Clay Paver Laying Guide & Technical Advice for Industry Professionals
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.
It is imperative that solid edge restraints are provided along the perimeter of all paved areas. They should be adequate to prevent the escape of the bedding-course material from beneath the paved surface.
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 inaccordance 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
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 widthsmay 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:-
1. 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.
2. 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.
3. 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
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.
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 permittedto use the pavement. DO NOT USE HIGH POWERED SUCTION CLEANERS ON NEWLY LAID AREAS OF FLEXIBLE PAVING.
Providing the paving has been laid in accordance with BS 7533 and the joints are completely full of sand we would not envisage any planned maintenance programme.
Chelmer Valley clay pavers are resistant to chemicals and cleaning fluids. Pavements using clay pavers can be high-pressured cleaned using ecologic detergents without affecting the colour or texture, however, any mechanical or high-water pressure cleansing operations may require re-sanding of the joints.
A joint sealant may prove beneficial to stabilise the jointing sand in the very early stages following construction, particularly if high powered suction street cleansing machines are employed, for example, to public realm/city centre pedestrianisation schemes where a high profile street cleansing regime is required.
Provided the joints are full following completion of the paved area these will self-seal within a period of 6-8 weeks. Joint stabilisers perform the specific task of sealing sand into the joints and products similar to Resiblock 22, being particularly suitable for clay paving, fulfil the task of sealing the joints together with the whole paver surface.
Sealants are generally promoted by the concrete industry as they prevent water/dirt ingress into the surface of the concrete paver and may go some way as to maintaining their initial colour which is achieved through pigments. Any dirt/oil/fats etc. are prevented from penetrating clay surfaces and becoming ingrained into the clay body due to the low water absorption and durability of clay. Dirt/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 and rinsed off with clean water.
Please be aware that sealants may slightly alter the colour of any surface they are applied to and as such we would strongly recommend a trial area is determined.
Should you require any further guidance please contact Technical Support at Chelmer Valley Brick Company Ltd.