Better Roads

October 2013

Better Roads Digital Magazine

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as wind resistance. It's estimated that a 5- to 7-percent reduction in rolling resistance – typically attributed to tire air pressure or baldness – only increases automobile fuel efficiency by less than 1 percent. Pavement stiffness's impact on fuel efficiency is minimal, APA says in Smoothness Matters. Stiffness, an indicator of strength, accounts for between 0.005 percent to 0.5 percent difference in fuel consumption, depending on the vehicle type (e.g., automobile vs. tractor-trailer). Pavement smoothness is more significant than the other factors in boosting fuel economy, at 5 percent or better, APA says. Pavement smoothness affects rolling resistance by influencing friction between the tire and the pavement. "Some experts estimate it is possible to reduce fuel consumption by as much as 10 percent by rehabilitating the roughest pavements," APA notes, adding driving on rough roads costs the nation's motorists $65 billion annually in extra vehicle operating costs, according to The Road Information Program (TRIP). Decline of the profilograph A variety of devices has been used to measure pavement smoothness. The most familiar is the California profilograph, but now inertial profilers and non-contact lightweight profiling devices are supplanting the former. "Profilographs have successfully been used for construction quality control on thousands of miles of pavements during the past several decades," says the Federal Highway Administration's (FHWA) Joseph Budras, P.E. The two most commonly used smoothness indexes are the International Roughness Index (IRI) and Profilograph Index (PrI or PI). With the early popularity of the profilograph, the profile index became a standard index for smoothness measurement in construction specifications, Budras says. Profilographs are relatively inexpensive, simple to operate and maintain and provide a trace of pavement surface that users can easily understand. However, because of its wheel linkage, it cannot be used for high-speed network pavement smoothness data collection. In contrast, the vehicle-mounted inertial profiler measures pavement smoothness while driving at highway speeds. The California profilograph it replaces is a non-motorized piece of equipment that moves at walking speed and exposes workers to traffic and construction hazards. To make the profilograph more useful, the "blanking band" was instituted, which is a band that is placed over a profilograph trace to "mask" the portion of the trace that is not counted as roughness. "PrI is determined by counting the number of scallops in the profile trace that fall outside of a specified blanking band," notes Federal Highway Administration (FHWA) engineering consultant Transtec Group. "0.2-inch (5 mm) and 0-inch (0 mm) blanking bands are most commonly used in the United States, although a few states use a 0.1-inch (2.5 mm) blanking band as well. PrI is sometimes called Profile Index (PI), but the former is more specific." For a long time, the profilograph has been used to measure the longitudinal profile of a pavement. Its principal use has been for the construction control of pavements. But because its front and rear wheels are in contact with the pavement surface, the profilograph can't accurately measure the pavement profile, and the profile index doesn't reflect the riding comfort of a roadway. "No claim is made that the roughness or riding quality of a pavement is directly or completely reflected by the profile index," said the profilograph's inventor, Francis N. Hveem, in 1960. "It should again be emphasized that, strictly speaking, the devices reported herein do not furnish a direct index to 'riding qualities'." Hveem is a titan of the asphalt industry and is enshrined in the National Asphalt Pavement Association's (NAPA) Hall of Fame. Enter inertial profilers While the California profilograph was used extensively after the 1960s, it's being supplanted by the inertial profiler, which can measure and record road surface profiles for network use at speeds between 10 and 70 mph. This means use of the PrI is declining in favor of the more nuanced IRI. "Many agencies are increasingly turning to inertial profilers as the most reliable instrument for construction acceptance testing and verifying pavement smoothness," says the National Highway Institute (NHI) in its course description for FHWANHI-131100, Pavement Smoothness: Use of Inertial Profiler Measurements for Construction Quality Control. The inertial profiler, also called a profilometer, measures and computes the longitudinal profile of the pavement through the creation of an inertial reference by using accelerometers placed on the body of a measuring vehicle. Relative displacement between the accelerometers and the pavement surface is measured with a non-contact light or acoustic measuring Better Roads October 2013 5 RoadScience_BR1013.indd 5 9/30/13 3:39 PM

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