February 27, 2009

Investigators look at segments of pavement to determine what went wrong on various roadways. Results can be used to improve techniques and technologies.

Ontario Hot Mix Producers Association

OHMPA seminars probe reasons for asphalt failure

IAN HARVEY

correspondent

TORONTO

Call it the Ontario Hot Mix Producers Association version of CSI: Miami. Perhaps CSI: Asphalt.

Those attending one of the organization’s recent seminars got a close up look at some of the detective work which goes into probing why paving fails and the lengths investigators often go to get to the bottom of the problem – literally.

Issues such as poor patch performance, thermal cracking, polishing, pavement segregation, shoving and teasing, rutting and longitudinal deformation are all common issues usually caused by a combination of workmanship, environment and materials, Sandy Brown, the OHMPA engineer explained.

He urged contractors to ensure they picked the right paving grade of asphalt according to the location, and in patch work to make sure they adequately compacted and treated the substrate to ensure there would be no shifting.

Polishing for example, he said, is a dangerous condition which can cause tires to lose traction and is usually caused by the wrong aggregate, while longitudinal distortion and flushing are often the result of the wrong mix for the traffic density and could be a case for superpave if traffic is particularly heavy.

Problems with longitudinal joints can be averted with the MTO’s new handbook on joints, he said, the result of a task group which looked at many of the problems.

Sometimes the mystery goes much deeper, noted Gerry Huber, technical director of the Heritage Group, chairman of the American Association of State Highway and Transportation Officials (AASHTO) expert task force on Recycled Asphalt and a leading forensic expert in pavement failure.

He walked people through the puzzling pavement cracks in a stretch of Colorado highway which defied logic. There were three longitudinal cracks across the centre lane running for several meters but core drill samples failed to produced any real cause.

Investigators decided to cut a slot sample across the width of the lane, encompassing all three cracks, lifting it out in one solid chunk with the assistance of a couple of lag bolts, a chain and a front-end loader.

Even then, results were murky because the mix seemed good throughout the core as was the aggregate.

It wasn’t until they looked at how the pavement was put in place things became clearer. The culprit was the paving machine which required some modifications to the screed and other parts to prevent it happening again.

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