Better Roads

October 2013

Better Roads Digital Magazine

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RoadScience system mounted with the accelerometer on the vehicle body. A new generation of lightweight non-contact profilers has emerged for construction quality control and quality-acceptance purposes. Smaller and lighter than network-level profilers, they provide the benefit of use immediately after asphalt placement and much sooner than would be possible with the network level devices on new PCC pavements. A typical lightweight profiler features state-of-the-art measuring equipment mounted on an all-terrain vehicle. A noncontact sensor collects data as the profiler travels the pavement surface. The raw data are stored in an on-board computer for processing. These data are used to calculate an IRI of a pavement. "The International Roughness Index (IRI) is a scale for roughness based on the simulated response of a generic motor vehicle to the roughness in a single wheel path of the road surface," according to the Minnesota Department of Transportation (DOT) at its Bituminous Smoothness Training workshop in 2007. "Its true value is determined by obtaining a suitably accurate measurement of the profile of the road, processing it through an algorithm that simulates the way a reference vehicle would respond to the roughness inputs and accumulating the suspension travel It is normally reported in inches/mile or meters/kilometer." "IRI can be determined using measurements from any valid profiler (inertial profiler, inclinometer-based device, rod-andlevel, etc.) which generates a profile trace showing the 'true' shape of the pavement surface," says Transtec, a pavement engineering and design firm. "This pavement profile is fed into an algorithm that determines the IRI value for the pavement. IRI can also be crudely measured by response-type systems using correlation to a reference profiler." However, the IRI is attuned into the wavelengths that cause humans to feel discomfort (body bounce and axle hop). IRI can be calculated from any accurately measured pavement profile but that rules out calculation of IRI from a profilograph trace. States make the change Figures 1 and 2 show the status of adoption of IRI versus PrI among state DOTs as of a few years ago, for bituminous pavements (Fig. 1) and portland cement concrete pavements (Fig. 2). Minnesota began its switch from PrI to IRI in 2007, with Fig. 1 Specified pavement profiling indices by state, hot-mix asphalt pavements Fig. 2 Specified pavement profiling indicies by state, PCC pavements Photo courtesy of Transtec Group/ a new specification for measurement and payment of surface smoothness on new PCC pavement construction. The report reviewed smoothness specs for PCC pavements in other states and analyzed the effect on smoothness incentives by comparing the existing PrI spec with the proposed IRI specification. The Florida DOT annually evaluates more than 40,000 lane miles of pavement across the state, it reported in December 2012. The work entails evaluating the surface condition of the pavement in terms of roughness, cracking and rutting at the network level as well as at the project level. "Although various roughness indices exist, the International Roughness Index (IRI) and Ride Number (RN) are the primary ones used in the United States," reports Precision of High-Speed Inertial Profilers for Asphalt Pavement Smoothness Measurement, by Alexander Mraz, Hyung S. Lee, Charles Holzschuher, Abdenour Nazef and Bouzid Choubane of the Florida State Materials Office. 6 October 2013 Better Roads RoadScience_BR1013.indd 6 9/30/13 3:40 PM

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