AC 20mm Limestone Open Graded Binder Course

This is a layer made up of 20 mm crushed rock aggregate. AC 20mm Limestone has less bitumen compared to binder course and is used as the first bound base layer in road construction where high strength and rigidity in the total construction is required. We’d recommend a layer thickness of between 100 to 230mm, depending on the volume and weight of traffic that will use the road.

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.