Experiences with thin and ultra thin HMA pavements in the United States

A number of transportation agencies in the USA have adopted or are adopting the strategy of preventive maintenance for their pavements. The Federal Highway Administration is providing the emphasis for this change and is funding preventive maintenance activities in current legislation (TEA-21). The use of preventive maintenance is designed to prevent major pavement distress from occurring by applying a nonstructural treatment early in the pavement life, which protects the existing surface and mitigates further deterioration. Several types of thin (less than 40 mm) and ultra thin (less than 25 mm) hot mix and cold mix systems have been used since the early 1980s. This paper focuses on the hot mix applications where experiences have indicated that these systems can be cost effective when placed on the right pavement at the right time. This paper describes these thin and ultra thin systems, their characteristics and special construction requirements as well as their performance history to date. The systems considered include the following: (1) Dense-graded mixes; (2) Open-graded or porous friction courses; (3) Gap-graded mixes - asphalt rubber and SMA mixes; (4) Specialized products, for example, Novachip, Gussasphalt. The paper also points out that additional work is needed to quantify the cost effectiveness and to determine the optimum timing and existing pavement condition for applying these treatments.