In 2008, Minnesota tested pervious concrete on a road with light traffic; long-term monitoring found ride quality held up, while vacuuming helped maintain drainage

When rain falls on a typical road or parking lot, it has relatively few options for where to go. Because standard concrete and asphalt protect the ground, water flows into storm drains that frequently empty into rivers untreated after sliding off the surface and picking up oil and grit along the way. To address that, pervious concrete was created. Rainfall flows directly through the surface and soaks into the soil below because it is filled with many tiny, linked spaces.The idea made sense in theory, but engineers in cold climates had concerns. A surface full of holes may break up if water freezes within, and salt, snow ploughs and passing traffic could permanently block up the pores. So, to find out, Minnesota erected test portions on a low-usage route in 2008. Monitoring at MnROAD found that ride quality remained adequate for roads with light traffic over the years. MnDOT's research summary reports that regular vacuum maintenance, more than twice a year, helped maintain the pavement's benefits, including drainage.The results are most relevant to village lanes, driveways and residential streets that carry light traffic in snowy climates. The MnROAD tests found that ride quality remained suitable for roads with light traffic, while regular maintenance helped protect drainage. Researchers also recorded differences in smoothness and permeability between the test cells, so the findings should not be treated as a guarantee that every pervious concrete surface will perform the same way.Why Minnesota did the trialTo appreciate why the trial mattered, it helps to understand what officials were seeking to learn. Pervious pavements had promise for crowded metropolitan areas where enormous quantities of contaminated runoff end up in drains, but no one knew how they would handle Minnesota’s harsh environment, how much maintenance they would require or what their effect would be on groundwater quality. We got an early glimpse with a pervious concrete driveway completed in 2005.In autumn 2008, the team extended the work by installing three test sections on MnROAD's Low Volume Road, a two lane research loop about 2.5 miles (about 4 km) long. The design and early performance of the sections are documented in MnDOT's 2011 final report.Two of the cells were full-depth pervious concrete, one constructed on sandy soil and one on clay. The third was a porous concrete overlay about 4 inches (10 centimetres) thick on an existing slab. The full-depth cells are 7 inches (approximately 18 cm) of pervious concrete atop 4 inches of railway ballast and 8 inches of crushed stone.The loop functions like a laboratory. A five-axle semi-trailer weighing 80,000 pounds (about 36 tonnes) repeatedly travels along the inner lane, while the outer lane remains exposed to the weather. Crews plough and salt the route much like they would any low-volume road. Thermocouples and moisture sensors embedded in the cells track the frequency of ground freeze and thaw, the movement of temperatures through the layers and the effectiveness of water drainage.What years of testing showedResearchers repeatedly measured ride quality, noise, friction, surface roughness, surface condition and permeability across seasons. During the first full year, friction readings taken with a smooth tyre ranged from 45 to 55, above the threshold of 25 that the report identified for corrective action.Noise readings on the pervious concrete cells ranged from 97 to 100.5 decibels, compared with about 103.5 decibels on a neighbouring concrete cell with transverse tining. The test results suggested that the porous surface absorbed more noise from the interaction between tyres and the road. Ride quality varied during the testing period. Reported roughness values ranged from 3.10 to 6.40 metres per kilometre, depending on the season, wheel path and measurement method. The report also noted that hand finishing during installation affected surface smoothness. Even so, MnDOT's longer-term analysis found that the test cells retained ride quality suitable for roads with light traffic.The cells did not behave identically. Researchers poured buckets of water onto the pavement and found that the wetting reached the full width of the inner lane but only about half the width of the outer lane. Separate density measurements suggested that the voids were unevenly distributed, with calculated porosity averaging from 13 to 19% in the cells. The report identified differences in construction and material inside the pores as possible explanations.Why vacuuming mattersGrit, leaves and winter sand can clog the connected pores, making maintenance important for drainage. In a demonstration about a year after construction, researchers used a vacuum truck with its brush switched off because mechanical brushing could push debris deeper into the gaps. The cells appeared relatively clean before the demonstration, and the drainage results changed only slightly after cleaning. The sandy subgrade cell continued to drain in 6 seconds, while the clay subgrade cell improved from 17 to 15.5 seconds.These results describe one demonstration and do not establish that regular maintenance is unnecessary or determine the ideal cleaning schedule. MnDOT's broader evaluation found that regular vacuuming more than twice a year helped maintain pervious pavement performance. Vacuuming and mechanical brushing are different procedures, so leaving the brush switched off during this test does not contradict the recommendation for recurring vacuum maintenance.The Minnesota trial does not establish that pervious concrete will work equally well on every road. It indicates that the material can be suitable for roads carrying light traffic in cold climates when installation is carefully controlled and drainage is maintained. The results also show why construction quality matters: differences in the distribution of pores and in surface smoothness affected the test cells. Anyone considering the material should account for local weather, traffic levels and the cost of regular vacuum maintenance.You use AI every day. Now get your AI Quotient. Take the AIQ test.