ARCHIVED – Motorists on the move with latest IRC research v2n4-13

Volume 2, Number 4, Spring/Summer 1997

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Motorists and municipalities alike stand to benefit from IRC's research into why pavement crack sealants fail. The discovery that the use of a hot-air lance in the crack-repair process can shorten the life of the repair will encourage the use of alternative methods, ultimately reducing traffic obstruction and road-repair budgets.

Sealing pavement cracks is known to be cheaper than applying a new coating of asphalt; however, the failure rate of sealants can approach 60 percent, frustrating both drivers and taxpayers. IRC's research into this high failure rate led it to examine the role of the hot-air lance in sealant adhesion.

In regions with severe winters, crack sealing is often preceded by routing, cleaning and heating with the hot-air lance. Despite uncertain evidence of the lance's effectiveness, the belief that it improved sealant adhesion has led to its wide-spread use in crack-sealing work. IRC researchers, however, have now produced concrete evidence that using the lance in areas where crack sealants are exposed to cold winter temperatures may not be advantageous and may, in fact, cause premature sealant failure.

The secret lies in the fact that the sealant needs to remain flexible at sub-zero temperatures. IRC research conducted in 1995 showed that heating a sealant can cause the more volatile components, on which its flexibility depends, to break down resulting in a loss in elasticity, tensile strength and mechanical strength.


Workers use a hot-air lance to seal pavement cracks.

This discovery focused recent research on the effects of the hot-air lance on sealants. Prior to IRC's work, little had been done to try to understand how the lance affects sealant adhesion to routed joint faces; as well, little was known about the high application temperatures to which these routs are subjected.

IRC researchers began by monitoring the use of the hot-air lance in the field, working alongside three crack-sealing crews and determining the temperatures at the surface of a rout being treated. They reproduced the observed effects on asphalt-concrete pavements by using an automated hot-air lance in a series of laboratory experiments.

They (researchers) found that if the sealant itself is correctly selected... , the lance becomes unnecessary because it does not increase sealant adhesion.

In those experiments, researchers compared the adhesion strength of three sealants applied to dry unheated, heated and overheated asphalt-concrete substrates that had been prepared with quartz or limestone aggregates.

They found that if the sealant itself is correctly selected for cold-temperature conditions and remains elastic, the lance becomes unnecessary because it does not increase sealant adhesion. Furthermore, if the wrong sealant is selected, and is prone to becoming rigid at low temperatures, the use of the lance will actually accelerate debonding.

IRC research therefore suggests that the hot-air lance not be used in crack-sealing work when the pavement is dry. While it can be useful for removing excess moisture from a wet crack surface, its disadvantages in heating dry routs mean that simply cleaning the rout with a blast of compressed air may be just as effective, if not more so, in the long term.

This IRC research has the potential to produce longer-lasting, better-quality crack repairs: a boon to municipalities, repair crews, and drivers.

For further information, please contact Dr. Jean-François Masson at (613) 993-2144, fax (613) 954-5984, or e-mail jean-francois.masson@nrc-cnrc.gc.ca