TRB's National Cooperative Highway Research Program (NCHRP) Synthesis 338: Thin and Ultra-Thin Whitetopping summarizes available information to document how state departments of transportation and others are currently using thin and ultra-thin whitetopping overlays among various pavement rehabilitation alternatives. The report covers all stages of the proper application of whitetopping overlays, including project selection, design, materials selection, construction, maintenance, and eventual rehabilitation or replacement.

Concrete overlay of deteriorated asphalt pavements (whitetopping) has been a viable alternative to improve the pavement's structural integrity for over six decades. The thickness of such overlay usually exceeds five inches. In the last few years, however, a newer technology has emerged which is commonly known as Ultra Thin Whitetopping (UTW). UTW is a construction technique, which involves placement of a thinner (than normal) thickness ranging from 2 to 4 inches. he intent of this research study is to identify and address important factors that contribute to the performance of the UTW pavement system. It is also the goal of this research to present an interim design procedure fine tuned by further observation of UTW systems.

A suburban city street in Kansas was rehabilitated with a 50 mm (2 in.) Portland Cement Concrete (PCC) thin overlay, commonly known as ultra-thin whitetopping (UTW). The construction and performance of this UTW project have been described in this report. The project, constructed in the Spring of 1995, incorporated the following design features: 0.9 m x 0.9 m (3 ft x 3 ft) panels versus 1.2 m x 1.2 m (4 ft x 4 ft) panels, plain versus fiber reinforced concrete, and sealed versus unsealed joints. The project has performed fairly well to date although some test sections needed periodic maintenance and all of the test sections except two have been overlaid as of October 2001. Experience on this project shows that the UTW overlay can be easily built with conventional equipment and locally available materials. UTW also permits a skid-resistant finish to be applied. Excellent smoothness can also be obtained although the slab thickness is very small. Corner cracking appears to be the most dominant distress type, though it was observed that bond existed between the concrete and the asphalt layers even for the cracked panels. The bond appeared to degrade with time. Joint spacing has a significant effect on performance. The sections with smaller joint spacing appeared to perform better. The performance of the sections with fibers in concrete was inconclusive. Also, joint sealing did not appear to affect the performance.

Whitetopping is a pavement rehabilitation construction practice where portland cement concrete (PCC) is placed over an existing asphalt concrete pavement as an overlay. Ultra-thin whitetopping (UTW) is generally a thin overlay with a thickness between 2 and 4 inches. UTW is usually of high strength and made with fibers for improved tensile strength, ductility and enhanced fatigue life. UTW differs from conventional whitetopping because of the design and construction procedures that ensure substantial bonding between the UTW and the underlying asphalt. UTW also employs much closer joint spacing than conventional whitetopping; this reduces the load-induced stresses within the UTW. UTW does not make use of steel reinforcement. A literature and technology review found that UTW is a proven means of asphalt pavement rehabilitation for improved serviceability. The mechanistic design concepts for UTW are clearly established. The construction methodologies have been developed and are in place. The material technology for fiber reinforced high strength PCC is available. Specifications have been written and successfully used on numerous highwayand airport pavement projects. The initial cost of UTW is more than the common asphalt concrete pavement rehabilitation used in New Mexico. However, life cycle costs of UTW compared to asphalt overlays are considered competitive. It is recommended thatthe NMSHTD design and construct UTW test sections at an appropriate selected airport in the state of New Mexico. Such a project will allow for the determination of the relative initial costs of the UTW versus asphalt rehabilitation, and long term monitoring will allow for the determination of the life cycle costs and long-term performance of UTW versus asphalt.