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In highway and airfield pavements, aggregates are used in various types of unbound or bound materials (standard nomenclature for pavements as used in the United Kingdom is illustrated in Fig. 10.1). This chapter is concerned with both primary (naturally occurring) and secondary (artificial or recycled) aggregates which are not bound by cementitious or bituminous binders. Unbound layers are used in the UK mainly for sub-bases or capping, but elsewhere may be used for bases or, in the case of low volume roads, the whole structure. Cement bound aggregates are discussed in Chapter 8 and bitumen and tar bound aggregates are discussed in Chapter 11. Figure 10.2 shows bitumen macadam being laid over unbound sub-base.

Unbound layers in pavement construction may fulfil some or all of the following functions:

  1. (a)

    a working platform for construction;

  2. (b)

    a structural layer (load spreading and resistance to rutting);

  3. (c)

    a replacement for frost-susceptible subgrade (if necessary);

  4. (d)

    a drainage layer.

The main use in the UK is as sub-base for which (a) is probably the most important function. Guidance is given in Powell et al. (1984).

The ability to spread load (high stiffness) and to resist rutting (low permanent deformation) is usually associated with closely graded materials whereas open gradings are thought necessary for good drainage. The apparent conflict between these requirements has received considerable attention over recent years (Roy 1981; Jones & Jones 1989). These and other aspects of the

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