Commercial Clay Paver Laying Guide & Technical Advice
Chelmer Valley Clay Paving is manufactured and tested in accordance with BS EN 1344:2013
The sub-grade and the sub-base
The overall strength of the pavement is determined by the strength and suitability of the subgrade (usually stated as a CBR value) together with the expected traffic loadings determining the type and depth of sub base required.
Any unsuitable material should be removed from the subgrade and replaced with a suitable material and compacted. The sub base should be sufficiently wide and must take account of edge restraints and adjacent structures. The layers should be as level (2-cm tolerance) and as compact as possible – dependent on the sub-base structure and the material used for the site. The purpose of a meticulously structured sub-grade and sub-base is to keep the bedding course of the pavement as dry as possible. A draining foundation may be required at points where the groundwater level is within 50cm of the surface.
Further direction can be given on foundation and pavement thickness design for various traffic loadings and CBR subgrade values, however, we would recommend guidance on pavement construction be determined by the engineer.
Should a bound (rigid) pavement be required the principle behind this method of construction is that the layers above the sub-base are rigid and transmit loads to the underlying layers. The pavers will then require laying on a mortar bed with 10mm mortar joints between the pavers and may require consideration of expansion joints.
Sufficient cross-fall should be provided to prevent standing water on all block paved surfaces.
Bedding course and paving
Bedding course material should be washed naturally occurring silica sand and should be selected in accordance with the recommendations given in BS7533 : Part 3 :1997 – Code of Practice for Laying Precast Concrete Paving Blocks and Clay Pavers for Flexible Pavements.
It is established that the best bedding course sands for flexible clay pavements are naturally occurring silica sands from the quaternary geological series and sea-dredged sands. Crushed rock sands should only be used in domestic applications. Crushed stone with excessive fines, such as dolomitic ‘crusher dust’, are unsuitable because although they can be compacted, they fail to provide a capillary break. Water entering this material carries with it fines providing a ‘lubricant’ for further breakdown of particles in heavily trafficked areas resulting in eventual bedding course failure.
Pavements subjected to heavily channelised traffic, such as a bus lane, require a sand of high stability-a Category 1A or 1B sand.
Pavements receiving severely channelised traffic
Adopted highways and other roads
Petrol station forecourts
Pedestrian areas with regular heavy traffic
Car parks receiving some heavy traffic
Footways regularly overridden by vehicles
Pedestrianisation schemes receiving only occasional heavy traffic
Car parks receiving no heavy vehicles
Areas receiving pedestrians only
Footways likely to be overridden by no more than occasional vehicular traffic
Bedding course material will be 30-45mm compacted thickness over the specified thickness of Type 1 sub-base, usually a minimum of 150mm compacted thickness. Use of a geo-textile below bedding course to prevent migration of sand may be advisable if the sub-base is open textured. The sub-base should be allowed to drain without loss of sand. Consistent bedding course thickness must be maintained as varying thicknesses may give rise to differential settlement producing ‘low’ spots under heavy channelized traffic.
Unpacking the pavers
In order to obtain correct blend of colour and is advisable to mix from a minimum of 3 packs. The paver packs require to be unpacked diagonally – not horizontally layer-by-layer. This will equally distribute minor size variations of the pavers over the entire pavement, and will help to maintain the correct bond pattern. Damaged or broken pavers must not be used – although they can be used as cut pieces for infilling.
Herringbone pattern is recommended for paved areas receiving traffic. Linear bond patterns such as running bond and basket weave can be considered for footpaths.
To maintain an acceptable joint line within the desired bond pattern during paving, a string line should be placed at regular distances as a line to work to ensuring a pleasing finish is achievable. The clay pavers should be laid with a minimum joint width of 2-3mm – although if necessary larger joint widths may be used to maintain the bond pattern. Pavers must not be laid touching each other as this will damage the edges. Individual pavers can be moved slightly and adjusted to re-align them. To prevent pooling of water, any unevenness of the laid surface must be confined to 7 mm maximum and should be checked with a 3m straight edge. The difference in height between adjacent paving bricks should no more than 2 mm.
Infilling pieces should be cut using a bench mounted diamond blade saw in order achieve accuracy of the cut in order to ensure an aesthetically good finish.
Pavers should be laid close butting in the designated bond pattern working from an edge restraint or existing laying face edge. Mechanical force should not be used to bring pavers into intimate contact and should be laid such that a joint width of 2 to 5mm forms between each paver with a target joint width of 3mm thus ensuring there is no point contact between units. The laying of any clay paver, with or without nibs, will require the opening or closing up of joints to maintain good lines throughout the work due to the tolerances of a natural clay product.
During construction, the action of the vibrating plate compactor will open up any close laid pavers thus allowing sand to fill the voids.
Joints and interlock
Interlock is developed during the installation of the pavers and can be defined as the inability of an individual paver to move in isolation from its neighbours. It can be divided into 3 components:-
- Vertical interlock prevents a loaded paver from sliding down the sides of its neighbours and is developed by the sand that enters the joints from below. This sand rises by approximately 20mm during vibration of the pavers and becomes wedged tightly between them. Vertical loads applied onto an individual paver are transferred into neighbouring pavers as a shear force through this sand, so generating vertical interlock.
