Citation: Alves, R.; Rios, S.; Fortunato, E.; Viana da Fonseca, A.; Guimarães Delgado, B. Mechanical Behaviour of Steel Slag–Rubber Mixtures: Laboratory Assessment. Sustainability 2023, 15, 1563. https:// doi.org/10.3390/su15021563 Academic Editor: Fausto Cavallaro Received: 11 October 2022 Revised: 12 December 2022 Accepted: 6 January 2023 Published: 13 January 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). sustainability Article Mechanical Behaviour of Steel Slag–Rubber Mixtures: Laboratory Assessment Rubens Alves 1,2, * , Sara Rios 1 , Eduardo Fortunato 3 , António Viana da Fonseca 1 and Bruno Guimarães Delgado 1 1 CONSTRUCT-GEO, Faculty of Engineering, University of Porto, 4200-465 Porto, Portugal 2 Federal Institute of Education, Science and Technology of Rio Grande do Norte (IFRN), Natal 59015-300, Brazil 3 Transportation Department, National Laboratory for Civil Engineering (LNEC), 1700-066 Lisboa, Portugal * Correspondence: rubens.alves@ifrn.edu.br Abstract: Slags and rubber from end-of-life tires represent a liability to the steel and tire industry, causing economic and environmental problems that are difficult to manage. Transport infrastructures can use these industrial by-products instead of extracting natural raw materials, but the adequate mechanical performance of the materials needs to be assured. This paper addresses the mechanical behaviour of slag–rubber mixtures in the laboratory with CBR, monotonic and cyclic triaxial tests. In addition, light falling weight deflectometer tests were also performed in a physical model. The results were analysed to meet technical specifications from Brazil, Portugal and Australia using railway sub-ballast layers, capping layers or road pavement layers as the base and sub-base to identify the applicability range of slag–rubber mixtures for transport infrastructures. Concerning the analysed parameters, it was demonstrated that slag–rubber mixtures can show resilient behaviour and strength adequate for the support layers of transport infrastructures provided that the rubber content is below 5% in weight and that the slag is milled to comply with the grain size distribution ranges available in the technical specifications of the cited countries. Keywords: CBR; triaxial tests; resilient modulus; industrial by-products; physical model 1. Introduction In Portugal, electric arc furnace slags have been used as aggregates for construction, being dense, stiff, clean and resistant to abrasion. Laboratory tests, corroborated by full- scale field trials, have demonstrated that these steel slags have better mechanical properties (stiffness and resistance against permanent deformation) than standard base course materi- als. In addition, leaching and lysimeter tests have not revealed any environmental or public health risks [1]. More recently, this material has been studied as an alternative aggregate for railway ballast, demonstrating enhanced performance under the higher loads of heavy-haul trains [2]. This is in agreement with other studies, including both experimental evaluations and numerical modelling of the performance of coarse unbound materials containing steel slag [3,4]. Rubber from end-of-life tires can be reused as a whole or after shredding in differ- ent applications as specified in ASTM D6270 [5], benefiting from its resilient properties. According to Downs et al. [6], scrap rubber tires do not pollute ground water tables. The different applications for slags and rubber contribute to reducing the amount that is landfilled or stockpiled, which has associated economic and environmental costs. Moreover, the use of these industrial by-products in transportation infrastructures avoids the extraction of natural raw materials, benefiting from all the advantages of a circular economy [7,8]. Several research studies have been conducted on the properties of scrap tires when mixed with sand, such as density, compactness, compressibility, stiffness and shear strength. Sustainability 2023, 15, 1563. https://doi.org/10.3390/su15021563 https://www.mdpi.com/journal/sustainability