Workability of the 25 mm-NMAS Stone Fill HMA Mixes

As of 2001, the Texas Department of Transportation (TxDOT) has been using a 25 mm nominal maximum aggregate size (NMAS) Stone Fill HMA (SFHMA) mix as the main structural rut-resistant layer in perpetual pavement structures. The SFHMA mix-design basically consists of moderately low asphalt-binder content with a relatively stiff performance-graded (PG) asphalt-binder on the order of about 4.2% content and a 25 mm NMAS coarse aggregate gradation. To date, over eight different SFHMA mix-designs have been utilized on different in-service perpetual pavement projects in Texas. The objective of the work presented in this paper was to review and document the workability characteristics of these coarse SFHMA mixes as observed both in the laboratory (during mix-design and material property characterization) and in the field (during construction). In general, these SFHMA mixes were found to have poor workability characteristics. Remedial measures as employed both in the lab and field to improve the SFHMA mix workability and other material property characteristics are also presented in the paper.