PSI is a surrogate measure for PSR (present serviceability rating).Procedure was developed to determine the present serviceability index (PSI) of the pavement based on its roughness and distress which were measured in terms of extent of cracking, patching, and rut depth for flexible pavements.To quantify pavement performance, a concept known as the serviceability performance was developed.The main factor considered under functional performance is riding quality.

Functional performance: is an indication of how effectively the pavement serves the user.These factors include: cracking, faulting, raveling, and so forth.Structural performance: related to the physical condition of the pavement with respect to the factors that have negative impact on the capability of the pavement to carry the traffic load.Structural and functional performances.Pavement Performance (Loss of serviceability).This method Incorporates various design inputs including :.ِِInterim guide was published in 1961, revised in 1972, further edition 1986, then 1993ĪASHTO Design Method/ Design Considerations.Data were thoroughly analyzed and the results formed the basis for the AASHTO method.Pavement curvature at different speeds, stress imposed on the subgrade and temperature distribution in the pavement layers.Longitudinal and transverse profiles were obtained to determine extent of rutting, surface deflection caused by loaded vehicles moving at very low speeds,.Data were collected from pavement with respect to extent of cracking and amount of patching required to maintain the section in service.

Vehicles were driven for thousand repetitions (single axle ( 2, 000 – 30, 000 lb) and tandem (24, 000 – 48,000 lb).Flexible: HMA surface (1-6 “), well-graded crushed limestone base (0-9”), and uniformly graded sand-gravel subbase (0-6”).

Tangents sections with different lengths (> 100 ft).

Many types of test section were prepared and tested.

It is an effort that was carried out with the cooperation of all states and several industry groups.

Based on the results of AASHTO road test conducted on Ottawa, Illinois.

Chapter 16: Highway Engineering, by Paul Wright & Karen Dixon, 7th Edition, Wiley & sons Instructor: Dr. Flexible Pavement Thickness Design / AASHTO Method Source: Chapter 20: Traffic & Highway Engineering by Nicholas Garber and Lester Hoel, Third Edition, Brooks/Cole.

Pavement design aashto method