## 3 Challenges for Using Waste Materials in Pavements  
Although there are many advantages to using waste materials in pavement construction, a large initial investment is necessary to develop infrastructure for sorting the materials based on type, source and risk. In addition, waste materials should be processed to meet the requirements for use in pavements. The use of waste materials requiring advanced technology to process may appear unappealing at first glance. For instance, using waste materials incurs costs through necessitating the construction of facilities and training workforces, however, the cost of pavement construction and rehabilitation decrease in the long-term.  
The potential difficulties of using waste materials in pavement construction can be illustrated by considering the recycling process of glass as an example. There are optical sensors that detect and sort waste glass based on colour [40], and since glass colour is related to its chemical composition, the durability of the resulting asphalt and concrete mix can be affected. A potential solution to such logistical issues could be investment by private sectors in the development of recycling infrastructure based on public-private partnerships. Additionally, contractors and pavement technologists could be incentivised to use waste materials through tax exemption. Such policies have been very successful in encouraging sustainable production in China [54]. Furthermore, pavement engineers, material technologists and paving crews should be trained to use different types of waste materials for pavement construction. To facilitate this, detailed technical guidance and practice codes should be provided. Although such technical codes were created for the use of RAP, there are no standardised practice codes for the use of waste glass, plastic or blast furnace slag.  
To produce more environmentally friendly asphalt and concrete mixes, it is recommended to use different combinations of waste materials. However, the long-term performance of such pavements is a cause for concern. To address this issue, optimal combinations of various waste materials should be determined through laboratory experimentation. A key factor in determining such a balance is the compatibility between different waste materials types. In other words, synergistic effects between the waste materials, binder and aggregate materials is desirable.  
Another challenge for using waste material is the potential production of leachate, which can be hazardous for eco-systems. Therefore, the chemical composition of leachates should be investigated.  
There are conflicting reports and research results on the composition of leachate from pavement made using waste materials, and further investigation is required.  
In addition, a pertinent question lies in how many times the waste materials can be recycled and reused for pavement construction. The appropriate number of uses depends on the type of waste material, its chemical components, energy feedstock, environmental emissions and economic justifications. A definitive answer that encompasses every composition is impossible. Therefore, it is necessary for various waste materials to be ranked based on their potential for recyclability. Further research is necessary to establish a multi-variable criterion for ranking different waste materials.