Recycled asphalt as sub-base in the A2/M2 Cobham to Junction 4 Widening A21

Asphalt, planed from redundant carriageways within the works area, was crushed ready for re-use on-site as unbound sub-base for the new road.

Being the main contractor for the design-and-build of the A2/M2 works Costain Skanska Mowlem Joint Venture (CSMjv) was the principal champion behind the use of asphalt arisings, although scheme designers WS Atkins also provided strong encouragement.

No specific financial calculations were made at an early stage, but CSMjv was aware that a considerable saving was possible if the use of asphalt arisings as Type 4 sub-base was permitted.

The direct costs of these operations, and the savings made when these costs are compared against those of using comparable primary material, are shown in the table below.

The £5.50 per tonne for the recycled material included:

• the cost of planing (milling) the redundant carriageway;
• stockpiling if necessary;
• adding water to achieve correct moisture content, prior to laying and compaction;
• transport by road wagons (£2 per tonne); and,
• material compliance and compaction testing (approximately £0.20 per tonne).

The approximate average market cost, and the local market cost, of the equivalent primary material would have been £8.82 per tonne, plus £1.60 per tonne for the Aggregates Levy (totalling £10.42).

This therefore represented total direct savings of £4.92 per tonne.

Savings

There were no indirect savings.

Costs

Instead of realising indirect cost savings, this case study has actually shown indirect costs (although not overall net costs). This is because CSMjv would have sold the surplus asphalt arisings to the local market had they not re-used them. Had the material been sold it would have been valued at £2 per tonne; which, for the 130,000 tonnes, results in £260,000 in lost revenue.

Other indirect benefits

Other indirect benefits included:

• a significant reduction in local traffic congestion due to fewer HGV deliveries;
• a reduction in local air pollution and noise due to fewer HGV deliveries;
• a reduction in traffic impacts of transportation of material away from the site; and,
• promotion of CSMjv’s environmental credentials.

By re-using 130,000 tonnes of asphalt arisings CSMjv significantly reduced the number of lorry movements on local roads. Had this not been done, primary material would have to have been imported from supplier stockpiles at Northfleet and Chatham. Deliveries are normally made in 20 tonne units, and a total of approximately 6,500 two-way lorry journeys were therefore avoided.

In terms of learning-from-experience CSMjv would have been able to programme works more effectively and avoid the need for stockpiling if they had, at the start of the project in 1999, known that the use of asphalt arisings as Type 4 sub-base would be permitted. The re-use of asphalt arisings is now included in Specifications for Highway Works (May 2001), so CSMjv will use it in future.

Technical issues were the most important material factors at the early specification stage, because before CSMjv were allowed to use the asphalt arisings as Type 4 sub-base they had to demonstrate, via construction of trial areas and testing, that site-won asphalt arisings:

• were compliant with the Specifications for Highway Works (May 2000 Clause 806) Type 4 sub-base grading;
• when subjected to a 1,000 standard axle trafficking trial deformed by less than 30mm;
• produced a surface California Bearing Ratio (CBR) greater than 32%; and,
• produced a foundation of greater stiffness than an identical Type 1 sub-base area.

CSMjv’s contract and design were based on the Specifications for Highway Works (March 1998), and therefore the use of asphalt arisings as Type 4 sub-base required Highways Agency approval.

The Type 4 sub-base complies with the Specification for Highways Works, Clause 806.