Road Purpose
The purpose of a Flexible Pavement is to provide vertical movement during loading, normally traffic. The pavement is designed to last between 20 and 40 years with a limited amount of maintenance and usually by replacing the top layer of the pavement. Geofabric materials have been introduced to provide extra strength where the sub-grade needs stabilisation due to high water tables or weak subsoil materials that would not withstand the stresses during loading.

Road Structure
The road is made up in layers of compacted material to provide even load spread. There would normally be four layers built up one on top of the other. The layers are made up of either bound or unbound materials. The layer thickness is dependant on expected traffic loading measured in Millions of Standard Axles (MSA). The MSA is used to define the type or class of road in the Specifications for the Reinstatement of Openings in Highways. Reinstatements must therefore be in accordance with the type or class or road, i.e. Type 3 Flexible Road.

• Hot rolled asphalt
• 10 or 14mm close graded
• 10 or 14mm open graded
• Dense tar surfacing
• 6mm medium graded
• 3mm fine graded
• 6mm dense
• 10 and 20mm pervious
• Friction courses
• Pervious (open porous asphalt)

• 20mm open graded bituminous material
• 20, 28 or 40mm dense bituminous material
• Hot rolled asphalt basecourse
• 40mm size single course

Sub-Base
The sub-base is designed to assist in the load spread over the sub-grade. It also allows drainage into the sub-grade and is often used during construction as a temporary running surface and is the main platform on which to build the road. The sub-base is also used to make up the thickness of the pavement to at least 450mm to give frost protection to the sub-grade in frost susceptible soils. Sub-base is always made up with unbound granular materials.

Reinstatement of Flexible Roads
The 'Specification for the Reinstatement of Openings in Highways' is used when designing the reinstatement and deciding the materials to be used. The specification also gives the permitted tolerances and intervention limits on all reinstatements.

Main Considerations for Reinstatement
When reinstating the road there must be great efforts to see that the workmanship and quality are of the highest standard. This will ensure that the reinstatement will stand up to the expected loading and prevent further visits to the site thus more expense. Time should be allowed for when quality is to be maintained.

There must be an agreement with the local authority on materials used in areas of special needs. Using unsuitable materials, even if well compacted, may lead to a defect notice being served.

Where reinstating a flexible road it is important to find out if there are any special considerations to be taken into account, such as high water tables, geofabrics if existing, sub-grade strength, etc.

The time between excavation and reinstatement should be kept to the absolute minimum, especially in the summer months to prevent the sub-grade and adjacent exposed surfaces from drying out.

Foamed concrete is used with the permission of the authority. This is because of the flexible nature of the road. Using foamed concrete may cause failure of the structural integrity of the adjacent road due to vertical movement of the road. This would also apply to using lean-mixed concrete.

Lean mixed concrete may only be used on small reinstatements of less than 2m² and less than 300mm in width.

Where possible the reinstatement should be 'like for like'.

The cavity must be trimmed back where an existing edge, kerb, ironwork or reinstatement is less than 250mm away from the reinstatement.

When using hot-lay materials consideration must be given to the temperature of the material. If the material is too cold it will not compact properly and if too hot the material may ravel, wave, crack or produce a slippery surface with little skid resistance. Were practical the use of a hot box or heated vehicle bed should be used. Hot rolled asphalt must be given an application of pre-coated chipping to provide texture depth and skid resistance. The rate of spread for pre-coated chipping is approximately 1.8kgm². Pre-coated chippings should be covered to prevent oxidation and wetting.

Where hot lay material is used a tack coat should be applied to the base course when there base course material has been allowed to cool. This tack coat is not applied when laying both courses together. Tack coat must not be used as an edge sealant. The rate of spread for tack coat is 0.4 - 0.6 litre/m² with no pooling.

The bituminous binder used in the base course and wearing course should be of a similar viscosity. This will prevent softening and movement of the basecourse material during laying of the wearing course.

Material 6mm aggregate size should only be used on roads which are lightly trafficked. This material is mainly intend for use in footways.

All vertical edges must be cut cleanly with a pavement saw or similar and be painted with an edge joint sealant immediately before laying surfacing materials. Where the sealant has been 'scrubbed off' during the compaction of the basecourse it must be reapplied. The use of over banding is not considered as a substitute for sealant. Where overbanding is required it must not be wider that 50mm or stand proud higher than 3mm.

Where a trench has been cut across different types or classes of road, the reinstatement should be to the higher type or class.

The selection of suitable compaction equipment cannot be under estimated. the equipment must be able to compact the given layer thickness. Light weight plates used for block paving are not heavy enough to adequately compact bound and unbound materials and should not be used. Access to the site, depth and size of trench should also be considered. The specifications give the recommended number of compaction passes and compacted layer thickness for the different types of materials. Some compaction equipment is unsuitable for some materials due to the compacted layer thickness or being too light to compact adequately.

Tolerances
The Department of the Environment, Roads and Regions (Clause 701) sets the tolerances for the layers within the structure of the carriageway, these are set out below:

Top of Sub-grade = +20mm to -30mm
Top of Sub-base = +10mm to -30mm
Top of Road base = +/-15mm
Top of Base Course = +/-6mm
Top of Wearing Course = +/-6mm

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