Article December 28, 2000
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At two sites, full-scale accelerated load testing is under way to answer questions about ultra-thin whitetopping (UTW): a thin (2- to 4-in.), high-strength concrete overlay placed over an existing asphalt pavement in need of rehabilitation.

This research will help quantify several unique structural advantages UTW has over normal concrete pavements: bond to the milled asphalt and the use of very short joint spacings. These features substantially reduce pavement stresses due to traffic loads. The purpose of the research is to verify and develop analysis models that predict load carrying capacity. The accelerated loading gets results much faster than the years that would be needed on a normal road.

In April, at the Federal Highway Administration (FHWA) Turner-Fairbank Research Center, McLean, Va., previously tested asphalt pavements were milled and overlaid with 2.5 and 3.5 in. of UTW with and without polypropylene fibers, and with joint spacings of 3, 4 and 6 ft. Eight lanes were instrumented to measure concrete and asphalt deflections and strains under load, as well as the composite action of the system. Loading began in May using the accelerated loading facility (ALF) pavement testing machine, which applies a 10,000-lb dual-wheel load (half of a 20,000-lb axle load). ALF will apply 35,000 loads per week of testing. Individual pavement sections will be tested until they develop significant distress. The overall duration of the study is 18 months. The research is being conducted under a cooperative agreement between FHWA and the American Concrete Pavement Association (ACPA).

At Ohio University's new indoor accelerated load pavement facility (ALPF), similar research is being conducted. In March, 3-in. UTW, with and without polypropylene fibers, was placed on asphalt milled to 6-in. depth and joints were green-saw cut to 3-ft squares. The ALPFs 9,000-lb super single tire load traversed the pavement at the rate of 250 cycles per hour. Load testing has been completed and Ohio University Civil Engineering Department is currently analyzing the strain data obtained from instrumentation installed prior to paving.

These projects represent a major advancement in the technology by providing information to evaluate UTW design features and to validate and refine procedures for determining its load-carrying capacity.

About the author: 
Packard is director of engineering-design for the American Concrete Pavement Association. You may write him in care of the editor.
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