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Slab on grade crack cause and repair

Slab on grade crack cause and repair

(OP)
Good day,

I recently purchased a house and after removing the carpet i realized there were cracks at every corner of every room.
Some of the cracks sound hollow.
The house is a flat house constructed in 1991 and is approx 1600 sq.ft covered area, sitting on flat (reclaimed) land approx 4100 sq.ft area.
I am wondering:
1. What is the cause of this?
2. What the crack patterns mean?
3. How serious these cracks are?
4. The best method of repair?
Thanks!

RE: Slab on grade crack cause and repair

Probably restraint shrinkage cracks, which are of little consequence in a slab on ground. New floor coverings will conceal them.

RE: Slab on grade crack cause and repair

Tapping makes a hollow sound... can you use a 3/8" carbide drill and try to drill through the concrete to see if there is anything (soil, hopefully) below? You can patch the drilled hole with cementitious products or epoxy...

Reclaimed land... you may be experiencing differential movement with the foundations settling more than the slab...

Are there other homes in the area experiencing the same type of problem?

Dik

RE: Slab on grade crack cause and repair

Agree with dik and hokie66...probably just drying shrinkage cracks that were not controlled by the contractor. Common in residential construction.

Lay a straightedge across the cracks. If one side is higher than the other, then the crack is "faulting". This means there is possible movement or displacement of the supporting soils below the slab. There can be many reasons for this, but fill control on reclaimed land is often suspect.

Based on your photo, assuming this is at an outside corner of the slab, it is probably restrained in two directions (by the footing) and is shrinking in its mass of the remainder of the slab (toward the middle of the slab). As for the location of the crack, it could be a thickness anomaly in the slab (thinner at the crack), but not necessarily so.

Since the crack is relatively tight and has been there for 20+ years with no apparent adverse affect, I would either leave it alone or if you are concerned with insect intrusion (termites, ants, etc) then fill the crack with a flowable acrylic or epoxy adhesive. You can use a sealant, but the crack probably wouldn't accept much material.

RE: Slab on grade crack cause and repair

I agree with Ron, if the other cracks have a similar appearance and are all at exterior corners or , you are most likely looking at shrinkage cracks. The contractor could have placed reinforcing in this slab to stop/reduce these cracks or they could have detailed a construction joint. In general, it doesn't appear to be a significant structural concern.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

RE: Slab on grade crack cause and repair

My only concern was that it sounded 'hollow'... sometimes soil can be washed away by foundation/water problems.

Dik

RE: Slab on grade crack cause and repair

Is the house slab constructed using UNBONDED post-tensioning? If so, be careful with drilling and chipping!

The hollow sounds (and the photo shows a relatively isolated single crack only), may indicate a delaminated topping. Chain-drag other non-cracked areas of the slab and determine if they have hollow sounds too.

You need to trim your toe nails too smile

And wear boots before doing any construction-type work!

RE: Slab on grade crack cause and repair

(OP)
Thank you all for your advice! (even you Ingenuity- I have trimmed my nails since then lol)

dik:
A few of the neighbours claimed that they do not have any cracking whatsoever.

Ron:
The crack shown in the picture is in one of the bedrooms. All the other bedrooms all have similar cracks at all four corners.

Ingenuity:
I am uncertain but i believe it was constructed with only BRC re-inforcement if any.

It does seem as though the center of the rooms have sunken a bit because the crack is higher on the corner side.
I have a feeling though, that the cracks have opened a bit recently because when i first inspected the house i did not hear any hollow sounds but only after starting construction and demolishing the entire roof (there was a lot of rainfall some of which could have seeped through) I noticed the hollow sound.
Does this mean there is more cause for concern??

RE: Slab on grade crack cause and repair

Hard to say without knowing more about the type construction and the subsurface conditions. Because this cracking is typical of individual rooms, it suggests that there are footings under the walls of those rooms, which would be restraining the slab and leading to shrinkage cracking. This may also explain the slope in the floors, as the footing founded lower may be on better ground.

If possible where you are located, I think advice by a local structural or geotechnical engineer is warranted.

RE: Slab on grade crack cause and repair

If there are cracks in the walls too, I would start to worry. That would indicate a larger settlement issue rather than local shrinkage.

With the hollow sound, that already indicates local settlement to me.

As Hokie said, call in a geotech or structural to ease your concerns (hopefully). :) Good luck.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

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