Concrete is Porous! Testing Leaks Below Slab Foundations

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Waterlogged side of a concrete paver proving concrete is porous–but not so fast!

Concrete is porous.

I claimed water could make it’s way through a slab foundation in my old post: 12 Ways to Detect a Supply Plumbing Leak Under a Slab Foundation

But how long would it actually take for water to make its way through a slab foundation?

That’s the experiment(s) I tried to run to see how long this scenario might take.

Experiment #1: More realistic with a water hose under a back patio concrete slab.

Setting up the “leak” below the concrete patio–it actually extended 22 inches under the concrete.

It seemed like the 5-6” thick slab was a good real life candidate for a “leak” with a water hose left on for an extended amount of time. I started with a slow drip for three days. Three days later I bumped up the flow rate. After three more days I ended the experiment:

Reason #1: I was worried about the very real phenomenon of expansive soil.
Reason #2: The water was just getting wasted.
Reason #3: It was terribly hot and if anything happened to make it up through the slab, it would burn off in the sun/heat.

Cracks in expansive soil.

So if I want to do this test again, it needs to be in the winter. The only result was a cooler spot on the concrete and weeds popping up along the edge of the patio.

No water showing in the concrete on day 6 after the “leak” started below the patio slab foundation.

Experiment #2: Water under a concrete paver.

Concrete paver on water filled tupperware.

The second attempt was a concrete paver set on top of a water-filled tupperware. Several times a day I would refill the tupperware to the brim, ensuring the bottom of the concrete paver was in contact with the water.

Filling the Tupperware with water before placing the concrete paver on top.

This certainly was not realistic as the paver is rated for foot traffic only. As such, it seems it would be more porous because it isn’t as strong as, say, the concrete in your slab where you can clearly drive a car.

Proof!

But it used a heck of a lot less water, it could go on for an extended time (I have a second shower to use for personal cleanliness, to keep curious little hands away from the experiment, the walk-in shower is prettier for a timelapse).

*Lesson learned there–after a couple days I put another tupperware below the first to protect against hard water stains on the pebble mosaic shower floor. No need to ruin a beautiful shower we renovated for months on an experiment with a $1.38 paver.

After 3.5 weeks of more or less casually refilling the tupperware with water (at least once a day–usually more), there was no visual progress.

It was time…

#1. I was curious to see how far the water had seeped into the stone.
#2. I want to incorporate the results into two ongoing video series on water monitors and foundation problems.
#3. I wanted to use my shower again.
#4. Busting the concrete paver seemed like a good way to finish off a frustrating second failed attempt.

Then I realized how cinematic it could be!

Breaking Concrete Paver Camera 1

Breaking Concrete Paver Camera 2

Breaking Concrete Paver Camera 3

Breaking Concrete Paver Camera 4 — ever present Wyze cam!

Frustrations aside, it’s gonna be somewhat epic when you have dust flying out from under your shoe.

Dramatic dust flying.

In any case, the results were abysmal.

No results on top of the 1.5″ thick concrete paver.

If you have a leak, it’s going to take a long time to see it come through. 3.5 weeks barely made it past the surface of the bottom. In 30 minutes, the paver dried out, leaving a nice hard water buildup stain.

Now, if there is a crack somewhere in the vicinity–it’s going to happen MUCH sooner.

One of the sayings about slab foundations is “it’s not if it’s going to crack, it’s when it is going to crack.

Also note, some slab foundations use plastic membrane technology between the soil below and the concrete. So in that situation, obviously, you’re going to have to resort to other means of detecting a leak because the water won’t be getting through.

Second, many of the new neighborhoods around here have PEX running through the attic. The only runs of pex below the foundation are through a PVC conduit from one line of PEX above to the kitchen island. In other words, there is minimal plumbing run below the slab.

So the good news is it will take a long time to see a leak below your slab just waiting for water to work it’s way up. Tiles won’t be popping off tomorrow if a leak starts today, last month, or maybe even last year.

The bad news is it will take a long time to see a leak below your slab just waiting for water to work it’s way up. You will have to use other means to find where a leak exists.

FYI, I’ve started serially watching all of Daniel Schiffer’s videos along with Peter McKinnon. So the videos are still improving with some motion, lighting, and very thoughtfully planned transitions. In fact, my very first cinematic light showed up on my doorstep last night–very excited about the new effects coming up in some unboxing videos and a couple special request videos for Alfred.

As a result, the video showing my results start out with a bang! Literally!