How many times have you thought about what’s underneath your shoes and tires? If the engineers had their way, you’d never think of about pavement at all.
“We don’t expect that the public would care,” Steve Lund told me. “They just want a smooth driveway or sidewalk, and it’s our job just to go behind the scenes to do that.”
Lund is a pavement engineer for MnDOT, which is (of course) the state’s department of transportation, with a massive budget and responsibility for the majority of the lane miles in Minnesota.
Zoom in on the pavement underfoot, and there’s a complex world of rocks and water that has huge impacts on your city’s budget, the quality of your stroll, the lakes and rivers all around us, and (of course) your car’s transmission.
Concrete vs. asphalt
Once you start talking to people about about pavement in the Midwest, you immediately find a schism.
“There are two types of pavement, concrete and asphalt, and these industries compete with each other,” explained Lev Khazanovich, an engineering professor at the University of Minnesota. “Each is always trying to improve their product, and so asphalt and concrete in Minnesota are much better than what was built 30, 40 years ago.”
The difference between the two pavements comes down to cost and lifespan. Concrete is harder and more durable, but asphalt is cheaper. In fact, considered philosophically, these two technologies might even be diametrical opposites.
“Fundamentally, asphalt is a mechanical compaction of the paving products,” Steve Lund explained to me. “It’s a physical compaction and pressing, whereas on the concrete side it’s more of a chemical reaction and hardening of the paving product.”
Concrete is composed of water, aggregate and cement (with occasional other kinds of material, such as coal ash from coal power plant filters). The chemical reaction between cement and water causes it to harden. It can thus be molded, like plastic, into a seemingly infinite variety of shapes.
Asphalt, on the other hand, is a form of petroleum mixed with aggregate (mostly sand and gravel), heated up and then pressed down to form our roads, bike paths and parking lots.
Minnesota’s extreme climate
Because Minnesota is a “continental climate” located very far from the moderating effect of oceans, our temperature extremes are some of the widest in the global north. And as any springtime driver knows, as water repeatedly freezes and thaws during our winters, it poses huge pothole-shaped problems for our pavements.
“The climate extremes are the number one dilemma we have to wrestle with,” Steve Lund said. “With extreme cold contraction we get brittleness, and with extreme heat we get expansion. That brings the buckling and the potential softening of the asphalt.”
Because of our climate, pavement in Minnesota has to be tweaked and modified to handle the pressure. For example, the “Minnesota mix” of concrete typically has less water than the mix used in other states, in order to minimize the potential exposure to expanding ice. And this means that Minnesota engineers have to be on the cutting edge of pavement techniques and technologies.
“MnDOT is one of the most technically competent, innovative and educated transportation departments in the country,” Kazanovich said, referring to their attempts to deal with our extreme climate. “The more water you place in the concrete, the more porous your concrete, and in this case, its resistance to freezing would be much lower. Our pavements are placed with special machines which allow for very stiff mixes more resistant to freezing and thawing.”
Trends in permeability
As it turns out, water is the key variable in the world of pavement. Not only do engineers worry about it freezing and thawing, pavement has a huge impact on our water systems because it affects storm water runoff. Each time it rains, all the roads, sidewalks and parking lots channel water away from soil and into storm sewers, lakes and rivers — with all kinds of consequences.
A full exploration of the storm water situation will have to wait until a future column. But you can’t really think about pavement without considering how it affects water, and that has led to one of the big trends in pavement: pervious paving. There are really two types of connected technologies here: permeable pavers and pervious pavements. (Say that 10 times fast!)
“Permeable pavers aren’t actually permeable,” Matt Kumka explained. Kumka is an engineer for Barr Engineering who specializes in permeable technologies.
“They can be standard brick pavers with spaces between them, [or] they’re special permeable pavers that have spaces built into the actual brick or paver unit so that water can flow in between the gaps,” Kumka continued.
Similarly, pervious pavements refer to concrete or asphalt that has more porosity built into their surfaces, that allow for water to flow into the subsurface soils underneath. For example, here in the Twin Cities, Shoreview has built a network of pervious concrete roadways that are the most extensive system in the metro area.
“There are several different types of pervious concrete,” explained Nathan Campeau, another engineer at Barr. “It has been around for quite a while now, 10 to 12 years, and installed in several locations around the state. And there’s a corollary in porous asphalt, special mixes that have a lot of void spaces in them. They’re basically like a sponge, and allow water to go from the surface down to the sub-grade below the pavement.”
