Laying Paving on a Mortar Bed
In a bound paving system, the slabs, flags, blocks, pavers or setts are laid on mortar and the joints are filled with mortar or grout. This is now a common method for natural stone and porcelain patios, as well as for many concrete paving products.
A bound surface creates a relatively stiff construction. When properly installed, it provides firm support and stable joints. However, its performance depends on strong adhesion between the base, bedding mortar and paving units.
Simply placing paving on small spots or rings of mortar does not provide reliable support. Each unit should be fully bedded, bonded where required and properly jointed.
What are the Bound Paving Systems
Bound System A and Bound System B
There can sometimes be confusion around what Bound System A and Bound System B actually mean within BS 7533-102, but the fundamental difference is relatively straightforward.
Bound System A is where the paving units are installed on a bound laying course, normally a cementitious bedding mortar, over a bound base such as concrete or another hydraulically bound material. This produces a relatively rigid pavement construction, with the paving, bedding and supporting base effectively acting together.
Bound System B uses a bound laying course and bound jointing material, but the construction beneath is an unbound granular base, typically a correctly specified and compacted Type 1 material.
This distinction is important because the two constructions behave differently.
With System A, the bound base provides a much more rigid and consistent platform and distributes loads across the pavement construction. With System B, considerably more reliance is placed on the bearing capacity of the subgrade and the correct specification, installation and compaction of the granular base.
Any movement or settlement within that unbound construction can be transferred into the bound bedding and paving above, potentially resulting in cracking, loss of bond or movement of the paving.
In simple terms:
System A – bound paving over a bound base.
System B – bound paving over an unbound base.
Both are recognised forms of construction, but they should not be considered interchangeable. The correct system needs to be selected according to the ground conditions, paving material, anticipated loading and intended use of the pavement.
Allow for Movement
Bound paving expands and contracts in response to temperature, moisture and movement in the supporting structure. Movement joints help accommodate these changes and reduce the risk of cracks developing.
They may be needed:
- Around the perimeter
- Beside buildings, columns and manholes
- At major changes in level or slope
- Across large areas
- Where movement joints already exist in the base
A joint in a concrete base should be continued through the paving above. Contraction joints are typically positioned no more than eight metres apart unless the design states otherwise. Joints formed where one day’s work ends and another begins can also act as contraction joints.
Movement joint positions should be settled before paving begins, as they need to work with both the base construction and the surface pattern.
Check the Base
Before laying the surface, check that the base is stable and constructed to the correct levels and falls.
The target bedding mortar thickness is generally 40 mm. For cropped or riven stone, the bed may need to be thicker to accommodate variations, but there should be at least 25 mm of mortar under the deepest part of any unit after it has been tamped into place.
If the available bedding depth exceeds 80 mm, the base should be brought up using compatible base material rather than an excessively thick mortar bed. If the base is too high, it should be reduced or regulated appropriately. The mortar bed should not be used to correct major defects in the base.
Clean Before Laying
Good adhesion requires clean surfaces.
An unbound base that has become contaminated should have the affected material replaced. Concrete, cement-bound and asphalt bases should be swept and washed to remove dust, loose material and debris.
Paving units should also be cleaned immediately before installation. Dust, dirt and manufacturing residue on the underside can interfere with the bond. Water and physical agitation, such as brushing or suitable pressure washing, may be used.
Reclaimed units need particular attention. All old mortar should be removed from their backs and sides before they are relaid.
Choose Suitable Mortar
A proprietary paving mortar can be used where it meets the required performance. Alternatively, a suitable site-batched bedding mortar may be prepared using one part cement to four parts appropriately graded aggregate by volume.
If using a proprietary product, follow the manufacturer’s mixing, thickness and curing requirements.
Mortar consistency will depend on the laying method and specification. It should remain workable, provide continuous support and allow the units to be firmly tamped into their finished position.
Only spread enough mortar to ensure it remains fresh when the paving is placed.
Bond the Paving Properly
Where paving is being bonded to a concrete base, a fresh bonding slurry should be applied to the base immediately before the bedding mortar. The slurry should be at least 2 mm thick and must remain wet when the mortar is spread.
The underside of each paving unit should also be primed immediately before laying. Bonding slurry should cover at least 95% of the back.
For porcelain, the bonding coat must fill the moulded profile on the underside and still provide a continuous layer at least 2 mm thick.
Each unit should be firmly tamped to produce continuous support. There should be no isolated mortar spots or significant voids beneath the paving.
Bedding mortar should not rise more than 10 mm into an open joint. Where joints need a deeper void for later grouting, prevent mortar rising to within the upper 30 mm of the surface, the full depth of the unit or the specified jointing depth, whichever is greater.
Keep the Work Clean
Inspect each freshly laid section. Open joints should be free from mortar and debris before jointing begins.
Wash cement and mortar residue from the surface promptly using clean water. Allowing residue to dry can cause staining that is difficult to remove later, particularly from textured, porous or dark paving.
Protect drainage systems from mortar, slurry and wash water.
Fill the Joints
Joints should normally be allowed to remain open until the bedding mortar has gained enough strength to support those carrying out the jointing. This will typically be at least 24 hours, although conditions and product instructions must be considered.
A mortar-filled joint should generally be:
- At least 5 mm wide for slurry grout
- At least 10 mm wide for plastic pointing mortar
- No more than 25 mm wide at any point
Slurry grout is suitable for a wide range of bound paving and is the required method for cobbles and naturally rounded stone. The paving should be thoroughly wetted before application and kept wet while the grout is worked into the joints.
Move the grout diagonally across the joint lines, repeatedly topping up areas where it settles. Clean the paving thoroughly before the grout hardens, ensuring waste slurry does not enter drains.
The finished joint should sit at the bottom of the chamfer or rounding, or approximately 2 mm to 3 mm below the paving surface.
Plastic pointing mortar should be forced into the full depth of the joint and properly compacted. It is suitable for steps, kerbs and paving that will not carry vehicles. Dry mixtures of sand and cement brushed into joints should not be used because they generally shrink, bond poorly and provide inadequate frost resistance.
Work in Suitable Weather
Mortar work is sensitive to temperature and moisture.
Bound paving should not be laid or jointed on frozen surfaces. Work should only proceed within the temperature conditions recommended for cementitious paving construction. Slurry grouting should not be undertaken below 3°C and specialist advice should be obtained in temperatures above 25°C.
If rain, frost, heat or rapid drying could affect the work, paving operations should stop and the installation should be protected.
During warm weather, newly completed joints may need covering with polythene or damp hessian to slow moisture loss. In colder weather, frost protection may be required. Completed areas should be covered for at least 24 hours where weather conditions could affect curing.
Keep Traffic Off Until It Is Ready
Mortar gains strength gradually. The time before the pavement can be used depends on temperature, weather protection, the product and whether the area will carry pedestrians or vehicles.
For pedestrian use, the mortar should reach at least 25% of its specified strength. Without test results, correctly protected work will typically require at least three days at temperatures of 10°C or above.
Vehicle areas should reach at least 50% of the specified strength. Without laboratory testing, this will typically require:
- Seven to ten days at temperatures of at least 20°C, or
- Around 14 days at temperatures of at least 10°C.
Lower temperatures slow strength development considerably. Proprietary rapid-strength mortars may allow earlier access, but the supplier’s instructions must be followed.
Mechanical sweeping and pressure washing should not begin until the jointing mortar has reached its intended design strength.
Bound paving performs best when every stage supports a strong and continuous bond. A clean base, fresh mortar, fully primed units, complete bedding, properly filled joints and adequate curing are all essential to producing a durable finished surface.
