Flexible Pavements

4.1 Applications
Various ranges of pavers have been successfully used in a wide range of applications covering the full spectrum of pedestrian and vehicular loadings. 52mm thick pavers are suitable for use in pedestrian applications and areas receiving light vehicular traffic. 65mm thick pavers can be used in all trafficked applications including industrial developments, lorry parks, bus stations and bus routes, town centre redevelopments and estate roads. However, Chelmer Valley concentrate mostly on domestic applications.

4.2 Laying

4.2.1 General
A flexible pavement consists of the following components (see Figure 1 and Section 2. Definitions):

• subgrade
• sub base/basecourse
• bedding course
• surface course
• jointing sand

The design of the lower courses is critical to the pavement’s final performance. With the possible exception of the domestic pathway, all flexible pavements should be designed around specific site conditions and the intended loadings, i.e. traffic use. It is up to the designer or contractor to ensure there is adequate information or that the required testing is carried out.

4.2.2 Subgrade
The California Bearing Ratio (see Section 2. Definitions) of the subgrade should be assessed when considered necessary by the designer or contractor. The CBR is an indicator of the shear strength of the subgrade material and widely used as a means of assessing subgrade strength for highway design. The CBR of a soil can be measured directly or derived from knowledge of other soil properties. Usually a figure of <2% indicates a soft, moist clay where >5% is more suitable for pavement construction.

The subgrade should be prepared to the required profile. It should be sufficiently wide to extend to the rear face of the proposed edge restraints or to the face of existing abutting structures. Any unsuitable material should be removed, treated or replaced with suitable material.

4.2.3 Sub-base
The sub-base should be designed on the basis of sound engineering principles and the CBR of the subgrade. The sub-base transmits loads to the subgrade and should be prepared to the required formation levels over an area which extends to the back of proposed edge restraints and abuts existing structures.
 
New sub-bases should be formed with materials such as Granular Sub-base Material Type 1 – BS EN 13285:2003 Unbound mixtures or Cement bound granular material (Categories 1-3) and wet lean concrete (DOT’s Specification for Highway Works).

The surface of a sub-base should be close textured to prevent any loss of bedding course material and it should be laid to falls, i.e. to accommodate drainage falls.

4.2.4 Bedding course
Geotextile materials
Geotextiles materials can be used to great effect as drainage layers and separation layers. In both applications it is recommended the fabric be placed between the sub-base and the bedding sand. In this position it can drain the bedding layer and prevent bedding material loss into the sub-base.

However, there are two school’s of thought that the use of geotextiles in a situation where heavy channelised traffic, such as buses or HGV’s constantly braking in a concentrated area, may create a ‘slip plane’ brought about by the geotextile having caused separation between the bedding sand and the sub-base.

Bedding course material
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.
Bedding sand is both the strength and weakness of a flexible clay pavement as it provides the interlock that ensures that pavers can accommodate extreme loading, however, it is also inherently less stable than any other component within the pavement.
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.

Sand Categories

 Moisture content of bedding course
The moisture content of the bedding course should be uniform and moist without being saturated. Stockpiled material should be covered.

Bedding sand thickness
Following compaction, the bedding sand should be uniformly 35mm thick with a maximum specified of 50mm and a minimum of 25mm.

Bedding sand construction
This is best prepared by spreading the sand in a loose uncompacted layer and then compacting it with a vibrating plate compactor. The compacted sand is then screeded to level to receive the pavers and must not be disturbed as any disturbance to this sand layer may lead to final surface undulations.

Edge restraints must be provided around the perimeter of the area to be paved. The restraint construction, together with any haunching, should be mature before any vibration of the paving takes place. Gaps between edge restraints or at the intersection with other pavements should be sealed to avoid loss of bedding sand.

4.2.5 Laying pavers

Laying patterns
The basic laying patterns are shown in Figure 2. Herringbone (both 45 and 90 degree) is recommended for wheel trafficked areas. Many other decorative laying patterns, based on basket weave and running bond are possible but are only suitable for pedestrian areas.

Figure 2 Laying patterns
45 degree herringbone	90 degree herringbone
Basketweave	Running bond

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.
Point contact between adjacent pavers produces high localised stresses that are known to cause edge chipping, reduce shear transfer and cause pattern distortion.

This nib helps to provide a physical gap between adjacent pavers when placed closely together for the joint filling sand to fill the void. The nib also offers protection of adjacent arrises coming together as any direct contact between pavers is taken up at the interface of the nib, which is obviously hidden. The nibs are not to be used as spacers.
Manufacturing tolerances for clay pavers and variations in surface profiles do not permit the achievement of perfectly straight lines when laying pavers, however, joint alignment can be refined with the aid of string lines.
Infill units should be incorporated as the pattern develops and should be included as soon as possible after laying and in any case by the end of the day.
Clay pavers can only be cut accurately using a bench mounted diamond bladed saw.

4.2.6. Joint filling

Procedure
At the end of the working day or as necessary, dry jointing sand, a fine dry rounded silica sand, should be spread over the surface and brushed into the joints between the pavers. The use of kiln dried sand will assist in rapid joint filling.

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.

4.2.7 Compaction of the surface course
Compaction should follow laying and joint filling as soon as possible but should not occur closer than one metre to the unrestrained working edge of the pavement under construction. No area of paving should be left uncompacted at the completion of the day’s work, apart from the laying face.

