Field versus Laboratory Volumetrics and Mechanical Properties
Most federal and state DOTs and many local agencies have adopted Quality Assurance (QA) programs for use on asphalt pavement projects. The QA approach requires the contractor and the agency to share testing responsibilities. Typically, the contractor conducts the majority of the testing for quality control and acceptance purposes and the agency conducts fewer tests to verify the contractor's test results.
Many agency specifications require the measurement of volumetric properties by both the contractor and agency as part of the QA program. Typically, the volumetric properties that are measured include air voids (Va), voids in mineral aggregate (VMA), and voids filled with asphalt (VFA). Volumetric testing is performed on compacted specimens of asphalt mixtures.
QA programs may also require testing of mechanical properties of the as-produced asphalt mixture. The same sampling and compaction equipment and procedures that are used to generate the compacted specimens for volumetric testing are generally used to produce specimens for mechanical testing.
There are three likely scenarios for production of asphalt mixture specimens for volumetric and mechanical property testing: (1) laboratory mixed and compacted, (2) plant mixed and laboratory compacted, and (3) plant mixed and field compacted.
The results of volumetric testing conducted by the contractor and agency are often compared statistically to verify the contractors test results. Such comparisons help the agency judge whether its QA test results are from the same population as the contractors test results. However, because the tests are performed by different operators using different equipment and with potentially different methods, variability of the test results is inevitable.
A further source of variability arises when the sampling and compaction of the specimens are not the same, i.e., when results from laboratory-mixed and compacted specimens are compared to those from plant-mixed, laboratory-compacted specimens, or plant-mixed, field-compacted specimens, or both, on a single project. A major barrier to conducting a sound QA program is quantifying the variability that arises when it is necessary to compare the properties of asphalt mixture specimens that may be (1) produced in a laboratory or at the plant, (2) compacted in different physical locations, and (3) compacted in the laboratory or in-place on the pavement.
The objectives of this research are to (1) determine causes of variability and the precision and bias for volumetric and mechanical properties of dense-graded asphalt mixtures measured within and among these three specimen types: (a) laboratory mixed and compacted, (b) plant mixed and laboratory compacted, and (c) plant mixed and field compacted; and (2) prepare a recommended practice for state DOTs to incorporate these results in specifications and criteria for (a) quality assurance, (b) mix design verification or validation, and (c) structural design and forensic studies.
The research will primarily address variability associated with volumetric and mechanical test results within and among the three specimen types defined herein and not the test methods within- and among-laboratory variability (i.e., repeatability and reproducibility or precision statement), which is generally well established and documented. However, the effect of this laboratory testing variability should be considered in accomplishing the project objectives.
