What are the AASHTO Specifications?

For years, the pavement profiling industry has had an urgent need for guidance in the development of standard specifications and protocols that will assure owner agencies that test results from inertial profilers are both repeatable and reproducible. In August 2003, the American Association of State Highway and Transportation Officials (AASHTO) Subcommittee on Materials (SOM) adopted Provisional Standards to address these needs, which were further revised in 2007 as the following full standards:

MP11 is an equipment specification to define the required attributes of an inertial profiler system to measure longitudinal pavement profile for construction quality control and acceptance. MP17 provides guidance for developing specifications when an inertial profiling system is used for measurement and evaluation of ride quality and compliance. PP49 describes minimum performance requirements for inertial profilers to be used for quality control and quality assurance of surface smoothness when the profile-based smoothness specification is applicable. PP50 describes the procedures for operating and verifying the calibration of an inertial profiler.

Key Components in the AASHTO Smoothness Specifications

MP11 Standard Equipment Specification for an Inertial Profiler

MP11 is an equipment specification that provides a template for profiler procurement language. It describes general hardware requirements such as the number of sensors and sensor performance, triggering capability, and data handling and reporting for an inertial profiler. It describes software requirements, such as IRI and RN calculation, as well as the software that is needed to perform calibration and troubleshooting. The specification also requires some key aspects of performance, such as the valid speed range and the valid roughness range.

Two important aspects of the specification that need review are the waveband and the sampling interval. While these were based on the best available knowledge at the time, these issues are still being researched. For example, it may not be necessary to measure profiles out to such long wavelengths. Furthermore, new footprint and sampling requirements are likely to be needed for coarse-textured pavements. The high-pass filter attenuation and roll off requirements are anticipated to be reviewed and revised. There will also be plenty to learn in this regard from the "Golden Footprint" study currently ongoing.

MP17 Standard Specification for Pavement Ride Quality when Measured Using Inertial Profiling Systems

MP17 (previously PP51 in the 2003 version) is a template for a smoothness incentive and disincentive specification. One important aspect of this specification is to define localized roughness based on continuous short-interval (25 ft) roughness reports (though it is still based on a relative value). This specification also provides a sample pay schedule based on the roughness histogram from Smoothness Assurance Module in FHWA's ProVAL software (www.roadprofile.com). The Standard Practice helps set expectations for smoothness, without imposing any constraints on the amount of money an agency must pay out. As such, an agency using the Standard Practice may retain their existing philosophy on the value of smoothness, or the cost of roughness. The Standard Practice also describes methods of auditing a contractor’s smoothness measurements and resolving disputes that might occur between the agency and contactor measurements. This is an often-overlooked aspect of smoothness incentive programs.

PP49 Standard Practice for Certification of Inertial Profiling Systems

PP49 provides technical instructions for certifying a profiler for quality assurance. This includes both static and dynamic performance measures of the system. This is an exceptionally important standard, because profiler certification programs are driving most of the improvements in profiler performance. One of the most important elements of this specification is rating a profiler's repeatability and accuracy based on cross correlation. A robust, efficient, and accurate analysis module called "Profiler Certification Module" (PCM) is included in FHWA's ProVAL software for this purpose. This powerful tool has helped drive improvements in profiler technology that are long overdue.

PP50 Standard Practice for Operating Inertial Profilers and Evaluating Pavement Profiles

PP 50 provides instructions for operation and maintenance inertial profilers. The instructions include important daily checks on the health of the system, periodic calibration and shakedown procedures, and general field procedures. This standard also describes the methods that are needed to store and analyze the measured profiles. One of the important requirements is to provide ProVAL-compatible data.

Summary of the AASHTO Standards

The four AASHTO Standards for using inertial profilers for smoothness measurement have been painstakingly conceived based on the best information available. They provide the majority of the information needed to help an agency launch and maintain a successful pavement smoothness construction quality control program. Further, the specifications are very well organized, and the content of each is properly targeted toward each essential part of the process. It is recognized, however, that the pavement profiling industry is constantly evolving, and these standards will need continual review since much is being learned about pavement smoothness measurement and interpretation, even since the time the specifications were written.

Active research projects being conducted by the FHWA, including a current project looking at ‘Smoothness Criteria for Concrete Pavements’ and an upcoming project looking at ‘Smoothness Criteria for Asphalt Pavements’ will provide answers to many of the questions concerning appropriate smoothness thresholds, the value of enhanced smoothness, and methodologies for identifying localized roughness.