The area to be compacted should be swept clean of joint filling sand and then receive at least 2 or 3 passes of a vibrating plate compactor with a neoprene sole plate to avoid scuffing see Figure 3. After the initial compaction, further jointing sand should be swept into the joints and recompacted until all joints are full, see Figure 4.

Figure 3  Vibrating plate compactor with neoprene sole plate

Figure 4 Well filled sand joints

4.2.8 Edge restraints
The perimeter of all paved areas should be provided with edge restraints in order to prevent lateral movement resulting in loss of interlock and failure of the pavement. These should be adequate to support the intended loads and prevent the escape of bedding course material.
Edge restraints should be formed before compacting adjacent units together with any concrete haunching. These should be left to mature before vibration of the surface course is undertaken.
Haunching to an edge restraint should be continued down to the underside of the bedding course. Where appropriate, drainage should be provided at edge restraints to prevent the build up of a head of water in the bedding course.

4.3 Flexible pavements for sloping driveways

4.3.1 General
This section deals with the general criteria to be used in the design and laying of clay pavers on sloping sites. These general design criteria may have to be further developed or modified for individual sites, depending on the volume and type of traffic, slope, drainage and local soil types etc.
The main concern on sloping sites is paver ‘creep’ and edge chipping of pavers no matter what material is used.
In normal use, pavers will only chip if they are able to touch and will only creep if the 2 to 3mm gap between the pavers is able to close, therefore creeping and chipping can be virtually eliminated if the gap between the pavers is maintained. On sloping and curved driveways this is difficult to attain as there is a tendency for the sand to be washed out during wet periods.

In order to minimise or eliminate creeping and chipping, the aspects previously discussed for flexible pavement construction must be rigidly adhered to. In addition, the use of a restraint system and ongoing maintenance in order to retain jointing sand between pavers must be applied.
The following sections detail these additional requirements.

4.3.2 Restraint system
Concrete transverse beams should be strategically placed along the driveway, generally at 90 degrees to the direction of traffic flow. The intention of the transverse beam is to reduce the effective length of the driveway and hence the potential for creeping. The spacing and size of these beams will be decided by the contractor or design engineer.

Figure 5  Transverse restraint beam on a sloping drive

4.3.3 Drainage
Surface drainage is not normally a problem on sloping driveways, however, inadequate or non-existent sub-surface drainage is a common cause of failure in sloping pavements.
Sub surface water will accumulate and will require drainage outlets otherwise the water will find its own way out eroding bedding and jointing sand which will inevitably result in creep and chipping. Sub surface water is likely to dam at the top side of transverse beams, the lower side of edge restraints and at grates. Figure 6 shows typical drainage systems that may be adopted.

Figure 6 Typical drainage systems for sloping driveways

4.3.4 Laying pattern
Pavers on sloping drives should be laid in 45 degree herringbone pattern as the interlocking action of this pattern offers the best interlock and resistance to creep.

4.3.5 Maintenance
It is critical to maintain the jointing sand between units and will require frequent ‘topping up’ of jointing sand, particularly following periods of heavy rain.
Alternatively, proprietary ‘binders’ or ‘paver sealants’ are available that effectively bind the sand particles together whilst maintaining the required flexibility to transfer load.

These sealants are not essential on block paved surfaces but they do reduce the instances of sand loss and minimise weed growth, however, experience shows that they can often leave behind a white residue on the surface.
Resiblock 22 is being promoted as being suitable specifically for clay paving.

Pavers around service covers and drains should be laid in header courses bedded on cement mortar.

4.4 Tactile, demarcation and deterrent paving
Tactile paving is an important element of any paved area and is designed to assist visually impaired pedestrians by providing specific detectable sensory indicators of crossing points, potential hazards, platforms and safe routes through large open areas.

4.4.1 Blister paving

Tactile blister paving is used to identify the location of a dropped kerb and an appropriate place to cross.
Guidelines regarding the use of tactile paving specify the use of red units at controlled crossing points and buff units at uncontrolled crossing points.Some relaxation of these guidelines may be allowed in conservation areas.
Guidelines also exist on the layout and positioning of blister paving units.

4.4.2 Hazard warning/corduroy paving


Hazard warning paving, often called corduroy paving, is used to identify any type of potential hazard, such as a change in level and advice pedestrians to proceed with care. The bars are laid at 90 degrees to the
hazard, indicating the location or area of the hazard and are a requirement of the Building Regulations Part M.
These units are often used at the top and bottom of steps, at level crossings and on shared pedestrian/cycle routes when the route meets a crossroad. These units are usually positioned 400mm from the hazard.

4.4.3 Lozenge paving

Lozenge paving is designed to warn users of the presence of a street platform edge for a supertram/monorail system. The lozenge runs parallel to the platform edge some 500mm away from the edge.

4.4.4 Directional guidance paving


Directional guidance paving is used to guide pedestrians through open areas, avoiding hazards and obstructions, providing them with a safe route. The round ended bars are laid in the direction of travel and turned at corners to give a warning of the change of direction.

4.4.5 Cycleway paving

Cycleway paving is used at the beginning and end of a shared pedestrian/cycle route which is not separated by levels. The units are laid transversely to denote the pedestrian path and longitudinally to denote the cycle path.

4.4.6 Platform edge paving (offset blister)

Denotes the proximity of a railway platform edge.

4.4.7 Demarcation paving

Available in yellow and white ‘line markers’

White line step pavers to assist partially sighted

4.4.8 Deterrent paving

Designed for use where pedestrian or vehicular access is discouraged.