Hot rolled asphalt surfaces
- HRA design mixtures give better resistance to deformation than recipe mixtures where queuing of heavy traffic is likely to occur, and it may be more economical to lay.
- 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, more prone to fretting and less well suited to the reinstatement of small openings.
- HRA 55/14F is a high stone content asphalt (HSC). HSC asphalts can be difficult to hand-lay and are relatively prone to segregation. If these risks are not mitigated, they can lead to problems including failure at joints and surface fretting. Consideration should therefore be given to the options below:
- Reduce nominal aggregate size.
- Softer binder grade.
- Use of permitted materials in A2, by agreement with the authority.
Stone mastic asphalt (SMA) and thin surface course systems
Suitability of a SMA or TSCS for reinstatement works its appropriateness for small volume works and hand laying. This is a consideration when designing for maintenance in highways and when the authority is requested to identify a suitable material for reinstatements. SMA and TSCS can be difficult to hand-lay, especially if they contain highly modified PMBs. If risks related to hand-laying are not mitigated, they can lead to problems including failure at joints and surface fretting. As a risk mitigation, a reduction in nominal aggregate size is included in S6.4 for SMA and TSCS. The suitability and availability of SMA or TSCS also need to be considered Considerations include availability of a low volume supply of mixtures that use specified PMBs (link to tankage and supply from the asphalt plant) and whether any modifications to the mix can be adopted, such as workability agents, to mitigate installation risks. The underlying logic is that the performance of the reinstated surface is not detrimentally impacted and that materials are available for the works. Increasing the surface area of reinstatements to enable use of paved material options only should be avoided unless by mutual agreement.
Other bituminous materials
Highway authority requirements for the use of specific materials within street works need to take into account the suitability of the material. In general terms this may mean that a highway authority specification for a SMA (or other material) that has been designed for paver installed resurfacing works may not be suitable for use within trench reinstatements without modification. NG6.4 gives guidance on this topic and the underlying logic is that the performance of the reinstatement surface is not detrimentally affected and that materials are available for the works.
High friction surfaces
- As high friction surfaces are usually laid for safety reasons they should be reinstated as soon as is practicable.
- Performance on concrete may not be as good as on bituminous surfaces and the suitability of a system should be checked by reference to its Product Acceptance Scheme certificate.
- Suppliers of some high friction surfacing systems may have specific requirements relating to the condition of the surface on which it is to be applied. Guidance on this should be sought either from the supplier or the Product Acceptance Scheme certificate.
- It is recommended that high friction surfacing systems are applied to a surface course that has been trafficked for some weeks. This is to help prevent cracking of the surface course induced by the application of a newly laid high friction surface.
- Edge sealing requirements specified in S6.8.7 may not be appropriate with porous asphalts because the free-flow characteristics of the material may be impeded.
- Some application rates in this Code are quoted in kg/m2 of residual bitumen. This differs from rates in l/m2. For example, the rate of 0.50 kg/m2 residual bitumen equates approximately to 1.2 l/m2 of K1-40 or 0.80 l/m2 of K1-60 emulsion.
- Coloured surfaces 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 coloured surface can be restored.
- It may not be possible to obtain surfaces in a wide selection of colours so authorities may have to accept limitations in colour matching, especially as colours fade with age.
Other specialist surfacing materials
Texture depth requirements specified in S2.6.2 may not be appropriate for the increasing number of specialist surfacing materials currently being used by some authorities.
In all roads, where the overall quality of existing surface treatments is to a high standard, it may be difficult to produce small openings or narrow trenches with surface dressings or other surface treatments that closely match the existing adjacent surfaces. Under such circumstances, if the requirement to reapply the surface treatment is only for aesthetic reasons, some localised variation in surface appearance should be acceptable to the authority.