huangyhg

waterproofing - equivalent to concrete cover?
here's the situation.
a large base slab for a rectangular tank has been poured. the vertical dowels projecting out of the slab for the future walls have been cast, but unfortunately they have been improperly located such that they have 1.5" of clear cover as opposed to the specified 2" as required by aci350.
per the civil engineer, there is no flexibility with regards to being able to bump out our wall thickness; i.e., we can't simply tell the contractor to bump out his forms 1/2" on each side to compensate for the inadequate cover because doing so has an effect on the function of the tank that is undesireable in the eyes of the civil guy (specifically, the inside volume of the tank would decrease).
obviously, it would be a huge task to tear out all of that mat slab concrete. similarly, it's a pretty big undertaking to come up with some sort of solution requiring the dowels to be cut off and replaced with properly-located epoxied dowels.
so here's the question.
is anybody aware of literature that outlines the application of a given surface-applied sealant that can essentially act as an "equivalent" thickness of concrete cover? aci318 commentary sectionr7.7 alludes to such a thing, but off hand, i'm not very familiar with what it would be.
thanks in advance for your help folks!
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the commentary for aci 318, section 7.7 states:
"alternate methods of protecting the reinforcement from weather may be provided if they are equivalent to the additional concrete cover required by the code. when approvide by the building official under the provision of 1.4 (just a section that says alternates can be submitted) reinforcement with alternative protection from the weather may have concrete cover not less than the cover required for reinforcement not exposed to weather."
so aci seems to have alternatives in mind. this may include painting (in the field of course) with some type of epoxy coating. but that would compromise your splice length to a certain extent and you would have to verify that you have excess splice length for the particular condition at the base of the walls.
i think your civil guy is being unreasonable. i don't know how large your tank is, but if you're dimensioning in ft., the 1/2 inch can't make that big of a difference.
if your tank is 10 ft. square and 10 ft. high (pretty small), the difference would be 1.7 percent. for larger tanks, the percentage difference is even smaller.
make him explain why moving the walls in is such a problem. unless there's some equipment that won't fit in, i'd advise changing the forms.
don't know of any data out there showing that a barrier coating can be a partial substitute for concrete cover, although aci 201.2 does say that coatings can help increase durability. aci 201.2 references aci 515.1 which apparently discusses barrier systems in greater detail, but regret i do not have a copy if it.
a dumb question- do you really need every dowel? you still have all the dowels on the other face of the wall. if you cut off the as-built dowels on the inside face and put back new ones with epoxy adhesive, but at a wider spacing, will the cost of the doweling be more reasonable? how does your resistance to shear across the ftg/wall joint look with fewer dwls? do you have a key you can take advantage of?
i agree with jedclampett. can't be that big of a deal.
barrier coatings require maintenance (moreso than exposed concrete) and their weatherability is short term (relative to structure life), so with that in mind, i would not recommend them as an alternative to cover.
for durability, there was a study done about 20 years ago in marine applications by a german group. their conclusion was the best protection for rebar was quality, dense concrete with sufficient cover (over any other method of rebar protection, coating, or mitigation).
(recommendations of the committee for waterfront structures, society for harbor engineering, eau 1985).

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