The first asphalt roadway in the United States was constructed in 1870 at Newark, New Jersey.

As experience was gained throughout the years, various methods were developed by different agencies for determining the thickness of pavement required.

As experience was gained throughout the years, various methods were developed by different agencies for determining the thickness of pavement required.

Prior to the early 1920s, the thickness of pavement was based purely on experience. The same thickness was used for a section of highway even though widely different soils were encountered.

Although pavement design has gradually evolved from art to science, empiricism still plays an important role even up to the present day.

Following are the five Flexible Pavement Design methods, evolved our many years:

The use of the empirical method without a strength test dates back to the development of the Public Roads (PR) soil classification system in which the sub-grade was classified as uniform from A-1 to A-8 and non-uniform from B-1 to B-3.

The PR system was later modified by the Highway Research Board (HRB), in which soils were grouped from A-1 to A-8 and a group index was added to differentiate the soil within each group.

Steele (1945) discussed the application of HRB classification and group index in estimating the sub-base and total pavement thickness without a strength test.

The empirical method with a strength test was first used by the California Highway Department in 1929.

The thickness of pavements was related to the California Bearing Ratio (CBR), defined as the penetration resistance of a sub-grade soil relative to a standard crushed rock.

The CBR method of design was studied extensively by the U.S. Corps of Engineers during World War II and became a very popular method after the war.

With the ever increasing speed and volume of traffic, pavements should be designed for riding comfort rather than for barely preventing shear failures.

The major properties of pavement components and subgrade soils to be considered are their cohesion and angle of internal friction.

The major properties of pavement components and subgrade soils to be considered are their cohesion and angle of internal friction. Barber (1946) applied Terzaghi's bearing capacity formula (1943) to determine pavement thickness.

The limiting shear failure method is used to determine the thickness of pavements so that shear failures will not occur.

The limiting deflection metho disused to determine the thickness of pavements so that the vertical deflection will not exceed the allowable limit.





The Kansas State Highway Commission (1947) modified Boussinesq's equation (Boussinesq, 1885) and limited the deflection of sub-grade to 0.1 in.





The U.S. Navy (1953) applied Burmister's two-layer theory (Burmister, 1943) and limited the surface deflection to 0.25 in.





The use of deflection as a design criterion has the apparent advantage that it can be easily measured in the field. Unfortunately, pavement failures are caused by excessive stresses and strains instead of deflections.