Posted By How on 22 Apr 2010 11:13 AM
I live a spot where it rains and rains and rains!!!

Using one website (http://www.worldclimate.com/) it appears your yearly average rainfall (for Battle Ground, WA) is approximately 52".  About 43" where we live in VA.  I wouldn't consider 52" (roughly 20% more than our average) a lot, but I suppose that is in the eye of the beholder.

Would you elaborate on why freezing doesn't present any problems for pervious concrete in this application?

I too wonder why pervious concrete doesn't turn to gravel if it freezes while saturated, but it is becoming somewhat common for 'green' driveways, etc, so there must be an answer to that issue.

Pervious might offer better conductivity than gravel since its structure is more continuous, offering higher cross-sectional areas of connection than gravel, but that's a wild guess.

I will start by saying that I am a homeowner, not an installer. Based on what I have read here and some reading on pervious concrete elsewhere, I would not be convinced that such a setup is superior without seeing a direct comparison (with and without pervious concrete) that includes data. I would also want to see longevity testing of pervious concrete in this type of below ground application. It's a shame this loop isn't two distinct loops (each with monitors), half with, and half without this concrete.

I imagine that water would drain/evaporate more readily in a sidewalk configuration since roughly half of the sidewalk, and the surrounding landscape is exposed to air. This should help lessen the impact of freeze/thaw cycles (due to less water being present.) Not much air 4' down, so drainage of the surrounding dirt may be very important with sub freezing temperatures. Too much (fast) drainage with the surrounding dirt and any potential benefits may be lost. Too little (slow) drainage with the surrounding dirt and freezing temperatures may wreak havoc on the pervious concrete over time.

I'm not trying to be negative. It just seems the OP is treating this setup as if it is inherently better than dirt alone. This has not been proven either way.

The rainfall in Battle Ground is mostly in the wintertime.

In the end, thanks for making me aware of the existence of pervious concrete. I may consider it for a sidewalk.

Posted By joe.ami on 16 Apr 2010 09:45 AM
What you did was improve the size of your loop system without increasing loop size and dropped it in a footprint that was less than design standards. Since most standards have fudge factor, you may get away with it, but if you do I'm suggesting you could have done it without the concrete by simply adding loops in a condensed pattern (cheaper).
j

Like grout, the middleman (material between the loop and the soil) may increase surface area in contact with surrounding soil, but it may simultaneously insulate, particularly in saturated soils. Pervious concrete when dry may add air gap to the insulation. Grout is used to ensure good contact with soil that typical backfill may not in vertical bores. For Horizonantal applications it has little value IMHO.
So, saturation offers high soil thermal conductivity and pervious concrete offers much more loop surface in contact with soil.......couldn't we have simply added loops for more loop contact with soil (since thermal conductivity is so high)?
This also takes advantage of low load in dry season, lacking appeal in many applications. We often get DIY's searching for ideas here and I wouldn't want someone in a cooling dominated climate, with rainy winter months, thinking this was a good idea for them.
j