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