Residential Foundation Slab Cracks

[SteelPE]

About a week or so ago I was asked to go look at some “foundation” issues for a client who owns a residential property. The property was built in 2016/2017. No structural drawings were provided for my review, and I have been told no structural drawings are available for review. Based upon historical construction practices for the area I would anticipate an unreinforced poured in place foundation wall to extend 4’-0” below frost elevation…. This wall would be supported by a 20” wide footing. However, this is just a guess I have no other information to go by.

While onsite I met the owner of the property who informed me that the cracks that were observed in the foundation/slab. The owner is concerned because the cracks have only been observed in the past 2 years (appearing 3+/- years after she had moved into the property). Attached you will find some of the notes that I made while I was onsite.

While onsite I observed some rather large cracks that I observed in the slab on grade (larger than what I have seen in the past). That being said, I did not observe any locations for shrinkage relief in the slab on grade…. And with a 50’ slab pour I would anticipate some amount of shrinkage cracks to form right after the concrete is placed… and at point of restraint (which was where the observed cracks were forming) but to have them show up 3 years after construction in an enigma.

I also observed cracking in the exposed foundation walls. Most of this cracking was observed at points of restraint in the wall (at change in directions) and at locations where the wall steps (varying support restraint of the wall). Cracks were observed under one beam bearing locations; however, this was also at a change in direction of the perimeter wall (points of restraint).

Does anyone have any insight as to what might be cause the issues observed onsite?

Part of me wants to say that the cracks are not much of a concern as they were observed in a slab, but then again, they are quite large.

 

[KootK]

That being said, I did not observe any locations for shrinkage relief in the slab on grade….

That one. If you build unreinforced and don't make provisions for planned cracking to occur as a result of shrinkage restraint, there's a pretty good change that you'll get unplanned cracking as a result of shrinkage restraint. None of this looks structurally concerning in my opinion.

 

[KootK]

...but to have them show up 3 years after construction in an enigma.

One might envision a process like this:

1) Slab initially has some tension in it due to shrinkage restraint but not enough to crack.

2) Slab tension is added as a result of moisture changes in the supporting soil (settlement) and/or surrounding air. Or, simply, load applied to the slab.

3) 1 + 2 = crack time.

Even without environmental change, I believe that concrete subject to significant tension will often wind up cracking over time even if it doesn't right out of the gate.

Regarding the "bigness" of the cracks, that is also to be expected when individual cracks are made to relieve the tension developed in a slab over large distances.

Lastly, homeowner estimates of when things occurred is notoriously unreliable. The basement looks pretty empty here. Any chance that he home is up for sale or recently sold and, therefore, was full of boxes engine manifolds a few months ago?

 

[MotorCity]

The fact that the cracks radiate from posts, wall corners, re-entrant corners, etc. is generally a telltale sign of restraint cracking that results from shrinkage. Based on your "quite large" description, I was actually expecting the cracks to be wider. While I agree they are not unnoticeable, I wouldn't say they don't appear unreasonably wide due to shrinkage. Mother nature usually does a pretty poor job at laying out orthogonal relief joints.

 

[jayrod12]

Not a residential example, but on a project I had a call from the contractor. Similar thing, large cracks but it was relatively early in a project, i.e. the owner hadn't taken possession of the space yet. It was shrinkage cracking, and in my case we hadn't been forwarded the concrete tests. So I requested the concrete tests, all looked good except for the tester's comment at the bottom of report "After sample was taken, contractor added water to the mix."

Why no one told us that was done is beyond me, what are we having the concrete testing companies come out for? But that's not the point of this story, the point is the added water increased the shrinkage potential and in my situation resulted in 1/2" wide cracks. I also know from experience that residential contractors are the first to put a hose into the truck when the concrete is a bit too thick.

 

[epoxybot]

Just to be clear, you're saying there are no plans on file with the county building dept.
This looks like settlement cracking.

 

[SteelPE]

I remember reading somewhere that concrete is expected to contract .1%-.05% of it's length... which equates to 1/16"-1/8" for every 10 feet..... so across 50' you would be looking at 5/8"-5/16" of contraction. Which is in line with what we are seeing.... but I don't want to just blow this off to shrinkage.

When I breached the subject of shrinkage they were dismissed because of the time it took for them to appear... which I agree, is coming from an unreliable source.

 

[RPGs]

It's much more likely that they are experiencing the Frequency Illusion: "increased awareness of something creates the illusion that it is appearing more often" (Wikipedia definition).

Not always easy to convince clients that "you're most-likely imagining the cracks are all appearing at the same time because you noticed them once" but those cracks are all very consistent with shrinkage at restraining points and I would not be very concerned with them.

In this scenario I always break out a laser level/tape measure and see if I can pick up any differential settlement in the slab, but even then--some settlement of the slab is not super concerning or an indication of a severe deficiency, unless located in a suspect area, like around a column or point load in foundation wall. That said, lightweight residential homes can tolerate quite a bit of differential settlement and, if they're not seeing drywall cracks upstairs that scare them (or you), it's not something to lose sleep over.

 

[SteelPE]

Not always easy to convince clients that "you're most-likely imagining the cracks are all appearing at the same time because you noticed them once" but those cracks are all very consistent with shrinkage at restraining points and I would not be very concerned with them.

I completely understand this. I was quite skeptical when I was onsite and I didn't want to offend the owner. She had an architect friend there who believed her story 100%. I am not saying I don't believe her story, I just find it hard to believe that you wouldn't expect to get any shrinkage cracks in a 51'-0" long residential placement with no contraction joints when I place contraction joints in my slabs at 12'-15'intervals. So in this slab I would have at least expected to have a minimum of 4 joints in both directions.... maybe more.

I just walked a slab on Friday on a 360' square building 7" thick with joints every 13'-4" o.c. The slab was placed a little over a month ago and from what I can tell, it's as perfect as you can expect (no a single crack).