Guideline for the Design and Use of Asphalt Pavements for ...

TRAFFIC (ESALs)
The destructive effect of repeated wheel loads is the major factor that contributes to the failure of roadway
pavement structure. Since both the magnitude of the load and the number of repetitions are important,
provision is made in the design procedure to allow for the effects of the number and weight of axle loads
expected during the design period.
The traffic data figures to be incorporated into the design procedure are in the form of 18 kip equivalent
single axle load applications (18k ESAL). All vehicular traffic on the design roadway is projected for the
design period in the categories of passenger cars, single unit trucks and combination trucks. The actual
projected traffic volumes for each category are weighted by the appropriate load equivalence factors and
converted to a cumulative total 18k ESAL. Adjustments to this value will be made for directional and lane
distribution and then entered in the pavement design equation.
As discussed in Chapter Two, the design procedure separates traffic into six classes (I through VI). Each
class is defined by the number of autos per day, the average daily number of heavy trucks expected on the
facility during the design period, and the type of street or highway. Each traffic class has been assigned a
total ESAL value range. A comprehensive traffic study should be performed to determine the class of
roadway or highway that will be needed for the design. The total ESAL value for a required class of roadway
or highway is used for the design.
Caution should be used when measuring current traffic volumes. Normal growth, new development and any
other special impacts that might change the roadway usage and future traffic volumes needs to be
considered. Oftentimes, it is not possible to accurately predict some unknown future development that could
result in traffic in excess of an existing roadway design. There could be special provisions that exist in
planning and zoning regulations that allow a city or county to circulate a proposed new developments' impact
on existing roadways. Intersecting roadway volumes should be taken into account when designing
intersections. Not taking the cross traffic volumes into the design, premature failure of the
intersection pavement could result.
DRAINAGE CHARACTERISTICS
Drainage factors in flexible pavement design are taken into account through the use of modified structural
layer coefficients. A higher effective structural layer coefficient would be used for improved drainage
conditions. The factor for modifying the structural layer coefficient is called a mi value. This drainage
coefficient (m2, m3) has been integrated into the structural number (SN) equation (3-3), shown below, used
to calculate the thickness of the various layers of the pavement structure.
SN = a1D1 + a2D2m2 + a3D3m3 (3-3)
Where: a1, a2, a3 = structural layer coefficients
D1 = thickness of HMA surface course in inches
D2 = thickness of base course in inches
D3 = thickness of subbase course in inches
m2 = drainage coefficient of base course
m3 = drainage coefficient of subbase course
Drainage coefficients for different quality of drainage and the percent of time during the year the pavement
structure would be normally exposed to moisture levels approaching saturation are shown in Table 3-1.
Obviously, the latter is dependent on the average yearly rainfall and the prevailing drainage conditions. It is