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Minnesota Test Nukes Potholes

By August 9, 2011 July 8th, 2014 No Comments

Temperature extremes have a way of driving innovation. The Inuit, centuries before there were Ray Bans, would strap bone, tusks or bark to their faces, with a thin slit cut to allow their eyes to peer out. This would protect them from snow blindness, a real malady when a landscape of clean white snow and blinding sunshine would coincide. Perhaps it was something far ahead of its time: They looked like something that might easily be worn today by pop singer Lady Gaga.

Researchers in Minnesota, another snow-swept land (albeit a bit further south), are being equally inventive about how to fix potholes in the future. They are testing microwave technology to heat potholes in midwinter as a method to fix pavement before springtime – when those potholes will be larger, more dangerous and more expensive. Taking it a step further, the researchers are looking at using locally mined and recycled materials as a means of making the technology work even better.

Fixing smaller and icier potholes nearly impossible

To understand why this is important, think about a cold, icy pothole. Just about everyone knows that water expands as it freezes, which is why a pavement crevice will get bigger when temperatures hit 32 degrees (F) or cooler. Add to that the cycles between temperatures above and below freezing – which can happen 40 times or more in a season, such as during the months of February, March and April in places like Minnesota, Michigan, Illinois, the Dakotas, northern New York and New England.  Those little cracks of February become the potholes of death by April.

But with traditional pothole repair methods, only a temporary hot-mix asphalt is typically used in the winter months. These are the “throw and go” pothole fixes that are simply intended to mend road hazards for a few weeks or months, until a better filler can be used when the weather warms. Compounding the problem is how most hot-mix asphalt will not adhere when the hole is wet – it needs to be cleared of debris and moisture in order to last, which under most winter conditions is all but impossible to accomplish. The practice is wasteful, as both the asphalt filler and labor costs are doubled or tripled with return trips in the months that follow.

Another additional cost happens when the asphalt trucks carry hot mix that cools before it is used – it must be returned to the plant for reprocessing and reheating. In colder regions, the process is so ineffective that hot mix plants just shut down entirely for the winter.

Heat, dry and fill that pothole – for good?

This is why microwave technology proves to be tempting. Microwave Utilities, Inc. (MUI) of Monticello, Minnesota worked with Roseville, Minnesota consultant David Hopstock to test a pothole repair method this past winter at a handful of locations in the state, including Duluth, in the winter of 2010-2011. The test was conducted under the auspices of the Natural Resources Research Institute of the University of Minnesota Duluth.

MUI more typically is in the business of thawing frozen-solid ground during winter to service gas and electric utility companies. But in this project, a 950 megahertz transmitter was aimed at potholes to heat and dry them, then to heat patch materials that variously included standard hot-mix asphalt, recycled roof shingles and taconite tailings, a coarse waste product from Minnesota’s Iron Range taconite plants.

The taconite makes good pavement filler but also happens to interact well with microwaves. “We used taconite to keep microwave energy focused on potholes by attracting microwaves,” explains MUI’s Kirk Kjellberg.  “This helps heat surface area and the mix faster while helping to keep the microwaves from penetrating too deep into the ground.”

The researchers tested a variety of pothole filler mixes in Duluth and Anoka County, some distance away. While a full analysis will not be complete until later in 2011, all holes have reportedly held up without requiring a second or third application – already an improvement over standard mid-winter temporary fixes.

When their analysis is complete, we might discover these researchers are as forward thinking as the ancient Inuit.