Watch out for flying food analogies because here they come!
My kids love oatmeal. Every morning for breakfast, it’s oatmeal. Funny thing, my daughter is about three years older than my son, she likes her oatmeal dry – just enough milk to wet the flakes, and make them sticky, but no more. My son likes his “soupy” – as wet as we can get it, but to where it still holds together on the spoon. This is a lot like what’s happened in the pervious concrete world in the last three years.
Three years ago, pervious concrete was, “a zero-slump, open-graded material consisting of portland cement, coarse aggregate, little or no fine aggregate, admixtures, and water.” We thought pervious concrete was good if it had just enough water to have sheen, and maybe you could mold it into a ball in your hand. Hardened mixes looked like Rice Krispies Treats, where the aggregate particles touched each other with a coating of lumpy, sticky paste. We used terms like “PAR” and “Wet-Metallic Sheen”. These “meatball mixes” had limited admixtures, little workability, and a spotty track record of success.
Today, pervious concrete is a flowing, workable, slump-able material (even though we shouldn’t spec it by slump). If it holds in a ball, it’s too dry. In fact, producers should be maximizing water in their pervious concrete mixtures to maximize durability, and hypothetically reduce the impact of autogenous shrinkage. In warm weather, pervious concrete should be batched too wet at the batch plant, then allowed to tighten up on the haul to the jobsite. Hardened mixes of modern pervious concrete should look, maybe not like soupy oatmeal, but the particles should be densely packed together, with a smooth paste bridge connecting adjacent stones.
What makes modern pervious concrete different from pervious concrete of three years ago? Admixtures. (You know I’m biased; I work for an admixture supplier.) Whether you use Grace’s admixtures, or the competing products, you can’t make modern pervious without the help of chemical technology.
So, if you’re a specifier, maybe call your local concrete producers, learn which admixtures they use. If you’re a concrete producer, call your admix supplier, find out what they have that works best in modern pervious concrete mixtures. If you’re a researcher, use your resources wisely, and incorporate modern pervious concrete into your research program.