The last two decades have seen huge improvements in the quality of our public road network. This has been due to the implementation of detailed standards and practices which have been developed mainly by Transport Infrastructure Ireland (TII), formerly known as the National Roads Authority (NRA). While the TII are responsible for a relatively small percentage of our roads (5%approx.), their standards are being applied across the whole network in terms of maintenance and materials. Ireland has a legacy of a very large road network given its population. While the majority of journeys are taken on national routes, the eponymous ‘boithrin’ still accounts for an important element of the network.
Roughly speaking, we have about 100,000km of roads in Ireland. Only about 5,000km of these are National Roads i.e., Motorways (e.g. M4), National Primary (e.g. N4) and National Secondary (e.g. N59). This small proportion of our network, however, carries 45% of our traffic. In general, most of these are designed to TII standards and are constructed to a high quality and funded by the TII.
Legacy National Secondary routes, however, were often developed gradually over the years built up by layers of substandard materials incapable of supporting modern heavy goods vehicles. In many instances, these roads require the installation of enough layers of bituminous material to provide the structural strength to carry modern-day traffic. Bitumen is a by-product of the oil industry and is as such a finite resource. It is used as the material that binds the aggregate (stone) together to provide structural strength. It has for quite some time replaced coal tar (which is identified as a carcinogen) as the binder for road materials. Therefore, tarmacadam is no longer used, and the terms bitumen macadam or asphaltic concrete are more reflective of the upper layers of road-making materials.
In some instances, it is possible to recycle old bituminous layers to provide the strength required. This is particularly useful for our bog rampart where adding extra weight (additional bituminous layers) only compounds the problem and expedites the structural failure. Recycling surfacing materials is an important innovation that goes some way toward more sustainable maintenance practices.
The rest of the network (95%), consists of non-national roads which are funded directly by the Department of Transport. These are broken up into Regional (e.g. R294), Local Primary and Secondary (e.g. L-1401) and finally Local Tertiary (L-15501). Regional roads are beginning to be treated like National Roads with condition surveys being carried out and bituminous layers added for structural strength. Local road maintenance is generally less scientific and is more based on Area Engineers’observations, how many houses they are serving or political representations. This maintenance programme is set by the executive of the local authorities and approved by the local representatives.
The main issues that require maintenance, arising on all roads are either structural or serviceability issues. Structural issues include:
- LongitudinalDistortion (bumps)
- Edge break up
- Road Disintegration
There are a number of causes of these types of failures including subgrade (the grounds supporting the road) weakness, water penetration, excessive vehicle loading and poor lateral support. The key to the avoidance of structural problems is addressing the issues on time. Detailed structural surveys which assess pavement strength are carried out on National Roads annually and they identify the need for interventions before the damage is irreversible. If the interventions are too late, full road reconstruction is necessary while, if treated on time, a much more economical and sustainable intervention such as inlay or overlay can be utilised.
Serviceability issues include ride quality, linemarking and studs but most importantly skid resistance. Skid resistance is what keeps vehicles on the road and is one of the most important engineering interventions in road safety. Low skid resistance, particularly in wet weather, increases stopping distances significantly and can cause loss of control. The surface course of the road forms the interface with the tyres of the vehicle which is critical in determining how quickly the vehicle can stop in an emergency. Surface courses are chosen specifically to provide adequate skid resistance for the nature of the road. Different surface courses commonly used on national roads include Stone Mastic Asphalts and Hot Rolled Asphalts. On Non-National roads, surface dressing is the most commonly used treatment. This material is uniformly sized stone chips rolled into a bituminous spray. The treatment also seals the surface, protecting the underlying layers from water ingress.
In TOBIN, our Roads and Transportation team have completed several pavement repair schemes around the country using a multitude of techniques and materials to provide the most economical and efficient solution for the particular maintenance issue. Road Maintenance design is a very specialised skill that contributes to keeping the road network open and supporting the economic activity of the country.