BS EN Hydraulically Bound Material for Carriageway Reconstruction

The project, located on Canning Road, involved approximately 539.4m² of Type 2 carriageway construction on a heavily trafficked bus route. The objective was to strengthen the existing failed carriageway structure while working within strict budget constraints and maximising reuse of materials already on site.

The Engineering Challenge

The existing carriageway was found to be structurally inconsistent, with sections of asphalt laid directly onto Type 1 material. This construction was unsuitable for frequent heavy bus traffic and had resulted in premature failure of the 150mm bituminous base layer.

Although full reconstruction in line with original design drawings was recommended, budget limitations meant a complete dig-out, disposal and rebuild was not feasible. There was an existing thick layer of Type 1 granular material in situ, and the client wished to retain as much of this material as possible.

The challenge was therefore to create a structurally compliant carriageway capable of supporting heavy traffic, without incurring the cost and environmental impact of excavating, tipping and importing new aggregates.

The SMR PreMixd Solution

SMR UK introduced SMR PreMixd, supplied via our approved hub-site partner, where recycled and site-won materials are processed using SMR technology to create high-performance structural mixes.

SMR PreMixd is classified as a BS EN Hydraulically Bound Material (HBM) and is suitable for use in accordance with the Specification for Highway Works (SHW) 800 Series. Due to the exceptional compressive strengths achieved, it can be used as a structural base layer within carriageway construction.

For this scheme, the existing Type 1 layer was retained and re-compacted to act as the sub-base. A 150mm layer of SMR PreMixd was then installed as the new roadbase, beneath the binder and surface course layers.

The material was laid in two layers and compacted using a single drum roller, with increased rolling passes to achieve optimal density and performance.

Performance Results

Immediate testing following compaction demonstrated strong structural performance across the site.

Initial compaction achieved an average CBR of 36%, increasing to an average of 50% across the full carriageway area upon completion. The material was ready for surfacing immediately after compaction and will continue to gain strength over a 28-day curing period.

The result was a high-strength, compliant base layer capable of supporting sustained heavy bus loading.

Circular Economy and Missed Opportunity

This project successfully maximised reuse of existing in-situ material; however, it also highlighted the value of early engagement.

Had SMR PreMixd been specified at design stage, it could have been used not only as the 150mm base layer but also within the sub-base construction. This would have enabled reuse of excavated spoil across multiple structural layers, removing the need to excavate, tip and import virgin Type 1 aggregate.

Early adoption would have delivered even greater cost savings, reduced vehicle movements and lower carbon impact — fully aligning with circular economy principles.

Project Outcome

By working as a specialised subcontractor, SMR UK supported the main contractor and Harrow Council in delivering a cost-effective, structurally compliant and sustainable carriageway solution.

The Canning Road project demonstrates that BS EN Hydraulically Bound Materials such as SMR PreMixd can be confidently used within SHW 800 Series applications as a structural base layer in full-depth highway construction.

This case study reinforces how innovative material reuse solutions can strengthen highway assets, reduce waste and deliver long-term performance — without the need for full excavation and replacement.