Pavement Priming – The use of a prime or a primer seal is common practise in pavement construction where unbound or lightly bound base material is present. In a similar nature to painting, the use of a prime promotes cohesion between the underlying base/subbase material and the overlying layers.

The importance of priming is particularly relevant in the granular or lightly modified pavement applications with either a spray seal or thin asphalt surface where the effect of shear forces can be significant.

Whilst the term priming is conventional, it typically referred two one of two methods, including:

  1. Prime: The application of a primer binder to the base material which can be easily damaged if trafficked without a protective dry aggregate scattercoat
  2. Primerseal: The application of a primer binder followed by a single size aggregate spread at a nominal spread rate (similar to a spray seal with the addition of cutter)

The benefits of each form of priming are typically associated with a ‘fit for purpose’ approach in which a prime is generally preferred for construction expediency – commonly used on new pavement construction. Whilst a primer seal may be preferred on a road section to be constructed under traffic (i.e. a resurfacing or rehabilitation project) prior to overlaying.

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Another difference between the two is the type and application rate of the primer binder, typically the use of a primerseal warrants a heavier prime (i.e. greater bitumen to cutter ratio) so as to enable the binder to retain the stone under traffic. Whilst alternatively, a prime generally requires a light prime (lesser bitumen to cutter ratio) to facilitate greater penetration of the binder into the base.

Whilst similar in nature to spray sealing, the key factor of priming is to incorporate a cutting agent to temporarily reduce the viscosity of the binder to allow penetration and bonding into the underlying base. In doing so, conventional compositions of primer binder include;

  1. Cutback Bitumen – A blend of conventional bitumen (typically C170) and a cutting agent i.e. Kerosene blended to the desired ratios.
  2. Bitumen Emulsion – The dispersion of bitumen binder and water to create an emulsified solution, applied at the desired blend ratios. Another use of bitumen emulsion has historically been dust suppression.
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Traditional cutback bitumen has been widely accepted throughout the pavement industry and has been coupled with undesirable curing times. In practice, the use of cutback bitumen for priming applications requires a minimum of 5 days and for primer sealing a minimum of 3 months (depending on prevailing weather conditions) for the volatile cutter to evaporate, leaving the bitumen prime. If the overlying material is placed over the prime/Primerseal prior to curing, the volatile material can deteriorate overlying bituminous material.

To overcome this waiting period, the use of bitumen emulsion as a primer binder can limit the curing time of both primes and primerseals to 24 hours (upon favourable weather conditions), however this can often involve additional cost.

For more information on Global Road Technology or Pavement Priming please contact GRT