3.7 Compaction (Density and Moisture)
Compaction is the process by which the air void ratio of the granular material is reduced. There is an optimum moisture content (OMC) at which the maximum dry density (MDD) will be achieved for a particular compactive effort. It is usual to specify the density required for a material as a percentage of that achieved in a laboratory compaction test. The Standard Compaction test produces MDD and OMC values equivalent to field values produced with a medium mass-vibrating roller, and hence some road agencies restrict its use to subgrade. The Modified (heavy) Compaction test produces densities equivalent to those achieved with the use of heavy rollers, and hence some road agencies consider it more appropriate for granular bases and subbases than the
Standard Compaction test. Note that may be difficult to achieve modified compaction of fine-grained base and subbase material which are commonly used on low volume roads in arid areas.
Compaction is achieved in the field through application of a static or vibratory roller of sufficient mass and energy for the particular material.
As discussed previously, it is clear that the compacted layer properties (density, moisture content and degree of anisotropy) have a very high impact on material performance. A high level of compaction will result in a material having high strength, high modulus and low deformation under imposed traffic loading. However, caution must be exercised not to over-compact some materials as high levels of compaction can induce high particle breakdown which can lead to a reduction in strength and an increase in moisture sensitivity.
A high compactive effort is required if the required dense packed structure is to be achieved. Generally, the resistance of a material to compaction depends on internal friction, cohesion, and permeability. As both internal friction and cohesion increase with density, the necessary compactive effort increases as density increases until no further compaction is possible. Permeability is also a factor as air and water can be trapped within the granular mass, and this can prevent the achievement of a higher density with additional rolling. A material exhibiting the required properties with respect to strength, modulus, resistance to deformation and permeability can be difficult to compact and a compromise is sometimes needed between satisfactory materials properties and compactability.
For low-permeability, high-plasticity unbound materials, from which water is not displaced during construction, it is critical that the moisture content at placement is as close as possible to the OMC value if the MDD value is to be achieved. For highly permeable and non-plastic materials, from which water can be readily displaced during construction, the moisture content at placement is not so critical. In both cases, a dry-back period is required to reduce the base moisture content to an acceptable level before sealing. Different specifications of moisture condition at sealing are adopted for different material types to maximise the performance of both the seal and the base.
Other intrinsic and manufactured properties that need to be controlled in a compaction process include the following.