NG6 Flexible and Composite Roads

NG6 Flexible and Composite Roads

NG6.1 There are no Notes for Guidance

NG6.2 Sub-base Reinstatement

1) An adequately compacted sub-base should achieve an in-situ CBR value in excess of 30%.

2) It is expected that a bituminous sub-base will only be selected where the Base is also bituminous.

3) When placing bituminous material directly on to the backfill it is important to ensure that the exposed surface of the backfill has been compacted. This operation is essential to minimise the risk of a build up of pore water pressure causing the subgrade to become spongy. It is also imperative that construction is phased such that excavated areas are covered, on the same day, with the first layer of bituminous material, to prevent the ingress of water. Care should be taken in the compaction of this first layer. If pore water pressure builds up in the backfill at this stage, then rolling should cease and the material left overnight, or longer if necessary, prior to the placement of any further layers.

4) The condition of sub-base material can be assessed using Field Identification Test No. 3 in Appendix A1.

NG6.3 Base Reinstatement

Overlaid Modular Layers

1) This Specification permits the re-use of cobbles and setts for the reinstatement of the relevant layer. However, it is often extremely difficult to achieve a performance from such reinstatements that is similar to that of the original, i.e. well interlocked and 'stress hardened' layer. Failure to achieve this structural stiffness could result in failure of the reinstatement and particularly any surfacing materials laid thereon.

2) The Specification does not permit the re-use of penning, in which the layer of modules is laid upright, in an interlocking manner, exhibiting a greater stiffness than an equivalent layer of cobbles/setts.

NG6.4 Surface Reinstatement

NG6.4.1 Hot Rolled Asphalt Surfaces

1) HRA design mixtures give better resistance to deformation where queuing of heavy traffic is likely to occur and may be more economical to lay.

2) Type C mixtures use fine aggregates of a coarser grading than Type F mixtures, usually associated with the use of crushed rock fines. Such mixtures tend to be stiffer and less well suited to the reinstatement of small excavations.

NG6.4.2 - NG6.4.5.1 There are no Notes for Guidance

NG6.4.5.2 High Friction Surfacings

1) High friction surfacings will usually have been laid for safety reasons and should be reinstated as soon as is practicable consistent with the manufactures specification.

2) Performance on concrete may not be as good as on bituminous surfacings and the suitability of a system should be checked by reference to its HAPAS Certificate.

3) Suppliers of some high friction surfacing systems may have specific requirements relating to the condition of the surface to which it is to be applied. Guidance on this should be sought either from the supplier or the HAPAS Certificate.

4) If It is recommended by the manufacture that High Friction surfacing systems are applied to a surface course that has been trafficked for some weeks. This is to help prevent cracking extending into the surface course induced by the application of a newly laid high friction surface.

NG6.4.5.3 Porous Asphalt

1) Edge sealing requirements specified in Section S6.5.2.2 may not be appropriate with porous asphalts because the free-flow characteristics of the material may be impeded. (refer to manufactures specification requirements)

2) Application rates in this specification are quoted in kg/m2 of residual bitumen. This differs from the previous specification, which quoted rates in l/m2 of total emulsion. For example, the rate of 0.50kg/m2 residual bitumen equates approximately to 1.2 l/m2 of K1-40 or 0.80 l/m2 of K1-60 emulsion.

NG6.4.5.4 Coloured Surfacings

1) Coloured surfacings are sometimes used for marking bus lanes, accident prevention measures, traffic prioritisation schemes, etc. The use of warning signs, e.g. "Temporary Road Surface", should be considered until the special surface can be restored.

2) It may not be possible to obtain coloured surfacings in a wide selection of colours and Authorities may have to accept limitations in colour matching. In addition, coloured surfacings can fade or undergo other changes in colour as the materials age.

NG6.4.5.5 Other Specialist Surfacing Materials

Texture depth requirements specified in Section S2.6.2 may not be appropriate for the increasing number of specialist surfacing materials currently being used by some Authorities.

NG6.4.6 Surface Treatments

In all roads, where the overall quality of existing surface or surface treatments are to a high standard, it may be difficult to produce small excavations or narrow trenches with surface treatments that closely and uniformly match the existing adjacent surfaces. Under such circumstances, some localised variation in surface quality may be acceptable to the Authority.

NG6.4.7 - NG6.4.10 There are no Notes for Guidance

NG6.5 Base and Edge Preparation

NG6.5.1 Base Preparation

1) Tack coating materials are generally based on rapid curing anionic or cationic bitumen emulsions to BS 434, with approximately 40% bitumen content. New tack coating materials are becoming available and the trial use of more modern variants is recommended.

2) Application rates in this specification are quoted in kg/m2 of residual bitumen. This differs from the previous specification, which quoted rates in l/m2 of total emulsion. For example, the rate of 0.15 kg/m2 residual bitumen equates approximately to 0.35 l/m2 of 1-40 or 0.25 l/m2 of K1-60 emulsion.

3) Further guidance on application of tack coats and bond coats can be found in BS594987.

NG6.5.2 Edge Preparation

1) Edge sealant materials are generally based on rapid curing bitumen emulsions to BS EN 13808, typically in the range 40 to 100 pen and approximately 70% bitumen content, or hot bitumens to BS EN 12591 typically 50 or 70 pen. An increasing number of high build and rubberised edge sealants are becoming available and, in general, are preferred. Alternatives to these materials exist and, if there are any doubts as to their effectiveness, they may be used on a trial basis.

2) When using edge sealant materials, strict adherence to manufacturer's instructions is essential. Dependant on weather conditions brushed sealant drying times may vary between 5 minutes and 2 hours, whilst spray sealant times may vary between 1 and 15 minutes before reinstatement can take place.

Additionally prior to application:

  • All excess water and loose material should be removed from the cut faces of the reinstatement;
  • All bound vertical edges must be clean and free from slurry and dust etc. with the stone in the existing layers clearly visible.

3) The following case study data sheets illustrate the results of edge sealant application under different edge conditions:

  • Example 1 - dry and clean;
  • Example 2 - wet;
  • Example 3 - dirty and damp.

Example 1 - dry and clean

Example 2 - wet

Example 3 - dirty and damp

NG6.6 There are no Notes for Guidance