huangyhg

sacrificial anode protection for rebar??
i have a condominium project where lack of cover has resulted in the corrosin of rebar and spalling of the concrete surface of a few balconies. the floor is post tensioned with 10'-0" lengths of #4 bars placed at regular intervals perpidicular to the longitudinal joint between the balcony and the interior finished floor joint. (balcony is approx 5' in width) post tensioned cables are running in both directions in general proximity of the area of distress.
at this point, the corroision has resulted in less than 10% section loss of the bar. however, due to aesthetic concerns with regards to surface spalling, its adverse affect on the balcony waterproofing, and additional section loss expected if nothing is done, i feel like action is required. the presence of multiple p.t. cables in various directions scares me out of excavating to provide the cover needed or installing a new dowel with adequate cover and remove the exposed portions of the existing. the presence of the exterior wall over the exact mid-length of the bar also creates problems with regards to a remove and replace situation. door opening elevations will not allow additional topping to be placed over the bar.
that said does anyone have experience or suggestions with regards to attaching a sacrfical anode to the exposed bar to /reduce prevent continued deterioration. has anyone had long-term success with corrosion inhibiting coatings? suggestions regarding methods/procedures for protecting the bar and preventing further corrosion?
i know these guys do this sort of protection work:
in addition, check into sika. they have teamed up with vector, i believe, and have a lot of products and technologies.
i admit i haven't tried sacrificial anodes with post-tensioning in the vicinity but post-tensioning usually doesn't like cathodic protection.
you should consider contacting a corrosion consultant. just providing cover protection may not be enough if corrosion has already started. it may be required that a corrosion protection system be installed, i.e. anodes and all, including periodic maintenance.
just a cautionary note regarding repairs to post-tensioned structures (forgive me if i'm wading into waters already visited); the conventional steel reinforcing is often used for crack control and does not contribute substantially to structural capacity. has the integrity of the post-tensioned tendons been confirmed, specifically, the absence/presence of detensioned wires and water penetration? i only ask because the corroding conventional steel may not be important. regarding the use of sacrifical anodes, in our opinion and experience, the success depends somewhat on the extent of chloride contamination versus carbonation related corrosion. they are however, cheap insurance and help extend the life of the patch repair.
jw
there is no reason to suspect deterioration of the pt cables. it appears that the corrosoin is only of the rebar with a lack of cover. this is an ocean front condo in florida, so most likely, corrosion is related to chlorides in the salt air. appreciate the insight on crack control. that makes me feel a little better about the structural condition, however, i still need to remediate as a waterproofing and aesthetic measure.
seminole22,
extending from 'pinehurst' comment, unbonded pt systems of yesteryear were often paper-wrapped or heat-sealed sheaths which have significantly less durability than the current extruded seamless hdpe sheaths. additionally, it was common practice for 6+" lengths of un-sheathed strand behind the dead and live end anchorages - if such lengths are in exterior type exposures, with or without coastal chlorides, wire corrosion can occur.
with unbonded pt corrosion it often goes undetected for sometimes many years - ie minimal visual evidence of wire or strand failure. so whilst "there is no reason to suspect deterioration of the pt cables" we have seen many projects where a concrete repair contractor starts just rebar spall repairs and gets into some large $ pt repairs.
also, concrete repair contractors doing just spall repairs without pt repair experincne can be dangerous. we have seen several contractors chip at slab/balcony edges for rebar/concrete spalls only to accidently de-tension live tendons. be careful!

we have used fosroc galvashield xp and cc imbedded anodes successfully - vector corrosion technologies have the license in north america and sublicense to mbt, sika etc.
a branch of aci (concrete innovations appraisal service -cias) has done some technical research on these anodes with good success.
actually, we've been involved in corrosion in slabs that eventually rusted out all of the rebar and cracked the slab (12") in half horizontally.
core tests found that the salts had penetrated all of the slab, with the most concentration at the bottom of the slab and decreasing towards the top.
the lawsuit went on about ten years.

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