Question: I have a specification calling for #89 gravel in the concrete mix and #57 stone for the subbase. What do these numbers mean?

Answer: This all comes from ASTM C 33, “Standard Specification for Concrete Aggregates.” First, there's really no difference in this context between stone and gravel and aggregate. In C 33 there's a chart showing the grading requirements for different sizes of coarse aggregate from #1 to #8. For example, #1 aggregate is nominal size 3½ to l½ inches (90 to 37.5 mm). Aggregate size #8 is nominal size 3/8 to 0.094 inch (No. 8 sieve) (9.5 to 2.36 mm). There are also a few blended or graded aggregate sizes that combine the gradation of several aggregates.

A #57 aggregate (combining the gradation of #5 and #7 aggregates) ranges from 1 to 0.19 inch (No. 4 sieve—25 to 4.75 mm) and has the following gradation (amounts finer than the given sieve size): 1½-inch sieve—100% passing; 1-inch—95 to 100%; ½-inch—25 to 60%; No. 4 sieve—0 to 10%; No. 8 sieve—0 to 5%.

A #89 aggregate has the following gradation: 1/2-inch sieve—100% passing; 3/8-inch—90 to 100%; No. 4 sieve—20 to 55%; No. 8 sieve—5 to 30%; No. 16 sieve—0 to 5%; No. 50 sieve—0 to 5%.

So all this simply means that a #57 stone is a coarser 1½-inch-minus graded aggregate and #89 gravel is a finer ½-inch-minus graded aggregate.