The use of in situ recyclers to rehabilitate distressed pavements is increasing worldwide for three reasons:

  • Speed of construction.  The fast-track nature of in situ recycling reduces construction time and minimises traffic disruption. 
  • Environmental benefits that accrue from reusing material from the existing pavement, mainly due to the drastic reduction in haulage associated with road construction.
  • Significant reduction in cost, especially when utilising the latest developments in material stabilisation.

Constructing pavement layers using material recycled in situ from the existing pavement is, however, very different from constructing new pavements in multiple layers.  This is primarily due to:

  • in situ recycled layers are normally much thicker than those in new pavement structures;
  • material recycled from the upper horizon of the existing pavement is never homogeneous; and
  • the support provided by the material beneath the recycled layer can vary significantly.

The following sections explain the approach required to both set and achieve target densities when constructing new layers using the in situ recycling process.

Variability of the recycled material

Most distressed pavements are associated with old roads that have been subjected to numerous maintenance interventions over the years, including localised patches, asphalt overlays and reseals.  It is therefore not surprising that the material recovered by recycling the upper horizon will include a range of different materials over the length of the road (and even over short sections).

From a density perspective, it is important to recognise that any change in the material will result in a different moisture/density relationship and this will change the maximum dry density (MDD) that is used as the reference density for controlling compaction.  Thus, specifying a density requirement for the material in a newly recycled pavement layer (e.g. 100% of MDD) implies that the MDD needs to be determined for each and every density test.

Variability of support beneath the recycled Layer

Unlike constructing a new pavement in a series of layers where the quality of each layer is checked prior to constructing the overlying layer, layers of in situ recycled material reside on whatever is below the recycled horizon.  This means that the new layer must be compacted on the underlying pavement structure which is seldom visible and is assumed to have adequate strength.  This is generally a valid assumption because old pavements have withstood millions of repeated axle loads that have consolidated the material, increasing the density and the effective strength. 

This is, however, not always true.  Any weakness in the underlying pavement structure will prevent the recycled material from being compacted to a high level of density.  This scenario leads to arguments when the density target is not met and raises the question as to whether such a density target is appropriate for the existing conditions.

Intelligent compaction

An Intelligent Compaction system fitted to the primary roller that operates immediately behind the recycler is a useful tool for dealing with variability.  These systems use an accelerometer mounted on the frame of the roller to measure the response of the pavement to the applied compaction force, based on the premise that there is a direct relationship between the magnitude of response and the density of the material.  The roller makes multiple passes, measuring the response on each pass.  As the density increases, the magnitude of response increases, initially by a significant amount, reducing with each subsequent pass.  When no further increase in response is measured, it can be concluded that the density has reached the maximum achievable under prevailing conditions (sometimes referred to as the “refusal density”).

The pictures below show the basic principles of the system.  A roller fitted with a GPS antenna is following behind the recycler compacting the material in the cut width.  A computer in the cab shows a map of the section being compacted and uses different colours on the strip to show the operator:

  • density is increasing from the previous pass (shown in blue);
  • density is the same as the previous pass (shown in yellow); or
  • density is reducing from the previous pass (normally shown in red).

The roller makes several passes travelling forwards and reversing on the section being compacted and continues to do so until the majority of the section is yellow.  This indicates that maximum density has been reached and to continue to roll will result in material losing density due to breakdown.

It is, however, important to appreciate that, in addition to density, there are other factors that influence the magnitude of the measured response.  These include:

  • The amount and uniformity of moisture in the material being compacted.  Dry material will indicate a relatively low refusal density;
  • roller operation.  Changes in the speed of advance will affect the measurement due to the vector effect.  In addition, it is vital that the travel path adheres to the strip of recycled material behind the recycler and does not wander onto un-recycled or previously recycled/compacted material on either side; and
  • the accuracy of the system used to compare the responses measured from multiple roller passes.

It must also be appreciated that this system cannot detect the development of a bridge of high density overlying un-compacted material at the base of the layer.  It is therefore of paramount importance to ensure that the roller meets the minimum static mass requirement, that the vibration settings are appropriate and that the moisture content of the material is uniform and sufficient.

Intelligent Compaction systems should not be used as the sole arbiter for density control.  Their primary function is to improve the efficiency of the compaction process by indicating when refusal density has been achieved as well as indicating where there are possible problems in the existing pavement due to “soft spots”.  The main benefit to accrue from using Intelligent Compaction is the plethora of data collected whilst compacting.  In addition to the total number of roller passes made over each square metre that is compacted, the following information is recorded for each pass:

  • response measurement (in units specific to the system employed);
  • roller speed of advance (in m/min);
  • vibration characteristics (amplitude and frequency); and
  • the location of each measurement (GPS coordinate).

Where a traditional density test (e.g. sand replacement test) indicates a failure (i.e. insufficient density achieved), the first step is to check that an appropriate reference density was applied (i.e. that the MDD was determined from a sample obtained from that specific location).  Thereafter, data collected by the Intelligent Compaction system can be interrogated to ensure that sufficient compaction effort was applied and if so, to indicate the probable cause of a low-density measurement.

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