Q.: Do you have anyone on staff to provide a clarification on what is required by ACI 301-99 where a contract specification requires a smooth form finish in accordance with ACI 301? Is rubbing required per ACI 301, paragraph Reading paragraphs and, this is not very clear.

A.: I have a copy of an article on ACI 301-96 written by M. K. Hurd from the November 1996 Concrete Construction, but I would like a clarification based on ACI 301-99.

The Hurd article from November 1996 very clearly pointed to this rubbed finish requirement as being a significant change. I looked back at the April 1996 article by Timothy Moore that was referenced in the November article, and from that got a sense of how the entire 1989 document had been rewritten and reorganized for its 1996 release. It appears to me that in the 1999 version, specifically in the sections you're referring to, ACI tempered its requirements somewhat.

Hurd does not quote it verbatim, but in ACI 301-96 paragraph reads: "Smooth form finish—Patch tie holes and defects. Remove fins exceeding 1/8 in. in height."

The requirement to comply with the rubbed finishes section has been removed, and a note has been added referring to this section in the Optional Requirements Checklist (for 301-99): "Specify more restrictive tolerances for as-cast form finishes as needed based upon importance of surface appearance. See Optional Requirements Checklist for (which deals with formwork and formwork accessories) for additional guidance."

It appears some of the tight requirements adopted in the 1996 version, and pointed out in the Hurd article, were relaxed and clarified in the 1999 version.

The 8-18 rule

Q.: What is the 8-18 rule for percent retained and how do I apply this to the proportioning of normal weight concrete?

A.: Jay Shilstone replied to this question, noting that the 8-18 "rule" isn't really a rule. It is a guideline. The concept is that gap-graded aggregate mixtures do not perform as well as well graded mixtures. Gap grading requires more sand, which increases water demand, shrinkage, permeability, and cracking and decreases strength. The 8-18 concept says that, except for the top two sieves and bottom two sieves, there should be at least 8% of combined aggregate retained on each sieve and no more than 18% retained on each sieve. This results in a "well graded" mix.

Shilstone warns, though that the 8-18 guideline is meant for maximum aggregate sizes of 1 inch or 11/2 inch, not larger or smaller. It is meant to run on the sieve series of 11/2 inch, 1 inch, 3/4 inch, 1/2 inch, 3/8 inch, #4, #8, #16, #30, #50, and #100. Some areas of the country do not have aggregate of all sieve sizes locally available and transportation costs for the aggregate may exceed any price or quality benefit. Also, if the aggregate grading is heavy on the fine side, there will be problems. Shilstone uses the coarseness factor/workability chart to check the final blend and notes that he has found that you don't really need to apply 8-18 guideline to each sieve, but to a moving average of 2 consecutive sieves. The 8-18 guideline represents a simplistic specification for a simple concept. However, applying the specification without understanding the concept is asking for trouble.

Ramon Cook noted that wide gap-grading of aggregates for concrete has become more common, since aggregate producers are selling certain ranges of fines for asphalt production, deleting them from the concrete mix. Shrinkage and other troubles can be the result, often without the specifier knowing such range-limited delivery of concrete raw materials is happening.