Recycled and alternative materials are finding increased use in the construction of road pavements and embankments. Environmental concerns are leading to constraints on quarrying of the materials that have, conventionally, been used while tax incentives and legislative limitations are encouraging the uptake of wastes, by-products and recycled elements in their place. These materials do not, necessarily, behave in the same manner in the presence of water as many conventional materials do. Self-cementation due to pozzolanic activity may help to stabilise some, others may exhibit undesirable leaching, yet others may have much higher permeability than the material they replace. As a consequence, it is important for the road designer or manager to understand these characteristics and their implication for the hydrological and environmental performance of a highway that incorporates such materials. Relying on experience alone is likely to be insufficient.
It is very common for regulators and potential users to express concern about leaching when construction with such materials is proposed — yet this is a concern that often has no basis in fact! Many alternative materials come from an industrial process in which some chemical has been involved which no-one would wish to become widely distributed in the environment. Thus, environmental regulators often show particular concerns about materials deriving from metal processing industries — e. g. slags, foundry sands, etc. However, their real impact depends not on the actual content of the chemical of concern in the solid but on its availability to pore fluids, its solubility and its transportability. Many alternative materials have been through a hot process which vitrifies the solids making it extremely difficult for chemicals, now held in a glass-like matrix, to leave the solid phase. Alternatively, the pH level in-situ may render the contaminant essentially non-soluble. Chapters 6 and 12 discuss these issues further.
