James L Burati Jr

Accuracy and precision of typical quality measures

Computer simulation studies were performed to evaluate the accuracy and precision of the estimators for three quality measures: percent within limits (PWL), average absolute deviation (AAD), and conformal index (CI). Sample sizes of 3, 5, and 10 were simulated for various levels of actual population PWL, AAD, or CI. The estimators for all three quality measures exhibited similar trends for the variabilities of their respective individual estimates. For each estimator, the variability decreased as the sample size increased. For PWL, the variabilities increased as the actual population PWL departed from either 0 or 100 PWL, peaking at 50 PWL. For AAD and CI, the variabilities increased as the actual population value, AAD or CI, departed from 0. Both PWL and AAD were unbiased estimators of their respective population parameters. However, the CI appeared to be a biased estimator, consistently underestimating the true population CI. Since the CI offers no benefits compared to AAD, and since it appears to be a ...

Risks with Multiple Pay Factor Acceptance Plans

It is rare that a state highway agency (SHA) uses only a single quality characteristic to determine the pay factor for a lot. This paper deals with two specific types of risk associated with the use of multiple quality characteristics: remove and replace provisions and the effect on pay factor determination if the quality characteristics are correlated. First, computer simulation studies showed that typical acceptance plan provisions that call for lot removal and replacement at a given quality level, such as a percentage of material within specification limits of 60 or 50, can place much greater risk on the contractor when they apply to multiple characteristics. An approach to correct this problem was developed. Next, a total of 1,742 sets of asphalt content (AC), air voids (AV), and voids in the mineral aggregates (VMA) test results were analyzed for correlations. VMA values were positively correlated with both AV and AC values. Additional simulation studies showed that while this correlation does not affect the long-term average pay factor for a lot, it increases the variability in the individual lot composite pay factors. The positive correlation also increases the weighting of AV and AC in a composite pay equation. The use of multiple quality characteristics for determining the pay factor for a lot introduces sources of risk that are not present when only one characteristic is used. Any SHA that uses multiple pay factors should consider performing the types of remove and replace and correlation analyses presented in this paper.

Evaluating Specification Limits

The results of a study to evaluate the existing South Carolina Department of Transportation (SCDOT) hot-mix asphalt quality assurance specification are presented. When the existing specification was developed, assumed values were used for the standard deviations needed to establish the specification limits. Acceptance test results from 39 different projects were analyzed to determine standard deviation values that were being obtained for asphalt content, air voids, voids in mineral aggregate, and density. For each project, standard deviation values were calculated for each lot, and these were then pooled to obtain a typical value for the project. The target miss variability, that is, the ability of contractors to center their processes on the target, was also determined for each acceptance characteristic. The project and target miss values were then used to establish a typical process standard deviation value for each of the four acceptance characteristics. All these standard deviation values proved to be...

Estimating Percent Within Limits for Skewed Populations

Computer simulation, using samples of size 3, 5, and 10, was used to evaluate the percent within limits (PWL) estimates for populations with skewness coefficients from 0.0 to ±3.0. The average bias and variability were then determined for the 10,000 PWL estimates. The analyses show that even a moderate amount of skewness in the underlying population can affect both the accuracy (bias) and the variability of individual lot PWL values. The amount of bias increased as the amount of skewness increased, and the bias also increased as the sample size increased. The results show that with two-sided specifications the bias varies not only in its magnitude but also in its sign, that is, positive or negative, depending on the split of defective material outside the lower and upper limits (the PDLLPDU split). The amount of variability also increases as a greater percentage of the defective material is on the long tail of the distribution. Increased variability means that errors in individual lot estimates will be la...

A Statewide Program to Identify and Prevent Stripping Damage

A statewide pavement coring and testing program was conducted in South Carolina to determine the extent and severity of asphalt pavement stripping. Information from extensive and intensive coring and testing programs identified causes of stripping so that effective measures could be developed to eliminate or minimize future damage. Stripping frequency was related to the following factors: highway, aggregate source, mix type, cross-section type, traffic group, pavement age, and presence of open-graded friction courses. Stripping was pervasive but of varying severity in all soil provinces. Results of findings from indirect tensile strength tests of pavement core layers and a visual examination are given. Strength retention of moisture-conditioned pavement core layers was measured for specimen cores obtained from the outer wheel path.

Part 2: Quality Assurance: Risks with Multiple Pay Factor Acceptance Plans

It is rare that a state highway agency (SHA) uses only a single quality characteristic to determine the pay factor for a lot. This paper deals with two specific types of risk associated with the use of multiple quality characteristics: remove and replace provisions and the effect on pay factor determination if the quality characteristics are correlated. First, computer simulation studies showed that typical acceptance plan provisions that call for lot removal and replacement at a given quality level, such as a percentage of material within specification limits of 60 or 50, can place much greater risk on the contractor when they apply to multiple characteristics. An approach to correct this problem was developed. Next, a total of 1,742 sets of asphalt content (AC), air voids (AV), and voids in the mineral aggregates (VMA) test results were analyzed for correlations. VMA values were positively correlated with both AV and AC values. Additional simulation studies showed that while this correlation does not af...