There has been some confusion as to what is the best base material to use for pervious concrete. Over the past year we have noticed an influx of construction details calling for the use of "Class II Permeable Aggregate" as the base for pervious concrete sections. This differs from the standard practice of open-graded crushed rock, and it is misguided.
The main confusion stems from a misunderstanding of the properties of Class II Permeable aggregate. In its loose state the Class II Permeable agregate has a decent void content, however unlike open-graded crushed rock, it requires compaction. This compaction reduces the aggregate's void contents well below that of the pervious concrete.
It is very important that the base material have a higher void content than the pervious concrete for proper drainage and storage. Specifying a base material with less voids than the pervious concrete increases the potential for water to backup into the pavement layer during a rain event.
Watch the video below to see the difference between Class II Permeable Aggregate and Open-graded crushed rock.
The video clearly demonstrates how the void contents of Class II Permeable aggregate and open-graded crushed rock affects water absorption into the base. The crushed rock's 40% void content allows for water to pass through unrestricted, unlike the Class II Permeable which produces standing water. To provide the appropriate drainage and storage for your pervious concrete project specify a base of 3/4" crushed drain rock, or a no. 57 stone.
Should you like assistance on designing your pervious concrete system or section thicknesses contact us for a consultation.