Recycling
The adjustment of the regulation on the avoidance and disposal of waste (VVEA; SR 814.600, 2015) increasingly restricts the landfill options for removed asphalt. This legal development promotes the use of recycled asphalt (Reclaimed Asphalt Pavement, RAP) and contributes to closing material cycles, reducing waste, and conserving resources in road construction. At the same time, a clear trend towards higher recycling rates in asphalt mixtures is evident in Switzerland and abroad. While the current standards allow RAP shares of 20 to 80% depending on the layer type and load class, current developments aim to expand this to 100% for lower and middle layers (VSS, 2022a).
The recycling of SDA pavements is gaining increasing importance in Switzerland, especially given the large number of pavements that have reached the end of their lifespan. Although initial practical experiences, such as in Le Landeron, are promising, standardized procedures and strategies for managing SDA-RAP are still lacking. The specific grain composition and high durability requirements present central challenges.
In the long term, a structured plan for the collection, processing, and reuse of SDA-RAP is necessary to fully exploit the existing ecological and technical potentials and to enable the transition to a circular asphalt economy.
Ecological assessment of RAP use
Several life cycle assessment studies demonstrate the significant ecological benefits of using RAP. Kytzia and Pohl (2021) show that using 50% RAP in the surface layer and 60% in the binder layer can reduce environmental impact (expressed in environmental impact points, UBP) by 20–35% compared to new construction without RAP. This reduction includes the phases of construction, deconstruction, and maintenance. Compared to other environmentally relevant measures – such as lowering the production temperature (low-temperature asphalts) or adding alternative materials like used tires or slags – RAP achieves a significantly higher ecological effect (Poulikakos et al., 20191 ; Mikhailenko et al., 20202 ; Piao et al., 20223 ).
The essential environmental impact arises from the reuse of the bitumen-containing portion, which is particularly resource-intensive. Liechti et al. (2016)4 considered a potentially increased burden from polycyclic aromatic hydrocarbons (PAHs) in old pavements in their life cycle analysis (LCA). Even under this assumption, the ecological benefits of using RAP outweigh the potential environmental risks.
Technological developments and processes
To maximize the RAP content in the mixture, many mixing plants continuously optimize their production processes. In addition to in-house processing, the so-called In-Place Recycling is gaining importance, where the removed asphalt is processed and reused directly on site. This process, which often involves the addition of rejuvenators, is already being successfully applied abroad (Hafeez, Ozer, and Al-Qadi, 2014)5 .
Requirements and challenges in SDA pavements
The application of RAP in open-graded noise-reducing pavements presents special requirements for material quality and grain size distribution. In particular, the narrow sieve curves, the low maximum grain size (e.g., for SDA 4), and the low filler content complicate the addition of RAP. Therefore, a pure, layer-wise removal and optimized storage and processing in the mixing plant are prerequisites for use in high-quality SDA pavements.
If the obtained RAP does not meet the requirements for an SDA surface layer, it can – especially in PmB-containing formulations – be reused in the binder layer. In Switzerland, the recycling of SDA pavements is still in its infancy. Although the potential benefits are scientifically proven, extensive practical experiences and standardized procedures are still lacking.
Current research and practical examples
A central Swiss example is the test section realized in 2022 in Le Landeron (Canton Neuchâtel), where an SDA 4-16 was produced with 10% RAP. Three variants were investigated: a mixture with conventional bitumen, one with PmB, and one with rubber granulate (RmA). The results of the water sensitivity test according to EN 12697-12 showed equivalent or slightly improved ITSR values for the RAP-containing mixtures compared to the reference pavement. Only the Marshall void content (HR %) was lower in the rubber-added mixtures, which is attributed to the filling effect of the rubber particles or an overcompensation of the bitumen content.
The CPX measurements in the first year of operation showed that both the conventional and RAP-containing SDA sections exhibited good acoustic performance. Only the variants with rubber addition did not reach the typical ≤ 6 dB(A) for SDA 4-16, which could correlate with the different void contents. Overall, it was shown that a RAP content of 10% had no significant negative effects on mechanical or acoustic performance.
Alternative aggregates and life cycle assessment aspects
In addition to RAP, recent studies have investigated other secondary materials in SDA. The use of rubber (0.7–1.0 wt%) in the dry process led to comparable mechanical properties as conventional SDA and potentially allows for the omission of polymer-modified bitumen (Bueno et al., 20216 ). Recycled concrete aggregates (RCA) and electric arc furnace slag (EAFS) have also been successfully tested (Mikhailenko et al., 20227 ; Mikhailenko, Piao, and Poulikakos, 2023)8 . With a replacement share of up to 15 vol.-%, the mechanical performance and durability remained unchanged. However, life cycle assessment showed that RCA and rubber only provide minor improvements, while the use of EAFS offers the greatest ecological potential due to reduced landfilling (Piao et al., 2023)9 .
BioBitumen and BioAsphalt
Fact sheet BioBitumen and BioAsphalt Weibel AG
Bitumen, which is traditionally obtained from petroleum, causes high CO₂ emissions and requires a lot of energy in production and processing. To decarbonize road construction, biobased alternatives are being developed.
BioBitumen consists of plant residues such as cashew nut shells and natural resins. It causes only about one-third of the emissions of petroleum bitumen and can be climate-neutral or even CO₂-negative through biogenic CO₂ storage.
Tests by Weibel AG show that BioBitumen can be processed, recycled, and compacted better at lower temperatures with existing facilities. The main disadvantages are currently higher costs and limited raw material availability. Nevertheless, BioBitumen is considered an important step towards environmentally friendly, future-proof road construction.