(2) Porous Asphalt Mixes - The bitumen content in this type of mixture is
usually between 2 and 6 percent by weight and is less than the void volume of
the aggregates. Because of the underfilled condition, the mixture is perme-
able; and its mechanical properties are governed by the aggregates.
The
bitumen acts only as a binder.
An example is "lean sand asphalt" where the
sand grains are stuck together by a thin film of bitumen.
As a result, sand
asphalt may be almost as permeable as natural sand and is commonly applied as
a filter layer. Sand asphalt, when warm, behaves like loose sand. After
cooling, it behaves like a soft sandstone when subjected to short-duration
loadings.
Another example is "open stone asphalt" which contains approximately
80 percent (by weight) uniformly sized stone and 20 percent mastic asphalt.
Two-stage mixing is also required for the production of this mixture (see van
This open stone asphalt mixture can be used as an
Garderen and Mulbers 1983).
armor layer under light to moderate wave climate.
(3) Mastic Asphalts - These are mixtures of mineral aggregate and filler
in which the voids in the mineral matrix are overfilled with bitumen. The
result is an asphalt mix that can be applied by pouring or by hand-floating
into place. A typical mastic contains 60 percent (by weight) sand, 20 percent
filler, and 20 percent bitumen.
The mechanical properties of the bitumen are
dominant in determining mix behavior. At ambient temperature, mastic asphalt
It behaves like an elastic
is highly viscous under long-duration loading.
material when subjected to short-duration loading, such as wave forces.
Mastic asphalt can be used in several ways including waterproofing,
erosion and scour protection, and reinforcement or stability enhancement to
rubble structures. Hot mastic mixes can be placed underwater through tremies,
chutes, or by simply dumping in masses.
For reinforcing, mastic asphalts are
used as grouts to fill and plug the voids in stone structures such as jetties,
b r e a k w a t e r s , and revetments (see Figure 1).
The binding action of mastic
asphalt tends to produce a more firm mass.
This binding ability could
possibly reduce the stone size required to produce a stable rubble structure
but may increase wave runup and reflection. Also, such filling may increase
maintenance costs due to undermining. Grouting techniques with asphalt
mastics are discussed by Smith (1962) and Visser (1969).
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