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Recycling
Structural
maintenance of roads using Hydraulically Bound Mixtures
For many
years now, recycling using Hydraulically Bound Mixtures has been used for the structural
maintenance of roads to produce new base layers in the region
of 150 to 300mm deep. This has involved both in-situ and
ex-situ techniques.
Deep-lift
insitu recycling has been around at least 50 years but has
become more prevalent and recognized in the last 20 years
with the advent of more powerful mixer-rotovators capable
of pulverizing existing asphalt layers prior to the addition
of liquid or powder binders to the ‘granulated’ pavement.
Essentially
and typically with the in-situ process;
- the
existing pavement is pulverized to the required depth say
200mm,
- new
binder is added and mixed using the same machine that carried
out the pulverizing,
- compaction
and shaping is carried out and
- depending
on traffic, a surfacing is applied varying from as little
as a surface dressing or 40mm of asphalt to 100mm asphalt.
Depending
on the quality and grading of the pulverised material, the
binders/additions used might typically be as follows:
- 6%
cement sometimes with coal fly ash (also known as pulverized
fuel ash or pfa) to improve grading
- 3%
foamed bitumen with 2% cement and usually 5 to 10% coal
fly ash for grading correction.
The ‘rules’ for
what is known as ‘cold deep-lift in-situ recycling’ are
well established and ‘laid-down’ in TRL Report
386.
More recently
however, TRL report 611 has been produced. As well as in-situ,
TRL 611 also embraces the use of ‘cold ex-situ recycling’,
which is becoming more widespread, and which describes the
central production of recycled mixtures for transportation
to the point of use.
TRL 611 also covers a broader range of binders than TRL 386 including the lime/fly
ash combination and the use of lime with granulated blast-furnace slag.
References
TRL 386. Design Guide and specification for structural
maintenance of highway pavements by cold in-situ recycling.
1999. www.trl.co.uk
TRL 611.
A guide to the use and specification of cold recycled materials
for the maintenance of road pavements. 2004. www.trl.co.uk |
