Unit, Bound or Continuous: Choosing an External Surface

Unbound paving works through friction and confinement: separate units in a loose bedding layer, joints packed so neighbouring units share what lands on them, and a restraint at the perimeter that is not supposed to move. Nothing is stuck to anything else, and the entry is clear that this is the intention rather than a shortcoming.

Bound paving is stiff: a bound base spreads load by bending across a soft area rather than conforming to it, with units mortared on so the whole acts as one. Bituminous surfacing sits between them, bound like the first and deformable like the second, with continuity achieved by compacting a continuous material rather than by filling gaps between pieces.

The entries name their own boundary tests, which is unusually useful when identifying an existing surface. Try to lift one unit: in unbound paving it comes out and goes back into the same space, while in bound paving removing it destroys the unit and the bed beneath.

Look for a gully within the field: conventional unbound paving is shaped to move water across its face to an inlet and will have one, while a permeable pavement has no inlet within the field because the field is the inlet. A continuous surfacing answers both tests at once, having no unit to lift and shedding everything across its top to whatever catches it.

This is the difference that persists. Unbound units are lifted and reset, and the entries treat lifting and relaying as a normal operation rather than a repair, provided spare units from the same production run were kept and somebody recorded where services run.

Opening a bound surface for a buried service means breaking it, and the repair is a new bound patch bonded to an old one. A continuous surface has to be cut and patched, and a patch introduces edges and joints into a construction whose whole argument was that it had none.

Unbound paving absorbs small ground movements by distributing them across many joints instead of concentrating them into one fracture, and the price of that tolerance is a dependence on joints staying filled and the restraint staying put.

Bound paving cannot spread, so accumulated movement resolves itself as a fracture, and the remedy is a pattern of joints deciding in advance where that fracture is permitted. Bituminous construction answers movement by deforming, so what is beneath it reads through as a shape rather than as a crack.

Unbound paving is not closed, so some water always enters the assembly, and what matters is whether it can leave the bedding and the sub-base as readily as it arrived. Bound and bituminous construction offer no downward route at all, so the shaping of the surface is the whole of the drainage and everything depends on water leaving before it finds a defect to enter through.

Permeable construction inverts the question entirely: the assembly is intended to be wet, and what matters is not keeping water out but knowing where it is being held and what is standing next to a layer that is full of water after heavy rain.

A field of separate units needs a continuous restraint with its own foundation, because the field presses outward and a restraint that moves takes the surface with it. A continuous bound surface has no tensile edge of its own, so an unsupported arris ravels inward under traffic and confinement replaces restraint.

A bound stiff surface treats every perimeter as a restraint condition, which is where the accumulated movement of the field arrives first. A confined loose fill is restrained cell by cell, with the outer row having neighbours on one side only.

Concrete units, clay pavers and stone appear across more than one construction, so choosing an appearance does not choose a mechanism. The material comparison guides in the library answer a real question, and they answer it better once the construction is settled.

What is common to every one of these is the part nobody looks at. The foundation beneath is the same system in each case, and the entry carrying it observes that rocking units, ponding, ruts and a settled wheel line are almost never complaints about the surface, even though the surface is the only visible part when they appear.

Each construction hands over a different recurring task. Unbound paving needs its joints kept filled and its restraint checked, which the entry treats as structural rather than tidying. Bound construction needs its joints and outlets attended to precisely because it cannot drain internally. A permeable surface needs cleaning at an interval nobody can guess from looking. A confined fill is topped up, because fill is consumable.

Which construction suits a particular use, ground and loading is a determination for whoever is designing the pavement as a whole. What an owner can settle in advance is how often the surface is likely to be opened, what crosses it, and who will do the recurring work.