Hydraulically Bound Mixtures
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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.

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

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