Specifications for the construction of new pavement layers using unbound or stabilised material will always include a minimum density requirement. Achieving this density requirement is of primary importance because the performance of a pavement layer under load is largely dictated by the density of the material in the layer.
The density / strength relationship for a typical granular material is shown in Figure 1 below, which plots density against the California Bearing Ratio (CBR), an indicator of the material’s strength. Using this graph as an example, if a design engineer was to specify a density requirement of 95% of the Maximum Dry Density (MDD) for a subbase layer constructed from this material, the CBR value assumed in the design would be > 25%. If this layer is subsequently compacted to only 93% of MDD during construction, the CBR actually achieved will be < 20%, implying that the strength of the layer will be less than that assumed in the design and premature failures can be expected.
Two variables play a major role in determining the density achieved when a material is compacted:
The moisture / density relationship of a material is of paramount importance because it defines the maximum dry density (MDD) and optimum moisture content (OMC) of the material subjected to a specific amount of compaction energy. The figure below shows the moisture / density relationship for a material subjected to three different levels of compaction energy:
The increase in MDD / reduction in OMC resulting from an increase in compaction energy is fundamental to achieving a specific target density.
In practice, the modified Proctor test (or an alternative, depending on the test method adopted by the relevant Road Authority) is carried out on a representative sample of the material that will be used to construct a pavement layer. The MDD thus determined becomes the “reference density” for specifying the target density for the material in the layer (e.g. 95% of MDD). The OMC provides a guideline for controlling the moisture content during construction (normally ± 70% of OMC using modern compaction equipment).
Utilising a roller that provides sufficient energy is therefore critical, as is controlling the moisture content of the material. The companion note entitled “The Importance of Compaction, Part Two: Roller Selection” provides guidelines for choosing appropriate rollers for a specific job.