Concrete Cracking 4

[M40A]

There was a new building constructed (less than 2 months old) and the reinforced concrete slab has considerable surface cracking near the control joints in some instances and nearly in the middle of others.

There are no records of control testing during the construction of this bldg - nonexistent (i.e. no In-place densities on the subgrade and base materials, no documentation of the concrete testing - slump, air, W/C ratio, mix design, etc...)

What are some possible root causes that may have contributed to this type of failure?

What is a good approach in determining what actually caused this failure and what are some possible lines of resolution? (tearing the whole building down and starting over - not really an option.

 

[bimr]

Plastic Shrinkage Cracking: When water evaporates from the surface of freshly placed concrete faster than it is replaced by bleed water, the surface concrete shrinks. Due to the restraint provided by the concrete below the drying surface layer, tensile stresses develop in the weak, stiffening plastic concrete, resulting in shallow cracks of varying depth. These cracks are often fairly wide at the
surface.

http://www.cement.org/tech/faq_cracking.asp

Can you post a picture?

 

[M40A]

Thanks bimr - excellent information.

 

[M40A]

Thanks bimr - excellent information. Please see attached picture. This is one of the most severe instances of the cracking along the control joint.

 

[hokie66]

Those are not plastic shrinkage cracks, but rather drying shrinkage cracks. The ones in the middle of the bays are probably due to excessive distance between joints, joints cut too late, too little reinforcement, little or no curing...the list goes on. At the joints, the cuts may not be deep enough, cut too late, etc. As there are no records to use in assessment, suffice it to say that these type cracks in slabs on ground are common when quality control and supervision are minimal.

 

[bimr]

I agree with hokie66, your picture appears to show drying shrinkage cracks which is slightly different than what you described. See the pictures on the link.

 

[Ron]

Agree with hokie66..these are drying shrinkage cracks. Further, your crack VII clearly indicates late sawcutting. When a crack runs parallel to a cut joint, it is an indication that the crack was there (but perhaps not clearly visible at the surface)when the sawcut was made.

Your other crack photo is more indicative of joints too far apart in both directions. This was likely a crack occurring in a slab panel interior to the building.

Although you have a finish on the slab and that might be obscuring my interpretation, it looks like there might have been excessive bleed water in the concrete. This is indicated by two things...first, some fine map cracking at the surface and secondly, the appearance of dark reflections of the coarse aggregate just below the surface. I think I'm seeing both in your photos, but could be wrong without additional photos to prove.

As hokie66 noted, there are numerous reasons for such cracks to occur, but almost all of them are related to poor mix control and placement by the contractor.

 

[dik]

when was it sawcut?

 

[beton1]

You mention (at the top) that it is a reinforced concrete slab, but then state there was a lack of testing etc.regarding the concrete and subgrade. I think it has been well covered by other contributors; however, the reinforcing placement may also play a role , if there is any in fact.

 

[kieran1]

Had these exact cracks on a large floor slab a few years ago. Was a result of the contractor not carring out the saw cut joints in time.

Kieran

 

[M40A]

I don't have the infomation as to when the control joints were cut.

I appreciate everybody's input. If any recommendations on how to resolve/alleviate this problem could be suggested - also greatly appreciated.

 

[Ron]

M40A...I would suggest that you sawcut joints in the locations that they should have been at the time of construction, usually 10 to 12 feet apart in each direction for thin slabs.

Make the sawcuts to a depth of 50 percent of the slab thickness. This step is important as the slab will continue to shrink.

Epoxy inject all "random" cracks. Then apply a pigmented epoxy floor finish for all exposed areas.

 

[M40A]

Thanks, Ron - Excellent information.

Any particular epoxy (recommended manufacturer)?

 

[bimr]

Sika makes a number of concrete repair products. Sikadur 55 SLV would probably suffice:

http://www.sikacorp.com/bro-cpd-SikaInformationCivilEngineeringBuildings-us.pdf

 

[Ron]

Agree with BIMR...SIKA products are high quality. For injection, I would use Sikadur 33. Sikadur 55 SLV is good for sealing surfaces, but not for sealing full-depth cracks and creating a contiguous structural entity. Inject the cracks. That requires an injection epoxy such as Sikadur 33.

After you're done with the crack injection, coat the entire area with an epoxy surfacing to hide the repairs. You might also need to broadcast sand into the top layer of coating to provide slip resistance if the floor has forklift or foot traffic on it.