Before describing the basic requirements for the supporting layers beneath various types of pavement construction it is important to consider some words which are in widespread use throughout the industry and which are frequently misunderstood.
An unbound pavement, constructed using aggregates and no mortar, is commonly referred to as being of 'flexible' construction. Quite correct, the structure is 'flexible', however, it is also 'plastic'.
A bound pavement, constructed using bedding and jointing mortar, is commonly referred to as being of 'rigid' construction. Quite correct, the structure is 'rigid', however, it is also 'flexible' although it is not 'plastic'.
The three types of supporting layer are:
Unbound Sub-Base - this is flexible and plastic.
Bitumen Bound Roadbase - this is also flexible and plastic but stiffer than an unbound base.
Concrete Roadbase - this is rigid and flexible.
It is important to understand that all structures are flexible, even stone, mortar and concrete.
Unbound aggregate and bitumen bound aggregate can behave plastically, that is they can permanently deform in response to load and fatigue.
Stone, mortar and concrete cannot behave plastically, their response to excess load and fatigue is to fail, in fact, to break.
What type of supporting layer is relevant and suitable beneath various types of pavement?
Unbound pavements subjected to light loading can be laid over an unbound Sub-Base.
Unbound pavements, using large setts and subjected to loading no greater than Site Category 2, can be laid over a Bitumen Bound Roadbase.
NB - the supporting layer beneath an unbound pavement must be water permeable, so a Bitumen Bound Roadbase must be open, or gap-graded.
Bound pavements, using full-depth setts of a size commensurate with the traffic loading, can be laid over a Bitumen Bound Roadbase.
Bound pavements, using shallow or relatively small setts must be laid over a Concrete Roadbase.
Bound pavements, using paving flags and slabs in trafficked areas must be laid over a Concrete Roadbase