The Criteria

In order to obtain the best value for money a number of factors are taken into account to prioritise the annual programme for highway improvement works. These factors include :
* Road Condition Surveys
* Accident Statistics
* Public referrals i.e. service requests
* The road classification - level of use
* Bus routes
* Principal access to centres of population or employment
* Alternative routes available
The choice of the treatment carried out largely depends upon the footway or carriageway condition but consideration is also given to the location i.e. urban or rural area, traffic flows etc.
 
Road Design Basics
A modern road is designed to withstand the effects of the weather and the volume of  traffic using it. The UK is one of the few countries in the world where almost the entire road network has some form of all weather surfacing.
Most rural minor roads, however, have evolved from un-surfaced tracks which have been shaped with stone and overlaid with tar and chippings.

The diagram below illustrates that a road consists of a number of layers of varying thickness and of different materials - a system designed to distribute the load imposed by traffic.

The surfacing layer provides skidding resistance and 'ride quality' and prevents water undermining the lower, 'base' layers that provide the structural strength of the road. The strength of the sub-grade and an estimation of traffic volume determines the thickness and composition of the different layers.

Treatments

The most common treatments used on the Authority's network are :
* Roads
o Micro-asphalt Surfacing
o Surface Dressing
o Thin Surfacings (Stone Mastic Asphalt)
o 'Traditional' Resurfacing
o Reconstruction
* Footways
o Paving Slabs
o Block Paving
o Flexible Construction (Bitumen Macadam)
o Slurry Seal

Micro-asphalt Surfacing

Micro-asphalt is a relatively cheap material consisting of a bitumen emulsion and aggregate slurry, which is laid cold by a paving machine.  It seals the surface and improves skid resistance and has very good stress absorbing characteristics. It will normally last between 5 and 10 years.

Micro-asphalt Surfacing is typically undertaken on the following sites

a) Roads which have been lacerated with utility company (gas, electric, water etc.) trenches or where ruts in the wheel tracks has occurred.  Micro-asphalt will regulate, or 'level out' out trenches in binder and surface courses to a thickness of approximately 10-30mm.
b) Kerbed roads where kerb upstand is minimal.  Micro-asphalt can be feathered out to 10mm thick at the channel or can be stopped short of the channel so that existing kerb height is maintained, so avoiding channel planning.
c) High stress sites e.g. roundabouts, severe bends, steep gradients where traditional surface dressing would not adhere.
d) Micro-asphalt surfacing is a rapid process and the material can be trafficked within 20 minutes.  Traffic disruption is therefore minimal.
e) Micro-asphalt is a water based slurry and can be laid throughout the year so long as freezing conditions do not prevail but is generally only available from April - October.

Micro-asphalt should generally only be applied to roads which are of good shape and structurally sound, as it has no inherent strength.

Surface Dressing

Surface dressing is another relatively cheap and cost effective means of sealing the carriageway from the ingress of water thereby protecting the road layers and structural layers from further deterioration. In addition the application of chippings improves the skidding resistance of the carriageway.  Life expectancy is approximately 5 years.

Generally, surface dressing is only used in rural and non-residential areas due to the problem of chipping loss onto footways etc.

Surface dressing is suitable in the following circumstances :
* The road surface Is showing signs of oxidation and tension cracks (crazing) are apparent. A sign of oxidation is when the road surface becomes grey in colour - also termed 'hungry'
* Skid resistance tests show the existing surface is slippery
* To prevent the road deforming due to ingress of surface water and where resurfacing/reconstruction is not affordable.

Surface dressing should generally only be applied to pavements which are structurally sound and of good shape.  Pre-surface dressing patching should ideally be done during the previous year to avoid the bitumen rich surfacing from 'fatting up' through the surface dressing.

Surface dressing is not usually suitable for high stress sites e.g. roundabouts, steep gradients etc. unless specialised binders are used and should be carried out between June and September.

Thin Surfacings (Stone Mastic Asphalt - SMA)

Due to its open texture SMA has similar characteristics to porous asphalt with noise levels and spray considerably less than for conventional surfacing material. It is therefore appropriate for use on high-speed roads, and environmentally sensitive areas e.g. the Wye Valley.

'Traditional' Resurfacing

Traditionally using dense bitumen macadam (DBM) or hot rolled asphalt but recently supplemented by High Stone Content (HSC) asphalts (60% stone). Life expectancy can be up to 20 years.

Surface course can be applied as an overlay i.e. to add extra strength or as an inlay following the removal of the existing surface. It is normally laid 40-50mm thick using a 14mm aggregate.

HSC Asphalt is less pervious than DMB macadam and has a greater resistance to 'cow manure' attack and is therefore considered as an alternative to DBM macadam in rural minor roads.  Due to its low texture depth it is not suitable for roads where speeds are likely to exceed 50mph.

Carriageway Reconstruction

Where the carriageway pavement is beyond repair then conventional or retread reconstruction is necessary.

Pavement failure may be attributed to any one or a combination of the following:-

* Sub-grade softening due to poor drainage.
* Sub-grade softening due to ingress of moisture through cracked and pervious pavement layers as a result of progressive oxidisation.
* Sub-grade failure due to insufficient pavement thickness. 
* Frost Heave

'Retread' is a recycling process and therefore 'environmentally friendly'. The  process consists of adding bitumen and/or cement to the 'planings' taken from the existing road surface then regraded and rolled to form a re-vitalised pavement.  The surface is finally sealed with a surface dressing as part of the process.  Shallow recycling to a depth of 75mm is often sufficient on very lightly trafficked minor roads whereas 60m surfacing overlay would nearly double its strength and also considerably improve its regularity.
Deep Cycling up to a pavement depth of 450mm can be undertaken as an alternative to a conventional reconstruction.

Footway Treatments

Footways generally consist of paving slabs, block paving or flexible construction.

Paving Slabs

Existing footways are normally replaced by new or re-bedded slabs - particularly in conservation areas, town centre streets and residential areas where terraced houses abut the footway.

Block Paving

Block paving is often better able to withstand traffic loadings than paving slabs following utility works thus avoiding visual scarring of the surface.

Flexible Construction ( i.e. bitumen macadam)

Cheapest footway construction option.  Used in residential areas, rural areas and out of town areas.

Footway Slurry Seal

Footway slurry seal can be employed where the surface course is beginning to perish and look 'hungry'. It is used in order to seal the surface and prevent further oxidisation.

Slurry seal is a cheap treatment but not always popular in residential areas.  The workmanship needs to be continually monitored to ensure that good regularity is maintained and that kerbs and ironwork are neatly masked.

Slurry seal has no inherent strength and therefore any pavement failure should be addressed by the replacement of the structural layers.