Bohdan Dołżycki

The RILEM technical committee on Performance-based Asphalt Recycling TC 308-PAR is dedicated to advancing research, promoting knowledge exchange, and disseminating information related to asphalt material recycling. Within this framework, Task Group 1 (TG1) “Performance-based Evaluation of Cold Recycled Asphalt Mixtures” concentrates on aligning laboratory testing methods with the real-world performance of cold recycled asphalt mixtures. To this end, TG1 members collaborated to share the various cold recycling practices adopted in their respective countries. This collaboration aimed to compare specific procedures, technical specifications, and expected performance outcomes. The participants diverse backgrounds, spanning differences in road classifications, traffic loads, climatic conditions, and availability of materials, enabled the development of a comprehensive overview of current methodologies and provided insights into the potential expansion of cold recycling applications. The comparative study of national practices has been structured into two papers: Part I focuses on the constituent materials and mixture composition, while Part II addresses testing protocols and the construction process. In detail, Part II reports a critical comparison of testing procedures, mechanical characteristics, construction methods and quality control procedures used in Italy, Canada, Poland, Brazil and USA, either adopted by selected road administrations or established by reference standards. Accordingly, all initial assumptions, terminology, definitions, and the general context presented in Part I should be considered fully referenced and applicable. This paper focuses on recycling techniques using both cement and bitumen as binders. Conversely, although the use of cement alone is a recognized and widely applied technique, as reported in Part I, Part II does not consider it, since a greater degree of uniformity in procedures and knowledge has already been achieved and shared worldwide.

Hot recycling of asphalt mixtures in mixing plants belongs to methods that enable sustainable production of new hot-mix paving mixtures incorporating material reclaimed through milling of existing pavement layers. Whereas hot recycling of base and binder courses has become a typical solution, application of this technology to wearing courses has not yet been fully explored. Polish motorways and expressways constructed in 2010-2012, whose wearing courses generally comprise stone mastic asphalt (SMA) mixtures with polymer-modified bitumens, are currently nearing the stage of wearing course replacement. Primary issues related to hot recycling of asphalt mixtures include evaluation of reclaimed asphalt (RA) in terms of its quality and suitability for production of new hot-mix SMA mixtures for wearing courses. This pertains both to the composition of the RA material itself and to the quality of the aged binder it contains. The article presents the results of investigations performed under the RID II rSMA2 research project, whose aims included, among others, quality evaluation of 6 different RA materials originating from milling of SMA8 and SMA11 wearing courses. In five cases RA material contained polymer-modified bitumens; in one case RA contained neat paving-grade (unmodified) bitumen. Quality evaluation encompassed determination of RA composition and particle size distribution, as well as assessment of the extracted bitumen and recovered fine asphalt mixture material (FAM – a mixture of bitumen, filler and sand). The obtained results are analyzed in the article. Extracted bitumens were evaluated based on European and American procedures; recovered mineral mixtures were assessed using both “black” and “white” gradation curves. For the first time in Polish literature, FAMs were separated from RA and evaluated.

In recent years, cold recycling using mineral-cement-emulsion (MCE) mixtures has become a common practice in the reconstruction of the old, deteriorated roads. The course made of MCE mixture, similarly to other materials used in pavement engineering, changes its properties with time. While the properties of such mixtures are currently quite well known, the biggest issue of their properties after many years on the pavement, when the layer is subjected to traffic and climate, remains. This article presents an evaluation of the properties of six road sections with an MCE base layer after around 10 years of service life. Sections located on Voivodship Road No. 172 differed in the amount of binding agents (cement/emulsion) used. Results of mechanical properties determined in 2023 in the laboratory on core specimens were compared with properties determined during the construction of the sections. Additionally, the results of the Falling Weight Deflectometer (FWD) test (deflection, deflection bowl, and backcalculated modulus) were analyzed considering the combinations of binding agents. Results and analysis indicated that stiffness moduli did not decrease significantly during ten years of pavement service life; simultaneously, Indirect Tensile Strength ITS increased up to twofold for some sections. The obtained results show that the behavior of the MCE mixtures in the field is not as unambiguous as in the laboratory and needs further research.

Gap-graded asphalt mixtures like stone mastic asphalt (SMA), porous asphalt (PA), and asphalt mixtures for very thin layers (fr. Béton Bitumineuse Très Mince—BBTM) are usually made with the use of SBS (styrene-butadiene-styrene) polymer-modified bitumen. This is a binder that allows one to achieve the required parameters, but at the same time, its use increases the costs of making pavement layers. An alternative to polymer-modified bitumen (SBS) is rubber-modified bitumen. The research presented in this publication includes an assessment of the resistance to permanent deformation and susceptibility to aging of SMA and porous asphalt (PA) mixtures containing both SBS polymer-modified bitumen and rubber-modified bitumen, where the modification process was carried out directly in the refinery. The laboratory tests of resistance to deformation were assessed based on the rutting test and on the assessment of the dynamic modulus (SPT). The changes in the tested asphalt mixtures after aging in laboratory conditions were assessed based on the changes in the stiffness modulus (IT-CY) and the changes in the indirect tensile strength (ITS) after the short-term and long-term aging processes. The presented research results clearly show that the use of rubber-modified bitumen produced in industrial conditions (i.e., in a refinery) allows one to obtain gap-graded mixtures that are as resistant to permanent deformation as mixtures containing SBS polymer-modified bitumen. Similar conclusions resulted from the study of susceptibility to aging. Changes after aging for both types of asphalt mixtures were at a similar level. The presented results clearly indicate that, in the case of gap-graded mixtures such as SMA- and PA-type mixtures, they meet the rutting and aging expectations when either expensive modified bitumen or a cheaper, more environmentally friendly alternative (rubber-modified bitumen) is used.

Deep cold in-place recycling is the most popular method of reuse of existing old and deteriorated asphalt layers of road pavements. In Poland, in most cases, the Mineral-Cement-Emulsion mixture technology is used, but there are also applications combining foamed bitumen and cement. Mineral-Cement-Emulsion mixtures contain two different binding agents – cement as well as asphalt from the asphalt emulsion. Asphalt creates asphalt bonding (responsible for flexible behaviour), whereas cement generates hydraulic bonds (responsible for stiffness of the layer). Final properties of Mineral- Cement-Emulsion mixtures are a result of a combination of both binding agents. While the stiffness of the material is unaffected by asphalt bonding, an increase in stiffness with time is visible for materials treated with hydraulic binders. This publication presents the change in stiffness modulus of Mineral-Cement-Emulsion mixtures by Simple Performance Test. For the analysis, two batches of specimens were used: the first was tested 28 days after compaction (period stated in Polish recommendations) and the second was tested 1.5 years after compaction. Analysis showed that after 1.5 years the stiffness modulus increased by about 10% in comparison to the 28 days after compaction. The change is minor but still significant. Unexpectedly, the level of the increase was unaffected by the combination of the binding agents (asphalt emulsion, cement).