Laying Surface Course Paving Unbound
In an unbound paving system, the paving units are laid on granular material rather than mortar beds or reinforced concrete slabs. The joints are also filled with loose aggregate. Typical examples include concrete block paving, clay pavers and some natural stone slabs or setts.
This form of construction allows the units, laying course and joints to work together as a flexible system. Its success depends on good edge restraint, an accurately prepared laying course, suitable jointing material and thorough compaction.
Although unbound paving can often be lifted and repaired more easily than mortar-bedded paving, it should not be regarded as a simpler or less exacting approach. Small errors in levels, bedding thickness or joint filling can affect the stability of the whole area.
Prepare the Base
The laying course should be installed over a completed bound or unbound base. That base must already provide the required levels and falls.
Laying course material should never be used to form the main drainage gradient or as a general levelling material. If the base is too high, base material should be removed. If it is too low, it should be corrected using material with similar properties to the existing base.
The target compacted laying course thickness is generally:
- 30 mm for concrete, clay and sawn-sided stone units
- 40 mm for cropped or riven stone
The laying course beneath the deepest unit should remain at least 25 mm thick after the paving has been compacted. Excessive bedding depth can allow deformation, while insufficient depth can contribute to cracked or broken paving.
Where the base is impermeable, drainage holes, weepholes or other secondary drainage measures should be in place before the laying course is installed.
Use the Right Laying Course Material
The aggregate should be clean, hard, durable and free-draining. It should not contain cement, lime, bitumen, resin or anything else that acts as a binder.
The type of material depends on the paving:
- Appropriately graded sand may be used beneath concrete blocks, concrete flags, clay pavers and sawn-sided sandstone setts.
- Crushed igneous rock is recommended beneath natural stone slabs and cropped natural stone setts in the domestic and light-use categories covered here.
The base should be protected from soil, mud and other contamination during the work. If contamination occurs, the affected material should normally be removed and replaced.
Control Moisture
Laying course material should be moist without being saturated, with no visible free water.
Material that becomes saturated before the paving is laid should be allowed to dry or be removed and replaced. Stockpiles may need covering to maintain a consistent moisture condition.
This matters because changes in moisture can affect how the aggregate compacts and therefore alter the final level of the paving.
Screed the Laying Course
For concrete blocks, clay pavers, concrete flags and sawn stone units, the laying course may be prepared in one of two ways.
It can be lightly pre-compacted before screeding, or it can be spread and screeded while loose. In either case, an allowance must be made for the paving and laying course settling during final compaction.
Clay pavers and mechanically installed units should be laid over a pre-compacted course. Where the paving units vary significantly in thickness, the laying course may require minor local adjustment.
After screeding rails are removed, the channels they leave should be carefully filled and lightly compacted. Any disturbed area should be corrected before paving is placed.
Only prepare as much laying course as can be protected and covered with paving. At the end of the working day, the prepared course should not extend more than necessary beyond the laying face and should not be left exposed overnight.
Cropped and riven stone units require individual bedding adjustment and should be laid as the laying course progresses.
Lay to a Consistent Pattern
Start against a straight kerb, restraint, temporary edge or accurately positioned string line. Continue in the agreed pattern while keeping the laying face open.
Check alignment regularly. Both straight and curved lines should be deliberately set out rather than adjusted by eye as work progresses. On a slope, paving should generally be laid from the bottom upwards.
Where the paving varies in appearance, intermingle units from several packs or batches. This distributes natural colour and texture differences across the finished area rather than concentrating them in visible patches. Colour banding, due to selecting paving from single packs can lead a patchwork quilt effect, often resulting in the lift and relay of the paving, something I fell foul of many years ago.
Maintain the Correct Joints
Joint width depends on the product:
| Paving Material | Typical Unbound Joint Width |
| Concrete Blocks | 2 mm to 5 mm |
| Clay Pavers | 3 mm to 6 mm |
| Sawn Stone Slabs and Concrete Flags | 2 mm to 5 mm |
| Stone Slabs with Fettled Edges | 6 mm to 12 mm |
| Cropped Setts up to 100mm Wide | Up to 10 mm |
| Cropped Setts over 100mm Wide | Up to 10 mm |
Except for cropped-sided setts and products designed with nibs, paving units should not be laid in direct contact. A suitable joint helps prevent edge damage and gives the jointing material space to perform.
Cropped setts should touch neighbouring units at points along their irregular sides. Fettled slabs should not touch adjoining units.
Fill and Compact the Surface
Freshly laid paving must be fully restrained before compaction. Concrete supporting kerbs, drainage units or other fixed features should normally be allowed to cure before adjacent paving is vibrated.
Before compaction, check:
- Finished levels and falls
- Surface flatness
- Joint widths and alignment
- Differences in height between units
- Broken, chipped or damaged paving
- Cleanliness of the surface
Correct any defects first. Fill the joints with the appropriate material, remove debris and use a suitable vibrating plate compactor. Except when compacting cropped setts, a protective neoprene mat should be fitted beneath the plate where required to avoid marking the paving.
Make at least two passes and compact the paving promptly. Do not compact within approximately one metre of an unfinished laying face. Apart from this working edge, the paving laid each day should be compacted before work finishes.
Top Up the Joints
Compaction causes jointing material to settle. Top up the joints and carry out further compaction as required.
Wide joints in cropped or cleft stone may require a finer topping material. This can be spread approximately 5 mm to 10 mm deep and washed gently into the joints with a fine spray of water. Joint filling may need to be repeated during the early life of these pavements.
Inspect the work soon after completion and at regular intervals. Replace lost material promptly. High-suction sweepers and aggressive pressure washing can remove aggregate from unbound joints and should be used with caution.
Unbound paving should only be installed above 0°C and should not be laid over saturated bedding. Joint filling is difficult in damp conditions and should be completed at the earliest suitable opportunity.
A successful unbound pavement depends on the whole surface working together. Accurate bedding, full restraints, consistent joints and careful compaction create the interlock needed to keep it stable.
