Stone Mastic Asphalt

Molly Nause-McCord

What is it?

• A type of road surfacing mix• Mix made up of 70% coarse aggregate• Nominal size of 10 mm or 14 mm

– 10mm for urban areas– 14mm for national routes

• Mix is not “well graded” but “gap graded”– Above the 2.36 mm fraction

• Bound by the ‘mastic’ made up of crushed rock fines and bitumen

History

• First mixed in Germany in 1960’s

• Initially used to reduce wear from studded tires

• Studded tires outlawed in 1975

How it’s made

• Mixed in conventional mixing plants– Performs better when mixed in a batch mixer– Can be mixed in a drum mixer if a high

bitumen level

• Laid on the surface the same way as hot mix asphalt with the same equipment– Conventional paver and equipment should be

used vrs a rake

Why is it so great?

• High stone to stone contact ratio– More durable, resists deformation better

• More bitumen and mastic means low void content– Resists age hardening, cracking and other

moisture related damages

• Aesthetic reasons– Reduced noise levels from traffic– Lower spray levels– Better skid resistance

Why it isn’t so great

• Higher costs– More coarse aggregate and filler content

means higher costs

• Longer mixing time