Pervious Concrete: The Road Less Raveled

Check Dams for Pervious Concrete

Originally Posted 7/17/09

The city of Olympia, Washington is a leader in the municipal implementation of pervious concrete technology.  On their website they show a good detail for check dams under a pervious concrete pavement.  There is also a presentation with a few good photos of check dam installation here.

The Last Load of the Day

What causes the last load of the day to show more raveling than normal?

First things first. -- what causes raveling?  Among other things, raveling in pervious concrete may come from weak cementitious paste in the mix, due to a low water-cement ratio, which prevents complete hydration of the cementitious materials.  

Three things might reduce the w/c to such low levels at the end of the day -- more so than during the course of the day: the subbase, the coarse aggregate, and the crew.

THE SUBBASE: Most pervious concrete crews are good at wetting down the subbase at the beginning of the day.  As the day wears on, the subbase tends to dry if not repeatedly sprayed.  Therefore, if a contractor wets the subbase in the morning, but not the rest of the day, by the time the last load rolls in, the subbase is both hot and dry.  A hot and dry subbase will suck moisture out of the fresh concrete and heat it up, thus speeding evaporation and reducing the overall w/c.

THE COARSE AGGREGATE:  Most concrete producers wet their rock while it’s in storage (stockpiles or silos), but have no means to keep it wet once it’s up in the plant.  During the morning hours, this works just fine because the rock is moving through the plant so quickly that is doesn’t have time to dry.  As the day wears on however, typically concrete production slows, so the aggregate isn’t moving through the plant as quickly.  When it comes time to batch the last load, the rock may have been sitting in the sun heating up and drying for some time.   Dry rock batched into pervious concrete will suck water out of the cementitious paste, thus reducing the effective water-cement ratio.

THE CREW: Most crews are good at covering up the concrete quickly after it's compacted and jointed.  As the day wears on, the crew wears out, and the process moves more slowly.  On a cool, cloudy day this won’t hurt the pavement much.  But, on a hot, sunny day, evaporation is so rapid, that 10 or 20 extra minutes where the pavement is exposed can reduce the water-cement ratio of the paste at the surface.  

Understanding these three factors makes prevention of distress easy.  Keep the subbase and coarse aggregate wet, and keep the crew moving.  Or, just don’t place the last load of the day.

Pervious Concrete Pollutant Removal Study

The results from the Florida DOT’s study on pervious concrete’s pollutant removal capacity can be found here. 

I was lucky enough to have worked with this research team on some of their earlier work on pervious concrete, but was not heavily involved in this one.

Proper Batch Water for Pervious Concrete

   

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.

Adding Water To a Load of Pervious Concrete

The first post is always the hardest to write, and this has been no exception.  This entry was written over a year ago, and has been changed many times - partly because the technology has advanced over that time, but partly because my opinion has changed too.  So here it is, my first formal blog post, enjoy!

Early in the learning curve, most contractors are told not to add water to a load of pervious concrete.  The fear is that the concrete will become too wet, and not end up truly pervious. 

Relax.  It's okay to add water on the jobsite.  Without enough water, the cement will be starved, and the concrete won't develop the strength it needs to hold traffic.  So, if the mix is too dry, add water.  To quote the NRMCA's text reference for their Pervious Concrete Contractor Certification, "Adjustments can be made to a load that is too dry by simply adding a little water." 

Now that I've told you it's okay to add water, understand it must be done carefully so that the mix doesn't become too wet.  To understand this better, there are three possible problems with adding too much water: the paste can become too thick and close the voids, the paste can become too slick and allow the concrete to compact too easily, or the paste can become too loose and bleed down through the pavement causing it to seal below the surface.  Understand these, and you'll have a better feel for how much water you can add on the jobsite.

First, let's look at paste thickness.  Paste in this situation is made of portland cement, water, and any other powder like fly ash or slag.  A typical pervious concrete mix is designed with about 20% paste and 20% voids.  They say that below 15% voids, the concrete isn't truly pervious.  In a typical mix, then, you have about 5% extra voids as a safety factor.  As you add water to a pervious concrete mix, the ratios change - paste content goes up, and voids go down.  Going through the math, you'd have to add about 10 gallons per yard to add 5% paste, and lose 5% voids.  I'm not saying go ahead and add 80 gallons to a load, because the next two issues might pop up at lower water contents.

Next, let's look at compaction.  The voids in a pervious concrete mix are designed using the standard compaction of the Dry Rodded Unit Weight.  If the aggregates are lubricated with cement paste, then those voids might close more easily through compaction.  The wetter a mix is, the slicker the paste will be, and it will be easier to reduce that void space.  Adding water to a mix that has no sheen won't impact this, but adding water to concrete that already has sheen and is compacting easily could lead to trouble.  Unless you are using very heavy compaction equipment, though you won't be likely to see this - more often you'll see one of the other two issues pop up first.

Lastly, we need to talk about paste bleed.  This happens when the paste is too wet and seeps down through the voids.  You won't notice this at the surface unless you watch carefully as the concrete sits, to see the paste oozing down off the aggregate.  From my experience, this happens when three to four gallons of water is added to a good load of fresh concrete.  If a viscosity modifying admixture is in the concrete it will probably be able to take an extra gallon or two per yard without bleeding, giving you more freedom to add water without worrying.

As always, before the job starts, the contractor and producer need to test the proposed mix to determine its range of water sensitivity, and come to an agreement as to how much water can safely be added to a load.  Until we have a standardized procedure for testing the water sensitivity of a load of pervious concrete, here are a few rules of thumb to try in the field (assuming the mix has 600 pounds of cement per yard):

• If the concrete has no sheen, add three to four gallons of water per yard to the truck before it leaves the batch plant and check it again. 
• If the load had sheen, but is drying up, add two to three gallons per yard
• If the load has good sheen and is compacting well, leave it alone
• If the load takes more than five gallons per yard, it was probably batched too dry (We'll talk about this more in a later post.)

In the end, don't be afraid to add water to a load of pervious concrete that truly needs it.