he coarse aggregates readily available are
sufficiently durable or uniform for the wearing surface of the
concrete road, but a suitable aggregate may be obtained at relatively
high price by shipping considerable distances. In such cases what is
known as the two course type of concrete road is employed. The wearing
course usually is about 2 inches thick and is constructed with
selected aggregates of good quality shipped in for the purpose. The
lower course is constructed of aggregates which do not possess the
desired qualities for a wearing course, but which are satisfactory for
concrete not subjected to abrasion. The aggregates for the wearing
course will be selected with the same regard for uniformity and
durability that would be the case if they were for the one course
pavement.

Bank run gravel, or run of the crusher stone, is generally not
sufficiently uniform as regards proportion of fine and coarse material
to produce uniformity in the concrete, and the use of aggregates of
that character is not permissible for the wearing course, but under
proper inspection they may be used for the lower course of two course
pavements.

=Fine Aggregate.=--The fine aggregate is generally natural sand, but a
mixture of natural sand and stone screenings is sometimes employed.
The fine aggregate of whatever character must be clean, free from
organic matter and sand, must contain no appreciable amount of mica,
feldspar, alkali, shale or similar deleterious substances and not
exceed two and one-half per cent of clay and silt. The sand is of such
a range of sizes that all will pass the one-fourth-inch sieve and that
not exceeding about five per cent will pass the 100-mesh sieve.

=Proportions.=--Various mixtures for the concrete are employed because
these may properly vary to some extent with the exact character and
grading of the aggregates. Experience seems to have shown that the
concrete used for the wearing surface should have a crushing strength
of at least 2500 pounds per square inch, and the mixture adopted is
based on the requirements that will give the desired crushing
strength. The common mixture for the one course pavement is one part
cement, two parts sand and three and one-half parts coarse aggregate.
For the wearing course of the two-course type of pavement, a mixture
of the same kind is very often specified.

While these are perhaps the most widely adopted proportions, many
others have been used, especially where the