HMA Sample Splitting Variability Using Mechanical Quartering Device per California Test 125

In 2019, the Pavement and Materials Partnering Committee (PMPC) work product group, made up of Caltrans and industry representatives, revised California Test (CT) 125 to include provisions for random sampling and minimum sample size. The mechanical quartering device, which is one of the methods used in CT 125 Appendix B for reducing the size of hot mix asphalt (HMA) samples, may introduce variability in the testing process. Minimizing testing variability is a crucial component of quality and contract compliance. It has been observed in many cases that only a single quadrant sample of the material may be secured and used for testing while it is required to use opposite quadrants of the mechanical quartering device. There is limited data available to justify if this practice is advisable. To address this issue, the California Department of Transportation (Caltrans) Independent Assurance and Reference Sample Program conducted an interlaboratory study on the 3/4″ HMA Type A mix sample reduction using four mechanical quartering devices. Eight laboratories were selected to study the variability in the reduction process and were requested to perform a single test on four samples obtained from each mechanical quartering device. This study showed that the combined samples from opposite quadrants (either A-C or B-D) are statistically similar for AASHTO T 209, T 308, and T 30 using the t-test analysis. In the case of AASHTO T 308, all the individual quadrants were statistically similar, indicating that a single sample can be made up of any single quadrant box. In contrast, using a single quadrant box for AASHTO T 209 or T 30 proved to have statistical difference in results. This validates CT 125 requirement for mixing and reducing samples to use a combination of opposite quadrants or corners of the mechanical quartering device.