HFS stands for High Friction Surfacing. Also known as anti-skid surfaces, these are pavement surface treatment systems that are composed of “tough” polish-resistant, abrasion-resistant aggregates bonded to the pavement surface using a resin. Although both “hot applied” and “cold applied” HFS processes have been used throughout the world, cold applied HFS are the most commonly used in the U.S.
Aggregates are critical to skid-resistance of the HFS system. The surface of the aggregate provides the microtextue for the pavement, which directly affects skid resistance. Two of the key properties of aggregates used for HFS are polish resistance and abrasion resistance, generally reported in terms of polished stone value (PSV) and aggregate abrasion value (AAV) or LA Abrasion Value. The polished stone value is a measure of how resistant an aggregate is to polishing under traffic wear. The higher the PSV, the better the aggregate can keep its surface texture (microtexture). Abrasion resistance is a measure of how resistant the aggregate is to material loss under traffic. Aggregates used for HFS are generally 3-4 mm nominal maximum size. Specifications generally require a minimum of 95 percent passing the No. 6 sieve and maximum of 5 percent passing the No. 16 sieve.
Calcined bauxite has traditionally been the most commonly used aggregate material used for HFS. However, other aggregates such as flint, granite, and steel slag have also been used.
Several varieties of resin binders are used in HFS systems, including epoxy-resin, rosin-ester, polyurethane-resin, polyester-resin, and methyl methacrylate binders. A description of the more common binders is provided below:
- Epoxy-resin: This type of binder is a two-component system. One part of the system is composed of the resin and an oil to reduce the viscosity of the resin, and the second part is composed of the curing agent, bitumen, oil and accelerators. Both parts are mixed on-site.
- Rosin-ester: This type of binder is “premixed”, meaning the resin and the chippings are bagged as a dry powder. Once on-site, the powder is heated and spread.
- Polyurethane-resin: A multi-component binder (two liquid and one powder, respectively) which cures chemically and is generally hand applied. The aggregate is then applied after. These binders help with achieving a quicker curing time during cold weather applications.
- Acrylic-resin: Similar to epoxy-resin, however it has a faster curing time.