A pervious concrete pour is also affected by external environmental factors. The ideal environment for a pervious placement is moist, cool and wet, Fisher says. "A drizzly day is perfect to pour pervious concrete because you can double your time for pervious placement," Fisher says. A hot and dry environment will make the mix dry quicker, shortening the window of working time. If hot weather is inevitable, Fisher suggests pouring early in the morning or overnight. Fisher says pervious concrete can be placed in temperatures ranging from 40 to 90 degrees Fahrenheit.
Temperature isn't the only factor to consider. Humidity also plays a roll. No humidity will cause the pervious concrete to dry out faster, Fisher says. A hydration stabilizer admixture is critical when pouring in dry, hot environments, he adds.
And when it's cold, Fisher tells contractors never to batch the pervious concrete with hot water. "The hot water is going to dry up too fast and the pervious won't have sufficient moisture for proper cure," he points out.
"Curing is probably the least focused on portion of the whole pervious concrete system, and it's probably the most important," Fisher says. "It's the make or break part of the process."
Curing pervious concrete helps hold in moisture to allow full hydration. And since pervious is a porous concrete, the top 1 to 11/2 inches are critical when it comes to proper curing, Fisher says. An improper cure may result in raveling on the top or the edges of the slab. Plus, improper curing will prevent the pervious concrete from reaching its desired properties and strength. Although an improper cure does not mean the pervious concrete is sure to fail, it does greatly increase the chance for failure.
Knowing how important curing is to pervious concrete means contractors should know how to properly cure it. The key step to curing pervious is covering the slab with plastic sheeting. "The cement in pervious needs good hydration to cure properly," Fisher says. "The plastic sheeting maintains a moist, humid environment for the pervious concrete mixture to cure. The plastic needs to go on quickly because if the little moisture that is in the mix is allowed to evaporate there will not be enough moisture to create that humid environment."
Pervious concrete needs to be covered while it still has a glistening sheen, Fisher adds. The edges need to be covered, too. Fisher says pervious concrete should have a "Tupperware" seal covering the entire slab. The plastic should remain on the slab for seven days. That time should be increased to 10 days or more if fly ash or other supplementary cementitious materials are used in the pervious mix, Fisher adds.
Although the ASTM standard for pervious concrete curing does not specify a recommended thickness for the plastic sheeting, Fisher says the norm right now is a 6 mil plastic. A plastic this thick can raise some concerns. For one, it means there is more plastic going to the dump after the pervious concrete has cured, Fisher says. Also, the thicker plastic is susceptible to lots of wrinkles when it is placed over the concrete. That leaves space for any wind to reach under the plastic and even lift it up during the curing process. Fisher suggests using a thinner plastic because it is more likely to stay down when placed on the pervious.
Pervious concrete is considered a greener product than traditional concrete. But the need for plastic during curing may not be the greenest process. Fisher points out that perhaps the biggest need in pervious concrete is alternative curing methods. Currently he says there is not much research for alternative methods, but he expects that will change in the future. Internal curing admixtures help in the curing process. At this point they do not serve as a replacement for the plastic, but they do allow for use of a thinner plastic. The internal curing admixtures also serve as risk management in case the plastic accidently blows off the concrete during the curing process.
Admixtures can be a great help when placing and curing pervious concrete. Here are a few of the most common admixtures used with pervious concrete:
Hydration Stabilizer. A hydration stabilizer admixture helps slow the setting time of pervious concrete, resulting in another 10 to 15 minutes of working time during the placing process.
Viscosity Modifier (VMA). A viscosity modifier admixture acts like a paste thickener for the pervious concrete mix. It allows for an increase in the water/cement ratio without making the paste too thin.