Recycled materials get the green light

Sally Ellis, Group Manager, Sustainable Construction, TRL Ltd, reviews the use of recycled materials used in road construction which are making a major contribution in the drive towards sustainable construction

In situ recycling
In situ recycling occurs when the existing pavement is either planed or pulverised and treated on site to be used in the new pavement structure. There are possibilities for all layers of the bound pavement structure to be recycled in this way. More specifically, Highways Agency specifications permit by means of hot mix recycling the use of up to 10% by mass in the surfacing layer and 50% by mass in the lower asphalt layers, of recycled aggregates. Additional new aggregates and bitumen binder are then used to complete the mix.

Cold in situ recycling for the structural maintenance of highway pavements has also been developed and used in the UK. TRL Report 386 provides a specification and design guide that considers deep cold in situ recycling by applying a cement or bituminous binder. The advantage when using cold mix techniques is that less energy consumption is required than for hot mix methods.

Further in situ recycling is that of soil stabilisation by means of application of lime or cement to create soil/cement or soil/lime mixes. By carrying out this technique the need for substantial pavement foundation layers is removed and therefore the volume of imported materials is less.

Ex situ recycling differs from in situ recycling in that the materials are removed from the site to be used as aggregate in plant mixed materials. Although this still makes good use of materials it can increase vehicle movements above those required for in situ recycling.

Alternative materials
The use of alternative materials is a form of ex situ recycling. In general alternative materials are industrial by-products. The materials are china clay sand, colliery spoil, demolition debris, glass bottles, incinerator bottom ash, pulverised fuel ash, rubber tyres, slag, slate waste, spent oil shale and it does seem likely that there is a future for sewage sludge. Some of these can be used as aggregates in conventional mixes whilst others have pozzolanic properties that can used as part or all of the binder required in bound applications. TRL has also developed specification clauses that allow performance rather than recipe use of materials, encouraging contractors to use unconventional materials to achieve performance criteria, for example, rutting tests. More specific material clauses have also been developed, for example the use of bound china clay sand or slate waste. These specification clauses developed by TRL are included in the Highways Agency Specification for Highways Works. Many local authorities also have their own guidance on the use of industrial by-products.

More obscure applications of recycling in road construction are those applied to crack and seat, haunching, and trench reinstatement activities.

Crack and seat is an innovative maintenance technique used on all levels of road including the motorway and trunk road network. It is also a great example of sustainable construction practices. Prior to the effective implementation of this maintenance method, through research and development by TRL, jointed concrete pavements that required structural maintenance were either excavated to landfill or low grade re-use, or overlaid with thick asphalt surfacing in excess of 180mm. The introduction of crack and seat allows the concrete to remain in place at as the structural layer of the new pavement and for the asphalt surfacing requirement to be reduced to 150mm. The impact of this treatment is that: the concrete is re-used at a higher grade, the volume of asphalt required is less and as a consequence the duration of the maintenance works is reduced, truck movements are reduced, traffic delays are reduced, and hence environmental pollution is less.

Haunching and trench reinstatement are examples of two smaller maintenance measures where it should not be forgotten that recycling techniques could also be applied. There is a haunching design guide that was developed by TRL in the 1990s and includes many options for in situ recycling or the use of alternative materials as described above. The Highways Authorities and Utilities Committee (HAUC) Specification for the reinstatement of openings in highways also allows recycling techniques to be used in trench reinstatements.

Summary
In summary, consideration of materials, their design and specification, encourages the development of new solutions to the problems of pavement engineering and the increased use of recycling and more alternative materials. This reduces environmental impact and will preserve our limited resources for the benefit of future generations. It is feasible to build roads with unusual materials such as china clay sand and slate waste. They have performed well in full-scale trials over a long-term period and are recommended for use.

It is well to remember that there are more advantages to recycling in road construction than re-use of materials. Recycling contributes directly to the need to preserve our natural resources but in addition, recycling can have an impact on the duration of maintenance contracts, the number of truck movements, traffic delays, energy consumption, and other factors that can all contribute to increased environmental benefits for our society.

For further information on any of the topics raised or any aspects of recycling in road construction, contact Sally Ellis, TRL Limited, tel. 01344 770023, email sjellis@trl.co.uk