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IP
t
= 2,5 - 3,0 for major highway and p
t
equals to 2 for minor highway PSI
SW, FH
= serviceability loss because of soil swelling effect of moisture and frost ΔPSI
SW, FH
= 0,00335 . V
R
. P
S
. 1 – e
- t
2.5
= swell rate constant as a function of moisture supply and soil fabric V
R
= maximum potential heave as a function of plasticity index, compaction and subgrade thickness, inch.
P
s
= swelling probability,
2.3.3 The Relationship between PSI and IRI
The loss of serviceability ΔPSI is the difference between the initial serviceability of the pavement when opened to traffic and the terminal serviceability that the pavement will reach
before rehabilitation, resurfacing or reconstruction is required. The present serviceability index PSI, also known as the present serviceability rating PSR, is a subjective measure by
the road user of the ride quality, ranging from zero impassible to five perfect ride. Studies conducted at the AASHO Road Test found that for a newly constructed flexible pavement, the
initial serviceability p
o
was approximately 4.2 AASHTO, 1993. The value of a terminal serviceability p
t
was ranging between 2.0 and 3.5. The 1993 AASHTO Design Guide recommends the selection of p
t
based upon the same traffic levels used for reliability selection: for low traffic, 2.5, for medium traffic, 3.0, and for high traffic,
3.5. To demonstrate the subjectivity of the measurement, studies from the AASHO Road Test found that an average of 12 of road users believe that a pavement receiving a rating of 3.0 is
unacceptable for driving while 55 of road users believe that 2.5 is unacceptable AASHTO, 1993.
Due to the subjective nature of serviceability measurements, most current road roughness measurements are now standardized to the international roughness index IRI.
This index provides a measure of the longitudinal wavelengths in the pavement profile in inches per mile or meters per kilometer. These measurements are taken by inertial profilers,
and can be closely replicated from machine to machine Sayers and Karamihas, 1998.
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The use of this index can remove the subjectivity of assessing the ride quality, and therefore is a more accurate measurement. However, since the AASHTO flexible pavement
design procedure still requires serviceability levels as inputs, a conversion must be made from IRI to PSI Hall and Munoz, 1999.
In 1999, Hall and Munoz developed relationships for relating IRI and PSI for both asphalt and concrete pavements. They analyzed data from AASHO Road test that included
parameters of slope variance SV and PSI, and then developed a correlation between SV and IRI for a broad range of road roughness levels. Their finding for flexible pavements can be
expressed mathematically as
2.6
Where: 2.7
2.8
in which all variables are as previously defined. Based upon the similarity of the Al-Omari and Darter 1994 and Holman 1990
equation, it was decided to focus on those relationships for this study. Since the equation developed by Al-Omari and Darter 1994 could produce much larger performance database,
therefore, this equation was selected to convert the IRI data to present serviceability values.
2.3.4 Reliability R and Standard Deviation S
