Evaluation and analysis of flexible pavement structures designed by conventional methods

Two most accepted conventional methods are evaluated to help highway engineers select appropriate procedures for designing flexible pavement structures: AASHTO Guide for Design of Pavement Structures in the U.S. and Overseas Road Note 31 from Britain. It is found that pavement thickness designed by both methods is approximately the same; however, AASHTO procedures come up with a thicker asphalt concrete layer whereas Road Note 31 with higher base and subbase values. Critical stress and strain locations within the pavement structure for analyzing design methods employed in this research are as follows: (1) deflection at the top of AC, base, subbase, and roadbed; (2) vertical compressive stress at the top of AC, base, subbase, and roadbed; (3) vertical micro strain at the top/bottom of layers; (4) tensile micro strain at the bottom of AC layer; and (5) tensile stress at the bottom of AC layer. The first three values are related to rutting and permanent deformation in a pavement structure, the fourth to fatigue cracking, and the fifth to thermal cracking. Three computer programs are used to calculate these stresses and strains. Under traffic loading, pavement structures designed by AASHTO procedures are shown to induce low stress, strain, and deflection than the ones by Road Note 31. The analyses of pavement structures indicate that AASHTO pavements can serve relatively longer for public usage. Pavement thicknesses suggested by Road Note 31 may cost less to build because of few AC materials needed, but they may be subject to premature failure, consequently requiring more money to maintain and rehabilitate them. It is then recommended that flexible pavement structures be designed according to the AASHTO procedure.