Editorial: Special Issue on Recycled Materials for Sustainable Geotechnical Construction
Authors: Cetin Bora , Edil Tuncer B. 
Published: December 2023

Abstract
Infrastructure serves a vital role in resolving some of the major challenges faced by societies: supporting economic growth, meeting basic needs of citizens, and facilitating mobility and social interaction. Establishing a long-term development and maintenance plan is a critical priority for many nations. This motivates the design and construction of more resilient and sustainable infrastructure systems that are also able to overcome emerging challenges, particularly given the extreme weather conditions that have recently led to several large-scale failures on these systems. Collectively, these investments require the use of large volumes of earthen materials in construction. This has led to the rapid depletion of natural aggregate materials. Recycled materials have been used in a variety of construction applications, for example, roadways (e.g., embankment, base, subbase, asphalt, and concrete), airfields, and railways, among others. Such materials are attractive to construction and transportation industry stakeholders and taxpayers because they generate millions of dollars in savings while producing environmental benefits. Legislations are being developed/promulgated that remove barriers to large-scale beneficial use of these materials to reduce construction costs and increase sustainability. In addition, the construction industry, including the transportation systems sector, is facing ever-increasing demands to control the cost and environmental impact of waste removal and disposal. The use of recycled materials in construction would reduce energy consumption and emission of greenhouse gases, and recycling provides a means to address these demands. Recycled materials avoid energy and emissions associated with mining and processing construction materials. Energy has already been expended in the first life of recycled material. The use of recycled materials allows for the avoidance of natural materials such as sand and gravel, limestone, and oil. Often, service life can be increased by stabilization using industrial by-products as binders such as self-cementing fly ash, waste gypsum, and other ashes. Waste reuse/recycling practices can be incorporated in new construction projects, renovation/rehabilitation projects, and ongoing operations, and they can be applied to both agency and contractor activities. The evaluation of recycled materials used in construction applications has been the main focus of scientific studies over the last few decades. However, the effectiveness of waste reuse/recycling efforts in such applications still could not be implemented fully because of the lack of comprehensive reviews and evaluations of the limitations and advantages of recycled materials used in construction applications. Therefore, the collection and critical review of such information is necessary. This special issue of ASTM International’s Geotechnical Testing Journal brings together 23 papers that cover the use of a broad range of recycled materials in construction applications including steel slag, rice husk ash, ferrochrome slag, recycled concrete aggregate, recycled asphalt pavement, lignin, recycled binders, recycled plastics, and quarry fines. Moreover, the papers in this issue provide state-of-the-art information on geotechnics of recycled materials, including their material characterization, performance evaluations, and environmental suitability assessment. The field has evolved in terms of new material types and applications that must be evaluated. A variety of laboratory tests are adapted from the standards for natural earthen materials, sometimes with modifications, and there is material for the development of new testing protocols and standards. ASTM standards are particularly needed for sample preparation of recycled materials. There were not many papers on in situ testing, but an increase is expected to be seen in this area in future studies. Thus, the need continues for future investigations to increase our understanding of recycled materials that are a crucial element of sustainable construction. The editors wish to express their appreciation to all those authors who submitted their papers. Particular thanks are extended to those who contributed papers and to the reviewers of the papers and also to the publication staff in producing this special issue.