Q: We've got a dispute on a poured wall placement regarding acceptable bug hole size. ACI 116R-90, Cement and Concrete Terminology, defines bug holes as small regular or irregular cavities usually not exceeding 15 mm (0.60 inch) in diameter. Does that mean any bug hole larger than 15 mm is unacceptable?
A.: ACI 116R-90 isn't a specification or standard. It simply describes bug holes and is silent on the subject of acceptable size.In some specifications, permissible bug hole size is related to the end use of the concrete. Current U.S. Army Corps of Engineers guide specifications require patching 1/2-inch-diameter or larger bug holes in Class A surfaces (those prominently exposed to public view, where appearance is of special importance).
But for Class C surfaces (permanently exposed surfaces not requiring other finishes), only 1 1/2-inch-diameter bug holes or larger must be patched.Naturally, contractors want to avoid patching bug holes because of the cost. Owners may also want to avoid patching for aesthetic reasons. ACI Manual of Concrete Inspection contains the following advice:
"Consider whether the repair will be less apparent and have a more pleasing appearance than the original blemish. Unfortunately, many repairs do not. For example, surface voids (bug holes) are common and their repair, except for sack rubbing or similar treatment, may be less satisfactory than no repair."
Unfortunately, many specifications don't clearly state an acceptable size for surface cavities, and disputes can arise when owner's or architect's expectations about formed surface appearance are unrealistic.
Help is on the way, however. The American Society for Concrete Construction is preparing a Guide for Surface Finish of Formed Concrete to help contractors and owners agree, in advance, on what constitutes an unacceptably large surface void on formed surfaces. The report should be available in early 1999.
In our answer, we stated that the acceptable size depends on the project specifications, which, unfortunately, aren't always clear on the subject. Following are letters from readers offering additional viewpoints.The answer to the question of acceptable bug hole size has to do entirely with the subjective wishes of the owner or owner's representatives, which should have been stipulated unambiguously in the specifications for the work. The question I believe to be more interesting is: What should the owner and the owner's architect/engineer require?
I believe that for most exterior concrete, the vertical surfaces should be cast against architectural form liners that have bold irregularities so bug holes of up to, say, 2 inches are indistinguishable from the intentional roughness provided by the form liner. Smooth, bug-hole free vertical concrete surfaces are appropriate in hospitals, kitchens, parts of breweries, and nuclear facilities, but bug-hole free surfaces often are specified in situations where they aren't needed.
--Bryant Mather Department of the Army, Waterways Experiment Station, Corps of Engineers Vicksburg, Miss.
The answer to the question on bug holes was correct, as far as it went. However, it did not touch on the major problem at present concerning the treatment of bug holes.U.S. Army Corps of Engineer specifications are never seen in building construction, and ACI 301-96 insists that all as-cast finishes other than "rough form" be rubbed. Therefore, the answer to the question is simple--the American Concrete Institute allows no bug holes.The American Society of Concrete Contractors does have a guide for surface finish of formed surfaces, but without ACI cooperation in adopting such a guide, the vast majority of jobs will not only continue to specify no bug holes, they will also call for rubbing, which in most cases is not what the specifier or contractor envisioned.
--Ronald Simonetti Cleveland Cement Contractors Inc. Cleveland
Mr. Simonetti is correct. Mary Hurd discussed this problem in the article "What Is a Smooth Form Finish?" (Concrete Construction, November 1996, pp. 817-819). ACI Committee 301 has revised the wording to restore the smooth form finish as an alternative to rough form or rubbed finishes. The revision is currently making its way through the ACI standardization process. Until the revised version of ACI 301 is printed, we recommend following Hurd's advice: "The contractor who is bidding work for which ACI 301-96 is incorporated by reference in the contract documents should check carefully to see what finish, if any, is specified. Where no job-specific finish is spelled out, the default values of ACI 301-96 apply, and all concrete exposed to public view will require a rubbed finish as defined in section 126.96.36.199. Since the new ACI 301 requirements may not yet be fully understood, check with specifiers to verify their intent."