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

In this Design Guide, pavement section thickness is determined using the following information:
! Subgrade strength, Resilient Modulus (MR)
! Traffic analysis, ESALs
! Reliability level of pavement performance
! Structural Number, SN
In Chapter Three, strength coefficients are provided for various types of materials available for use in the
pavement section. Chapter Three also provides thickness design procedures for determining alternate
pavement sections.
TRAFFIC ANALYSIS
Because the primary function of a pavement is to transmit and distribute wheel loads of vehicles to the
supporting subgrade, information about the traffic stream is required. Pavement should be designed to
serve traffic needs over a period of years. The volume of traffic and the various types of vehicles using the
facility should be estimated for the pavement's anticipated performance period.
A traffic assessment for an existing roadway, that is going to be improved, should be based on a detailed
traffic count of the existing traffic. The existing traffic should be determined for the following three
categories:
· Passenger cars and pickup trucks.
· Single unit trucks.
· Combination trucks.
Anticipated changes in traffic type should also be considered.
Based on appropriate growth factors, the traffic volume is
projected or inflated out to the end of the performance period,
usually 20 years.
Local agencies should also consider the type and extent of
construction traffic loads in new developments. Most often, the
heaviest loadings occur during the build-out phase of the
development. If these loadings are not taken into account in
the pavement structure design, a shortened performance period
of the pavement structure could result.
Since pavements are usually designed for periods ranging up to 20 years on the average or more, it is
necessary to predict the ESAL’s for this period of time, i.e., the performance period. The performance
period, often referred to as the design period, is defined as the period of time that an initial pavement
structure will last before reaching its terminal serviceability. Any performance period may be used with this
Guide since design is based on the total number of equivalent single axle loads (ESAL’s). However,
experience may indicate a practical upper limit based on considerations other than traffic. The ESAL’s for
the performance period represent the cumulative number from the time the roadway is opened to traffic to
the time when the serviceability is reduced to a terminal value (e.g., pt equal to 2.5 or 2.0). If the traffic is
underestimated, the actual time to pt will probably be less than the predicted performance period, thereby
resulting in increased maintenance and rehabilitation costs.