Residential Slab Cracks 7

[mferg318]

I am inspecting some cracks in a residential concrete slab. The crack is a little more significant than I usually see in the house slabs I inspect, but I was not able to find any other issues with the house that would indicate a significant foundation issue; no cracking in interior drywall or exterior brick walls, no racking of doors/windows, etc. My guess is that there was some settlement at some point that caused the crack, but it has since stopped moving. The owner says the crack has been the same for years. Has anyone ever seen a similar situation with a significant settlement crack but no other issues? My general experience is that these cracks don't typically result in any significant structural issues. In fact, I think there are a lot more of these cracks that are never seen because they are hidden under flooring or carpet/

 

[r13]

IMO, this is a settlement crack. The subgrade may contain clayey material that has experienced volume change, it settled somewhere and push the slab upward. This type of crack usually is located away from the stiffer elements (wall, column).

 

[JoelTXCive]

I think all you can do is note the crack on your report.

I might also throw a comment about any future hard surface flooring (tile/wood) needing to be isolated from the slab.

That floor needs to have carpet always; and not tile/wood on it.

 

[XR250]

Run a laser or a level on the floor to verify if it is settlement. Looks like shrinkage to me - especially w/o any Sheetrock cracks (unless they have been nicely patched) 1/8” crack every 20 ft.can be considered normal.

 

[dik]

A steel straight edge may be more appropriate... and can use feeler gauges to see 'how much'... that has to be the biggest shrinkage crack I've seen (if shrinkage).

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[XR250]

A steel straight edge may be more appropriate... and can use feeler gauges to see 'how much'... that has to be the biggest shrinkage crack I've seen (if shrinkage).

A steel straight edge can be deceiving due to potential slab curling at the crack. I see shrinkage cracks like that all day long in tract houses.

 

[JoshPlumSE]

Looks very similar to a crack I found in my house when I pulled up my kitchen tile. Mine was a settlement crack for sure as there were other signs of settlement where the house was added onto over what must have been non-engineered fill.

Settlement can happen for all kinds of reasons, obviously. I've seen it happen because of a leaking pipe or sewage line, poor soil, or nearby excavation.

Is this a garage slab? My tendency is to say that garage slabs are the worst. They may be poorly built (WWF that wasn't pulled into the middle of the slab) or under reinforced. They may not have expansion joints at appropriate locations resulting in cracking. et cetera.

To me the important thing is the safety. If the slab has been like this for years and isn't directly supporting the structure above then this is probably a serviceability issue. There are ways to repair it or make it look better. But, what the cost vs benefit.

 

[dik]

To me, that's a huge shrinkage crack... must have lived a sheltered life... if curling is an issue, I have a 8' straight edge that could indicate curling... I usually use a 1' or 18" steel straightedge or whatever I have with me. unless you are going to use a total station with the precision, a laser level 'dot' may be larger than the difference in elevation.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[darthsoilsguy2]

i wonder if it's more than shrinkage. for a crack of that size to be only shrinkage, i would think you would have cracks intersecting it over such a length that we're not seeing. When no better info is available and you're seeing an old slab cold.... the most important thing for me to figure out if a large crack is shrinkage is to have a map of the entire slab pour with the cracks, understand the slab restraint points, and imagine the slab mobility during uniform volume loss during plastic state.

i would suspect initial crack was shrinkage that became weakest link and led to settlement by loads. also hard to tell from a photo but seems like the mating surfaces don't line up consistently, so the crack might be smaller than it appears since the shearing off of the top edge-line of the crack doesn't count as crack.

 

[mferg318]

Thanks all for the replies. Seems that my initial diagnosis as a settlement crack is likely confirmed. This house has all exposed concrete floors, and this crack unfortunately runs right through the living room. While the crack is unsightly, I cant find any other symptoms that would indicate a real structural issue, so I will give the homeowner some cosmetic fixes. As I said in my original post, I suspect that if the carpet and flooring was pulled up, you would likely find similar cracks in a lot of houses.

