Failure Portrait: Rust Jacking

by Don Friedman on January 14, 2016

There’s more to the cracking of masonry from embedded steel than simply calculating the amount of tension force it would take to create a crack. Cracks propagate through existing small imperfections in the masonry and also through prying, which magnifies the effect of the tension that actually exists.

Enough preface. Check out this crack:


That’s unreinforced-concrete encasement around a steel-angle post, probably put there as fireproofing. That crack runs about ten feet top to bottom (the full height of the post) and is about a quarter of an inch wide and an inch deep to the steel surface. If we said that the crack was created solely by expansion of the steel as it rusted, and taking the tension strength of the low-quality concrete as about 100 to 150 psi, we get 12,000 to 18,000 pounds of force required to create the crack. That sounds worse than it is, since that force was distributed along the crack, but it’s not nothing.

One of the problems with examining this type of situation is that the damaged, non-structural concrete may be unintentionally contributing a significant amount of strength to the post, so creating probes to see the condition of the steel is potentially dangerous.

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