AC 10mm Hardstone Close Graded Surface Course
This is a is a layer made up of a graded mix of the smaller of crushed rock aggregate, up to 10mm. Aggregate size / layer thickness varies according to end-use. Due to the larger surface area of the aggregate to be coated, bitumen content of surface course is slightly higher than in the binder course.
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.