Important performance information is emerging from UTW projects in service;
but each project is only a single point of reference, usually not quantifying
systematic comparisons of different design features. However, six comprehensive
research projects are intended to shed light on UTW design questions: Louisville,
St. Louis International Airport, Iowa Route 21, and three projects in Colorado.
Variables include concrete thickness (2-6 in.), joint spacing (2-6 ft),
milled and unmilled asphalt surface, and fibers and no fibers in the concrete.
All six projects have been, or will be, instrumented with strain gauges.
Other tests conducted at most of the sites are: falling weight deflectometer,
profile and temperature measurements, condition surveys, and cores to measure
bond between concrete and asphalt.
So far, the short-term performance of UTW has been outstanding, much better
than would be predicted by conventional design procedures. This success
is attributed to two factors: concrete bond to asphalt, and the use of short
joint spacings. The bond at the concrete-asphalt interface creates a composite
section so that load stresses in the concrete are substantially reduced.
Short joint spacings also reduce stresses because the slabs are not long
enough to develop as much bending moment.
Iowa State University will analyze data from the Iowa project that includes
65 concrete overlay sections. Construction Technology Laboratories (CTL),
Skokie, Ill., will monitor the data from the St. Louis Airport and the Colorado
projects. Initial strain measurements indicate high bond or friction at
the interface so CTL has developed three-dimensional (3D) finite-element
software because conventional computer techniques can't model variations
of this condition.
Performance data from in-service pavements and detailed data from the research
projects, along with the 3D computer model will be used to develop guidelines
for UTW design.
Packard is director of engineering-design for the American Concrete Pavement Association. You may write him in care of the editor.