Strange slab cracking 2

[JAE]

I have been investigating a building where the slab-on-grade was placed about a month ago and only a portion of it has some cracks. The building is a long, fairly narrow footprint (sort of like an apartment building - where the slab was placed in three separate pours. The center 1/3 is cracking.

Here's some relevent data:

1. 4" thick
2. Placed on 4" sand
3. Compaction tests of subbgrade all ok.
4. Center 1/3 area was placed with retarder in 1/2 of it (contractor said 30 minute retarder) and then the last half had 60 minute retarder.
5. f'c = 4500 psi (I agree its high for a SOG)
6. Concrete used a steel fiber reinforcing throughout (no WWF in the slab).
7. Contractor stated that in their area they have had a cement shortage and the source of the cement had shut down operations for a time, and then fired up again - he thinks that the cement they used in the center 1/3 was produced after the shutdown and he got some reports that the cement was "hot". Not the concrete being hot in the truck - but the cement was hot or too fresh.
8. The placement finished with a thunderstorm which thoroughly wetted the slab for the rest of the day - Curing compound placed the next day after the slab dried a bit.
9. No idea on aggregate size.

I've never heard of "hot" or too-fresh cement before causing cracks - is this a valid reason for excess shrinkage or are there any other thoughts on this? Cracks are fairly spaced out and fairly large (1/8" to 3/16"). Many project out of re-entrant corners of the building and other floor openings so I'm pretty confident its shrinkage.

 

[SlideRuleEra]

JAE - I bet that the combination of rain on fresh concrete and the delay in application of curing compound is the real problem. When we were pouring bridge decks, and the rains started soon after placement - you covered the concrete immediately to preserve structural integrity (even if the premature covering damaged the concrete finish).

About the delay in applying curing compound. Most products specify that they be applied "as soon as the surface water has dissipated and the concrete has been finished", here is a link to typical products

http://www.symons.com/products/brochure/chemical/02cc.htm

As you know, a lot depends on the workmanship of the application of curing compound. Would be interesting to know if the delay caused the application to be on a holiday or weekend.

http://www.SlideRuleEra.net

 

[LCruiser]

Sounds like the rain - too much water, lowering the concrete strength. Check some cores.

 

[JAE]

The rain came after the placement and finishing - so no water got into the concrete itself - breaks came out OK.

 

[LCruiser]

Cylinder breaks or core breaks?

 

[henri2]

JAE,

I concur with your conclusion. Clearly plastic shrinkage cracking is the culprit.

By the way, were the breaks based on cylinders fabricated at time of placement or were they based on cores taken in the questionable area afterward?

Since the SOG was not protected from rain toward the end of placement, the water content/unit volume and W/C ratio in areas where the concrete was still fresh would have increased. Increase in water content/unit volume increases plastic shrinkage which could lead to cracking. In areas where concrete had just been deposited, the top surface could be compromised leading to cracking, laitance, scaling, discoloration etc.

Cores may still indicate the f'c is okay but satisfying f'c does not mitigate the deleterious effects of shrinkage cracking potential caused by excessive increase in unit water content that much.

What about the interface between the mix with 30 min retarder and the one with 60 min retarder? Any cracking noticed there?

 

[Ron]

JAE...I would agree that it is a shrinkage issue, but not necessarily plastic shrinkage as henri2 notes.

"Hot Cement" usually doesn't cause this issue any more than any other cement. Yes, low cement supply has increased the incidence of "hot" cement being placed on the market (this is cement that has not had an opportunity to cool after the burning/grinding of the clinker and is loaded into silos while the temperature is elevated).

You can experience more "flash set" with these materials, though the mass of the cement relative to the mass of the other constituents in the mix is sufficiently small such that it doesn't greatly affect the mix temperature.

The pattern of the cracks will tell you a lot. The fact that they are 1/8-3/16" wide already is a clear indication of inordinate mix shrinkage. Next is the orientation of the cracks. If they are long and continuous, with relatively uniform width, they are not likely plastic shrinkage cracks (just plain ol' shrinkage cracks caused by restraint of either subgrade or re-entrant corner conditions, or mix variations). Plastic shrinkage cracks are generally short (less than about 12 inches long each), discontinuous and wider in the middle than at the ends. They have no preferential orientation in large slabs, but will follow linear orientation in footings and beams. Plastic shrinkage cracks form prior to full setting of the concrete and are usually caused by wind blowing across the concrete surface thus causing the evaporation rate to exceed the bleed rate. Plastic shrinkage cracks sometimes heal autogenously, but more often they remain and become joining segments with normal shrinkage cracks. This makes for very erratic crack patterns, not seeming to follow any of the typical shrinkage crack patterns such as halfway between two sawcut joints. Since you had a thunderstorm occur during/at the end of placement, again the likelihood of plastic shrinkage is low.

One key piece of info is the non-uniform use of retarder. When two different mixes are placed within the same pour and contiguous with each other, the properties of each are different and they will shrink differently. Having two different dosages of retarder in the same contiguous placement of concrete can cause the shrinkage at two different locations to be significantly different. This occurs when the first placement's retarder dissipates and it sets up while the 2nd placement's retarder is still active....they will shrink away from each other, thus causing more and wider cracks.

 

[henri2]

Ron,

Thanks for pointing out my terminological error. I should have stated drying shrinkage cracking rather than plastic shrinkage cracking.

 

[BigH]

On a recent job we found that the plastic shrinkage cracks on concrete road slabs were rather uniformly spaced (no reinforcement so not plastic settlement) were typically longer than 12 inches - say nearly 30 or so and were all tending diagonal to the pour panels (10ft x 15 ft).
Neville Properties of Concrete (4th and final edition) has table 10.8 and Figure 10.12
- Plastic shrinkage cracks are diagonal in pavement and slabs, randon in reinforced concrete slabs and over reinforcement in RC slabs (like plastic settlement) - due to rapid earlly drying or low rate of bleeding - appears 30 minutes to 6 h and indicates a need for better early curing.
- For Early thermal contraction - the restraints are important and for thick slabs due to excess temperature gradient or rapid cooling - appears 1 day to 2-3 weeks after placement.
I always learn something with Ron - thanks. Suggest reference be made to Neville - it is a very useful reference.

 

[JAE]

LCruiser, I believe they are cylinder breaks - I'm not the EOR, I was just brought in to review this and numerous other issues in the structure.

Ron, thanks, as usual, for your concrete expertise....and thanks to all the other responders.

The cracks did seem to show up around re-entrant corners.

One final question: Is it correct to assume that the owner's best recourse at this point would be to monitor these cracks, verifying that the crack width expansion has terminated over time, and then, at the latest possible time, repair the cracks or replace segments (or all) of the slab?

 

[LCruiser]

If they were cylinder breaks they would not have been subjected to the extra moisture that the slab was due to the rain. Cores could be in order.

 

[SlideRuleEra]

As an Owner's representative, I would have the Contractor propose a course of action (probably by hiring His own Consultant), and present this plan to the Owner (and you) for evaluation. If it sounds reasonable, accept the plan, if not replacement of the concrete would be the Owner's best option.

Since you don't have all the facts, it would be risky for you to establish a set of rules to determine acceptability.

http://www.SlideRuleEra.net

 

[Ron]

JAE....yes, I would agree that watching/monitoring the cracks for a while then repairing is appropriate. As long as there is no faulting at the cracks, they can be easily repaired with a gravity-feed epoxy.

Thanks to all for kind remarks.