Use of the Shear Box Compactor for Porous Asphalt Mix Property Assessment

The most commonly used compaction methodologies for specimen preparation for the assessment of porous asphalt (PA) mix properties are gyratory and Marshall. Even though several individual blocks prepared by one of these test methods are usually considered to be replicates, the heating and handling histories are never identical as these are individual specimens prepared at different times. In contrast, the Shear Box Compaction (SBC) technique allows fabrication of a single slab, at a given time, which is then used to extract multiple specimens that can be considered as true replicates. The SBC can offer significant advantages in preparation of PA specimens for various tests typically performed on gyratory or Marshall specimens. The recently released ASTM D7981-15 test method on SBC of asphalt mixes is written around compaction of dense-graded (DG) mixes, not PA mixes, even though the specific mix type is not stated in the test method. Furthermore, there is a very limited literature on the SBC, with most of the published work focusing on DG mixes. This paper aims to address this gap. In this study, the SBC was used for PA mix compaction with a nominal maximum aggregate size of 10 mm using two different types on aggregates and four bituminous binders. Slab preparation and specimen extraction methodology developed were discussed in details. Variation of the air voids (AV) between the replicate specimens extracted from one slab were within ±0.5% from the average AV value regardless of the aggregate or binder type, or slab height. The total variation of AV in all the 64 extracted specimens tested was 1.9%. This gives the confidence that performance testing such as Dynamic Modulus testing, which requires very stringent control of specimen dimensions, uniformity, and AV consistency among the replicate specimens, can be performed reliably on PA. Also, some recommendations were provided on the changes needed in the ASTM D7981 for PA compaction using SBC.