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Concrete Crack Tolerance
2

Concrete Crack Tolerance

Concrete Crack Tolerance

(OP)
I'm looking for a standard for acceptance of concrete structures pertaining to concrete crack widths or severity.  For example,  a concrete lined channel has developed severe cracks while curing.  Is there a typical rule of acceptance?  

RE: Concrete Crack Tolerance

I have read that cracks in water retaining structures should not be more than about 0.01". However, your problem sounds more severe. The importance of the channel should dictate the relative severity of the cracks.

I am assuming that the channel is used for storm water run-off, and that it is not reinforced (or very lightly reinforced) and well-sloped. You should try to etablish whether the cracks are all the way through the concrete. If they are, you could end up with some washout below the concrete. Surface cracks are not necessarily an immediate problem, but will probably escalate to full-depth cracks, especially in freezing climates.

One way to test if the cracking is severe enough to warrant repairs is to block the water flow through the channel and fill it with water. The rate that the water escapes or leaks out of the channel should not visible, or more than is judged acceptable. This could be repeated at several intervals to test the entire channel.

Let me know if my assumptions are wrong and if the use is more critical than I have guessed.

RE: Concrete Crack Tolerance

2
General rule for crack width tolerance is:

Interior - max. 0.014 inches
Exterior (normal exposure) - max. 0.010 inches
Exterior (aggressive exposure)- max. 0.006 inches

This was put forth by Kesler, et al in a Bulletin from the University of Illinois Engineering Experiment Station in 1965.  This is consistent with current ACI guidelines.

Your application would probably fall in the "aggressive" exposure range.  I have used this tolerance scale for evaluation of Sewage Treatment plants and other aggressive exposures.

RE: Concrete Crack Tolerance

(OP)
Yes we've cored the cracks and they do penetrate the entire thickness of the lined channel.  The cracks are extensive in a uniformed diagonal formation & inches apart.  As if the channel had been stretched while the concrete was still wet.  Compaction tests were with in spec, and concrete mix designs were accurate.  Soils were wet before placement, Concrete temps were about 82deg averaged - "Chilled Water".  This only happened once in a 200' area - just in the invert.

RE: Concrete Crack Tolerance

Sounds like shrinkage cracks from significant restraint condition coupled with excessive shrinkage of the mix.  Check to see that there was consistency in the mix from batch to batch.  Does the grade change in this section?  Any other features different for this section?  

Since the cracks were closely spaced, could have been initiated/exacerbated by "plastic" shrinkage, though they are likely following a rebar pattern.  Look also for slumping directly over the rebar.  This causes a reduced cross section, susceptible to early cracking.

RE: Concrete Crack Tolerance

(OP)
We are looking into consistency from batch to batch now.. The grade has a .67% slope from side to side in the invert, and the channel end to end is real flat "Low Feet Per Second" "Large Volume". The channel slopes are at a 2:1 slope but didn't crack as bad as the invert. We are using #3 Welded wire at 6" centers.  

Your Classification of "Aggressive" Exposure range was not looked at until your comment.

Thanks for the help!

RE: Concrete Crack Tolerance

(OP)
The described cracks are from 1/100" to 1/4" in width.

RE: Concrete Crack Tolerance

(OP)
and range from anywhere from 3" to 2'

RE: Concrete Crack Tolerance

Was there a provision for any control joints in the channel? What is the channel section thickness?  You could have been a victim of "self restraint" in the concrete in that the previously placed concrete had sufficiently set to "lock" the welded wire and start shrinking, while the new placement was doing the same in perhaps an opposing direction.

Your crack spacing implies that perhaps the welded wire is "wavy" or sits variable in depth of the cross section.

RE: Concrete Crack Tolerance

Was the pour continuous, or "checker board"? Shrinkage due to a large continuous pour combined with insufficient curing and causing plastic shrinkage, also possible insufficient reinforcement, could cause this.  

I have another question along the same lines.

I am constructing a reinforced concrete box culvert outlet works for a dam.  The specs call for control joints with waterstops every 40'.  Adjacent pours must wait for a full 7 day cure prior to pouring the next section.  The contractor is asking if 5 days is sufficient (to get back on schedule).  My question is - How critical is the 7 day curing period for drying shrinkage?  Research I have read studied drying shrinkage and plotted from 1 month to 33 months.  So how much actual shrinkage occurs in the first few days?  The concrete is spec'd at 4,000 psi with .45 w/c and 1" max agg.  Type I/II cement with air entrainment.  Daytime temperatures will reach up to 110 degrees F.  3 and 4 day compressive strengths of the trial mixture are over 4,000 psi.

RE: Concrete Crack Tolerance

ACI 224R-80 - "Control of Cracking in Concrete Structures"
gives the following for tolerable crack widths:

exposure                   tolerable crack width (inch)
=======================================================
dry air or protective membrane   0.016
humidity, moist air, soil        0.012
deicing chemicals                0.007
seawater, wetting and drying     0.006
water retaining structures       0.004



RE: Concrete Crack Tolerance

(OP)
Thank you for all your help!  You've shed a lot of light on this problem..   You've got a excellent program here and we all would like to thank you from our Org.

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