Ex Situ HBM Carriageway Reconstruction (AWCCT)

  • Client:  KierWSP & Surrey CC
  • Road Class:  Class 4 (0.5msa)
  • Depth:  200mm
  • Surfacing:  14mm Close Graded
  • Site:  Various Urban Sites, Surrey (5 month programme)
  • Works Type:  Ex Situ Road Recycling
  • Material:  Hydraulically Bound Material (HBM) H2 Class
  • Specification:  TRL611
2 Surrey Planed out
As part of Project Horizon, Surrey County Council and KierWSP highlighted a need to reconstruct various urban sites across the County. The sites suffered overall from poor and limited construction, with the upper bound layers containing coal tar bound material and as such increasing the costs of conventional construction significantly. Ex-situ recycling of the existing pavement and encapsulating the tar bound material was the chosen treatment. A pre site investigation, testing and design was provided on all sites.


Mix designs that were used incorporated PFA and OPC to produce a H2 class CBGM, with each site having a bespoke design. The inclusion of Pulverised Fly Ash (PFA) reduces the risk of reflective cracking commonly associated with cement bound materials by protracting the rate of hardening. This is achieved by utilising the pozzolans available in the PFA to form part of the cementitious binder. Allasso Recycling mobilised an ex-situ mixing plant and associated equipment to a client depot to form a temporary processing depot for the work. Planing was completed in a 2 stage process separating the coal tar material to be recycled for the site and material to be recycled for use elsewhere.The material was then crushed and screened to a 0/20mm product, mixed according to mix design and returned to site to be paver laid by Allasso gangs. This process enabled the same vehicle bringing in the site Planed arisings to return to site with prepared HBM, reducing vehicle movements. Bituminous waste containing coal tar from other sites in the County was also used to ensure disposal costs from other Surrey schemes were reduced.Quality was monitored by an onsite Technician to ensure compliance with the mix design. A running surface was maintained at the end of every shift to allow access to the adjacent properties. Surfacing was provided by Allasso as a single source provider for the whole operation, improving programming and communication. Positive feedback on ad-herence to programme, safety and quality of the works were received from the Client.


Process Overview

  1. Plane to required depth and return to processing area (coal tar bearing material)
  2. Plane inert and return to recycling depot
  3. Crush/Screen and Mix material according to mix design
  4. Paver lay material
  5. Application of Curing membrane and Sealing grit to provide a temporary running surface
  6. Surfacing


Key Benefits

  • Ex situ processing allowed every load to be inspected for quality enabling a consistent, high quality material to be produced.
  • Sustainable Construction
  • Significantly reduced costs.