THE IMPORTANCE OF COMPACTION. PART 2: ROLLER SELECTION

Roller selection:

Thanks to on-going research and development in compaction technology, layers of granular and stabilised material are usually compacted using single-drum vibrating rollers.  Pneumatic tyred rollers (PTRs) are used extensively to compact layers of hot mix asphalt but confined to thin layers of fine-grained sandy material when used to compact granular material.

The following are all important factors that influence particle packing when selecting a specific single drum vibrating roller for a specific job:

  • The thickness of the layer being compacted.
  • The relative coarseness of the material in the layer.
  • The static mass of the vibrating roller.
  • The type of drum fitted to the roller (smooth or padfoot).

The figure below* summarises the relationship between the above factors when selecting an appropriate roller.

  • the top horizontal axis indicates the thickness of the layer (after compaction, in mm);
  • the vertical axis indicates the relative coarseness of the material in the layer (ranging from fine at the top to coarse at the bottom);
  • the bottom horizontal axis indicates the static mass of the roller required to compact the material;
  • three zones in different colours indicate the recommended type of roller for these conditions (yellow for pneumatic tyred rollers (PTR) to compact thin layers of fine material, blue for smooth drum vibrating rollers and green for padfoot vibrating rollers.

Thus, to compact a 200mm thick layer of relatively coarse material, this graph indicates that a vibrating padfoot roller with a minimum static mass of 15 tons should be applied.

USING SMOOTH DRUM VIBRATING ROLLERS

Smooth drum rollers are ideal for compacting thinner layers where the material is relatively fine.  As shown in the adjacent figure, the roller applies compaction energy at the surface, spread uniformly across the width of the drum.  This creates a zone of high density in the upper horizon, requiring multiple passes of the roller for the energy to penetrate through to the underlying material.  When insufficient energy is applied (too few rollers passes), the result is a density gradient with the upper horizon at a higher density than the lower.  (For this reason, it is recommended to measure the density of the full layer thickness and, independently, the upper half of the layer.)

RISK OF BRIDGING

Problems arise when the roller employed does not have sufficient mass relative to the layer thickness and coarseness of the material being compacted.  The energy applied compacts the upper horizon to the extent that it forms a “bridge” over the underlying material, resulting in an un-compacted (or poorly compacted) lower horizon that will subsequently, consolidate under repeated traffic loads.

 

USING PADFOOT VIBRATING ROLLERS

Provided the roller has sufficient static mass, padfoot drums prevent bridging by building density in the lower horizon of the layer first, and then the upper (the so-called “walking out” phenomenon).  The upper portion of the layer is disturbed by the pad imprints, thus preventing a horizon of high-density material from forming.  Once the roller has walked out, the upper horizon can then be shaped by a grader and sufficient water added so that it can be compacted using a relatively small smooth drum roller (only the upper horizon of the layer needs to be compacted).

OTHER FACTORS THAT INFLUENCE DENSITY DEVELOPMENT

In addition to selecting an appropriate roller for the job, the following factors also influence the density achieved:

Vibration characteristics (amplitude and frequency).

Thick layers require high amplitude vibration to assist in penetrating the energy downwards into the material.  To prevent “over compaction” (breaking up the layer), low amplitude is applied when compacting thin layers.  If transverse shear cracks develop when using a smooth drum vibrating roller to compact a layer of relatively fine material, the amplitude should be reduced and vibration applied when rolling in one direction only (the direction that minimises cracking).

The amount and uniformity of moisture in the material. Water is the lubricating agent that promotes particle packing when compaction energy is applied.

  • If the moisture content is too low (e.g. < 50% of OMC), density requirements will only be achieved by increasing the amount of compaction energy applied (e.g. heavier roller and/or additional roller passes).
  • If the moisture content is too high (e.g. > OMC) then density cannot be achieved because, as the density increases, the voids will fill with water that acts as a hydraulic medium to prevent particle packing.
  • If the moisture content is not uniform, the density achieved will vary.  The material should therefore be thoroughly pre-mixed with water prior to placing and compaction.  This is best achieved using a recycler.

The advance speed of the roller.

The frequency of vibration and the advance speed of the roller dictate the distance between “strikes” applied to the layer.  The frequency of vibration is normally pre-set to ±30Hz which means that the speed of advance largely dictates the strike interval.  As a general rule, vibrating rollers should maintain a constant speed of advance of 2.5km/hr (40m/min).

Variability of support provided by the underlying material.

The support provided by the underlying pavement provides the “anvil” on which to compact the new layer. Where the support is good (e.g. a cement stabilised layer), a high level of density can be achieved with ease. However, if the support is weak, a high density cannot be achieved regardless of the amount of compaction energy applied.

Such a scenario is seldom encountered when constructing a new pavement because the density specifications for subsequent layers ensure sufficient support. This is not the case when rehabilitating an existing road using the in situ recycling process and is covered in the companion note entitled “The Importance of Compaction, Part 3. Compacting Layers of In Situ Recycled Material”.

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