Quality Control–Quality Assurance Sampling Strategies for Hot-Mix Asphalt Construction

Abstract

The effects of sampling strategies and size on statistically based decision making in hot-mix asphalt (HMA) construction are presented. For sample sizes agreed on by the agency and the contractor, an acceptable level for an HMA mix parameter is determined with risk balanced between the two organizations. With increased emphasis on specific performance requirements, the use of performance tests on HMA specimens prepared from in situ samples is developing. Examples include direct measures of mix stiffness and permanent deformation characteristics. A measure of rutting resistance, the stabilometer S-value, is used by the California Department of Transportation (Caltrans) for quality control–quality assurance (QC/QA) projects. Although the S-value was used for this simulation because extensive tests were available, this approach is applicable to any performance measures already in use, such as HMA thickness or compacted air void content. A sampling scheme and sampling size through statistical simulation of a fixed length of a one-lane-width placement of HMA are discussed. Sample size is based on the combination of the sample size of the contractor and that of the agency to balance the risk to both organizations and results in a mix that will meet the minimum performance requirement. An example is given for the placement of 15,000 tons of HMA according to Caltrans QC/QA requirements. For this total tonnage, the contractor and agency are respectively assumed to perform a specific number of stabilometer tests for QC and QA. Results of the simulation yield the average stabilometer value that the contractor must obtain to ensure that the quality requirement, that is, the minimum S-value, is met.