"Chip sealing" of streets generates complaints

On Madison's west side, City Engineering is in the process of "chip sealing."  This is a maintenance program that extends the life of pavement which is still in good condition.

Steps in the process we observed
  • Workers clean out cracks between pavement and curb, and place filters on the stormsewer inlets.
  • Asphalt is laid down to seal the cracks in pavement.
  • Bottom slag is liberally applied to cover the asphalt, provide traction, and surface protecton.
  • A pickup truck drags a rake to make sure the slag is evenly distributed.
  • The excess slag is removed in several passes by sweepers.

Complaints are coming from the use of a black, sandy material called "bottom slag."

The loose slag is creating a nuisance for pedestrians and a minor hazard for cyclists.  It's also creating a lot of dust, either when vehicles pass by, or when the contractor drags a rake over the slag--as they are doing in multiple passes (below).

The City claims chip sealing will save millions of dollars over the years.  They also claim that it's environmentally friendly, because it saves natural resources.  In other words, when slag is used on streets, sand doesn't have to be mined.  And, keeping the slag--a waste product--out of landfills saves landfill space.  I don't doubt these assertions.  I just wonder if all the long-term costs of using slag have been accounted for in the "cost-benefit" calculations.

The details
Bottom slag is a glassy ash from coal-fired power plants.  It's properties vary, according the the source of the coal, so it's hard to make generalizations without testing the ash.  It can often contain small amounts of toxic heavy metals like mercury.  Therefore, it's important that the City know the toxic properties of this particular ash product.  Have they done the testing?

Effects on groundwater and the lakes
Bottom slag contains significant amounts of salt and phosphorus, which have well-known harmful effects on the lakes.

"Leaching of metals and air quality during construction are two environmental issues associated with using bottom ash or boiler slag in asphalt pavement. Bottom ash and boiler slag consist of the same chemical components as fly ash; therefore there exists the potential to leach trace elements. Because bottom ash and boiler slag have larger particles and less surface area per unit volume, the potential to leach trace elements is reduced. In addition, coal combustion products mixed in asphalt pavement is considered an encapsulated application that further reduces the potential to leach elements. A recent leachate study was conducted on test strips of asphalt concrete with bottom ash. Although trace elements were observed in the leachate, there was no evidence that the use of coal ash in asphalt pavements was the source."  Source

For short-term dust in the air, Madison ranks as the 24th most polluted among cities in the US--so it's imperative to reduce dust from construction activities.

The largest component of slag is silica.  Hence the dust produced by chip sealing operations and traffic is the toxic silica dust. (Silica dust is most toxic when the silica has been recently fractured--which we can assume is the case.) 

Engineering claims that the bottom slag is "non-carcinogenic."  That might be true if you were to eat or drink food contaminated with slag.  But if you breathe it, it IS carcinogenic "...The International Agency for Research on Cancer (IARC) reviewed the medical data and classified crystalline silica as "carcinogenic to humans."  Source

"Another study reviewed the toxicity and health hazards of coal fly ash compared to coal mine dust, which is known to cause pneumoconiosis and emphysema.  Researchers concluded that exposure to high concentrations of coal fly ash can cause chronic bronchitis and air flow obstruction—typical of the common effects seen after inhaling a variety of different types of dusts."

But a study by the Electric Power Research Institute found no harmful effect on workers exposed to routine levels of coal ash.

How is the contractor performing?

Things we liked...
  • Multiple passes of sweepers to clean up excess slag.
  • Use of water spray to suppress dust while sweeping.
  • Trucks driving slowly to reduce dust.
  • Trucks removing excess slag covered their load with a tarp, to suppress dust.
  • The City's letter sent to residents, explaining chip sealing.
Needs improvement...
  • Between spreading of slag and the removal of excess, much dust was produced by traffic and the contractor's equipment.  This dust is hazardous.  I observed a teenager on a swing, next to the road while clouds of dust were drifting her way.  There should be signs to warn people to say indoors.
  • The workers in sweepers were protected from dust by closed windows (and presumably air conditioning).  But other workers were not protected....
The worker towing a rake--while producing clouds of dust--had the window open, and was not wearing a respirator.


The issues are complex, and there are environmental benefits to using bottom slag on streets, instead of putting it in a landfill.  EPA doesn't consider bottom slag to be a hazardous substance, but it does advise caution in its use.

The City should ensure through testing that this particular bottom slag does not leach harmful phosphorus, salts, or heavy metals into the groundwater or lakes.  Over the years, much of the chip seal layer will be pounded into dust by traffic, and then washed to the lakes.  The City should also test bottom slag DUST for its potential to leach harmful chemicals.

More efforts should be taken to reduce the exposure of the public and workers to the construction dust, especially given Madison's already dirty air.

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1 comment:

  1. More than a minor hazard for cyclist, this stuff is basically a lot of tiny pieces of glass. The largest of these can easily puncture a bicycle tire. Use of this material on streets is a big step backwards for a purportedly 'bike friendly' city.


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