Pavement Priming – Prime vs Primer seal

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: is an The application of a primerbinder to the prepared granular base material which can be easily damaged if trafficked without a protective dry aggregate scattercoat it usually consists of a bitumen and cutter oil, or specially formulated bitumen emulsion primer and is placed without a cover aggregate. Examples include AMCOO, AMCO and AMC1.
2. Primerseal: is an application of primerbinder sprayed onto a prepared crushed rock or gravel pavement surface and covered with a layer of aggregate. 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) It allows for immediate trafficking and permits a delay in placing of the final surfacing. Examples include AMC3, AMC4 and AMC5

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. 

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. Higher wetting of the aggregate by the prime allows excellent contact, adhesion and moisture resistance. Whilst alternatively, a prime generally requires a light prime (less bitumen to cutter ratio) to facilitate greater penetration of the binder into the base. Emulsion particle size should be small enough to allow the prime to have good penetration into the capillary size pores of the compacted granular base. 

In general, the benefits to applying a primerseal to an existing granular surface include providing a temporary waterproof and sealing to protect from water ingression. Ability to consolidate and bind the surface material to preserve the profile of the road, promote adhesion between existing surfaces and subsequent pavement applications such as surface treatments and binding of fugitive dust particles on the roadway surface. 

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. They utilize solvents as a carrier for the bituminous binder. The solvent confers low viscosity, facilitates wetting and coating of the aggregate with the residual binder. Known to be expensive due to scarcity and high cost of petroleum solvents, toxic and environmentally unfriendly owing to high flashpoint volatile organic compounds released into the atmosphere on evaporation. 
2. Bitumen Emulsion – The dispersion of a neat or polymer-modified bituminous bitumen binder and aqueous solution of surface active agent water to create an emulsified solution, applied at the desired blend ratios. Water can be used as an effective carrier for bituminous binders. Preference for bitumen emulsions is due to them being safer, cleaner, cheaper and better performing than cutback bitumen. Another use of bitumen emulsion has historically been dust suppression, although this is now considered poor practice due to safety and environmental concerns.

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. The future for priming is emulsion technology driven in being able to reduce delays to paving operations and seal operations which saves the final construction costs. Chemically, the reduction in hydrocarbon solvent emitted will be important for environmental purposes with hope that this will not take place at the detriment of pavement penetration and strength. The critical properties of emulsion based prime would be low viscosity for surface penetration, adhesion to enable coating of fine particles in the process sealing surface pores and bonding between pavement layers. The storage stability requirements involve periodic agitation to avoid phase separation and settlement of bitumen in the continuous aqueous phase. 

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REFERENCES 

Kucharek, A.S., Davidson, J.K., and Kennedy, M. 2008. Development of a Solvent-Free Asphalt Emulsion for Prime Coats and Granular Sealing. Canadian Technical Asphalt Association. 

Pickering, J. 2009. Best practice in spray seal application. Fulton Hogan.