Better Roads Digital Magazine
Issue link: https://read.dmtmag.com/i/307110
HighwayContractor 20 May 2014 Better Roads Dense-graded mixes Dense-graded mixes are often classifi ed as either coarse or fi ne. Coarse mixes have a maximum aggregate size of 19 mm (3/4 inch) or larger. Coarse mixes are usually laid down in fairly thick layers of 75 mm (3 inches) or thicker. Mats that consist of coarse mix are less likely to move around under heavy compaction energy. You can use vibra- tory compactors in higher amplitude ranges and pneumatic compactors with higher ground pressures. Some dense-graded mixes are classifi ed as fi ne mixes. Fine mixes have a maximum aggregate up to 13 mm (1/2 inch) and typically a fairly large percent of fi nes and asphalt cement. Some fi ne mixes can be unstable during the com- paction process, especially if layer thickness exceeds 50 mm (2 inches). Static compaction may be needed to help stabilize fi ne mixes prior to vibratory passes. High-compaction energy can damage layers of fi ne mix. Lighter compactors, either steel drum or pneumatic, are recommended for use on fi ne mixes. Open-graded mixes Open-graded mixes have relatively uniform-sized aggregate typifi ed by an absence of intermediate sized particles. Mix designs typical of this structure are permeable friction lay- ers and asphalt-treated permeable bases. Because of their open structures, precautions are taken to minimize the amount of asphalt cement draining to the bottom of the layer by using modifi ed asphalt cement, usually latex rubber or fi bers. Stone-on-stone contact with a heavy asphalt cement particle coating is typical for open- graded mixes. Gap-graded mix Gap-graded mixes use an aggregate gradation with particles ranging from large to fi ne with some intermediate sizes miss- ing. Gap-graded mixes are also typifi ed by stone-on-stone contact and are more permeable than dense-graded mixes. Stone-matrix asphalt Stone-matrix asphalt (SMA), like other open-graded mixes, will be missing most intermediate aggregate sizes. However, stone-matrix asphalt mixes have a much higher proportion of fi nes. Modifi ed asphalt cement combines with these fi nes to produce a thick mastic coating around and between the large aggregate particles. Shape matters, too Aggregate shape also affects compaction. The shape of the aggregate determines the amount of internal friction be- tween the particles. Rounded aggregates have low internal friction and move closer together within the layer under less compaction energy. However, mixes with rounded aggregates tend to be unstable and move around under the weight of the compactor. There- fore, when you know the shape of the aggregate is rounded, select low vibratory amplitude or a light static compactor. Angular aggregates, on the other hand, have high internal nominal maximum aggregate intermediate aggregate small aggregate fines asphalt cement nominal maximum aggregate small aggregate fines asphalt cement nominal maximum aggregate small aggregate fines asphalt cement intermediate aggregate nominal maximum aggregate fines asphalt cement ROUNDED AGGREGATES low internal friction ANGULAR AGGREGATES high internal friction