When I help a concrete producer proportion a mix, I'm often asked, "How many gallons of water are in that mix?" The question always stumps me because when it comes to pervious, my brain is more conditioned to be concerned with w/c than total water.
Thinking back to the discussion of "A Mix for Every Aggregate", the aggregate will dictate how much paste it will allow in the mix, and the cement (or cementitious blend) will dictate how much water it can tolerate, from there, the numbers work themselves out. So an aggregate that only can carry 500 pounds of cement per cubic yard at a 0.28 w/c would have much less water than a mix with an aggregate that can hold 580 pounds of cementitious materials at a 0.35 w/c, but both could yield well-performing pervious mixes at however many gallons of water that comes out to be.
I'm a big fan of fines (sand) in pervious concrete. Five years ago we would have thought this blasphemy to add sand to no-fines concrete, but experience has shown us it makes a lot of sense.
When I proportion mixes, what I see is that fines can help cut powder content without sacrificing durability. So if you have two mixes, at the same void content, the fines mix will require less cementitious material, but perform the same in the Los Angeles machine with respect to mass loss. Fines in the mix will also help increase the w/c -- which is a good thing in pervious, to help improve workability and durability. In the real world, adding fines to a mix adds a little risk for the producer, because now they have to make sure the sand moisture is correct when batching. But, if they have good QC practices anyway, then this shouldn't be an issue.
One of the fundamental differences between plain concrete and pervious concrete is in the water-cement ratio.
With plain concrete, more than anything the aggregate blend dictates the w/c of a mix. As Walker and Bartel (1947) state it, "...within practical limits, for given aggregates, the quantity of mixing water per unit volume of concrete required to produce a given consistency is the same for any cement content." This is counter-intuitive to what your mother taught you when you were a kid making pizza dough -- if the dough was too dry, add water; too wet, add flour.
Pervious concrete, then, is more like pizza dough. For a given consistency, the cement (cementitious) quantity controls water demand.
I'll use the data from some lab work for the ASTM round robin on surface durability as a example. As the protocol states, three mixes were put up in the lab with voids of 18, 20, and 22%, respectively. Using the same coarse aggregate weight for each mix, the cement contents of the mixes were 655, 602, and 550 pounds, with the corresponding water contents at 216, 199, and 182 pounds per cubic yards. Doing some quick math will tell you that each mix had a w/c of 0.33.
Consistency of these mixes was measured by slump. The same old slump we use for plain concrete -- 3 lifts, 25 rods per lift. Each mix measured a 7-1/2" slump. Of course, we don't know the precision and bias of the slump test for pervious concrete, so this isn't 100% credible. However, the three mixes looked, and behaved very similar. Specifically, blow counts in compacting the specimens ranged from 15 to 20.
For concrete producers, don't get hung up on trying to hit a certain w/c out of concern for quality. Make a mix that works with your raw materials and your customer's equipment -- that will lead to a quality field installation.
If you remember the previous discussions of Gravel vs. Crushed Rock for coarse aggregate for pervious concrete, you know I'm not a fan of rounded aggregates for traffic applications.
Looking at the unpublished data from the ASTM round robin testing of the raveling potential standard, some interesting trends are present:
1) Crushed aggregates have a lower potential mass loss than rounded aggregates, but rounded aggregates seem to perform better at higher void contents. So if I was designing a mix to be as durable as possible, I'd still use a crushed aggregate to push the mass loss as low as possible. 2) In general, rounded aggregates showed a very narrow range of mass loss over a range of void contents. So whether designed at 15 or 25 percent voids, the performace will not change significantly. 3) There seems to be no correlation between mass loss and mass loss COV. This is a opposite of expected -- you'd expect as the material is weaker, the more variable it would be.
A few words of caution about the data: 1) These mixes are all old meatball style mixes, so I'm not sure how the more modern mixes would perform. 2) This was the first time for the many of labs and operators to be running the tests, so I'm not sure if we'd see lower variability on another round of testing.
3) The proposed test method is undergoing some changes as we speak, so this data should not be considered absolute scietific truth. For example the revolutions used in the test have changed from 50 to 500 to reduce the COV. Specimens are tumbled in triplets now instead of singles. However, the trends shown here are expected to have some reflection in reality.
Relationship between Voids and Mass Loss in Pervious Concrete with Crushed and Rounded aggregates
Relationship between Mass Loss and Mass Loss COV in Pervious Concrete with
Crushed and Rounded aggregates
If you remember the previous discussion of Modern vs Meatball pervious concrete mixtures, I thought some photos might help demonstrate the difference. The first photo below is the old Meatball style mix. You can see the paste was sticky when wet, and rough. The second photo is a modern mix, you can see how smooth the paste is indicating good flowability and workability.
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
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.
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.
This isn’t supposed to be a place to plug specific products, but a
forum for exchanging the latest in pervious concrete technology.
Today’s blurb, falls under both. So, to be fair, I’ll keep it short.
Construction Products has released a new admixture specifically for
pervious concrete, V-MAR VSC500. Among other things, it was designed
to improve truck discharge time, batch water tolerance, and thus
overall durability of pervious concrete. From what I've seen, this
stuff really works!