Neil Leake, national technical manager at Holcim UK, explores the value of recycled asphalt pavement (RAP) and argues that uptake is essential for reliable network maintenance.

Global floor area – the total usable internal floor space of buildings worldwide - is expected to double by 2060; that's the equivalent of building New York City every month for the next 40 years. 

This staggering undertaking demands vast quantities of natural resources not only for new construction but also for maintaining existing infrastructure. Indeed, 69% of the 238.4 million tonnes (Mt) of aggregates supplied in 2023 was primary material, illustrating the strong dependence on natural assets for the continued development of our national infrastructure.  

As of 2023, permitted mineral reserves stood at 5,106Mt. This rate of extraction raises serious, long-term questions for the security of our national supplies, as well as the impact of large-scale construction on natural environments. In short, there's a desperate need to move away from an overreliance on primary materials and towards circularity across the sector. 

The case for RAP

RAP has an integral part to play in making sustainable construction a reality, being one of the few construction materials that is 100% reusable. It can be added to new asphalt mixes, where the aggregates and bituminous binder perform their original functions.

Encouragingly, advancements in R&D and technology, coupled with investments in asphalt plants, are increasing the use of RAP within the construction industry, with current standards (PD 6691, Table 1) permitting up to 20% to be used in surface courses and up to 50% in base/binder courses. 

To ensure pavement longevity, it should, however, be observed that rigorous quality control is essential. RAP must maintain a verified penetration of >15 to prevent performance degradation. Furthermore, exceeding 10% RAP in surface layers, or 20% in lower layers, triggers mandatory testing for aggregate properties, including polished stone value and aggregate abrasion value. 

The move from 10-20% thresholds to the 20-50% allowed by PD 6691 represents a huge shift in binder management. As mentioned, the >15 verified penetration requirement is the ‘red line' for preventing premature fatigue cracking. However, as RAP percentages climb, the industry is increasingly investing in advancements in bitumen and blending, ensuring that the aged and new bitumen actually ‘talk' to each other, rather than merely coating the aggregate.

Since performance and durability are the ultimate drivers of carbon and cost reduction, the integrity of the final product cannot be compromised. To ensure long-term stability, a robust quality control system is crucial. This requires rigorous monitoring at two critical stages: the characterisation of incoming RAP, and the performance testing of the finished asphalt.

CRBM has a role to play

Another tried and tested method is using cold recycled bound materials (CRBM) – such as Holcim's Foamix low carbon recycled asphalt solution – which is engineered primarily from RAP, crushed to a 20mm grading, and mixed using a cold process. Combining foamed bitumen with hydraulic binders like cement or lime, they create a high-performance base and/or binder course. Notably, CRBMs use up to 85% RAP, reduce carbon emissions by 50% compared to warm mix asphalt (WMA) and provide a sustainable way to encapsulate hazardous site arisings. 

What's more, CRBMs are a proven solution that aligns with the Specification for Highway Works (SHW) Clause 948, as well as the latest Manual of Contract Documents for Highways Works (MCHW) standards. They are also compliant with the SHW's CD 226 standards for traffic loadings up to 80 million standard axles. 

Unlocking RAP's true potential

We must work differently to extract the most value from RAP.

While it's true that asphalt is already recycled in large volumes, significant challenges remain: with underinvestment in material technology, as well as legacy plant networks and working practices, serving to threaten RAP's continued growth across the construction industry.

By investing in process improvements and innovative materials, we can unlock the full circularity potential of our existing roads, ensuring a durable, sustainable network that supports our economy and urban infrastructure development.