EFFECTS OF HIGHER TIRE PRESSURES ON STRAIN IN THIN AC PAVEMENTS

Since the oil embargo of 1973 and the attendant increase in fuel prices, pressures to increase truck sizes and weights have intensified. A second factor, which has also been on the increase but which has been given little attention, is tire inflation pressures. Inflation pressures have increased from in the vicinity of 80 psi to a typical value of 120 psi found in Texas tire pressure surveys in 1984. The objective of this paper is to evaluate the effect of this increase in tire pressures on tensile strains in thin asphalt concrete pavements. To determine the stress distribution between the tire and the road surface, the researchers used a finite element computer program developed at Texas Transportation Institute to study the effect of tire parameters on road surface friction forces. This computer program was used to develop the vertical pressure distribution and the horizontal surface shear forces for a free-rolling truck tire inflated to both 75 and 125 psi. A series of computer runs was made using ILLI-PAVE to determine the horizontal tensile strains for asphalt concrete surfaces 1, 1.5, 2, and 4 in. thick over an 8-in. granular base with three different moduli and a subgrade soil that is stress sensitive with an initial modulus of 10 ksi. This series of runs showed that increased truck tire pressures increase tensile strain and attendant fatigue cracking dramatically, that some thin pavement structures cannot provide adequate service, and that design procedures must be upgraded to consider pavement materials that can resist these high tensile strains. In general, to provide adequate service, materials should be either thin and flexible or thick and stiff.