Britpave: The British In-Situ Concrete Paving Association

HBM boosts sustainable road construction

Following research published last year and the November 2004 update of the Specification for Highway Works pavement engineers are now in a much better position to use the existing subgrade or pavement materials for insitu or off-site recycling, and to use waste or by-product materials such as pulverized fuel ash (PFA) or granulated blast furnace slag (GBS) as binders alongside the traditional cement, lime or bitumen.

Writing in the Surveyor, Ian Walsh explains that the results from the recent SMART (recycling) project reported in TRL 611 coupled with the new versatile pavement design method reported in TRL 615 mean that engineers can now access a much wider range of materials in the capping layer, subbase and base.
What is particularly new is that the foundation, which is the combination of capping and subbase or subbase alone where no capping is used, can be designed to different strengths or stiffness and the performance checked on site. So now, unlike previously, the stiffer the foundation the thinner the pavement above it


What is also different is that the new hydraulic-bound materials using slow setting, slow hardening binders based on PFA or GBS were investigated in TRL reports 611 and 615 and these are now included in the SHW. An additional benefit from the ’performance’ approach to pavement foundations is that a wider range of permitted materials can be used, including processed highway maintenance arisings and demolition material, and crushed concrete or asphalt millings/planings.

The other development that has proved its worth on reconstruction and development sites in the South East has been the successful stabilisation of clays with lime to make a pseudo-granular material, and then adding cement or other binders to give strengths in excess of the normal capping soaked CBR requirement of 15%. CBR values in excess of 100% have been achieved to produce subbase.
Where the clay is naturally occurring, and the contractor can be satisfied that sulfate will not be a problem, dramatic savings in use of imported granular material can be made. This has a great sustainability benefit in reduction in fuel for transportation, especially in the South East, where naturally occurring hard stone subbase is rarely available locally.

The author makes the point that these materials will enable a local authority to satisfy its best value performance indicators for sustainability because most of these new materials use recycled or secondary materials and less energy in their production.

References
TRL 611 (SMART project).D Merrill, I Carswell & M Nunn. A guide to the use and specification of cold recycled materials for the maintenance of road pavements. TRL 2004 TRL 615. M Nunn. Development of a more versatile approach to flexible and flexiblecomposite pavement design. TRL 2004. Available from www.trl.co.uk

The immediate trafficking of cement bound materials
Reviews the results of a joint Highways Agency/Britpave project to examine the immediate trafficking of a range of cement bound materials. The report identifies those mixtures that can be trafficked early and those that require a curing period. Date 2005; Price £10.00


Soil stabilisation: Guidelines for best practice
Provides guidelines for all involved in soil stabilisation. Pays particular attention to the responsibilities of the parties to the contract, and can be used to assess the information that is required to specify a soil stabilisation project and schedule the detailed testing required. Date 2005; Price £10.00


Stabilisation of sulfate-bearing soils
Explains the mechanisms that cause sulfate heave, before recommending methods for sampling and testing for sulfates and sulfides and describing measures to minimise the risk of sulfate-induced disruption Date 2005; Price £10.00


Stabilised soils as subbase or base for roads and other pavements
Gives guidance on the two stage stabilisation of cohesive soils such as clay using lime followed by cement, pfa and/or ggbs to produce a strengthened subbase or base. Covers specification guidance, site investigation, mix design, construction and testing. Date 2004; Price £10.00

Cement and other hydraulically bound mixtures
Describes the new European Standard, BS EN 14227 issued in November 2004 that covers hydraulically bound mixtures for road and other pavements. Explains how the new mixtures compare with those specified for use before that date, and gives guidance on the selection and specification of the new mixtures. Date 2005, Price £10.00