A pervious concrete pour is a three-step process - placing to elevation, compaction and curing. Unlike traditional concrete which looks for a smooth, finished surface, pervious concrete contractors strive for compaction, aggregate interlock and a porous surface.
Like traditional concrete, there are many different methods and equipment for pouring pervious concrete. However, since pervious concrete is a low-slump mixture, it cannot be pumped into place like traditional concrete. The most common methods of placement are directly from a ready-mixed truck, with a concrete buggy or by conveyor.
Pervious concrete is commonly poured in formed-out strips, says Dale Fisher, CEO of PCI Systems, LLC. Strips are poured every other to allow the pervious the proper amount of curing time. Current documents specify contractors pour strips between 12 and 20 feet wide. However, this can be a challenge when pouring very large areas that have tight scheduling deadlines. New equipment, such as laser screeds, allows contractors to pour larger areas without strips for situations like these.
Some pervious concrete can be placed with a concrete slipform paver. Slipform pavers do not use forms when pouring, so Fisher says most pervious placements using this equipment is done on slabs that do not need a definite straight edge, such as trail pathways.
Fisher likens a pervious pour to that of a long assembly line with the only difference being the crew members are working much closer together during a pervious pour. "Everything happens at the same time with pervious concrete. With regular concrete it's spread out," Fisher says. "With traditional concrete the whole slab is placed before the finishing operation even begins. With pervious concrete your placement, finishing and curing is all done at the same time."
Typically, there is one crew member near the ready-mixed truck's chute (or by the buggy or conveyor) directing the pervious concrete where it should go. The screed operators are the next in line, screeding the pervious concrete to the proper elevation and compacting it. Contractors can use roller, truss or laser screeds with pervious concrete. Plate compactors can be used instead of screeds, but a plate compactor may not provide the same consistency of compaction that screeds can, Fisher says. Contractors also need to avoid over-compaction when using plate compactors.
The screed is followed by the finishing crew, which has multiple jobs. After screeding the pervious, it needs to be cross rolled. Typically, one crew member follows behind the screed with a cross roller, rolling in a perpendicular direction to the screed. Cross rolling gives the finish a nice continuity, Fisher says.
Joints are also placed during this stage. Fisher says they are placed just like traditional concrete joints by either using a roller jointer while the pervious is still wet or sawing the joints after it has cured.
The final finishing crew members follow with hand floats to touch up the edges. "Don't use trowels because trowels tend to damage the surface," Fisher says. He adds that edging is not encouraged with pervious concrete. "When the pervious concrete is wet the 'paste' sticks together. But when it's dry it doesn't. Once that paste has gone from its plastic state to a dry paste it loses its bond. If you were to edge it when it was dry all the aggregate is going to ravel off," Fisher says.
Another difference you see with pervious concrete over traditional concrete placement is the window of workable time. Pervious, because it is a drier mix, has a much shorter window of time for placing. Although that window is dependent on the type of mix being used (a wetter mix will have a longer working time than a drier mix), Fisher says there is generally a 15- to 30-minute window once the pervious concrete hits the ground. So getting the right mix is essential. Admixtures can offer some assistance for workability. (See the "Pervious Admixtures" sidebar below)