Concrete Cracks formed just after finishing 3

[N3M4N]

Hi all,

I am working on a project where we recently poured a suspended slab, cracks similar to the attached photo was observed (5-7 hours after pour) at several locations, cracks do appear superficial. I would like to know whether there should be a structural concern?

Concrete was cured 8-9 hours after placing using curing blanket,next day when I checked temperature beneath the curing blanket it read 38.5 degrees celsius, has the concrete cured sufficiently at this temperature?

Thanks in advance for feedback.

 

[oldestguy]

Looks like a high w/c ratio. What was the slump? When was finishing done with respect to placement? Could have been on it too early bringing up water.

 

[N3M4N]

Concrete strength 35 Mpa
concrete temperature at time of placement 27 degrees celsius
slump 120mm to 180mm (Tested avg 140mm)
air 5-8%
Finishing of concrete was approx 4-5 hours after placement

 

[TehMightyEngineer]

Looks like classic plastic shrinkage cracking.

https://www.nrmca.org/aboutconcrete/cips/05p.pdf

It shouldn't be a structural concern but it's definitely not acceptable without being addressed by the concrete contractor.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[oldestguy]

I've seen the workers add water before or during the first part of placement, some time after the slump was measured. My stepson drives a ready mix truck and he says that habit is very common. As a driver he just takes orders from the job foreman and has no control of the adding of water.

 

[XR250]

As a driver he just takes orders from the job foreman and has no control of the adding of water.

Hopefully, he keeps documentation of how much water was added.

 

[JAE]

If you fine trowel air entrained concrete (5-8% as you state) you could be looking at some blistering there.

Fine troweling air entr. concrete will densify the top thin layer of concrete and work water and air up to just below that densified surface and create void areas there.



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[dik]

I hope that slump is with superp... What was the weather like... hot and windy?... exterior exposure?

Dik

 

[dik]

As a driver he just takes orders from the job foreman and has no control of the adding of water.

If the supplier is responsible for the mix... the driver should not be taking instructions from the foreman... foreman should contact the supplier for instructions.

Dik

 

[oldestguy]

dik: Sounds nice, but with a busy schedule, everyone occupied, not very practical.

 

[hokie66]

TME is correct. They are plastic shrinkage cracks, caused by atmospheric conditions, poor finishing technique, and perhaps to a lesser extent the concrete mix. These cracks should have been closed during finishing.

 

[dik]

In these environs, without the supplier's OK, water is often never added to a mix.

Dik

 

[dik]

hokie, that's what it looks like to me...

Dik

 

[Ron]

Yes, those are plastic shrinkage cracks. They will develop into full depth shrinkage cracks due to the reduced cross section.

Look to the right at 4-1/2 inches, you see a long jagged crack, then more to the right a short discontinuous crack, then further to the right, another longer jagged crack......classic plastic shrinkage.

 

[oldestguy]

A quote from National Concrete Precast paper "Reducing Shrinkage Cracking". Is one sure all these precautions were followed? The waiting required for sufficient setting is an irritation easily affecting the job. Note one of the specs. says finishing to wait a given number of hours. Now what do you expect the guys to do when there is not sufficient setting to allow getting on with a power trowel. Lets say an experienced worker will want to wait two or three more hours to do it right? Writing the spec with given hours is likely to cause problems. My "spec" says wait until a trowel rings when moved across the slab. In hot weather that might be much sooner than in colder weather. I suspect the problem here is the specs.

A common mistake to watch out for is bleed water that is worked back into the top layer of concrete. This will increase the w/c ratio of the top layer of concrete, which causes a reduction in strength among other critical surface issues. This also creates a problem by eliminating water that would normally evaporate, increasing the temperature of the concrete and subsequently increasing the occurrence of cracking. Do not begin finishing the concrete before most of the bleed water has evaporated.

 

[charliealphabravo]

I agree that these look like plastic shrinkage cracks, but since this is a suspended slab you might confirm that these cracks are not systematically located over supports. I think excessive load deflection at negative moment areas could cause similar cracks while the concrete is plastic.