Importance of compaction


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 amount of compaction energy applied; and
  • The amount of moisture in the material.

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 thin dotted curve utilised “standard Proctor” compaction energy of 600 kN-m/m² that reflects the rollers available in 1933 when Ralph Proctor, the “father of compaction”, introduced the test.
  • The solid line is the “modified Proctor” from 1958 that applies 2700 kN-m/m² of energy, reflecting the improvement in compaction equipment.
  • The thick dashed line represents the compaction energy applied by a modern heavy roller that applies energy far in excess of 2 700 kN-m/m².

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.

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