Stone Mastic Asphalt (SMA) is a gap-graded mix laid between 30mm and 40mm thickness. It may be manufactured using a polymer modified binder (PMB), giving the aggregate skeleton additional strength. SMA is used in the construction of new roads and overlaying existing roads and offers superior durability and resistance to deformation. SMA must be designed and manufactured in accordance with the TII Specification for Road Works when used on national roads. All coarse aggregates must have a PSV value of 60 or greater when skid resistance is required.