Using Construction and Demolition Waste Recycled Aggregate as a Sustainable Solution in Hydraulic Bound Mixtures and Composite Pavements

Abstract

The growth in worldwide waste production has resulted in severe negative effects on the environment, with continuing disposal of items causing long-term health and environmental challenges. Construction and demolition (C&D) waste comprises 25–30% of total waste in the European Union. In Saudi Arabia, it has emerged as a substantial waste stream after the huge increase in C&D waste amount led by the government mega redevelopment projects which required the demolition of all slum districts within the country . Many countries aim to increase the amount of recyclable material and achieve a sustainable waste disposal system. For example, Saudi Arabia’s 2030 Vision aims to improve various sections, including infrastructure, industrial lands, and research, and provide important ways to build and enhance sustainable roads and infrastructural systems. HBM plays a crucial role in road infrastructure, offering a sustainable and cost-effective solution for various pavement applications. It can be used in road foundations as a stronger alternative to traditional unbound sub-base in flexible pavement or as a replacement for the unbound sub-base layer or the asphalt base layer in a composite pavement. The research aims to investigate the possibility of using C&D waste as a replacement for natural aggregate to produce sustainable hydraulic bound mixtures (HBM) and use them in road construction applications as a base layer in composite pavement. To achieve the research aims, the research is divided into different phases which are summarised here. The first phase aims to study the physical properties of C&D waste to prepare and produce a recycled aggregate (RA) and then, understand the differences in physical properties between the recycled and natural aggregates (NA). In this phase, the study employed three aggregate types, namely two RAs (Crushed Concrete (CC) and Construction and Demolition (C&D)) and one NA (limestone). The second phase includes designing and preparing a range of HBMs and then investigating their mechanical properties. The results from this phase show that, using RA instead of NA resulted in a substantial decrease in compressive strength by 45 % to more than 70 %. However, although NA-HBM presents better mechanical performance, the compressive strength values for RA-HBM were in the range for base and subbase design for HBMs in pavements, which are 5-20 MPa and 2–10 MPa for base and subbase design respectively. The third phase includes developing a methodology to study composite pavement behaviour under wheel load in the laboratory which includes designing and acquiring a new mould to study the fatigue cracking and the distress in the HBM layer in composite pavements and then developing a KENLAYER simulation model to simulate the wheel track test. The result after arranging the wheel tracking test indicates an evident degradation in all HBMs but also a higher and more significant degradation in RA-samples compared to NA-samples. The compressive strength measurement showed significant deterioration among all HBMs, with the greatest reduction for M3 which contain C&D RA by around 37% , around 17.5% for M2 which contain CC RA and only 3% for M1 which contain NA. Also, the deflection measurements indicate a degradation in all HBMs but higher and more significant in M3 than M2 and M1, with average deflection measurements of around 897, 642, and 382 µm. Moreover, the visual crack examination made it evident that the difference between HBM bases had a major effect on the crack growth in the HBM samples, most significant for M3 and M2 and less noticeable for M1. On the other hand, The KENLAYER simulation models show that there were only minor variations in the initial transverse strain calculation for the majority of strain gauges and the simulation model's calculation of elastic modulus showed significant deterioration in all HBMs, with the greatest elastic modulus reduction for RA-samples around 32% and 20% for M3 and M2 respectively and less reduction in NA-samples around 5% only for M1. The low cycle count in the test had a significant negative impact on the life prediction for HBMs in the KENLAYER simulation models, especially in the NA-sample. However, RA-samples show efficient and appropriate results, with life predictions that outperformed those predicted using the Australian models. The last phase includes the sustainability assessment including economic and environmental assessment for all HBMs and for different composite pavements that contain NA and RA and have the same functional work. The result from this phase indicates that the most negative impact on the sustainability assessment in HBM was caused by the presence of limestone NA as an aggregate and cement as a binder while, in pavement assessment, the asphalt materials show the worst sustainability impact. The total costs were reduced by 48% to 70% when limestone NA was replaced with RA in HBM while it was reduced by 23% to 34% when flexible and composite pavements were compared. Also , the embodied energy (EE) values decreased by 69% to 80% when limestone NA was replaced with RA in HBM while it decreased by 28% to 43% when flexible and composite pavements were compared. Moreover, the embodied carbon (EC) values decreased by 48% to 70% when limestone NA was replaced with RA in HBM while, it decreased by 2% to 10% when flexible was compared with the majority of composite pavements with C&D RA in their HBM. To sum up, the research project indicates that, using C&D RA rather than NA in HBM for use in road construction is considered to be a durable and sustainable solution that can solve many sustainability problems around the world.