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.
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