minimum concrete cover tolerance

huangyhg

minimum concrete cover tolerance
per aci 381-05, section 7.5.2.1, the tolerance on minimum cover reads -3/8" for d<=8" and -1/2" for d>8".  for tilt-up wall panels, the delegated engineer on my project has specified a minimum clear cover of 3/4".  we have found locations where the reinforcing is very close (<=1/2") to the face of panel, with even some locations the rebar showing.  
the way i am reading this tolerance is subtract the tolerance value from the minimum clear cover.  so for 3/4" minimum clear and a d<=8", this would allow the minimum clear cover to be as small as 3/8".  this to me seems unreasonably close.  shouldn't these tolerances be (+) instead of (-)???
nick deal, p.e.
michael brady inc.
i'm sure you are meaning aci 318-05.  you must also read the text, not just the table, where the text provides a tolerance for "cover shall not exceed minus 1/3 the minimum concrete cover required in the design drawings and specifications."  reiterated in aci 117 where  "reduction in cover shall not exceed 1/3 the specified concrete cover."  this seems reasonable to me.  it seems to me if you are missing cover concrete, you may also have an issue with consolidation.     
greg
the tolerances given in 318 are in reference to the design capacity of the member and as such, locate the tension steel such that a reduction in d is minimized, but an increase in d is allowed, such that cover is not compromised. as noted above, an increased d does not relieve the requirement for cover and aci 117 provides a more in depth review of the requirements.
the tolerance for d and the tolerance on allowable clear cover are seperate items. in typical construction, they are at adds, but both must be satisfied.
it appears in your case, the cover tolerances have been violated in construction and a means of remediation must be determined.
with 3/4" minimum cover, reducing that by 1/3 would give a minimum allowable cover of 1/2". if you are able to see bars at the surface and tell that bars are 1/2" below the surface, you have additional issues as noted by greg.
the revised 117-06 does a pretty good job of giving more detailed explanations of tolerances noted. take a look.
daniel toon