Q: What is the metric equivalent of the #4, #8, and #16 stone and the #467 blend in a recent Concrete Perspectives in Concrete Construction, August 2001?

A: In this article, Edward Finkel and Gary Herron discussed aggregate gradations that are usually missing from typical concrete mixes, including #4, #8, and #16 stone as well as using a blend of #467 stone and mid-sized coarse aggregates. The purpose was to avoid the adverse effects of gap grading on durability.

Aggregate stone numbers refer to aggregates that meet the grading requirements listed in Table 2 of ASTM C 33, "Standard Specification for Concrete Aggregates." The larger the number, the smaller the size of the stone. Stone number is defined by the amount of an aggregate that will pass through specific sieves by mass percent. The numbered sieves refer to the number of square openings per inch (25.4 mm-so a #4 sieve has 1/4-inch (4.75-mm) square openings; a #16 has 1/16-inch (1.18-mm) square openings.

Stone size and sieve size are related (the sieve size is basically the minimum size the stone can be), but a stone size is actually an allowable range of sizes. For example, for a #4 stone, 100% will pass through a 50-mm or 2-inch sieve (2x2-inch or 50x50-mm square sieve openings) and at least 95% must be larger than the opening in a #4 sieve. Here's the complete gradation requirement:

 Sieve size (square openings) Amount by mass percent that will pass through 50 mm (2 inch) 100% 37.5 mm (1 1/2 inch) 90 to 100% 25.0 mm (1 inch) 20 to 55% 19.0 mm (3/4 inch) 0 to 15% 9.5 mm (3/8 inch) 0 to 5%

This effectively translates into a #4 stone ranging from 19.0 to 37.5 mm (3/4 inch to 11/2 inches), but again, this lists the amount of stone (by mass) that passes through each size of sieve.

A #8 stone ranges in size from 2.36 to 9.5 mm (#8 sieve to 3/8 inch). A #16 stone isn't actually a stone but rather fine aggregate larger than a #16 sieve (1/16-inch or 1.18-mm square openings).

#467 stone (as defined by ASTM C 33) ranges from 4.75 to 37.5 mm (#4 sieve to 11/2 inches) basically a blend of #4, #6, and #7 stone. Finkel and Herron were advocating a smooth #467 gradation, that is, a range of aggregate sizes with no "gaps."

A gap is where the aggregate mix completely passes through one or more of the sieves within the aggregate range. This is a stricter requirement than ASTM's. For example, ASTM C 33 allows the #467 stone to leave no material retained on the 25.0 mm (1-inch) and 12.5-mm (0.5-inch) sieves. Further, the authors advocate an aggregate gradation that includes all the listed sieve sizes, from a 50-mm (2-inch) sieve down to a #100 sieve (150 ¦m), thereby providing a more complete gradation than that allowed by ASTM for #467 stone alone.