Which statement best describes Stone Matrix Asphalt (SMA) theory?

• A. Stone skeleton to resist permanent deformation (Stone on Stone Contact), Mortar for thick binder films
• B. Stone skeleton, One size aggregate, Thick binder films, No fine aggregate, High air voids
• C. Open Graded Friction Course properties
• D. Stone Matrix Asphalt (SMA) Theory: Stone skeleton, One size aggregate, Thick binder films, No fine aggregate, High air voids

Answer: (D)

Stone Matrix Asphalt relies on a rigid stone-on-stone skeleton created by interlocking coarse aggregates, achieved with a gap-graded mix that uses a narrow or single-size aggregate to maximize contact between particles. The binder film between the stones is intentionally thick, so the asphalt cement coats and binds the skeleton, providing durability against traffic loads. Fines are limited or essentially absent to preserve the open skeleton and help prevent drain-down, while air voids are kept relatively high to accommodate the thick binder and ensure proper coating. Put together, these features—stone skeleton, one-size/coarse interlock, thick binder films, minimal fines, and higher air voids—best describe SMA. Other descriptions either emphasize different materials or mix designs that don’t match SMA’s characteristic structure.