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Improving the technical properties of recycled aggregates for road construction

Solomon Adomako (photo)

The findings contribute to maximizing sustainable use of resources in Norway and may help end -users with increased understanding of handling techniques and the use of recycled aggregates from excavation masses.

Solomon Adomako

PhD Candidate

Solomon Adomako will defend his PhD thesis Improving the technical properties of recycled aggregates for road construction 18 October 2023.

Summary of the thesis

Sustainable construction, operation and maintenance of road infrastructure are currently of high priority in Norway. The development of the highway standards and specifications (e.g., N200) plays an important role, so that optimized use of recycled excavation materials and crushed concrete is achieved in the sub-base and base layers.

Generally, factors that hinder the use of EM include geological complexity (i.e., composition of mechanically weak rocks), the absence of declaration policies, and a standard framework to characterize the testing frequency of general properties. Considering their application in permitted quantities in the base layer, it is expected that compliance with the mechanical performance, that is, Los Angeles (LA) and micro-Deval (MD) and other geometric properties designed for conventional materials will be met. In addition, given the related geological complexity, it is essential to identify the effects of chemical and mineralogical features, as this may enhance the opportunity to classify geological variations and optimize the performance by mixing the masses with other materials.

In this study, the first approach involved the identification of knowledge gaps pertaining to the relationship between the local geology and the mechanical performance (LA and MD) of the aggregates. Considering this, state of the art studies on the geological influences on the properties of rock aggregates were per-formed. It was demonstrated that the global synergy of the influence of weathering on geological parameters (mineralogy, grain size and crystal size, grain shape, and porosity) and the extent to which they affect the overall performance remain important criteria in material selection. This pertains to excavation materials given the large geological make-up and may help end-users to understand the different roles played by mineralogy and other textural properties.

Second, this study presents a documented baseline for the mechanical stability of excavation materials and assesses their mineralogical and chemical influences. The approach consisted of mixing protocols, such as intermixing with other materials as a mechanical stabilizing technique. The experimental protocol in-volved LA and MD tests and repeated load triaxial tests (RLTT). Generally, in the cases of LA and MD, the local excavation materials met the current technical criteria for the base layer. However, if weak rocks, such as phyllite, make up the composition, a limited target of ≤ 40% would be tolerable. The findings on the mineralogical influence demonstrated that phyllosilicates (mica and chlorite) pre-dominantly contributed to the low mechanical performances. In addition, it was demonstrated that if excavation materials are potentially composed of soluble materials such as masonry rubble, careful consideration should be given to the mixing level.

Regarding the stiffness and deformation properties, as analyzed by Hicks and Monismith’s model and Uzan’s model, respectively, it was shown that as the bulk stress increased excavation materials produced higher stiffness response than phyllites. At the intermix level, the stiffness was higher at 25% substitution by phyllites compared to 50%. Hence, the performance was sensitive to the increased phyllite content. On the other hand, the permanent deformation behavior did not show significant variations despite the reported values of the degree of mobilized friction angle ρ(°), and incremental friction angle φ(°) which describe mobilized and maximum shear strength, respectively. These findings provide valuable input to future guidelines in predicting the bearing capacities for roads constructed with recycled materials.

Finally, this study demonstrated the extended performance of recycled aggregates derived from concrete sludge by the Re-Con Zero dry washing technology (RCZ) applied as feedstock in wet recycling of excavation materials to increase material circularity. The results showed a general trend of increased LA and MD with increased feeding of RCZ. The acid solubility results indicated that cement paste remained on the particles after the wet recycling process and parts of the cement paste was size reduced (< 1.6 mm) in the LA-testing process. The content of chemical species of potential concern was generally low and complied with Norwegian waste regulations. In addition, the low Cr(VI) content indicates low leaching of Cr upon carbonation and a decrease in pH when used under real conditions.

These findings contribute to maximizing sustainable use of resources in Norway and may help end -users with increased understanding of handling techniques and the use of recycled aggregates from excavation masses.

Find more about time and place for the doctoral defense.