Terra E-News, November 2011
Earlier this year at the Center for Transportation Studies annual research conference, University of Minnesota civil engineering researcher Mary Vancura presented more findings from a TERRA-initiated study of the cold-weather performance of pervious concrete. Vancura discussed the ways pervious concrete can go wrong and what solutions may help prevent failure.
Pervious concrete allows water to seep through it via tiny passages and holes in the concrete. This is helpful in areas where water tends to accumulate or could not return to the ground with regular concrete in place. But the special nature of pervious concrete also presents a unique set of problems. Vancura explained that three things can go wrong with pervious concrete: impermeability, raveling, and cracking. Impermeability is usually the result of dirt and debris, usually from trees, getting lodged in the holes of the concrete. This is easily avoided with regular maintenance using any machine with a high-powered vacuum or a power washer to dislodge the debris. Raveling, which is when the top aggregate layers separate, is often due to debonding of the concrete and aggregates and can affect just the first two layers or deeper layers. Cracking is similar to what happens to regular concrete, although the nature of pervious concrete means that it cannot withstand heavy loads, like garbage trucks, on a regular basis. This is why pervious concrete is best used in low-volume applications, such as alleys, parking lots, or recreational paths, she said.
As part of her research, Vancura took samples of pervious concrete from 22 sites around Minnesota. She found that deeper raveling tended to happen when poor quality aggregate was used or when the concrete got too dry. She also noticed that air was often not properly entrained in the concrete.
Vancura and her colleagues also conducted a small experiment by placing pervious concrete at the edge of a parking lot. One section was a standard mix pervious concrete, one section contained extra viscosity-modifying admixture, which made a stiffer paste, and one section had light-weight aggregates, which can absorb water and regulate hydration of the paste. Luck was not on her side the day Vancura and two other installers laid the concrete. It was a cold day in November with winds up to 40 miles per hour. The truck with the cement got lost, and the concrete sat in the truck for approximately an hour and a half. The team also had to contend with equipment malfunctions. After a harrowing day, the team laid insulating blankets over the concrete to trap heat (as the temperature was predicted to dip to 32 degrees F after dark) and prevent evaporation (from the high winds).
When Vancura revisited her concrete after five months, she found it had not only survived the winter—it looked great. She attributed the outcome to the use of the insulating blankets during the curing period, as well as the extra time the cement spent in the truck, allowing the even distribution of water.
University of Minnesota civil engineering professor Lev Khazanovich is the principal investigator of the study along with co-investigator Kevin MacDonald of Cemstone. The Aggregate & Ready Mix Association of Minnesota (ARM) partnered with the University of Minnesota Department of Civil Engineering to obtain a research grant from the Ready Mixed Concrete (RMC) Research & Education Foundation for the study. Much of the research has been conducted using pervious concrete systems at MnROAD. ARM, the RMC Research & Education Foundation, and the University of Minnesota are TERRA members.
Research on pervious concrete and its applications is ongoing, as it is still a relatively new approach in cold climates.
Related resources:
- Download presentation slides (3.2 MB PDF)