Dual tires have traditionally been used to limit pavement damage by efficiently distributing axle loads over a larger contact area than single tires. However, in recent years, the trucking industry, stating economic and safety benefits, has promoted the use of wide-base single tires. The Super Single tire, an early type of wide-base tire, proved inadequate and induced excessive pavement damage. By contrast, the new generation wide-base tires have contact areas that approach those of dual tires and offer the potential for improved performance. The Florida Department of Transportation investigated the pavement damage potential of four tire types, including a conventional dual tire (11R22.5), a Super Single (425/65R22.5), and two newly designed wide-base single tires (445/50R22.5 and 455/55R22.5 respectively). A controlled accelerated pavement testing program, in addition to theoretical modeling, was performed to determine critical pavement response parameters. Pavement damage was measured in terms of rutting and fatigue cracking (bottom-up or top-down), the predominant distresses in Florida. The investigation revealed that the 455-mm wide-base tire performed as well as the dual tire. By comparison, the 445-mm wide-base tire was shown to create more rut damage on a dense-graded pavement surface and was predicted to create more bottom-up cracking than a dual tire. As expected, the Super Single induced the most damage to the pavement.

Resilient modulus and Young's modulus are parameters increasingly used to characterize the behavior of pavement materials, both in the laboratory and in the field. The small-strain modulus response of unbound aggregate base coarse materials to various moisture environments was documented. Modulus is not constant, even when held at constant moisture, and significant changes in modulus occur with drying and wetting of the material. The response to drying and wetting cycles appears to be repeatable and suggests that the underlying mechanism that controls the response is reversible. This behavior can have a significant effect on the use of modulus for pavement design and quality acceptance.

He has worked for 35 years in government, industry, and education. Besides teaching during the last 14 years, Dr. Najafi has conducted research, has been a participating member of several professional societies including ASEE, has published numerous refereed and non-refereed articles, and has presented many technical papers to international, national and local organizations.