Paving within the Limits

How PWL is changing the face of Michigan roads

Recently, representatives of the Federal Highway Administration, Michigan Department of Transportation, and the Asphalt Pavement Association of Michigan agreed to a method of acceptance of hot-mix asphalt (HMA) placed on the National Highway System in the state of Michigan. This method of acceptance is known as percent within limits (PWL). In the past, a very similar method was used for HMA pavement acceptance on airport projects. According to the United States Department of Transportation, this statistically based method was devised to give the owner adequate assurance of quality achievement.

It’s understandable that a gasp seems to go up from the crowd whenever statistical measurement is mentioned. Statistics, however, are just a way to analyze numerical data. We all use statistics every day. When you want to calculate your car’s gasoline mileage, for example, you divide the gallons of fuel used by the number of miles driven. The numerical inputs are miles driven and gallons used. The analysis results are the miles per gallon for your vehicle.

PWL Calculations

To use the PWL method of acceptance, the owner provides upper- and lower- specification limits for specified mixture criteria and the mean and standard deviation for a Lot of (n) tests are then computed. The mean and standard deviation are used to compute the upper quality index (Q_u) and the lower quality index (Q_l). Using the Q_u and Q_l numbers, we can create a table for estimating the percentage of mixture that is within the upper- and lower-specification limits — Pu and Pl, respectively. The PWL is therefore (P_u + P_l) – 100. The following is an example of the PWL calculation:

Percent of materials within upper-specification limit (P_u) = 100

Percent of materials within lower-specification limit (P_l) = 96

PWL = (100 + 96) – 100 = 96

This analysis states that for the lot of mixture sampled, 96 percent of the placed mixture is estimated to be within specification limits, while 4 percent is estimated to be outside specification limits.

Michigan PWL Requirements

In the Michigan PWL specification, the numerical analysis is performed on a lot basis — a lot consists of three to seven sublots — by using the HMA mixture sublot test results for voids in mineral aggregates (VMA), air voids, and in-place density of the finished pavement. Tests have shown that each measured criterion contributes at a different level to finished-pavement quality. Therefore, each criterion is assigned a level of contribution, with the total contribution equaling 100 percent. For the Michigan PWL, air voids are assigned 20 percent, VMA 20 percent, and in-place density 60 percent. For each criterion, a PWL number is calculated and a pay factor determined using a formula contained in the specific special provision of the project proposal. With the pay factor for each criterion determined, the overall lot pay factor is calculated.

Consistent quality in any product is important, and the PWL method of acceptance for HMA is a technique that determines and rewards consistency. It also provides a fair way to adjust payment based on a calculated percentage of acceptable material.