Figure 13-27.-Simplified flow chart showing production of road tars from bituminous coals.
rod is cast into the concrete to be tested. Then the force
required to pull the rod from the concrete is measured.
The measured strength is the direct shear strength of the
concrete. By correlating the shear strength with the
compressive strength, a measurement of the in-place
strength is made.
BITUMENS AND
BITUMINOUS-MATERIALS TESTING
Bituminous pavements are a mixture of well-graded
mineral aggregates, mineral filler, and a bituminous
cement or binder. They are used as the top portion of a
flexible-pavement structure to provide a resilient,
waterproof, load-distributing medium that protects the
base course from detrimental effects of water and the
abrasive action of traffic. This chapter discusses the
materials used in the construction of bituminous
surfaces and the methods of identifying and testing these
materials.
BITUMINOUS-PAVEMENT MATERIALS
Bituminous-pavement materials are made up of a
mixture of coarse and fine aggregate, bound together by
a liquid or semisolid bituminous binder. The coarse
aggregate is stone or gravel that is too large to pass the
No. 8 sieve. The fine aggregate is fine gravel and sand,
small enough to pass the No. 8 sieve, but too large to
pass the No. 200. Fine rock dust that will pass the No.
200 sieve is called mineral dust. A small amount of this
may be included in a paving mix, or a small amount of
mineral filler may be added to the mix. Commonly used
mineral fillers are portland cement, pulverized
limestone (called limestone dust), silica and hydrated
lime.
Aggregates
Aggregates may consist of crushed stone, crushed
or uncrushed gravel, slag, sand, and mineral filler, or a
combination of some of these materials. Aggregates
normally constitute 90 percent or more, by weight, of
bituminous mixtures, and their properties have an
important effect upon the finished product.
Bituminous Cements
Bituminous cement is the adhesive agent in the
bituminous mixture and may be either an asphaltic
material or a tar. Asphalt may sometimes be obtained
from natural deposits but are most generally obtained
from the distillation of crude petroleum (fig. 13-26).
Tars are obtained from the destructive distillation of
bituminous coal (fig. 13-27). The functions of
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