Yield....the dirty little secret

It's somewhat of a balancing act to develop a pervious concrete mix that places easily, performs well, and has good durability.  Finding this balance requires the producer to develop mix designs for each of his aggregates and also have the ability to adjust the mixes on-the-fly based on field density tests.  Unfortunately, a concrete producer or contractor that doesn't have the experience or ability to make these adjustments, will have problems on the job site as the aggregate gradations will typically change during the project.  There may be yield issues, mixes that wont drain, mixes that ravel, and the list goes on.

As for yield issues in pervious concrete, understand that changes in aggregate gradation always affect the yield.  I'll say it again for emphasis....changes in gradation  always affect the yield.  If the gradation becomes more uniform in the mix, the yield increases...if more fines are introduced, the yield decreases (it could be as much as 5% and still meet common specs).  I should note that pervious concrete mixes rarely become more uniform during the course of the job, but typically mixes increase in density as the gradation changes.  

This is really a lose/lose scenario from the producer and contractor's perspective.  Not only will the producer waste material like cement and admixtures (I'll explain later in another post), but the contractor will work harder to place a mix that will under-yield (I'll explain this later too).  This is why I encourage good communication between the producer and the contractor, good quality control, and do your density tests.

Remember pervious concrete's dirty little secret......keep the gradation under control or your yield will be out of control!