While the benefits of new pervious pavements are clear, they also present technological challenges. First, it can be tricky to install them, and they require specific equipment and time-consuming construction processes. Many contractors struggle with pervious technologies when using them for the first time.
Second, cities like Shoreview have to carefully maintain their pervious pavements, so that sediment and grit does not clog up its “void spaces.” To do this, cities and property owners have to actually vacuum the sediment out of their pervious roads and parking lots.
“Permeable pavers are a little easier to maintain in terms of getting sands and silts out of the spaces between the blocks,” Campeau told me. “It’s easier to vacuum that out than vacuum out the silt that migrates partway through the porous asphalt or pervious concrete.”
Because of the clear environmental benefits of permeable techniques, the new pavements are appearing in lots of parking lots and sidewalks around the state. For example, the Green Line along University Avenue has an extensive system of permeable pavers and catch basins for water that will allow for trees to grow and prevent rainwater from running directly into Saint Paul’s lakes and rivers.
The future of paving surfaces
Even though it seems invisible, the world of pavement has been changing for years. In St. Paul and Minneapolis, you can see some of the different eras of pavement as you drive or walk around the city. Many of St. Paul’s older historic districts (like Cathedral Hill) retain some of their original brick pavers, though most of these have been replaced at great expense over the years.
Likewise, industrial neighborhoods in both core cities have concrete streets that can be decades old. And until recently, Duluth still had century-old pavements made from hard stone.
While the asphalt and concrete used in reconstructed roads might seem the same as a road from a few decades ago, underneath the tires there are a lot of differences. On the one hand, roads are becoming thinner as new asphalt and concrete technologies are developed that last longer and require less maintenance. Similarly, newer roads are often quieter than roads built a few decades ago.
“In the past, concrete roads could last a long time, but they were noisier,” explained Kazoanovich. “By changing the pattern of the texture, MnDOT is working on making them much quieter. [Steel] dowels reduce bumping noise significantly.”
I also talked with Rebecca Embacher, an advanced materials and technology engineer for MNDoT, who told me about new technology trends at the agency. MnDOT is developing “intelligent compaction rollers” that use GPS technology to ensure uniformity so that, as Embacher explains, “uniformity in our compaction efforts [that avoid] lower density measurements.”
Thermal profiling
Similarly, MnDOT has been working on developing new thermal profiling technologies that can very accurately measure the rate at which asphalt cools as its shipped from the plant to the construction site.
“Now, you can look at the temperature segregation as they’re placing the asphalt,” Embacher explained. “Thermal profiling increases uniformity and density measurements,” which ensures that they will last longer.
Increasing the lifespan of a pavement is one of the key goals, because it reduces maintenance and replacement costs, and minimizes construction delays.
Finally, Minnesota’s pavement engineers are working on developing more resilient “Minnesota mixes” of colored concrete that might be used to better demarcate pedestrian, bicycle and road spaces to calm traffic in cities. So far, attempts to use colored concrete have not stood up well to the extreme Minnesota climate, but that might be changing.
“There is no reason why concrete pavements shouldn’t last for 60 years,” Lev Khazanovich told me. “This is a new technology and it will take time to work it out for applications. You have to start somewhere. People drive and see failures, and MnDOT will study those failures and eventually it will be resolved.”
Doubling down on awful
I don’t understand why Saint Paul is covering concrete streets (Hamline, Pascal) with a thin layer of asphalt. It will be nice and smooth for a couple of months, and will then deteriorate into the complete embarrassment that Hamline was just north of I-94 this past spring.
Within two years they will be milling the destroyed asphalt off the streets in early March, and then will hopefully fix them correctly by cutting out the damaged seams and pouring new cement to replace the gaps.
Cement roads last decades. They should be repaired with new cement, not covered up with asphalt that will just fall apart in a couple of years.
Interesting Article
There’s quite a vast technology concerning concrete. Just a few incidental observations: a veteran walker or runner will tell you that an asphalt surface is kinder to feet and joints than concrete. I’ve even noticed a difference between physical activity on a terrazzo floor as opposed to ordinary concrete; the terrazzo seems friendlier.
The old wooden pavers can be seen in some alleys in St. Paul’s Crocus Hill neighborhood; if I’m not mistaken they were of cedar, laid vertically so the end grain shows on the traffic surface, like a butcher’s block.
yup
there are some of those in an alley by my father’s house. i’m amazed they held up so well!
