Volume 38, Issue 6 (November 2010)
Evaluation of Statistical Validity of Some Asphalt Binder Grade Systems
The performance grade (PG) and the penetration (pen) grade systems were examined for their statistical validity. It is found from this limited study that the pass/fail high-temperature of the PG binder follows a normal distribution. Since PG system has no gap between each grade, the binder falling out of limit trespasses into another binder grade. The Korean pen grading system has the same problem. Therefore, many products can have similar property levels with an adjacent-grade even though the mean value is within the specification limit. The PG binder will have a high chance of passing the rheological requirements at above the upper limit but the least chance of falling below the lower limit because the means were above median and variations were higher than they should be. Since PG is characterized without gap between each grade, the producer must maintain the coefficient of variation below the current level to satisfy the quality control limit of the product. Similarly, in other grading systems such as the Korean pen specifications, due to its lack of a gap between grades, higher percentage of products will be duplicated even if the mean is at the median of the specification limits due to material’s quality uncertainty. Especially if the mean moves toward the upper limit, one-half of the binder grade will be overlapped with the higher binder grade. Therefore, these specifications should be statistically examined for further modification.