Citation: Sapkota, K.; Yaghoubi, E.; Wasantha, P.L.P.; Van Staden, R.; Fragomeni, S. Mechanical Characteristics and Durability of HMA Made of Recycled Aggregates. Sustainability 2023, 15, 5594. https:// doi.org/10.3390/su15065594 Academic Editors: Allex E. Alvarez and Vladimir Merchan Received: 22 February 2023 Revised: 16 March 2023 Accepted: 18 March 2023 Published: 22 March 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 Characteristics and Durability of HMA Made of Recycled Aggregates Kiran Sapkota 1 , Ehsan Yaghoubi 2, * , P. L. P. Wasantha 2 , Rudi Van Staden 2 and Sam Fragomeni 2 1 Institute for Sustainability Industries and Liveable Cities, Victoria University, Melbourne, VIC 3011, Australia 2 College of Sport, Health & Engineering, Victoria University, Melbourne, VIC 3011, Australia * Correspondence: ehsan.yaghoubi@vu.edu.au; Tel.: +61-3-9919-4804 Abstract: The application of recycled aggregates in the asphalt industry has been investigated in recent decades. However, low percentages of these materials have practically been used in asphalt mixtures because of the limitations set by the relevant specifications due to their performance uncertainties. This research investigates the feasibility of increasing the percentage of recycled aggregates to 100% in hot mix asphalt (HMA). Recycled concrete aggregate (RCA), recycled glass (RG), and reclaimed asphalt pavement (RAP) were used to develop HMAs suitable for roads with light to medium traffic. First, potential mix designs were proposed using an innovative approach considering the industry’s needs. Next, the volumetric properties, tensile strength, moisture sensitivity and resilient modulus response of the mixtures under different temperature conditions were determined and compared. In general, the proposed recycled material HMA exhibited superior mechanical and resilient modulus performances, i.e., 45 to 145% increase in stiffness, and up to 99% higher in Marshall stability. Furthermore, higher tensile strength ratios of the recycled material mixtures indicated a greater resistance to water damage, and hence greater durability. The findings of this research provide evidence-based insights into the increased proportion of recycled materials in the construction of asphalt pavements, thereby promoting sustainable pavement construction materials. Keywords: green asphalt; recycled aggregates; indirect tensile modulus; moisture sensitivity; sustainable pavements 1. Introduction During the construction of pavement, the following six broad phases occur: raw material production, mixing, transportation, laying of materials, compaction and the curing phase. Out of the mentioned phases, the highest amount of greenhouse gas emission is produced during the mixing phase, followed by the raw material production phase [1]. The raw material production phase includes sourcing aggregates, primarily natural aggregates, and bitumen required for the production of asphalt. Hot mix asphalt (HMA) typically consists of 90 to 95% mineral aggregates by weight and 75 to 85% by volume [2]. The gradation and physical composition of the mineral aggregate determine the mixture’s hardness, durability, and stripping potential [3]. The surface course aggregates play a critical role in the short- and long-term performance of the asphalt mixture as they occupy a substantial proportion of the HMA. Similarly, bitumen’s role in an asphalt mixture is vital in bonding aggregates of different sizes [4]. The properties of the used aggregates dictate the quality and performance of pavement structural layers [5]. The aggregates used in the asphalt mixture for the wearing course (or surface course), and intermediate course (or binder course) in flexible pavements, preferably exhibit high internal friction, low water absorption (and hence, more effective bitumen content), low Los Angeles (LA) abrasion loss, and higher strength to achieve a cost-effective and high-quality mixture. The extraction and processing of natural aggregate are expensive and associated with environmental impacts, although they generally meet the Sustainability 2023, 15, 5594. https://doi.org/10.3390/su15065594 https://www.mdpi.com/journal/sustainability