Technical information

3-39
responses were shown in Figures 3.5.2 to 3.5.4. In contrast to the NCAT case, each
APT pass resulted in one stress (or strain) pulse; also, the pavement was allowed to rest
for 8 seconds between passes. It should be noted that the APT was operated on
weekends also. Graphically, the response traces from the two experiments are very
much different and any attempt to directly compare them is futile given that they
embody many dissimilarities:
(i) Loading Speed: 45 mph at NCAT vs. 5 mph in the APT. The loading speed
influences the duration of stress and stress pulses and affects the HMA stiffness;
(ii) Axle Configuration: NCAT had three axle types, single, dual and dual
tandem vs. one axle type in the APT, dual;
(iii) Axle Load: NCAT axles were loaded to 20,000 pounds except for the steer
axle which was loaded to 10,000 pounds vs. one load level in the APT of 15,000
pounds;
(iv) Location of Loads relative to the Measuring Gauge: NCAT loads were
applied near the gauge array but the exact location is unknown while in the APT study
the load could be positioned accurately;
(v) HMA Temperature: NCAT temperature changed with the environment while
it was constant in the APT at 15.5ºC. The prevailing temperature influences the stiffness
of the HMA and hence the resulting responses.
Based on the above discussion it may be concluded that a direct relation
between the two experiments cannot be established as both performance and response
are distinct. Consequently a more fundamental approach is needed to link the observed
behavior. This is pursued in the following chapters using mechanistic principals
assuming the pavement systems in the two experiments have similar material properties.
Mechanistic treatment can account for each of the aforementioned dissimilarities. As
put forward and explained in Chapter 1 (Section 1.2) the analysis will focus on resilient
responses.