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New Workshop Explains Percent Within Limits:

FHWA Recommends PWL for Measuring the Quality of Pavement Materials

There are many ways State departments of transportation can measure the quality of pavement material. Most State highway agencies accept pavement material from a contractor and pay that contractor through a system of incentives and disincentives. The Federal Highway Administration (FHWA) is recommending one particular method as its quality measure of choice. It is called "Percent Within Limits," or PWL.

PWL ties payment of pavement materials to a more statistically accurate measure of quality that is based on controlling both the center and spread of the test results to determine how much of the material represented by those tests is within the specification limits. Many State highway agencies use methods to identify quality that only control either the mean or the variability of the test results.

PWL uses basic statistical methodologies to determine the quality of pavement. After obtaining multiple random samples, PWL is computed, starting with the mean and standard deviation of the samples and tests, with the mean and standard deviation used to compute the quality index, and finally the quality index is converted to an "estimated" PWL using tables and computer software. PWL essentially estimates the total percentage of the material that meets the specification limits. A PWL of 98.3, for example, means that an estimated 98.3 percent of the material meets the project specification.

"Obtaining a 'true' PWL would require testing all of the contractor's material," says Dennis Dvorak, a materials engineer with FHWA's Resource Center in Olympia Fields, IL. "Because it's not practical and realistic to test all of the material, we need to estimate what is produced using a statistically valid measure of quality based on multiple random samples. PWL provides that valid measurement," he said.

"One sample doesn't tell us much," says Dvorak. "Two samples show us there's variability in the material. Three samples start to give us a sense of the magnitude of the variability, and additional samples improve the estimate of the variability. But multiple samples help us better define the actual quality that's present in the pavement materials."

"Once the PWL is determined, the contractor is paid based on meeting the requirements," says Ewa Flom, a pavement engineer from FHWA's Office of Pavement Technology. "A properly established PWL specification will motivate the contractor to improve quality and innovate," she says. "When specification limits are set by good performance and achievable production, then PWL is a positive approach for achieving these goals. FHWA believes it's a good way to go."

Workshops Provide Basic PWL Overview

FHWA is offering an introductory workshop to provide an overview of how PWL works and how to apply the methodology. The 1-day workshop, which debuted February 8, 2006, in Raleigh, NC, includes a series of hands-on exercises that give participants the opportunity to experience how PWL actually works. One module provides a review of basic statistical concepts, such as random sampling, averaging, and standard deviation. Other modules provide an overview and a comparison of various quality measures and how to compute PWL. Two additional modules focus on setting specification limits and payment plan issues. The workshop targets State highway agency and FHWA division staff responsible for developing and overseeing quality assurance specifications.

Percent Within Limits Workshop

The Federal Highway Administration (FHWA) is offering an introductory workshop to provide an overview of how Percent Within Limits (PWL) methodology works and how to apply it. The 1-day workshop includes a series of hands-on exercises that give participants the opportunity to experience how PWL actually works. In addition to the workshop, several resources are available to assist with implementing PWL. These resources include two FHWA publications:

  • Optimal Procedures for Quality Assurance Specifications (Publication No. FHWA-RD-02-095)
  • Evaluation of Procedures for Quality Assurance Specifications (Publication No. FHWA-HRT-04-046)

These publications are available online. Click on "Publications."

PWL has been around for many years, according to Dvorak, initially getting its start in the U.S. military after World War II. FHWA started encouraging the method about 30 years ago. PWL has slowly expanded into government and the private sector to the point where today more than half of State highway agencies use PWL in various ways, according to a recent study.

So why is FHWA emphasizing PWL now? "Computers have made using PWL much easier," says Jim Walls, a pavement design engineer from FHWA's Resource Center office in Baltimore, MD. "A variety of software packages are making PWL much easier to use by making all the calculations for you."

FHWA also wants contractors and State highway agencies to do the job right—to have the proper quality controls in place throughout the process so that pavement performance can be improved. "This emphasis on quality will go a long way in better serving our customers, the motorists out there who drive our highways everyday," says Walls.

For more information on PWL or scheduling a workshop in your State, contact one of the FHWA PWL team members. You can also visit the PWL Web site. The PWL team members are also available to provide technical assistance in conjunction with the workshop.


Dennis Dvorak
Materials Engineer

FHWA Resource Center
(708) 283-3542

Ewa Flom
Pavement Engineer

Office of Pavement Technology
FHWA Headquarters
(202) 366-2169

Lee Gallivan
Pavement Engineer

Office of Pavement Technology
FHWA Headquarters
(317) 226-7493

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