cracks in a rib-mat slab

[eatapeach]

Cracks occurred in a slab that was placed per drawings, protected & consolidated per ACI, everything was performed by the book.
The 8" slab is on grade with grade beams placed at 21'oc and saw joints at 10'6. The slabs were required to be placed in lanes no greater than 25' in width. The cracks have occurred about 2' to 3' off the grade beams and are approx. 0.012 to 0.020. The cracks dont appear to be very deep. The base is select fill with 8" of small gravel under a 15 mil vapor barrier.

The design mix was an approved mix with HRWR which allowed up to a 8" slump and 5% air. My second placement I took the HRWR out and went to a 5" slump and reduced the air to 3%. This seems to have help but not totally convinced that this was the problem.

I have read through all the ACI books trying to understand the type of crack. It doesn't appear to be shrinkage but more of a stress crack. The specifications states no cracks of any size will be allowed. Supposedly this is why I'm forced to place the concrete in 25' x 100' lanes, yet cracking still occurred.

Does anyone have a suggestion what may have caused this? I have about 50 more of these lanes to place and dont want the same problem.

 

[JedClampett]

A picture would help. Also a sketch of the location of the cracks vs the grade beams.
You didn't mention percentage of reinforcing. The amount and lcoation is very important to crack reduction.
You're correct, the HRWR probably didn't contribute to the cracks. Higher slumps do contribute to shrinkage, so that might be it. They're more likely due to shrinkage of the concrete vs highly restrained area.

 

[hokie66]

Why do you have grade beams? What are they for, and are they in both directions? Were the slab and grade beams placed monolithically, tied together, or separated?

When were the sawcuts made in relation to when the slab was cast? What is the detail at the sawcuts? Where are the cracks relative to the sawcuts?

Restraint shrinkage cracking is the most likely cause. When slabs shrink and are not free to shorten, tensile stress causes cracking. Therefore, if you have restraint, it may not be possible to completely eliminate cracking. Any specification which says that no cracks are allowed is unreasonable, unless the design allows for freedom of the slab to shorten.

 

[SlideRuleEra]

I agree with JedClampett, probably too high slump concrete for your situation. A vapor barrier will keep any excess water from leaving the concrete through the bottom of the slab. Wet curing, preferably for seven days, will slow down uneven drying and help minimize this vapor barrier problem. Be sure to begin the wet cure as soon as possible after placement, don't leave the slab exposed to the air any longer than necessary.

Since saw cuts are included, I assume the concrete is not reinforced. It is important is to make the saw cuts within 24 hours of concrete placement. The cuts should be at least 1/4 the depth of the slab (2" deep for an 8" slab).
Note: Making saw cuts while wet curing is inconvenient, but it can be done.

In addition to the proper steps that you are already taking, I would wager that 3" to 4" slump concrete and continuous wet curing will give good results.

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[Ron]

Agree with hokie66...sounds like restraint shrinkage cracking. You mentioned the cracks are 2 to 3 feet off the grade beams, presumably parallel to the grade beams. This is characteristic of significant restraint offered by the grade beams, and the concrete shrinking between. Consider the following:

You have three cracks in the 21 feet. One near each of the grade beams on either side of the section and one within the sawcut. You measured crack widths of up to 0.02". So cumulatively, you have about 0.06" of crack width. If you consider a reasonable shrinkage strain of 200x10^-6 in/in, over a distance of 252 inches (21 feet), you would expect a shrinkage strain of about 0.05"...pretty close, huh? My point is there's nothing unusual here, it is just that you have not controlled the cracking to occur within sawcuts. As hokie66 mentioned, the sawcuts must be properly timed. Sawcuts should be made the same day as placement of the concrete...if you wait longer, the cracks will occur, but won't necessarily be visible for a few days.

The cracks probably don't extend through the concrete...yet. Eventually they will progress deeper.

 

[hokie66]

One addendum to what Ron said. With sawcuts at 10'6", I think you may have two planned cracks and two unplanned cracks, four in total.

If the grade beams and slab are monolithic or tied together, you may also experience cracking in the slab transverse to the beams, as the thicker sections shrink at a lower rate than the thin slab. The width of these cracks will depend on the amount of reinforcement which is present.

 

[eatapeach]

Thanks for the post. I too was leaning toward restraint cracking. The saw cuts were performed within 2 hours after the finish and double layer of saturated burlap was installed after that. I monitored the evaporation rate per ACI during the entire process. Wind breaks and misters were installed even though the evaporation rate never exceeded 0.2 so this was not an issue.
One concern I had from the very beginning was the height of the reinforcement. The engineer specified the slab reinforcement be within 1 1/2" of FF. A saw cut depth of 1" was specified and not the 1/4" depth that I'm use to which would have been 2" and was impossible with the reinforcement height.

The grade beams were installed with double #8 bars on the bottom and double #9 bars on top with #4 stirrups at 18"oc. The rib-mat slab reinforcemnt was #5 bars at 8"oc. The engineer gave the contractor an option of placing monolithically or placing a joint at 18" below FF. The contractor chose to place the concrete in the beams and make a second pour of 18" to include the slab. The total depth of beams and slab was 42".

The slab overall looks really well with the exception of the few cracks that occurred parallel with the grade beams. As far as % of reinforcement I would be concerned with too much vs. not enough. With saw joints at 10'6" it would seem this would be plenty to handle the cracking. The saw joints occur over each beam and then one in between.

I am the QC for the project and would like to eliminate any future cracking. The second pour with the lower slump and less air did make an improvement or it just was not going to crack, not sure.

Thanks!

 

[eatapeach]

Typical condition. See pic.

 

[SlideRuleEra]

hokie66 - Sorry if my post seems to ignore the comments that you made. When I started writing it your post was not visible. Only after I had submitted my post did I see yours.

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[hokie66]

SRE,
No problem. You were addressing the concrete issues, I was talking about the design and geometry. Both important.

eatapeach,
The sawcut on the beam centreline is doing no good at all. It won't move. Suggest you place two sawcuts within the slab section instead. Same spacing, but where they will work. And you are correct...none of the sawcuts are deep enough to do any good. What really needs to be done is to lower the reinforcement and curtail some of the bars, say half, at the sawcut locations.

With the design as it exists, no matter what you do with the mix design, placement, and curing, you still will have cracking. Maybe you will still get some cracks after the sawcuts are relocated and deepened, but the amount of reinforcement you have should control the crack width adequately.

 

[cvg]

the grade beams are restraining the slab and in effect, causing the cracking - not the sawcutting or steel placement. Is there a reason for the grade beams? If not, then why are you paying extra for heavily reinforced beams which are then causing your slab to crack?

 

[eatapeach]

Grade beams are required due to the soils. See pic of slab and grade beam.

 

[hokie66]

Since you have the whole thing set up, nothing you do with the concrete is going to eliminate cracking. Suggest you advise the owner that cracking is inevitable with this design, but should not affect the serviceability of the slab (unless there is some reason like storage of loose material that we don't know about). Sawcuts 1" deep are probably not doing anything. Unless you can rechair the reinforcement lower to allow at least 2" deep sawcuts, and relocate the sawcuts away from the beams, you may as well not waste your time sawing.