- Rotational interlock is developed by providing edge restraint to the paved area and is completed by vibrating fine jointing sand from above. An individual paver can rotate only if its neighbours move laterally to create the space needed for rotation. Edge restraints prevent this lateral movement and so generate rotational interlock. The inclusion of the fine sand in the joints shifts the potential hinge of rotation to the top of the paver and thereby adds further rotational interlock.
- Horizontal interlock is achieved by ensuring that either the laying pattern or the shape of the paver eliminates continuous straight lines through the pavement surface. Horizontal interlock is achieved most commonly by laying a rectangular paver in herringbone pattern. Though 45 degree alignment to the direction of traffic is often preferred, there is no structural preference for the alignment of the direction of the herringbone.
After compaction, a further application of fine kiln dried silica sand is brushed into the joints until all joints are entirely full. Failure to ensure joints are full before opening up the area to traffic may cause movement and loosening of the paved surface. Upon satisfactory completion traffic may be permitted to use the pavement. DO NOT USE HIGH POWERED SUCTION CLEANERS ON NEWLY LAID AREAS OF FLEXIBLE PAVING.
Compaction of pavers
When a sufficient area of pavers has been laid – and before starting the vibration – a fine kiln dried silica jointing sand must be brushed into the joints. The pavers are compacted onto the sand bed using a vibrating plate compactor with a rubber sole-plate to any avoid damage to the surface.
After compaction, any damaged pavers must be immediately removed and replaced. Any unevenness or differences in height must be re-adjusted.
Generally, clay paving requires minimal maintenance providing the paving has been laid in accordance with “BS 7533 – Pavements constructed with clay, natural stone or concrete pavers”, the Code of Practice for laying precast concrete paving blocks and clay pavers for flexible pavements.
Resistance to staining
One of the advantages of clay paving is its ability to resist discoloration from dirt/oil/fats etc., having a relatively low water absorption and high durability preventing stains penetrating and becoming ingrained into the surface. Oil/diesel etc. tend to lay on the surface of clay pavers and are dispersed naturally. Any stubborn build up can be removed by off the shelf de-greasers followed by rinsing down with clean water.
It is important to ensure sanded joints remain completely full and following completion of the construction process it is imperative not to carry out suction street cleaning for at least 6-8 weeks which will allow general dirt and detritus to naturally seal the sand in the joints. Should there be a high incidence of street cleaning required, for instance outside fast food outlets where the use of high pressure water jetting or mechanical sweeping is used, this will remove sand between joints and may weaken the integrity of the surface. If joints between pavers are void of sand these must be resanded using fine dried silica sand.
In any situation where a flexible/unbound construction method is used, whereupon jointing sand may be regularly removed by pressure washing or scouring, it may be beneficial to apply a joint stabiliser/paver sealant which will prevent jointing sand erosion. RESIBLOCK ‘22’ is a specialist jointing sand stabiliser designed for heavy duty small element flexibly laid clay paving www.resiblock.com/product/resiblock-22 , or similar approved.
Please be aware that some general sealants may alter the colour of any surface they are applied to and as such we would strongly recommend a trial area is determined. For further advise email [email protected].
Removal of moss/algae
To maintain a pristine appearance, particularly in north facing sheltered areas receiving little or no natural sunlight which may be overhung with trees or vegetation and very little footfall, algae may appear in the sand joints and paver surface. This can easily be removed by the cleansing operation described above followed by an application of an algae inhibitor, such as Agrigem’s Sapphire – https://www.agrigem.co.uk/sapphire-moss-killer-5l or similar approved. Please contact the supplier for further information regarding application guidelines.
To prevent the establishment of algae please ensure paved areas are laid to falls directing surface water into a drainage channel. Areas laid flat or have insufficient cross fall may retain moisture and detritus encouraging algae to gain hold and staining from dirt. In areas with a low footfall preventing the natural dispersion of dirt from the paving surface, it may be necessary to sweep these areas more regularly.
In specific problem areas retaining surface dirt, consequently promoting algal/moss growth, it may be beneficial following the cleaning and algal treatment to stabilise the joints and seal the surface. This will further inhibit algal growth and make subsequent cleaning easier. However, please check the compatibility of whichever algal treatment is used with the sealant.
Should any graffiti or accidental damage be caused by spilt paint etc., lifting of existing pavers and turning them over followed by recompaction and sanding of the joints should fix this problem.
It is possible, usually preceding a lengthy period of the clay paving being saturated, such as a long wet Winter followed by a warmer and drier Spring, the surface of the pavers may exhibit a white powdery substance known as efflorescence. This is simply the accumulation of soluble salt crystals on the surface of the pavers and is totally harmless. These soluble salts, often originating from within the bedding sand or jointing sand, travel through the paver in solution and crystalize upon the surface.
This temporary accumulation of salts on the paver surface will naturally blow away or can be swept away using a soft bristle brush. However, should this brief appearance be unsightly then the crystals may be dabbed off using a moist sponge using plenty of clean water which lifts the crystals off the surface.
Totally saturating the affected area with a hosepipe may appear to have removed the salts, but this saturation simply dissolves the soluble salts back in to solution only to be re-absorbed by the pavers and jointing material to re-appear once again as the surface dries out.
Under no circumstances apply brick cleaning solutions, such as diluted acids, as this may cause a chemical reaction subsequently ‘fixing’ the salts on the surface and possibly cause a more permanent discolouration.
Should you require any further guidance please contact Technical Support at Chelmer Valley Brick Company Ltd. Tel. 01277 219 634 or email [email protected]