Large Concrete Cracks in Basement 12

[cal91]

I was asked to look at some slab cracks in an unfinished basement. The house is coming up on a year old (when the warranty expires).

The builder was asked to come and look, and he said they were fine and they would not fix the cracks.

The cracks are near structural posts, and there is moderate elevation change across the crack. The side with the post is lower. I suspect there is not a thickened slab under the post and the post reaction has cracked through the slab. I have not seen any building plans however.

I am wondering if the IBC or some other source I could reference that defines acceptable crack widths and elevation differentials?

Any opinions would greatly be appreciated. Thanks!

 

[dik]

looks really ugly... more than shrinkage cracking... just a few questions: is the post and wall sitting on the slab? is the post and wall taking any load? what is the soil like under the slab? what is the construction of the slab?

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

-Dik

 

[KootK]

ADA and NFPA (egress) both have trip hazard limits at 1/4". I doubt either applies to this but it's a basis for comparison at least.

 

[cal91]

Thanks guys.

I'm not sure if there's a footing under the (4) ply post. There SHOULD be, but who knows? The (4) ply is a post that continues through the floor and up to the roof. The (3) ply is header studs.

 

[Rabbit12]

Yikes. Not surprised the builder would try to dodge this one. I've heard more horror stories than I care to admit about builders being really shady on warranty items.

Around me the go from a foundation to a framed enclosed house in two days. Can't imagine there is much if any oversight at all.

I'm with dik that's more than just shrinkage cracks. Any expansive soils in your neck of the woods?

 

[Rabbit12]

What does the connection of the wall look like at the top and base? Typically those should be allowed to float. While likely not the chief cause of the problem it could be exasperating the issue.

 

[MegaStructures]

Definitely has “yikes” factor to those cracks. I went to an ACI conference and got a crack card to measure severity of cracks. These are about the size of the whole card let alone the scale marks on the card. Can’t say I can really contribute to answers, but I have questions.

How would the post create cracks like this? It seems the top side of the slab would be in compression when considering flexure, would shear cracks present like that, or likely in a perimeter around the post?

Possibly there is expansive soil underneath?

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

 

[XR250]

Get yourself a hammer drill and determine the slab thickness, potential footing existence and soil quality.
That could certainly be shrinkage if a lot of water was added to the mix. The vertical offset might also be due to slab curling.

 

[lsmfse]

I remember when I was building my house, the contractor poured all the perimeter, interior continuous and interior spread footings.
The slab section consisted of 4" sand base with w.p. membrane.
When they came to put in the sand, they had this conveyor thing that shot the sand into the pad.
The sand covered up all of the interior footings although it was obvious where they were since slab dowels could be located.
We cleaned the sand and all was good. I'm with XR, get a hammer drill and see if you have solid concrete between the slab and footing (If one exists)

 

[BAretired]

You need to find the building plans to determine what was specified.

BA

 

[rowingengineer]

Wouldn't do any destructive investigations, without builder permission.

I would stick to non-destructive ways like ground penetrating radar or similar.

I would get plans like BA suggested.

And if possible find a reference that you can use similar to this from the Australian standards. Basically anything above damage level two is considered a defect in Aust.

 

[Canuck65]

In my jurisdiction there are not codified reference for crack widths in a residential concrete slab-on-grade. However, there are Construction Performance Guidelines published by Tarion that are a useful reference. Tarion is the agency that mediates between the contractors and new homeowners with respect to warranty work. Thus they publish a very comprehensive guide that determines what is considered a defect and what is considered acceptable construction tolerances, etc... It is by no means perfect, but it does contain some useful information. Download the guide and look up the concrete basement slab-on-grade section. In my area a typical residential slab-on-grade is 3" thick, unreinforced, and no control joints provided.

https://www.tarion.com/resources/construction-performance-guidelines

These can be difficult and costly to fully investigate. It looks like more than shrinkage cracks to me. Settlement? Settlement combined with part of the slab getting hung up on a strip footing? Heaving? (I looked at one place where we suspected the slab had cracked due to frost heave during winter construction), inadequate subgrade preparation (poor compaction, large stones under slab), poor soil, etc...

You could investigate this to death, or I have had success in the past A) listing as many potential causes of the deterioration (see above), B) proving the deterioration is beyond normal construction tolerances and then C) basically stating that all the potential causes were within the care and control of the contractor, therefore it is the contractors problem to fix.

 

[CarlB]

Looks to me that a hammer-tap or chain drag survey would indicate voids under the slab. Cracks could indicate settlement has occurred due to the column load and poor soil support. Maybe little, if any slab reinforcing.
You might be able to detect a depression at upper floor levels bearing on this column.

 

[racookpe1978]

My basement slab cracked through due to what we thought was simple external root invasion (lifting) at one corner.

Turns out the roots did invade, and were the focus of the cracks, but the original builder in 1977 has simply laid plastic over the hand-dug and hand-leveled basement slab. Places in the slab were as little as 1/2 inch thick, others were 4-5 inches thick where the concrete was used to "level" the irregular hole for the basement. Where the slab was thin, it cracked. There was no "connection" between the wall foundations (concrete block underground, or brick and frame wall where exposed) and the basement slab.

A simple drilled hole (1/2 inch or 3/4 inch dia) in several places will tell the average thickness, and confirm or deny the location of the needed pads under the vertical column load points. My opinion? No pad, or too small a pad under the 6x column load point, and the concrete slab is being either punched through, or has yielded already and is being driven down into the loose, but now-somewhat-compacted soil under the load point.

 

[oldestguy]

Likely shrinkage cracks. I'd leave them and expect a significant reduction in asking price. Enough reduction to replace the slab some day.

 

[dik]

I've never seen that much water...

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

-Dik

 

[racookpe1978]

No, not 1/4 cracks, all located on the load-side of the vertical column load, radiating from that point load. Too much vertical movement for shrinkage cracking.

 

[kostast88]

Blind Guess: it's cracking that started due to dry shrinkage (most of it happens during year 1), but the post load combined with poor (or no) soil support stressed the slab more than it could take. Reinforcement wasn't enough (or doesn't exist) to resist cracking so here it goes. If you have a Ferroscan in your possession it might be wort to scan locally, I suspect no rebar would come up.

 

[MegaStructures]

CarlB if from voids under the slab are we thinking flexural cracks started on the tension side of the slab and progressed through the cross section all the way to the top? I’m not seeing a lot of talk about where the stresses are in the slab and why the cracks would open where they are.

Again I’m just stealing a piece of this post to try to learn!

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

 

[cal91]

Thanks all. Trying to get building plans and will update soon.