurs. For a beam, many times the low point in the beam is near midspan and the corrosion appears on the bottom, where water and oxygen has infiltrated through the flexural cracks at midspan. Here, the concrete spalls off on the tension face and loss of section occurs. However, the bond issue doesn't apply as the development of the tension reinforcing occurs further on toward the supports where no corrosion occurs.

When loss of section occurs near the supports, the top steel is the primary concern, not the bottom steel. The stirrups also may loose section and bond - very difficult to see sometimes.

 

[SlideRuleEra]

See the writeup starting on page 75 and summary on page 104 of "Effects of Corrosion on Load Carrying Capacity of Structural & Reinforcing Steel" at this link

http://ntl.bts.gov/lib/5000/5700/5737/ecfl.pdf

(Weight loss of 5% to 10% causes dramatic changes in rebar properties)

Here is a research paper, "Combined Effect of Corrosion & Stress in Reinforced Concrete Beams", that documents Dinosaur's comments

http://www.ce.utexas.edu/em2000/papers/KWang.pdf

http://www.SlideRuleEra.net

 

[doejohn]

Thanks Dinosaur, JAE, and SlideRuleEra, for your insights! In particular, the reference in SlideRuleEra's first link (a 1994 report sponsored by Fed Highway Admin and Illinois DOT) was helpful.