By: Robert G. Packard, P.E.
UTW proves its worth in worldwide tests Ultra-thin whitetopping (UTW) is
a fast-growing resurfacing process where a thin 2- to 4-in. layer of high-strength
concrete is placed on top of existing asphalt. Since the 1991 research project
in Louisville, 44 UTW pavements were built in the U.S. Many of these were
placed at intersections where the asphalt had rutted or shoved. Mexico,
Sweden and Canada also have built UTW projects to bring the total to 68
projects.
Mexico is the volume leader in UTW use, placing 621,000 sq yd in 21 different
projects. The largest is in Ensenada, Baja California. It covers 299,000
sq yd, is 2.5-in. thick and has joints spaced on a 3 ft grid. Second largest
is a 2.5-in. thick overlay at the Ford plant in Hermosillo, Sonora, covering
120,000 sq yd.
Mexico has a UTW research project on an urban arterial in Tijuana. Test
sections include thicknesses of 2.5 and 3.5 in. and square-joint spacings
of 3, 4 and 6 ft. Initial results indicate the UTW is a rehabilitation technique
that significantly extends the useful life of flexible pavements.
Sweden also has done research with UTW in an effort to determine if they
could use concrete to stop studded-tire wear of asphalt. Projects in 1989
and 1993 found that concrete did stop the studded-tire wear and that the
concrete bonded to the asphalt, improving the structural capacity of the
roadway.
Two of the largest instrumented UTW experiments in the U.S. are those at
the St. Louis Airport and on Route 21 in Iowa near Belle Plain. The airport
is being used to develop a UTW design procedure. The Iowa project has 64
different test sections. Variables include thickness, maximum joint spacing,
and the use of fibers. To date, both projects are performing exceptionally
well.
Experience gained from these research and in-service projects will lead
to new design criteria and optimum use of UTW.
About The Author: Packard is director of engineering-design for the American Concrete Pavement
Association. You may write him in care of the editor.