The wooden alley pavers
I understand these were used to help deaden the sound of horses’ hoofs. But I think they would also be kinder to the limbs and joints of horses than the old paving bricks. I’m curious to know whether the well-off old-time residents on those alleys got the city to pay the cost of special pavers.
Pavers ?
Great article. Do you know how well pavers hold up compared to concrete and asphalt in our environment? With their new Tiger Paver, The Netherlands will often use pavers at crossings even when the bicycle path is asphalt and the roadway concrete. And expect quite high durability. A bit milder environment than ours though.
I wonder if pavers would work in a place like Shoreview where they just replaced the red crossings with standard color concrete that does not make the crossing stand out as much and that so far seems less safe.
Pavers ?
Last week I was at a presentation by Tom Brownlow, former City Administrator of Charles City, Iowa, where he talked about that city’s permeable paver program. They have paved several blocks with permeable pavers, saying that this is more expensive by around 30 to 40% over bituminous or concrete. However, he said there are several benefits – they are supposed to have a much longer life span, are more aesthetically pleasing, good for the river, and reduce other stormwater treatment costs. Brownlow mentioned that permeable pavers should be used only on slow speed city streets, because heavy trucks will displace pavers on high speed roads.
Pavers ?
Concerning Charles City, it has come to my attention that the 30-40% increase in cost is much lower than typical paving area cost comparisons. It appears that this low cost increase was obtained mainly from special federal ARRA subsidies, and possibly other subsidies. Not sure if lower costs for other storm water treatment was factored. So, without special subsidies, the high cost of permeable pavers is a big issue.
Pavements Article
Check the comment “–MnDOT has responsibility for the majority of the lane miles in MN”. Seems unlikely but Cities & Counties may make up the greatest share. Many of those miles are not eligible for gas tax funding. Also check the research being done at MNROAD on I-94 @ near Albertville with international partners.
Transmission? I think you mean suspension.
Transmission
That depends on how deep the pothole is.
reducing carbon input making cement
Bill: another subject for your pavement series could be informing us about advances in reducing the amount of energy needed to create cement. Our own I35 bridge over the Mississippi R is a model of lower inputs developed by chemists at our Cemstone plant. Fly ash is one way to decrease the need for cement in concrete. Another innovative approach pumps the emissions from coal or gas-fired power plants through sea water to yield calcium carbonate. This means that cement produced in this fashion could claim to reduce the concentration of CO2 in our atmosphere instead of having a carbon footprint. There was a great article or series in “NY Times” just after our bridge opened.
I work in landscape design/install/maintenance. Our industry likes to promote permeable pavers, yet rarely includes the cost of maintenance. Vast amounts of energy are expended hauled the soil away and sorted rock to convey water while supporting the weight of vehicles. Most of those systems fail due to just organic matter plugging the pore space in a short while. A church in White Bear Lake created a lake in their parking area surrounded by trees. Cutting trees down doesn’t seem the best solution.
My technique is to build swales into driveways to guide water into raingardens where native plants continue to improve infiltration as their roots grow deeper. Some die and become the straws that take the water 10′ or more into the soil column. Existing pavement can be modified with a bag of patching asphalt pounded onto the sloping drive to guide it off to either side instead of making it to the street.
A very simple solution for sidewalks is to remove soil from boulevard to get water to soak in vs. creating puddles after rain or snow melt. Of course winter is the worst because those puddles become hazardous smooth ice patches overnight.
We had done a number of examples I could show you if you are interested in viewing some projects already in the ground: in front of our office at 1708 Selby there are 2 small raingardens that keep our walk dry after even the heaviest rain vs. city walk to both E & W with deep puddles. We have a big project in Golden Valley that we installed a year ago with a vast rain garden fed by long sloping driveway. It has a big turnaround circle at front door/garage that doubles as guest parking with a raingarden in the center. The turnaround surface was pitched to flow into the middle. Site was pure sandy gravel once we removed the topsoil, so it has tremendous capacity, but we added an overflow pipe for unique situations.
You can get in touch with me via http://www.energyscapes.com or by calling our office 612-821-9797 (we brought our old # from Mpls when we moved to St Paul).
Keep up your good work. Douglas
oops
you are correct
thanks for the great info
i’d love to chat more about this when i come around to some of these topics
Concrete is better for highways because it consumes less fuel as you are driving fast. Asphalt is better for residential roads, county roads, and boulevards. Because you are turning and it has better traction.