Bank or creek gravel, which will answer the purpose of sand and gravel
combined, sometimes can be obtained, and frequently it is used in small
jobs of concrete work just as it comes from the pit or creek. Although
such gravel occasionally contains nearly the right proportions of sand
and gravel, in the majority of sand pits and gravel banks there is a
great variation in the sizes of the grains and pebbles or gravel and
in the relative quantity of each. It is advisable to screen the sand
and gravel and to remix them in the correct proportions, as well-graded
aggregates make stronger concrete and, ordinarily, enough cement will be
saved to pay for the cost of screening.
Experience has shown that it is advisable to screen bank gravel twice;
first over a screen with large meshes to eliminate particles too large
for use. The size of the mesh will depend upon the nature of the work
involved (see Coarse aggregate); then the material which has passed
through this screen should be sifted again over a screen with one-fourth
inch meshes. All material which passes the latter screen may be
considered sand and should conform to the characteristics discussed under
"Fine aggregate."