ASPHALT BINDERS AND ASPHALT MIXTURES - Materials for Civil and Construction Engineers - page 377

The specimens are divided into two lots: reference specimens and condi-
tioned specimens. A strength test is used to evaluate the strength before and
after conditioning; the retained strength, the ratio of conditioned strength to
reference strength, expressed in percent, is computed. Criteria are used to
determine if the retained strength is adequate. The different techniques for
evaluating moisture susceptibility vary, depending on the specimen prepa-
ration, conditioning procedures, and strength.
The immersion-compression test (ASTM D1075) has been used to evalu-
ate moisture susceptibility. The method evaluates the retained compressive
strength after vacuum saturation. Other methods use Marshall specimens,
freezing and water soaking to condition the samples, and determining diame-
tral strength and modulus values to evaluate the retained strength. Freezing
the samples greatly increases the severity of the test.
There are several ways to alter asphalt concrete's susceptibility to water
damage. Methods identified by the Asphalt Institute include the following:

1. increasing asphalt content
2. using a higher viscosity asphalt cement
3. cleaning aggregate of any dust and clay
4. adding antistripping additives
5. altering aggregate gradation

In addition, portland cement and lime have been used by some agencies
as antistripping agents. Generally, when water damage susceptibility is a
problem, the additive is added to the mix at three levels, and the water dam-
age test is performed to determine the minimum amount of additive that can
be used to increase the retained strength to an acceptable level. If an ac-
ceptable mix can be developed, Marshall or Hveem specimens are prepared,
and the mix is tested to determine whether it meets the design criteria.

9.10 Characterization of Asphalt Concrete

Tests used to characterize asphalt concrete are somewhat different from
those used to characterize other civil engineering materials, such as steel,
portland cement concrete, and wood. One of the main reasons for this dif-
ference is that asphalt concrete is a nonlinear viscoelastic or viscoelasto-
plastic material. Thus, its response to loading is greatly affected by the rate
of loading and temperature. Also, asphalt pavements are typically subjected
to dynamic loads applied by traffic. Moreover, asphalt pavements do not
normally fail due to sudden collapse under the effect of vehicular loads, but
due to accumulation of permanent deformation in the wheel path (rutting),
cracking due to repeated bending of the asphalt concrete layer (fatigue crack-
ing), thermal cracking, excessive roughness of the pavement surface, migra-
tion of asphalt binder at the pavement surface (bleeding or flushing), loss of
flexibility of asphalt binder due to aging and oxidation (raveling), loss of