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testing methods with the diametral resilient modulus test (8), indirect
tension test (9), and the triaxial resilient modulus test (1) being the
most popular. The latter is useful for unbound materials such as base
course and subgrade soils, while the other two are for bound materials
like asphalt concrete and stabilized materials. Monismith et al. (78)
studied the various factors that affect laboratory determination of the
moduli of pavement systems. They concluded that
. . it is extremely difficult to obtain the same
conditions that exist in the road materials (mois
ture content, density, etc.) and the same loadings
(including loading history) in the laboratory as
will be encountered in situ. . Thus the best
method of analysis would appear to be to determine
an equivalent modulus which when substituted into
expressions derived from the theory of elasticity,
will give a reasonable estimate of the probable
deformation.
(78, p. 112)
Destructive field tests include, among others, several different
plate load tests (8) and the California Bearing Ratio (CBR) test (8).
These tests require trench construction and subsequent repair of the
pavement, and like the laboratory test methods, usually call for an
elaborate and costly testing program. The delays associated with such
programs are prohibitive especially for routine pavement analysis
studies.
The third method involves the extraction of pavement-layer proper
ties from in situ nondestructive testing (NDT). NDT methods have gained
wide popularity in the last few decades because of their ability to
collect data at many locations on a highway or airfield in a short
time. Therefore, a great deal of research effort has been concentrated
on this area. A review of the various types of NDT equipment available
and the associated interpretation tools is presented below.