You might have noticed that a curb installed about 15 or 20 years ago is starting to crumble.
And you would be correct if you thought that was odd — that the curb was suffering a premature death.
It’s a problem that’s taken years to surface, but it is affecting cities across the metropolitan area. More importantly, it’s costing cities millions to repair.
“Kansas City is a big metropolitan area with a lot of streets built at a lot of different times in the past,” said Michael Ross, manager of technical and administrative services for Overland Park’s Public Works Department. “There’s a block of infrastructure that is old enough that has this problem and that will be very expensive to go back in and replace.”
To understand the problem, it helps to know the key ingredients of concrete: rock (called large aggregate), mixed with sand, cement and water.
Today’s problems, called D-cracking by the industry, can be traced back to bad limestone that often made up the large aggregate used in the late 1980s.
“Some of the areas from where they were getting the limestone was a soft limestone that was susceptible to water being in it,” said Joe Johnson, director of public works for Leawood.
In winter, the low temperatures made that water expand. When it warmed back up, it thawed and the aggregate would contract.
“As it would go through the (freeze/thaw) cycle, the rocks would disintegrate,” Johnson said. “At the same it was expanding against the cement paste and causing it to fail — eventually it just kind of turns into gravel.”
The problem appears to affect all the streets Leawood built from the late 1980s to 2000. In 2001, the city started using a different aggregate — quartzite or granite.
Leawood is not alone. Other cities throughout the metropolitan area are seeing that happen too.
“We probably really started seeing this deterioration of the concrete about 10 years ago,” said Dena Mezger, deputy director of public works and city engineer for Lee’s Summit.
“We started noticing that curbs didn’t seem to be lasting as long as we thought they should be,” she said. “We started seeing this cracking and deterioration happening. It tended to be on newer curb, not something that had been around for 20 or 30 years.”
Overland Park was one of the first cities to realize the problem.
The city had for many years done pavement inspections every other year, said Ross, the Overland Park manager.
“In those pavement inspections we were picking up distresses in concrete curbs,” Ross said. “By looking at the age at which those distresses occurred, we became convinced we had some significant problems with the concrete.”
In the late 1990s, the city got together with the concrete industry and came up with a solution. Now public works organizations from communities on both sides of the metropolitan area participate in the Kansas City Metro Materials Board, which standardizes material specifications for durable concrete.
At one time, limestone was a good choice to use in concrete.
“Years and years ago, you could get a good quality limestone for concrete and cement,” Ross said.
What cities didn’t know was that the quality of limestone had changed.
“What everyone is thinking is that we went through all the good rock,” Johnson said. “In the late 1980s, the ledge that was being mined for the limestone was just not as durable as what we had gotten in the past.”
It is difficult to tell the difference between the good and bad limestone.
“You could have a handful of really good limestone and you could have a handful of really bad limestone and they look the same,” Johnson said.
The solution was to switch to the new aggregate.
“Sitting here today, 10 years later, it’s my opinion that we have solved the D-cracking problem,” Ross said. “Changing the aggregate doesn’t solve all of the problems, but it solved the one that was killing most of our concrete.”
And there’s still the problem of all the bad concrete cities poured over the years.
Overland Park will spend approximately $2 million next year replacing curbs and gutters, predominately because of the D-cracking problem, Ross said.
Lee’s Summit tried to address the replacement through its annual curb program, but it wasn’t able to fix it fast enough. The city asked voters in 2010 to approve bonds for a large-scale curb replacement program to try to get ahead of the problem.
The city was able to fund a little more than $9 million through that program to replace about 400,000 feet of curb and gutter that was in the worst condition.
The city has enough left to do about $50,000 in repairs next year.
“It addresses the worst areas,” Mezger said. “Hopefully we will be able to keep up from there on through annual programs and maybe some supplemental funding from here or there in the future.”
Beginning next year, Leawood will replace curbs along its residential and arterial streets. The city is looking to spend $12 million to $16 million over the next four years.
The good news is the new curb should last 75 years or longer, Johnson said. There might be some areas that need patching because of ground movement.
“Time will tell, but we think we are going down the right path,” Johnson said.