With the “Design of recycled asphalt pavement with recycled glass”, our partner COMSA presents a methodology for designing asphalt pavements in which some of the natural sand is replaced by recycled glass. This is interesting as it addresses climate adaptation and circular economy in road construction in Mediterranean cities, in this case Barcelona.

This result addresses the need to integrate climate adaptation and circular economy into everyday road maintenance. Mediterranean cities such as Barcelona regularly resurface their streets, but local administrations often lack practical tools to include recycled materials such as glass in asphalt mixes while still ensuring adequate mechanical performance, safety and acceptable thermal behaviour under climate change and urban heat island conditions. As a result, resurfacing works usually follow conventional specifications, missing opportunities to reduce the use of natural resources, valorise waste and help mitigate heat accumulation in dense urban areas.
To fill this gap, a design and assessment methodology has been developed within MULTICLIMACT to create and evaluate an asphalt wearing course in which part of the natural sand is replaced by crushed recycled glass.

The finally chosen recycled glass has been selected for its consistent quality, colourless appearance, and its origin from 100% pre-consumer industrial scrap. Its soda-lime composition, primarily silica (70–80% by weight), with sodium oxide (12–18%), calcium oxide (9–12%), magnesium oxide (3–6%), and minor alumina (0.5–1.5%), that ensures the expected clarity, durability, and stable melt behaviour of standard clear glass. The material’s composition also provides good thermal mass, helping to moderate temperature fluctuations and contributing to the absorption of excess urban heat under summer conditions. These combined characteristics make it a robust and well-suited choice for the intended application.

In practical terms, the core outcome is a 5 cm AC16 D Surf mix with 50/70 bitumen, in which around 25% of the fine fraction is substituted by the glass. The methodology combines laboratory mix design and testing (granulometry, Marshall, water sensitivity and rutting), thermal modelling and full-scale prototypes. Three 2 × 2 m outdoor slabs were built and monitored, comparing a reference mix with two glass-containing mixes, and the concept is now being transferred to a pilot street section in carrer Carles Pirozzini (Barcelona), divided into three zones (reference mix, slag mix and glass mix).

At this stage, the tool takes the form of technical reports, validated mix recipes, performance indicators and monitoring protocols. In the next step, these elements will be translated into a practical design guideline and a simple parametric calculator, which could be hosted online or embedded in the MULTICLIMACT toolkit to make the solution easily accessible to other cities and road authorities.

The main users are municipal road departments, pavement and geotechnical engineers, and asphalt producers, as well as urban climate and resilience planners who need realistic, buildable materials when assessing adaptation strategies.

By providing ready-to-use mix designs with documented structural and thermal performance, the tool helps users include recycled glass asphalt in standard resurfacing contracts with a low implementation risk, improving resource efficiency and supporting moderate reduction in surface temperatures and heat storage compared to conventional mixes.

The solution is being tested in the Spanish demo site through the monitored slabs and the pilot street application, where temperature sensors, visual inspections and functional surveys will provide feedback on constructability, durability and comfort. This real-world validation will be used to refine the method and confirm its transferability to other urban contexts.