Cracks in foundation

[dtmilad]

Hello,

We have a 70 cm raft foundation, it was poured around 1 month ago, last week we have noticed cracks connected to its corners.

The image shows these cracks (red lines).

Are these cracks due to the shape of foundation, (knowing that no corner reinforcement were provided) , or it could be because of soil settlement.

Thank you

 

[hokie66]

Drying shrinkage cracks, which form at those locations due to stress risers, as you say due to the shape, reentrant corners.

 

[dtmilad]

Thank you Hokie66 for your replay.

The foundation is around 40 x 40 m and the crack continued along the whole foundation length, which made use worried about the possibility of soil-structure interaction since we have a limestone soil with small cavities.

 

[concretemasonry]

There is no surprise about the cracks. I am surprised that there were not a couple more with a lack of corner reinforcement. - That is a basic design/detail flaw.

The micro-management of the soil-structure interaction problem that also cannot be altered.

Expect some addition changes/cracks in the crack patterns as the concrete continues and experiences some thermal changes if you are in a cool climate.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[JoshPlumSE]

Likely shrinkage cracks. What's the approximate width of the cracks? The presence of cracks is not really a problem if the cracks are narrow enough that they won't cause other problems.

 

[dik]

The slab should have been sawcut at the proper time and likely your cracks would not have formed.

Dik

 

[ATSE]

Respectfully disagree with dik.
Never sawcut a structural mat (raft) foundation. Sawcuts and tooled joints are primarily for thin lightly reinforced slabs on grade.
These cracks are somewhat preventable [manageable and kept very small] by:
a) more X-Y reinforcing,
b) corner reinforcing, and
c) construction joints ("cold joints")

 

[dik]

ATSE: sorry, missed the 70cm thickness.

Since the shape has likely caused the cracking, I'd have poured it in separate 'chunks' and may have increased the reinforcing, too. Don't know how it's loaded.

Also, low slump and superplasticiser and larger aggregate size.

Dik

 

[Teguci]

As JoshPlum says, these cracks are not necessarily a problem. Unless there is a corrosion issue (ex. landfill, shallow raft near ocean, etc...), foundations are a great place for untreated shrinkage cracks to occur and to be ignored.

Assuming you have top and bottom reinforcement (of course you do for a 70 cm raft over karst topo), carry on. If these cracks were due to settlement, (in the unlikely event that you have a sinkhole at each corner of your new foundation) You should be able to verify by checking elevations. In which case, you should provide Mr. Magoo, your geotech, as an offering to this newest cenote.

 

[dtmilad]

Thank you all for your responses.

Could you please check the images for the cracks, we are about to cast another foundation next month and we need to try to avoid these cracks.

 

[dik]

Unless you pour it in 'chunks' and/or increase the reinforcing to distribute 'smaller' cracks, you will likely end up with cracking similar to the first one.

Dik

 

[Teguci]

let the concrete crack. Its a foundation slab that won't be seen.

If you must, you can add a top mat of welded wire reinforcement to redistribute the cracking over more area (more smaller cracks that no one will ever see again). Throw some small corner bars at the reentrant corners as well.