The increased use of recycled and secondary aggregates in highway construction is a key policy goal of the Highways Agency’s Environmental Strategic Plan, "Towards a balance with nature"
The following examples show where recent developments in the Specifications for Highway Works (SHW) facilitate the uptake of recovered materials:Reclaimed Bituminous Materials – Clause 902
"Reclaimed bituminous material may be used in the production of bituminous surface course, binder course and base. The maximum amount of reclaimed bituminous material permitted shall be 10% in surface course and 50% in all other layers."
The new clause allows 10% recycling without invoking the Clause. Additional performance requirements are required when the recycled content exceeds 25% by mass. The Clause has allowed manufacturers to adjust their feedstocks and optimise production and performance within their existing manufacturing plant.
The inclusion of 10% reclaimed content appears to be the norm (except for proprietary thin surfacings) and most suppliers do not exceed 30% recycled content due to manufacturing restraints within the plant.
However, some modern mixing plants, typically in the US, are now capable of handling 70% recycled content and cold/warm bitumen emulsion mix technology can incorporate up to 100% recycled aggregate content. This can be used in surface, base and binder courses with in-situ/ex-situ processing techniques. Unbound, Hydraulically Bound and Other Materials – 800 series
- Clause 803 (Type 1 sub-base)and Clause 804 (Type 2 sub-base) allow a 100% recycled and secondary aggregate content. The revised clauses also allow for the use of Recycled Asphalt Planings and waste (RAP) of up to 100% recycled content (within defined parameters), up to 50% recycled asphalt content in other mixtures, and for 1% content of other materials such as plastic and wood.
- Clause 805, Type 3 sub-base is open-graded and restricts the use of asphalt recyclate content for engineering reasons.
- Clause 809, Slag Bound Mixture is a granular aggregate (which includes crushed concrete and slag) bound by granulated blast furnace slag.
Highways Authorities Utilities Committee (HAUC) reinstatement specifications simply require sub-base to comply with Clause 803 of the SHW. Now that Clause 803 has been amended to allow recycled material, no special agreements are needed to use recycled sub-base in HAUC applications. Aggregates For Use In Cement Bound Applications – 1000 series
Materials used may include recycled aggregate, crushed concrete, blast furnace slag and pulverised fuel ash. There are some restrictions on the sourcing of such aggregates due to quality compliance and conformity requirements. Drainage Aggregates – 500 series
Aggregates for pipe bedding, surround to pipes, trench backfill and filter drains, include the use of recycled aggregate and recycled concrete aggregate within quality and conformity requirements. Earthworks – 600 series
Recycled aggregates and waste streams can be used in the formation of earthworks. These include Pulverised Fuel Ash (PFA), furnace bottom ash, unburnt colliery spoil and crushed/recycled aggregates.
Otherwise unsuitable material can be incorporated using lime or cement stabilisation. However, it does not make economic sense to use “high quality recycled aggregates” as fill material when higher value applications are available. In-situ Material Re-use SHW Clauses
For asphalt, in-situ recycling is generally introduced by SHW Clause 926, and foamed bitumen (SHW Clause 948) has made particular gains in popularity in recent years.
Other in-situ techniques include the crushing of concrete slabs to produce granular sub-base Types 1 and 2 (SHW Clauses 803 and 804), aggregate for concrete or cement bound material (SHW Clauses 1035 - 1038 or 1046), and crack and seat (SHW Clause 1047). Note:
- As a companion document to the SHW, the Design Manual for Roads and Bridges (DMRB) has a similar impact on the use of recycled and secondary materials.
- Design guidance was provided in 1995 (HD35/95) and has been updated in HD35/04.
- The published European standards for hydraulic bound materials are in the final stages of preparation and have increased the scope of materials usage that was already available with cement-bound materials (CBM1–CBM5).
- The DMRB is currently being updated to take advantage of the opportunities presented by this development.