ASPHALT BINDERS AND ASPHALT MIXTURES - Materials for Civil and Construction Engineers - page 381

■

9.10.3 Freeze and Thaw Test

The freeze and thaw test is performed to evaluate the effect of freeze and
thaw cycles on the stiffness properties of asphalt concrete. Cylindrical
specimens 102 mm (4 in.) in diameter and 64 mm (2.5 in.) high are used.
Three specimens are tested for resilient modulus as discussed earlier,
while the other three specimens are subjected to cycles of freeze and thaw,
after which the resilient modulus is determined. The tensile strength ratio
is computed by dividing the average resilient modulus of conditioned
specimens by the average resilient modulus of unconditioned specimens,
expressed in percent. A minimum tensile strength ratio is usually re-
quired to identify mixes that are not severely affected by freeze and thaw
cycles.

■

9.10.4 Use of Rheological Models to Analyze

Time-Dependent Response
Asphalt concrete is a viscoelastic material exhibiting a time-dependent re-
sponse under load. Rheological models consisting of combinations of
Hookean (spring) and Newtonean (dashpots) elements have been used to
analyze the response of time-dependent materials, as discussed in Chapter
1. The Burgers model illustrated in Figure 1.12 can closely approximate
the response of asphaltic mixtures (Mamlouk 1984). Laboratory tests, such
as the creep test, are used to obtain the parameters of the Burgers model
using a curve-fitting procedure. Once these parameters are determined,
the model can be used to predict the response of the material under dif-
ferent loading conditions. For example, Burgers model has been used to
predict rutting of asphalt concrete pavement under the action of traffic
loads.

9.11 Asphalt Concrete Production

Asphalt concrete is produced in either a batch plant or a continuous (drum)
plant (The Asphalt Institute 1989). In the United States, batch plants were
used extensively in the past; however, more energy efficient continuous plants
are now preferred.
In continuous plants (Figure 9.38), aggregates of different gradations are
placed in cold bins. The gradation proportions needed are taken from the
cold bins by a cold feed elevator. Aggregates are transferred to the first part
of the drum, where they are dried and heated. Hot asphalt cement is intro-
duced in the last one-third of the drum; then aggregates and asphalt are
mixed. Since asphalt concrete is produced continuously in this type of
plant, it is transferred to a storage silo until placed in a truck and transported
to the job site.