I want to make growing benches out of concrete and cinder blocks.
(Stop me here if this is the wrong approach.) Everything made of wood
will rot here in New Orleans. I want to make bench tops about
18inches deep and 30inches wide and 1.5 inches thick. I'll use cinder
blocks to support the bench tops.

Problem is that I don't think that Quickcrete is the right mix for the
job. The gravel is too large and the result is too heavy.

So I want to mix my own. I figure the Gravel is the only component I
can change to make the concrete lighter. Any suggestions on what I
should do here?

Bill.