For thin surfacings, a bond coat. BS 594987 is explicit that tack coats, though traditionally used, are no longer regarded as best practice. A bond coat uses a polymer modified binder with a specified minimum cohesion, because thin and ultra-thin surfacings put shearing forces through the joint that a traditional tack coat was never designed to carry.
Road Emulsion Association, Technical Data Sheet No.5
What a tack or bond coat does
Tack and bond coating is an established technique for providing a thin adhesive film of bituminous binder between an existing road surface and an overlay, or between courses in road construction.
Its job is not only adhesion. During paving or rolling of the new overlay, a bond coat reduces pushing and sliding, which allows better compaction and so increases the life expectancy of the surfacing. It also makes an important contribution to waterproofing, inhibiting the ingress of water into the structural layers beneath.
What changed, and why
Traditionally, the majority of tack coats were applied as thin films of C40B3 or A1-40 bitumen emulsion containing penetration grade bitumen, in constructions involving hot rolled asphalt or macadam.
Thin surfacings are more demanding than that.
While tack coats have traditionally been used, they are no longer regarded as best practice.
BS 594987, quoted in REA Technical Data Sheet No.5
To accommodate the shearing forces present in thin and ultra-thin surfacings, enhanced adhesive and cohesive strength is required of the binder. A bond coat therefore contains a polymer modified binder, with a minimum peak cohesion measured by pendulum test.
Sweeping still matters
A bond coat minimises the problems caused by surface dust on the road, but it does not remove them. It remains fundamentally important to sweep the surface before the bond coat is applied.
Related
All REA technical datasheets — twelve datasheets covering emulsion types, handling, surface dressing, bond coating and more.
The REA is the UK trade association for producers and users of bitumen road emulsion, and publishes these datasheets through its Technical Committee.