Roads are at the core of our national transport system. One third of all traffic and two-thirds of all freight use the Strategic Road Network. It is the backbone of the country’s economy and a lifeline for communities everywhere, big and small.
The state and structural integrity of the roads is therefore integral to the smooth running of the country. It is why Highways England is embarking on an extensive programme of maintenance and repair as part of its second Road Investment Strategy, prioritising safety, customer satisfaction and sustainability.
A brief history of concrete
The design and construction processes for concrete roads were derived from those developed to construct airfield pavements, particularly during the Second World War. But it wasn’t until the 1970s and the first oil crisis that concrete roads in the UK became more common. The shortage of oil led to rationing, including of bitumen, produced from the refining of petroleum and used widely for road surfaces.
Concrete roads rapidly expanded during the 1980s, supported by a breakthrough in concrete design in 1987, when the Transport and Road Research Laboratory (TRRL, and now known as TRL) published Research Report 87 (RR87) design guidance for concrete pavements. The document looked at different construction methodologies, analysed their performance in terms of design, thickness and types of foundations used. The research proved that the performance of concrete roads was particularly sensitive to the strength of the foundation underneath.
To achieve the best performance, RR87 advised that concrete foundations should be constructed with an equivalent foundation modulus of 270 megapascals (MPa) or above, which can only be achieved by using a cement stabilised subbase. This is a stiffer, more uniform material and reduces the risk of cracking and early slab failure. After the publication of RR87, the industry switched from granular foundation design to cement stabilised foundation design.
Today, about 4% of the national road network or 172 miles in England is made of concrete. Many concrete roads have now exceeded their design life, particularly the older concrete pavements built before RR87. They are deteriorating quickly and showing multiple defects, from minor cracking to severe spalling. Repair and maintenance work is required urgently.
Pursuing the circular economy
Working within a limited budget, the challenge is to make the most of what we already have. One of the infrastructure industry’s priorities is to improve sustainable practices, and one sure-fire way to do this is to follow the circular economy philosophy.
Principles of the circular economy dictate that we should reduce waste and pollution by limiting our reliance on natural, finite resources and instead maintain, recycle and reuse the waste materials we already have in circulation.
In the context of concrete roads, the asset is in its highest value state in-situ and priority should be given to extending the service life in the first instance. Life extension works – such as crack repairs, concrete bay replacements, full and partial depth repairs, joint sealing and surface grinding – can achieve great results, prolonging the life of concrete roads by several years.
Concrete is manufactured from aggregate, cement and water and is 100% recyclable. Once a surface is broken down, any steel reinforcement can be removed and recycled, and the concrete re-used as an aggregate in the production of new concrete products or as a granular aggregate material – a process that seamlessly turns a waste product into a valuable resource.
Quieter roads
Despite many advantages, concrete road pavements are disliked by many drivers because of the noise they make, which comes from the interaction between the tyres and the surface of the road.
Research in 2017 from independent watchdog Transport Focus reflected motorists’ discontent with our concrete roads, with driving on these surfaces commonly described as a stressful, noisy experience. Compared with the much smoother, quieter journeys on asphalt, this poses a huge challenge for Highways England to solve going forward.
The texture of pavement surfaces can be categorised as either positive or negative. Surfaces with a positive surface texture tend to create more noise, whereas those with a negative surface texture absorb it. Early on in concrete road construction, the texture created through the application of brushed finishes on the road surfaces presented a series of transverse ridges that reflected the tyre noise back into the environment and vehicle. Joints between the concrete slabs further added to the noise and discomfort when driving over them.
However, these challenges can be mitigated through the application of surface treatments, such as diamond grinding and grooving, which has been applied already to some sections of the road network. One of the most promising evolutions of this approach currently being explored is Next Generation Concrete Surface (NGCS).
NGCS involves grinding a thin layer off the surface of the concrete and then introducing longitudinal grooves – that is, in the direction. The grinding process smoothens the surface by removing irregularities, while the introduction of the grooves improves noise characteristics by introducing a negative surface texture and also achieves the required surface friction.