Load transfer between new and old concrete

[nuche1973]

Ok, here is an interesting one:
I am working on a 1930's concrete building. The foundation columns are spalling and deteriorated. Our exploratory investigation has shown that the foundation is undergoing a sulfate attack leading to carbonization in the columns and causing the reinforcing to corrode. Our fix is to remove the bad concrete around the core and repour a new encasement, after cleaning and installing galvanization measures. My question is in regards to the amount of new concrete added: I assume that I replace 2x what is removed from the original cross-section. How do I calculate the load transfer between the new concrete and the old? Surely, it is not just the coefficient of friction between concrete times the load, right? Any insight would be appreciated.

 

[Teguci]

Over time concrete creep transfers much of the load from the concrete to the reinforcing. The existing concrete confines the steel reinforcing which carries the bulk of the compressive load (ref. ACI 318 10.9 commentary regarding min column reinforcement).

For your repair, replace corroded steel with new and use a low shrinkage concrete mix for aesthetics. Unless you are replacing a significant height of column, the change in height should be just about negligible. And, because reinforced concrete is ductile, local yielding of the reinforcement will redistribute.

Is there any other failure mechanism besides compression? (Shear, bending, etc...)

 

[Ron]

You mention that sulfate attack is leading to carbonation. I'm a bit confused by this. Carbonation is caused by exposure. Sulfate attack is caused by the contact presence of sodium, magnesium or potassium sulfate, usually waterborne.

In any case, the affected concrete has to be removed, preferably by water blast or mechanical scabbling. I would not recommend cold galvanizing compound being spray on exposed rebar. This can create other problems. Use an epoxy coating on the rebar as a repair coating.

As for placing concrete back around the repaired section, it is preferable to use a relatively thick section, on the order of 2-1/2 to 3 inches. I would dowel and pin the old concrete to new and place a polymer modified concrete for the repair section.