×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Concrete Cracking
4

Concrete Cracking

Concrete Cracking

(OP)
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.

 

RE: Concrete Cracking

2
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?

RE: Concrete Cracking

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.

RE: Concrete Cracking

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.

RE: Concrete Cracking

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.

RE: Concrete Cracking

when was it sawcut?
 

RE: Concrete Cracking

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.

RE: Concrete Cracking

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
 

RE: Concrete Cracking

(OP)
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.  

RE: Concrete Cracking

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.

RE: Concrete Cracking

(OP)
Thanks, Ron - Excellent information.

Any particular epoxy (recommended manufacturer)?

RE: Concrete Cracking

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources