Testing of Unbound Materials in the Nottingham Asphalt Tester Springbox

Abstract

The current trend in mechanistic (analytical) pavement design is to use the mechanistic properties of pavement materials to optimize design. This is compatible with the move toward performance-based specifications and away from traditional empirically based design methods and recipe specifications. Other drivers—such as the Europeanwide adoption of aggregate mixture standards, which no longer differentiate on source, but moves toward sustainable construction—mean that a wide range of recycled, secondary, and primary aggregate sources can potentially be used in highway construction. The requirement for accelerated, performance-based testing is therefore coming to the fore. The U.K. Highways Agency has funded an accelerated testing program across a range of unbound capping and subbase materials. The performance parameters assessed, over a range of moisture and soaking conditions, are resistance to permanent deformation and resilient stiffness. The apparatus used during the unbound mixture assessments is the newly developed Springbox, which uses the standard Nottingham Asphalt Tester loading frame and software. This follows a simplification of the K-mold test and facilitates repeated loading of 170-mm cubic specimens under variable confinement. Samples are compacted into stainless steel liners. If required, samples can then be soaked before placement within the Springbox apparatus for testing. The following are discussed: the aggregate mixture performance parameters being measured, sample preparation procedures, the Springbox equipment and test procedures, a testing program of unbound capping and subbase materials, and results. The performance of the Springbox apparatus is discussed, and conclusions on the relative performance of the aggregates are presented.