Longitudinal joints critical to length of pavement life

Getting to the bottom of longitudinal joints will help lengthen the lifecycle of pavement, says a presenter at the annual technical seminar of the Ontario Hot Mix Producers Association.

Mark Buncher of the Asphalt Institute presented the findings of a study he’s been working on due for publication next month.

“This wasn’t a high-budget research project,” he said. “But it was produced in co-operation with the FHWA (Federal Highway Administration) and the Asphalt Institute to develop best practices around longitudinal joints. Hopefully it will spur more research, which is always good.”

Why longitudinal joints need research in the first place also struck him as a bit of a puzzler, he admitted.

“When I looked at the literature, I found there’s a lot we don’t know, at least when it comes to opinions on best practices.”

One doesn’t have to go far to find examples of longitudinal joints, he said. A drive down almost any highways will usually turn up evidence of long cracks or patches where a failed joint has been milled out and redone.

“The black eye for this industry is long joints, in my opinion,” said Buncher, pointing to a series of headlines maligning SuperPave projects when in fact it was a joint failure.

“You end up where the mat is in good, but the joint deteriorates.”

As part of the study Buncher looked at the available literature and surveyed both jurisdictions and 19 top-rated paving companies.

What they found was that half of the 51 owners surveyed were unhappy with the overall quality of their long joints. Some two-thirds have a long joint spec, he said, and they were split between a density spec and a methodology spec.

What was most telling was the data around the air voids in the density of the joint. The more air — even just a point or two compared with the density of the mat — the more deterioration could be expected.

If the joint was 88 per cent of the mat’s density then there would likely be a 33 per cent loss of pavement life.

The closer the joint density was to the mat density, the more the robust the pavement would likely be and the longer it would last, he said.

Indeed, variances of just a couple of points had dramatic effect on overall pavement life.

Good joint performance comes out at a density of 97 per cent of that while fair performances run at 93 per cent to 97 per cent and drop off quickly after that.

The question then, he said, was how best to build a longitudinal joint? Even among the 19 experts surveyed, opinions differed.

Some preferred butt over wedge joints, others preferred using a “fine” gradation.

However, he said, there were key points all agreed on such as getting the first pass laser straight and using a string line to make sure.

“Again they also agreed the vibratory screed should always be one,” he said adding quality control is a must at all stages. The use of surface sealers and joint heaters is also recommended, he said.