What is causing these spots on concrete -especially when the air humidity is increasing?

[MiniMe4Eng]

Hi guys

I have been watching this for around an year and I can't find any reason for what I am seeing.

this is on my front porch, the area has roof above it and it gets those spots when it is humid outside
What you see in the picture is an extreme situation.

On a related note, a year ago I sealed the gap shown in this pictures:

Today it looks like this

I am just wondering if now there is humidity trapped under the slabs and that is what is causing this problems
Ideally I would like to install tiles over the above area to give it a better look.

 

[TehMightyEngineer]

One thing most people don't realize is concrete and cement is extremely porous and is a great sponge for water. This is not a cause for concern but it is why you shouldn't have bare, untreated wood in direct contact with concrete. It's also why you should always have a 8" minimum gap of bare foundation wall between the soil and the siding. This provides a space for moisture to travel out of the foundation wall into the outside air (rather than traveling inside and making your basement humid). So, yes, there is moisture trapped in/under the slab but it's moisture from the soil/air and will always be there. For that picture in the second post (just post everything in one topic next time) you can see the outlines of the block wall in the foundation due to the grout being able to absorb more moisture.

As for why you have different colors of the concrete it's called efflorescence. You're correct in your other post that this is salt and other deposits. The water in the slab brings salts to the surface and when the water evaporates it leaves the salts behind. Google efflorescence in concrete and you'll get a ton of info along with examples of how bad it can get (you have it very little). Overall this wont hurt the concrete too much but the salt does contribute to a slow deterioration of the concrete surface (on the order of about 20+ years, so don't worry).

As for fixing it, I wouldn't install tiles just yet. Go hire someone with a pressure washer and have them scour the concrete surface. For such a small area this should be cheap. Don't try doing it yourself unless you know what you're doing, a high power pressure washer is not a squirt gun; they can remove paint, skin, and other things you don't want removed. But, this will be good so that you can remove the salts and any discoloring materials on the outer layer of the concrete. Looking at some of the minor concrete surface spalling you have they're going to need to be careful not to over do the pressure or they'll take of a layer of concrete. They also make special cleaning machines that look similar to a push lawn mower.

That would be where I would start and I would tile or coat the concrete only if that wasn't successful.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[Ron]

As TME alludes, concrete is not a homogeneous material in some respects. It has variations in density and porosity and even more so for residential concrete that is placed with fewer controls than commercial concrete. Further, the surface has textural variations that allow more or less water to be absorbed in to the surface than other areas. All of this can create variations in the appearance of the concrete. If you place tile over the area on a properly prepared surface and with proper thinset adhesive, the moisture variations should not be an issue. Since the surface will be covered with and impervious tile, the moisture content within the slab will likely equilibrate anyway.

 

[MiniMe4Eng]

TehMightyEngineer thanks for your reply.
I am not sure that I am following you here, are you talking about cleaning the porch area or about the vertical wall showed in the other thread?
In the above pictures I do not see efflorescence at all.
What I am worried about is that due this trapped humidity the concrete might crack
As you can see in the second picture the surface next to the slab is already chipped off

 

[MiniMe4Eng]

As TME alludes, concrete is not a homogeneous material in some respects. It has variations in density and porosity and even more so for residential concrete that is placed with fewer controls than commercial concrete. Further, the surface has textural variations that allow more or less water to be absorbed in to the surface than other areas. All of this can create variations in the appearance of the concrete. If you place tile over the area on a properly prepared surface and with proper thinset adhesive, the moisture variations should not be an issue. Since the surface will be covered with and impervious tile, the moisture content within the slab will likely equilibrate anyway.

Hi Ron

That is an interesting point of view for me for many reasons.
Are you saying that if the slab is "sealed off" by applying tiles on top of it practically the moisture does not tend to migrate through the slab anymore?
Is that because the tile will practically void the moisture gradient that currently exist from one side of the slab to the other (the exterior one).
I guess that this gradient is actually what makes the moisture migration possible.

From this perspective I have another question. I have a wet smell in the basement. It seems to originate from the furnace room where the ceiling is open (this is a finished basement) and where I have a couple of openings in the finish (drywall and insulation) that covers the cinder block walls. Based on what you are saying I suspect that the humidity otherwise trapped between the basement walls and the drywall and the insulation layers is migrating toward these openings causing the smell. The smell seems to disappear when the weather (more exactly the air pressure and the humidity) outside reaches a certain point. Does this theory make sense for a specialist like you?

 

[TehMightyEngineer]

As Ron and I indicated; concrete acts as a bridge between moist areas (typically saturated soil) and dry areas (typically the exterior surface of the concrete. This is why a basement with no standing water can still feel humid (the slab and walls are absorbing water and releasing it into the basement). When you seal off a surface of the concrete it will then expel the moisture in another direction.

As for the efflorescence, just because it's not a visible layer doesn't mean it's not there. Concrete will have different moisture areas (some more moist than other) and this will cause the surfaces to change color differently over time. This can result in a pattern of light and dark colors. Often, this is because the surface of the concrete is "stained" and cleaning the outer layer you can get everything back to square one.

As for your moist basement smell I'd say your suspicions are likely correct. Obviously there could be other factors but, all things equal, I suspect that the basement being finished means that there are very few areas for the concrete blocks to radiate moisture. Having a bare gap of foundation between the finished grade and the siding will help somewhat by allowing the foundation wall to expel moisture outside rather than inside.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[Ron]

One other point to consider is whether there was a curing compound sprayed on the concrete after placement. While most of them will dissipate with time, they sometimes yield a mottled appearance like you have. A light acid wash will take care it. It you want to do a very light acid wash you can use lemon KoolAid dissolved in warm water or get some dishwasher cleaning powder and dissolve that in warm water. They both contain citric acid which will attack the surface but will be consumed in the light reaction and not harm the concrete.

 

[Ron]

Oh...and use a stiff scrub brush to work it in, then rinse completely.

 

[TehMightyEngineer]

Oh, good point Ron. A light acid wash may be the trick and is relatively easy to perform. Google should provide you with plenty of good tips on how to acid wash concrete.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[MiniMe4Eng]

Thanks guys, I will give it a try and report back here