Tack Coat/Bond Coat Asphalt Treatments

Managers seeking to make an impact look for ideas with leverage, in other words, high payoff for the amount of effort expanded.   Bond coat is one of those ideas.   A recent study of hot mix bid tabs revealed that bond coat typically cost 1% of the total pavement costs, but a bond coat failure could cost 100% of the pavement cost, much more if you consider the cost of removal and replacement plus the time and money tied up in construction zones.  One study estimated that even a small decrease in bond strength 10 to 30 % could result in 50 to 70% loss in the fatigue life of the pavement.  For such a small cost offsetting substantial possible negative consequences, bond coat is a sound investment and doing it well requires relatively little effort.  It just makes good sense.  

Research backs the promise that correctly applied bond coat can significantly improve the lifespan of the surface layer of pavement.  Roffe and Chaignon reported that if a pavement displayed no bonding within its layers, a 60% loss of life could be expected. Similarly, Brown and Brunton reported that no bonding would cause a 75% reduction in pavement life, and at 70% bond strength, a 70% reduction in pavement life could occur. Moreover, King and May reported that with only a 10% loss of bond, a 50% reduction in fatigue life would be expected.

Costs will vary depending on the bond coat material and application technology chosen.  In general, emulsions will be the least expensive, hot paving grade asphalts the next most, and cutbacks the most expensive.  

Slow setting emulsions are generally a commodity product in most markets and will tend to be the least expensive.  RS emulsions will tend to be more expensive because of more expensive ingredients. They will offer better breaking performance than slow setting emulsions.  Non tracking or reduced tracking bond coats are the most expensive.  Non tracking bond coats are not yet widely available and their components are more expensive than other emulsions. They do offer rapid break times and more bond coat remains in place where it should be creating a superior bond. 

Spray Paver technology is relatively new to the market and is not yet widely available.  Generally, it will be more expensive than a traditional process.  Additionally the emulsion used by the spray paver is more expensive than commodity emulsions.  However, because of the placement technology, a superior emulsion can be used and the paver technology can guarantee that no one can drive in the material before the hotmix is placed.  This should lead to the best possible bond between the pavements.  

• Make bond coat a pay item in the specification
• Contact local emulsion manufacture to discuss the best possible emulsions available for bond coats
• Make sure the distributor is ready to do the job correctly
• Spray bar at the right height - triple overlap
  • The distributor should have computer controlled application rate
  • Nozzles should be the correct size and all the same
  • Nozzle angle set to 15 to 30 degrees and all the same, end nozzels may vary
  • All nozzles open all the time
• Inspect that good practice is carried out on the project consistently.