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


The methods and materials used for construction and maintenance of road pavements

in the UK changed substantially during the close of the twentieth century. This

was in no uncertain terms due to the need for a “sustainable” future.

For construction, industry has seriously considered “development which

meets the needs of the present without compromising the ability of future generations

to meet their own needs” and this has led to increased use of recycling

techniques and use of alternative materials.

Outlined below are the different aspects to recycling in road construction,

including reference to the way in which these methods can contribute to the

sustainable future that is needed.

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


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