Should pervious concrete be bought and sold by the cubic yard?
Maybe not. What we've come to find is that a yard isn't a yard.
If this hasn't confused you enough, let me just say this is another painful step in the learning curve for contractors and concrete producers new to pervious concrete. Even though a concrete producer has done a great job of making sure to proportion a mix to an exact cubic yard, the yield of a batch of pervious concrete will be significantly impacted by how much water is added to the mix, how much compaction the contractor applies to the fresh concrete, and how well the site is graded. What is batched as a cubic yard of pervious concrete may end up as a cubic yard of pervious concrete in a pavement, but it's not likely.
With pervious concrete, adding more water to the mix will increase how easily it compacts. A mix that compacts easily will not hold as much volume as a mix that is difficult to compact. This would push the yield to be short for the contractor. On the opposite side of the scale, a mix that is too dry would be difficult to compact and would tend to overyield.
Similarly, considering the contractor's equipment, heavier equipment - equipment that puts more compaction energy into the fresh concrete -- will tend to compact the concrete more. This is good for strength, but will shorten the yield of the load.
Lastly, coming from a concrete producer's background, I was taught that if a customer complained about yield, the first thing to check was how well the site was graded. Specifically, we wanted to see if the forms for a 6-inch thick slab weren't sitting on a rough site that would cause the slab to be 7 or 8 inches thick in some areas. This can have a significant impact on how much area a load of concrete covers. The same is true for pervious concrete. The site must be graded smoothly to prevent the slab from being too thick.
How is a contractor to bid pervious concrete work with this yield dilemma in mind? Assume the concrete will not yield a cubic yard, but less than that. Cast test panels with a concrete producer's mix and the contractor's jobsite equipment to get a feel for how much area a cubic yard will cover. Make sure the earthwork crew is preparing a site with tight tolerances on grading. Do all this, and maybe you'll be able to sleep at night -- without worrying about yield.