AC 10mm Limestone Open graded Surface Course

This is a layer made up of a graded mix of 10 mm crushed rock aggregate, but with slightly more bitumen than dense base. This more cohesive material is used as the supporting or base layer to the surface course. It also serves to even-out any undulations. Layer thickness is between 50 to 70mm.

Open graded asphalt concrete is made with less smaller aggregate and therefore has more space between the coarse aggregate pieces. They allow water to pass readily through their structure and are useful as a base layer for flat sports surfaces, such as artificial turf pitches and free-draining surface courses.

Asphalt

Asphalt is a layer of crushed rock, which relies on the locking and friction of the aggregate to provide strength. The aggregate is composed of a mix of various particle sizes, coarse medium and fine, dependent on specification and a bitumen binder is used to make the layer stick together and prevent it from falling apart.

Whilst modern asphalt uses a bitumen binder, tar used to be the binder of choice. From the 1800s, tar was produced as a by-product at gas works, and was used early-on as a binder in road construction, hence the word tarmac. The word “Tarmac” is still used to describe asphalt road surfaces, even though tar has not been used since the mid 1970s and has now been replaced with bitumen.