The sub-base layer is very important in terms of the expected performance of any pavement structure. The sub-base is often the main load carrying layer of the pavement, as opposed to the base, and is designed to distribute the stresses and strains exerted from passing traffic loads down to the foundation. This distribution needs to be replicated in any reinstatement. Crushed aggregates are generally recommended for sub-base materials. There are two types of material which can be used in the sub-base layers: unbound granular materials and hydraulic bound materials (HBMs), the latter being a commonly used material for pavement sub-bases and bases. HBMs are defined as mixtures that set and harden by hydraulic reaction. They include cement bound materials (i.e. mixtures based on the fast setting and hardening characteristics of cement) and slow setting and hardening mixtures made from industrial by-products such as fly ash (FA) and ground granulated blast furnace slag (GGBS). The introduction of recycled and secondary aggregates (RSA) in HBM design has reduced the need to use primary materials, with demand for RSA increasing due to environmental and sustainability concerns. The preferred material for pavement applications is the slow setting HBMs. Although fast setting HBMs show good strength after initial construction, they often experience thermal stress cracks, due to shrinkage during the curing period, which are seen in the asphalt surface due to reflective cracking. The slower setting and hardening HBMs, with a reduced tendency to crack, lead to less reflective cracking and thus the possibility of reduced asphalt overlay thickness. The performance of HBMs can be monitored by extracting cores for analysis, as well as by using the Falling Weight Deflectometer to determine the stiffness of the HBM layers. Although current literature indicates that in general pavement construction and in some utility trenches, HBM materials have shown to perform well, there are concerns regarding its use in pavements that are below current design standards. Further research of HBM applications and adjacent pavement performance would need to be gathered and analysed in order to attain a better understanding of what materials and techniques work best. This further research could include pavement trials.

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