 

[dik]

Epoxy injection and finish flooring...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[GeorgeTheCivilEngineer]

Suggest bright red epoxy to make it look like an opening to the underworld. Would be quite the talking point.

 

[Compositepro]

I deal with shrinkage cracking in composite materials. One important point to be aware of is that volumetric shrinkage can be almost entirely in the unrestrained Z-direction, when the X- and Y-directions are restrained by reinforcement.

 

[r13]

This picture depict the type of shrinkage well. It is helpful when plan the steel reinforcement.

Another interesting chart - shrinkage vs time and relative humidity.

Other than plastic shrinkage, drying shrinkage, here is another type:
Carbonation Shrinkage
Carbonation shrinkage is a phenomenon very recently recognized. Carbon dioxide present in the atmosphere reacts in the presence of water with hydrated cement. Calcium hydroxide [Ca(OH)2] gets converted to calcium carbonate and also some other cement compounds are decomposed. Such a complete decomposition of calcium compound in hydrated cement is chemically possible even at the low pressure of carbon dioxide in the normal
atmosphere.

Carbonation penetrates beyond the exposed surface of concrete only very slowly. The rate of penetration of carbon dioxide depends also on the moisture content of the concrete and the relative humidity of the ambient medium. Carbonation is accompanied by an increase in weight of the concrete and by shrinkage.

Carbonation shrinkage is probably caused by the dissolution of crystals of calcium hydroxide and deposition of calcium carbonate in its place. As the new product is less in volume than the product replaced, shrinkage takes
place

 

[Ron247]

1. To me this crack is much wider than a typical shrinkage crack. It is either extremely wet concrete or more than simple shrinkage.
2. Regardless of cause, it is probably not a true structural issue, i.e. not going to cause collapse or structural failure.
3. I would further investigate expansive soil possibilities.
4. The Owner saying it has been the same for years is not very reliable. Problems like this change so slowly that no one ever notices the change. Worst one I have been involved with is 14" of soil loss over 15 years. Owner honestly stated he did not believe that but I 100% proved it happened. A child is born and 16 years later they are hitting the parents up for a car, insurance and gas money. But on any given day, the newborn never looked any different.

 

[Ron]

This crack has nothing to do with settlement. It is a shrinkage crack that has progressed with time. This is not uncommon in residential construction where the concrete is placed very wet. I've seen this many, many times and have had the opportunity to watch the progression.

I've seen worse. There is no significant evidence of faulting at the crack. If you cored through the crack and looked at the crack face you would likely find that the coarse aggregate was separated from the paste near the top without fracturing the aggregate, which essentially creates a "control joint" where the concrete below will fail in tension as strength gain occurs. This is common in high slump mixes, often used in residential construction.

 

[XR250]

Thanks all for the replies. Seems that my initial diagnosis as a settlement crack is likely confirmed

No it's not. You have presented zero evidence that this is a settlement crack. Unless you have used a level of some sort to measure the floor elevations, how are you coming to this conclusion? There is no displacement across the crack.  What would be the mechanism causing the crack if it was, in fact, settlement? Let's say, for giggles, that the crack is 1/8" on top and tight on the bottom of a 4" slab. 10 ft away, you should expect to see over 3" of slab elevation difference. That would be pretty obvious without any measuring device. Even if it was 1/16" on the bottom and 1/8" on the top, that is still over 1 1/2" at 10 ft - again pretty obvious. As Ron mentioned, high slump concrete will easily shrink the amount you are seeing.Granted the repair in this situation is the same for settlement or shrinkage - do nothing. Maybe caulk to keep the radon at bay and be mindful of hard floor finishes.

 

[MIStructE_IRE]

To me that’s nothing more than shrinkage. If it was settlement you’d also be seeing issues elsewhere. Ron is spot on above!