In this article, University of KwaZulu-Natal Sustainable Transportation Research group head Professor Mohamed Mostafa writes about the growing use of sustainable materials in road construction projects.

Amid the dynamic realm of modern roads and infrastructure development, a significant paradigm shift is taking place. This transformative journey is anchored in the increasing prominence of sustainable materials, which represents a departure from conventional practices. As societies and industries increasingly embrace ecological responsibility and long-term viability, the ascendancy of sustainable materials in road construction represents a critical milestone.

INNOVATION DRIVEN BY SUSTAINABILITY
While traditional road construction methods have been effective, the pressing need for environmental sustainability has driven the emergence of innovative solutions. Today, governments, research institutions and civil engineering practitioners are collaborating to reimagine road construction through the lens of eco-friendliness and durability. This trend is now evolving in South Africa, with the South African National Roads Agency (Sanral), a pioneer of these initiatives through its research panels.

This paradigm shift is well-rooted in the growing awareness of resource depletion, carbon emissions and climate change. Sustainable materials such as recycled aggregates, bio-based binders and alternative asphalt mixtures are taking centre stage globally, offering promising solutions for mitigating environmental impact while maintaining road performance standards.

AN ECONOMIC IMPERATIVE
The adoption of sustainable materials in road construction is backed by a strong economic rationale. As governments grapple with budget constraints and increasing costs of traditional raw materials, using recycled and eco-friendly alternatives often proves cost-effective. Research published by MDPI on the recycling of construction demolition waste in concrete underscores the potential financial savings achieved by incorporating recycled materials into road construction. Furthermore, industries recognise the strategic advantages of aligning with sustainability trends, enhancing brand reputation and fostering social goodwill. However, challenges remain, including material compatibility, performance predictability and regulatory frameworks. In Africa, the challenges are no different from the global picture, however, the lack of legislation to enforce large-scale recycling poses a significant hindrance.

Among the many pioneering facets of sustainable road construction, a compelling innovation that has gained significant attention is the use of recycled aggregates to create robust roadways. The process involves sourcing and reprocessing materials such as reclaimed asphalt pavement (RAP) and crushed concrete to create recycled aggregates. These aggregates offer various benefits, ranging from reduced environmental impact to enhanced pavement performance, notably, by diverting waste from landfills and decreasing the demand for virgin aggregates, incorporating recycled aggregates aligns seamlessly with circular economy principles. Additionally, recycled aggregates reduce the carbon footprint associated with road construction.

MEETING THE RIGHT STANDARDS
While integrating recycled aggregates in road construction holds immense promise, challenges and considerations must be navigated. Quality control and proper processing methods are paramount to ensure the resulting recycled aggregates meet the required performance standards. Moreover, the variability in source materials and the regional availability of suitable recycled aggregates necessitate a tailored approach for each construction project.

As the push for sustainable road construction gains momentum, the spotlight turns to the realm of binders, where bio-based and recycled bitumen are emerging as transformative alternatives. Bio-based binders, derived from renewable resources such as plant oils and recycled bitumen from reclaimed asphalt materials, are at the forefront of this paradigm shift. Their promise lies in not only mitigating carbon emissions but also in fostering greater resilience within road networks.

A study into sustainable green pavement using bio-based polyurethane binder in tunnels, published by MDPI, have highlighted the environmentally friendly attributes of bio-based binders, showcasing their potential to reduce greenhouse gas emissions during production and application. Additionally, bio-based binders can demonstrate improved ageing resistance, enhancing pavement longevity and minimising maintenance requirements.

However, the transition towards green binders is not without its complexities. Challenges such as binder performance consistency, compatibility with existing infrastructure, and market availability must be navigated. Standardised testing protocols and specifications for these alternative binders are critical to ensure that road performance remains reliable and consistent. These are gradually gaining recognition in various regions and countries like the United States and several European nations have initiated efforts to develop and implement these standards. More efforts are needed locally to drive this change.

The adoption of sustainable alternatives to conventional asphalt entails a multifaceted approach that spans mixture design, among other factors. Alternative mixes such as warm mix asphalt (WMA) and cold mix asphalt offer distinct advantages in terms of energy consumption and emissions reduction.

SHAPING FUTURE PRACTICES
The insights gained from case studies of sustainable road projects play a crucial role in shaping future practices. These case studies provide a valuable opportunity to learn from real-world experiences, uncover best practices and anticipate challenges that arise when implementing innovative solutions.

Case studies are the living embodiments of the theoretical concepts discussed within the realm of sustainable road construction. In London, for example, bottom ash, a byproduct of waste-to-energy plants, has been incorporated into road construction. This sustainable approach not only reduces the environmental burden of waste disposal but also contributes to enhanced pavement performance. In the United States, recycled glass has been successfully used in asphalt pavements. The glass particles provide benefits such as improved skid resistance and reduced thermal cracking.

In South Africa, trials are taking place locally on a small scale to incorporate recycled materials. Notably, small sections of roads in Jeffreys Bay (Eastern Cape), Cliffdale (KwaZulu-Natal), and Tshwane (Gauteng) have successfully integrated recycled plastic. Additionally, Hazira (KwaZulu-Natal) used steel slag, Centurion (Gauteng) employed glass, Durban (KwaZulu-Natal) experimented with fly ash, and Cape Town (Western Cape) used crumb rubber in their road construction projects.

These pioneering trials demonstrate the feasibility of incorporating alternative materials into our road infrastructure. To expand and refine these practices, we can draw inspiration from international experiences and invest further in local research efforts. Several universities, including the University of KwaZulu-Natal (UKZN), the University of Pretoria (UP), and Stellenbosch University (SUN), are actively engaged in researching sustainable road materials. Collaborative efforts between academia and industry can drive innovation and pave the way for more environmentally friendly road construction practices in South Africa.

In conclusion, there is a need to emphasise the importance of robust project planning and stakeholder coordination when incorporating sustainable materials into road construction. Additionally, case studies shed light on the positive role of collaboration between governmental agencies, private sector partners, and research institutions.