One the American Association of State Highway Officials

One of the primary factors that affects
the pavement smoothness is International Roughness Index(IRI). IRI is the
measure for texture of pavement. IRI was established in the late 1970s as an
internationally accepted parameter to monitor roadway smoothness and
irregularities. Several studies 6,7 have shown that smooth roads costs
highway agencies less over the life cycle of the pavement, thereby decreasing
the overall cost of maintaining the roadway in addition to user operating
costs, delay costs, and fuel consumption. IRI is measured using specialized
vehicles with computer technology to monitor pavement roughness. These vehicles
record the displacement of the vehicle chassis, which is located on the rear
axle, usually in terms of irregularities per mile or foot 6.  

In addition to IRI, roadway smoothness
can also be quantified in the form of the Present Serviceability Rating (PSR),
which depends on a human subjective evaluation of ride quality on a road
segment. This was established by the American Association of State Highway
Officials (AASHO) on 1962. Research studies have been initiated to help state
highway departments establish new smoothness specifications. Most of the
research was focused on establishing correlations between the old and new
pavement smoothness indices. Models were developed to predict IRI using the
profile index PI5.1 obtained from the measurements of roadway profiles using
manual profilographs, computerized profilographs, lightweight inertial
profilers, ultrasonic–type inertial profilers and laser-type inertial
profilers. These models were developed by investigators such as Florida DOT
(1997), Fernando (2000) and Hossain et al. (2000), based on data collected from
specific climatic regions (e.g., Kansas, where conditions are met with winter
freeze), using specific equipment (e.g., lightweight inertial profiler) and
specific pavement type (e.g., Portland Cement Concrete). 8 Evaluation of
pavement smoothness using the PI5.1 results in filtering a portion of the
pavement roughness and therefore, shows smoother roads than in reality. This
situation led to a search for more acceptable measures of pavement smoothness.
With the advancement in roadway profiling equipment and technology, attention
is focused on the International Roughness Index (IRI) as the rational measure
of pavement smoothness. After measuring the longitudinal roadway profile, the
IRI is determined using a mathematical model by accumulating the output of
quarter-car model and dividing by the profile length. The IRI is expressed in
mm/km or inch/mile. The IRI is described as a rational indicator that reflects
the smoothness of pavements and the ride quality 8.  

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Pavement performance is a function of the
combined effects of traffic load, environment, age, and past major and routine
maintenance. There is a direct relationship between pavement age and pavement
serviceability for highway sections with similar ranges of traffic volume. In
such a case, pavement age can be used to represent pavement surface roughness
over time 13. In previous studies, pavements have been divided into several
groups depending on their age or time after last major improvement. Age Group I
includes pavements that are either fairly new (5 years) or it has been less than
5 years since last major repairs/improvements have been made. These pavements
are in very good condition with respect to surface roughness and do not require
maintenance works. In fact, it has been suggested that excessive improvement
works on such pavements may even increase the surface roughness. Age Group II
contains pavements in good to fair condition and of age between 5 to 15 years
of reconstruction or major maintenance works. Pavements on this group require
increasingly more frequent routine maintenance with time. Effectiveness of
maintenance is highest for pavements in this group. Age Group III contains
pavements in poor condition. Pavement age may range from 10 years to more than
15 years. The need for major improvement or resurfacing in this group is more
than the need for routine maintenance. So, the effectiveness of routine maintenance
would be little for this age group as for group I. From this classification and
the available information, it is clear that age is a major factor when it comes
to determining pavement smoothness. Therefore, the effect of age of pavement on
pavement smoothness needs to be considered.

A major concern today in many parts of
the United States is excessive permanent deformation (rutting) in heavy duty
asphalt-concrete pavements resulting from frequent repetitions of heavy axle
loads, many of which are operating with radial tires having pressures 20 to 25
psi higher than the bias-ply tires which they have replaced (e.g., 105 psi
versus 80 psi). Rutting gradually develops with increasing numbers of load
applications and appears as longitudinal depressions in the wheel paths. These
depressions or ruts are of concern for at least two reasons: 1) if the surface
is impervious, the ruts trap water and, at depths of about 0.2 in.,
hydroplaning (particularly for passenger cars) is a definite threat; and 2) as
the ruts progress in depth, steering becomes increasingly difficult, leading to
added safety concerns. Hence, it is safe to say that rutting has significant
effect on pavement performance 9. Therefore, if the effect of different
factors on pavement smoothness is to be studied, it is important that rutting,
a factor that highly determines the serviceability of pavements, be also be
considered in the study. Rutting in paving materials develops gradually with
increasing numbers of load applications, usually appearing as longitudinal
depressions in the wheel paths accompanied by small upheavals to the sides. It
is caused by a combination of densification (decrease in volume and, hence,
increase in density) and shear deformation and can occur in any one or more of
the pavement layers as well as in the subgrade. Trenching studies performed at
the AASHO Road Test (Highway Research Board, 1962) and test-track studies
reported by Hofstra and Klomp (1972) indicated that shear deformation rather
than densification was the primary rutting mechanism.

The fatigue resistance of an asphalt mix
is its ability to withstand repeated bending without fracture. Fatigue, a
common form of distress in asphalt-concrete pavement, manifests itself in the
form of cracking from repeated traffic loading. The fatigue characteristics of asphalt
mixes are usually expressed as relationships between the initial stress or
strain and the number of load repetitions to failure—determined by using
repeated flexure, direct tension, or diametral tests performed at several stress
or strain levels 10. Hence, fatigue has been identified as another parameter
whose effect of pavement smoothness needs to be studied.

Temperature stresses can cause transverse
cracking of asphalt pavements which can cost many millions of dollars in
maintenance cost each year. It was once thought that temperature induced
transverse cracking of asphalt pavements was entirely the result of low
temperatures causing the pavement material’s tensile strength to be exceeded by
tensile stresses – a mechanism now termed “low-temperature cracking”.
Although models for low temperature cracking have been used with some success
in northerly regions, where the temperature drops low enough to cause a
pavement to reach its “fracture temperature”, in many cases
transverse cracking is quite common even though relatively moderate
temperatures prevail. A mechanism that accounts for thermally induced cracking
of asphalt pavements in relatively moderate climates is “thermal-fatigue
cracking” due to temperature cycling that eventually results in the
fatigue resistance of the asphalt concrete being exceeded 11. Therefore, it
seems clear that temperature too plays an important role in the performance of
asphalt pavements. It seems reasonable then to assume that it might also have
some effect on pavement smoothness. Therefore, the effect of temperature on the
smoothness of pavement (characterized by IRI) will also be explored in this
report.

Truck traffic is the most important
factor leading to pavement deterioration and damage. In previous studies it has
been mentioned that Truck Traffic affects the texture of the pavement.
Traditionally, truck traffic has been aggregated into equivalent repetitions of
a standard axle load (ESAL) for pavement design. Recent developments in
pavement distress models, high-speed personal computers, and sophisticated test
equipment have advanced pavement design to the mechanistic-empirical procedures.
In Report 12 it is mentioned that California Department of Transportation (Caltrans)
has been installing Weigh-In-Motion (WIM) stations and collecting truck traffic
data on the state highways since 1987. It has maintained a very detailed
database of historical truck traffic information for over 80 highway sites
across the state. Examination of these microscopic-level (site-specific) data
can provide insights into the truck traffic flow pattern and help answer
questions.  With the help of the previous
study the AADTT was included as one of the parameter in this study.

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