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.
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.
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