John Merriss, Policy Section Manager555 13th Street NE, Suite 2,
Salem, Oregon 97301-4178
(503) 986-3466
This issue of Policy Notes was written by John Merriss, Policy
Sec-tion Manager, ODOT Policy Section, and does not necessarily
re-flect the views of the Oregon Department of Transportation or
thePolicy Section. Author can be reached by email
[email protected] or call (503) 986-3474.
New Research on Pavement Damage Factors
June 2003
Oregon Department of Transportation, Policy Section
notespolicyNumerous factors contribute to deterioration
ofpavements and the consequent need to reconstruct,rehabilitate, or
repair these pavements. These includeenvironmental (weather)
influences, dynamicinteractions between pavements and vehicle
speeds,numbers of tires per axle, tire pressures andsuspension
characteristics, and the frequency,spacing and magnitude of axle
loads. Sub-base soiland terrain characteristics, as well as
maintenancepractices and the structure, materials, depth
andconstruction quality of the initial pavement also play arole in
determining pavement life.
The available research indicates that of these factors,by far
the most important are the frequency, spacingand magnitude of axle
loads. These are therefore theprimary factors, along with
environmental influences,considered in assessing responsibility for
pavementdeterioration.
Over the years, various sets of equivalence factorshave been
developed to compare the pavement stressimposed by different types
and weights of vehicles.This article briefly compares the
load-related damagefactors from previous research with those from
thenew National Pavement Cost Model (NAPCOM)research.
Previous Research
The pavement damage equivalence factors used inhighway cost
allocation, truck size and weight, andother pavement studies prior
to the mid-1990s werebased on the results of the American
Association ofState Highway Officials (AASHO) Road Test of the
late1950s and early 1960s. The results of this research
were used to develop equivalent single axle load(ESAL) factors
relating the pavement stress imposedby an axle of any given weight
to that imposed by an18,000-pound single axle. This weight was
selected asthe benchmark axle weight because it was themaximum
legal weight of a single (dual-tired) axle atthe time of the AASHO
Road Test.
This research indicated that, on average, pavementwear increases
at approximately the fourth power ofincreases in axle weight.
Doubling the weight on anaxle, in other words, results in an
approximately 16-fold (i.e., two to the fourth power, or 2x2x2x2)
increasein the amount of stress applied to pavements. TheAASHO Road
Test research and resulting ESALfactors were also the basis for the
often-quotedstatement that it takes 9,600 automobiles to do thesame
pavement damage as one fully-loaded, 80,000-pound, five-axle
truck.
New Research
For highway cost allocation studies (HCASs), the olderAASHO Road
Test results have now been replaced bythose from the newer NAPCOM
research. NAPCOM isa complex simulation model developed by the
FederalHighway Administration in 1992 that further refines
thedeterioration-based method employed in the 1982Federal HCAS. It
uses information on specific,representative highway sections
supplied by the statesthrough the Highway Performance Monitoring
System.NAPCOM models eleven different pavement
Policy Notes June 2003
2
distresses such as faulting, fatigue cracking, thermalcracking,
rutting and loss of skid resistance to estimatewhen pavement
restoration will be required anddetermine expected pavement
condition at the end ofeach year of the analysis.
The NAPCOM results and load equivalence factors(LEFs) developed
from these results have been usedin the 1997 Federal HCAS and the
most recentnational Truck Size and Weight Study, as well as
the1999, 2001, and 2003 Oregon HCASs. The NAPCOMresults differ from
those of the older AASHO Road Testresearch in two major ways:
1. Unlike the approximately fourth-power relationshipestablished
by the earlier research, NAPCOMestimates separate pavement wear
relationshipsfor each different distress modeled. For most,
theexponent is considerably less than four. While theprecise
relationship differs depending on the typeof pavement and distress
being considered, theNAPCOM research suggests, on average,
theexponent relating pavement deterioration toincreases in axle
weight is closer to 2.5 than 4.This means a doubling of axle weight
results in anapproximately 6-fold, rather than 16-fold, increasein
load-related pavement damage.
As with the older ESAL factors, separate LEFshave been developed
for rigid (Portland cementconcrete) and flexible (asphaltic
concrete)pavements. Using a weighted average reflectingthe overall
mix of Oregon pavement types (about93% flexible and 7% rigid)
suggests it takesroughly 750 automobiles weighing 3,800 poundseach
to do the same pavement damage as onefully-loaded, 80,000-pound
truck. This issubstantially lower than the 8,000-10,000 to
onerelationship derived from the older ESAL factors.
2. The second major finding of the NAPCOMresearch, which has
received less attention thanthe first, is that weather and other
non-load-relatedfactors play significantly less of a role in
pavementdeterioration than suggested by the earlierresearch. This
largely offsets the impact of thelower exponential relationship
between pavementwear and axle weight, so that the
overallresponsibility of trucks and other heavy vehiclesfor
pavement costs is approximately the same asunder the older
research.
Use of the newer LEFs instead of the older ESALfactors in a
HCAS, in other words, results in a shiftof responsibility for
pavement costs within theheavy vehicle classes, specifically from
the heavieraxle weight classes to the lighter and medium-weight
axle classes, but does not materiallychange the overall
responsibility of heavy vehiclesfor pavement costs. This is
illustrated by the factthe 2003 Oregon HCAS found heavy vehicles,
as agroup, to be responsible for approximately 70% ofpavement
reconstruction and rehabilitation costs, aresult generally similar
to those of the past severalOregon studies.
Summary
New research on the relationship between vehicleweight and
pavement deterioration finds thisrelationship to be less extreme
than previouslythought. This research suggests that, on average,
theexponent relating pavement damage to increases inaxle weight is
closer to 2.5 than 4. At the same time,this research indicates
weather and other non-load-related factors play less of a role in
pavementdeterioration than suggested by the earlier research.The
combined effect of these two findings is to leavethe overall
responsibility of trucks and other heavyvehicles for pavement costs
relatively unchanged, butto shift more of this responsibility from
heavier axleweight vehicles to lighter and medium-weight
axlevehicles. These findings have been reflected in theresults of
the 1997 Federal HCAS and the past threeOregon HCASs.
