A graded aggregate manuafctured to Series 600 - Specification for Highway Works. Class 6F4 fine grading 0/32mm and 6F5 coarse grading 0/80mm.
The aggregates act as a capping material to sub grades providing improved strength and a lower level to Type 1 Sub Base. The requirement is mostly for Class 6F5 which is manufactured in crushed rock, recycled and seconday aggregates.
A capping layer helps to create a construction platform where the subgrade is not viable. It is granular product from a crushed rock quarry, and often incorporates recycled materials. Capping layer materials help to reduce costs and to protect the subgrade from rainfall and weathering.
In road construction the disruption caused by wet weather can be reduced significantly by using readily available, locally sourced capping materials.
A capping layer is required when the pavement is to be constructed on subgrade with CBR <5%.