Batching pervious concrete is almost a leap of faith for a concrete producer. A leap of faith in that we hope the batch comes out at the right moisture content - not too wet, not too dry.
What usually happens? Producers new to pervious concrete tend to batch the concrete too dry expecting they can add water to the mix to get it right.
Before we talk about this leap a faith, there are a few things to remember about pervious concrete: first, the load will dry up on the haul to the jobsite, especially on a hot day, even with a good hydration stabilizer; second, there is a range of water content that will make an acceptable mix; and third, if a load of pervious concrete is batched too dry, it may not be possible to fix, even by adding water.
When developing a new mix design for pervious concrete, a producer should take the time to understand how much the mix will change over time. This might be done in a controlled environment, like in a lab mixer, or might be tested in a ready mix truck. Either way, it is critical for a producer to understand this dynamic so they know how wet to send a truck to the jobsite.
Additionally, a producer should test a new set of mixture proportions to understand the range of water contents. A pervious concrete mix may work at a surprisingly wide range of water contents without negative impact on strength (we think). Understanding how wide this range truly is, and what impact admixtures might have on it is important for both the producer and contractor.
Lastly, producers need to be careful not to batch a mix too dry. The worst of this is when a mix is batched with dry coarse aggregate. Starving a mix for water will prevent complete hydration of the cement causing strength and raveling issues in the finished pavement. Further, if the mix doesn't have enough water to develop a good, wet solution, the admixtures won't be properly dispersed, and the set properties could be unpredictable.
To ease this leap of faith, the producer should know their aggregate moistures, and maintain their stockpiles in a way to maintain consistent moisture, as with any good concrete quality control program. Additionally, the mixture proportions should be well tested to understand the range of acceptable water contents in the batch. Finally, consider how a viscosity-modifying admixture can extend the range of acceptable water content in a mix.