Pavement Partnership

Working together to improve the sustainability of flexible pavements

By Darren Merrill and Paul Sanders, TRL

As traffic volume and vehicle loading on the UK road network increases, so does the challenge to manage and maintain the network in a sustainable manner. The construction and maintenance of roads must be carried out economically and in a manner which embraces as many aspects of sustainability as possible. One of the ways of managing the network in a more sustainable way has been developed for pavements with relatively ‘thick’ bituminous structures – long-life pavements (LLPs).

The characteristics of LLPs were recognized in the 1990s when it was noted that structural deterioration did not appear to occur, ie deflections on thick, well-constructed bituminous pavements tended to reduce with time, and pavement defects were largely confined to surface layers.

A maintenance strategy for long-life pavement has been implemented by the Highways Agency. This strategy is currently applied to a significant proportion of the trunk road network and involves routine monitoring of the pavement surface with more detailed investigation where appropriate.

In its present form it is already providing significant benefits over the traditional construction and maintenance strategies, namely reduced congestion and improved economy. Further opportunities for improving sustainability have since been recognized.

Accordingly, TRL have completed a study of some of these opportunities on behalf of the Highways Agency, the Quarry Products Association and the Refined Bitumen Association.

The three-year study looked at a number of aspects including: the introduction to the UK of a high-modulus bituminous material (Enrobe Module Eléve – EME); the concept of ‘robust’ pavements; and new maintenance treatments for the surfacing layers.
The common theme for these investigations was to provide a more sustainable paving solution by improving durability, reducing resource consumption and improving asset management.

EME has been successfully used in France for more than 20 years as the combination of aggregate grading and low-penetration binder forms a macadam with a rich binder matrix that is durable and resistant to loading. Although asphalt materials with low-penetration-grade binders have been available in the UK for more than a decade, EME mixtures provide improved properties as they are less permeable, due largely to a binder content of almost 6%.

The engineering properties of EME Class?2 material were assessed using the TRL Pavement Test Facility (PTF). The trials focused on aspects of constructability and deformation resistance. The trials demonstrated that the EME-II material was sufficiently workable so that special construction techniques were not required to obtain well-compacted dense materials in both narrow (1m wide) trenches and full-lane treatments. The trials also showed that deformation resistance and load spreading ability were superior to typical HDM binder-course materials. The results of permeability and stiffness testing after soaking in water showed that the material is impermeable due to the combination of high binder content and aggregate grading. Fatigue testing also showed the material to be more resistant than HDM.

Overall, the findings suggest that when used as a binder course in new design, as tested in this study, it is possible to reduce binder course thicknesses while still retaining good resistance to deformation, fatigue and moisture ingress. In addition, its high workability makes this material ideal for use in the maintenance of long-life pavements.

The study also considered the assessment procedures for LLPs. By consideration of what characterizes a ‘long-life’ type of pavement, the concept of ‘robust pavements’ was developed. This concept acknowledges that there are some pavements that are thinner than LLPs or have a greater deflection than LLPs, but no structural deterioration occurs and they can therefore be maintained in the same way as an LLP. Identification of robust pavements is made through a suite of tests aimed at confirming the characteristics of ‘long-life’ type pavements, namely: thickness; resistance to damage from traffic; soundness of the asphalt layer; and good support from the foundation.

The concept of robust pavements also covers a method of upgrading pavements to a long-life condition. Such upgrading is required for situations where a period of 40 years without structural maintenance is desired but where the existing pavements do not meet the current long-life assessment criteria, as detailed in the Design Manual for Roads and Bridges.

The amended upgrading approach links with existing methods while allowing more flexibility. Where deflections are high, an equivalent thickness can be found that is compatible with the current long-life pavement deflection criteria. The amended approach to upgrading is to develop the thickness of the pavement such that it is increased to the equivalent thickness of at least 300mm.

To maintain long-life pavements only the surfacing layers need to be treated in a timely fashion. This approach not only satisfies the functional requirements of pavements, but the duration of roadworks is minimized and, since only the surfacing layers are replaced, it is potentially ‘resource efficient’ compared with maintenance of determinate-life pavements. New maintenance treatments that further reduce the amount of material used (partial-depth inlays and wheel-track trench inlays) have also been considered.

Partial-depth inlays refer to inlays where deep, ‘top-down’ cracks are not fully planed out, which is currently advocated. A full-scale trial on a trunk road is under way with URS-Carillion. Following current advice, the inlay treatment would have been 170mm deep, but instead 80mm was replaced. This reduced the amount of material removed and reinstated by more than 50% as well as reducing the energy required for extraction, processing and transport.

Monitoring of this trial is ongoing.

The wheel-track trench inlay treatment, as tested in the TRL PTF, is to be used where deterioration predominantly occurs in wheel paths, such as rutting or longitudinal cracking. The main perceived drawbacks for this treatment are the difficultly in constructing well-compacted, durable, longitudinal joints together with good compaction within the trench. However, test results showed high levels of compaction in both the trench and along the joints. The trial illustrated that, compared with a full-width inlay, a saving of approximately 45% in material can be achieved.

The collaborative study presented a number of ways to improve the sustainability of roads using flexible pavements. Robust pavements offer significant benefits for the management of principal roads as well as trunk roads; a future challenge remains as to how best to implement these concepts for the UK road network. The novel maintenance treatments illustrated offer potential economic and environmental benefits. To help implement the findings of the study, a specification for EME is being developed by industry and the Highways Agency.