Comparison of quality control and quality assurance for asphalt concrete

A typical quality assurance (QA) is the process by which highway construction elements are sampled and tested by agencies to ensure compliance with specifications. The results of quality control (QC) performed by contractors for tests of mix properties of asphalt pavements are increasingly used in the acceptance decision by highway agencies. An analysis of the QC and QA results during the 2008 construction season was undertaken for quality test measures including asphalt content and aggregate gradation. Differences between QC and QA test results for hot-mix asphalt concrete were shown to be statistically significant for some of mix properties. On quality characteristics in which statistically significant differences occurred, the QA values were typically larger. QA and QC comparisons indicated less variable and more favorable contractor test results that would give more favorable acceptance outcomes. A process to determine a typical standard deviation value was developed for the quality characteristics of asphalt mixtures. Revised specification limits were recommended to take into account the variability of both QC and QA data. These findings would help highway agencies structure their quality assurance programs, specifically the use of contractor-performed tests for acceptance decisions.