How to prevent concrete slab curling? 1

[oldestguy]

The question deals with the material below the slab before concrete is placed and maybe something to do with concrete itself.

Inside structures it sometiems is a problem to have "dish shaped" slabs and in many highway applications it is my view that the progressive joint displacement of slabs is related to curling. We do know highway base course gradation also contributes, but what about wet or dry base course, etc?

Has anyone seen a standard for subgrade prep or other specifically to prevent curling. If so where.

Is there any positive experience comparing plastic barrier versus none?

 

[Ron]

oldestguy...I would disagree that most joint displacement in exposed pavement slab is related to curling. Most joint displacement is related to lack of load transfer capability between slabs, across the joint. Anyway....

Slab curl is usually caused by differential drying in the slab. The top dries more and faster than the bottom, thus causing a curvature in the slab.

Certain conditions exacerbate this. A wet subgrade, plastic vapor barriers, concrete placed too wet, high water-cement ratio, small coarse aggregate fraction, and several other mix parameters.

The proper placement of dowels and control joints can mitigate some curling, but not all.

 

[eric1037]

Also, wet curing can help out. Another thing to watch out for is temperature differential. If it's really hot out, then the top can cure faster than the bottom.

 

[oldestguy]

OK, good comments, but I think theories.

I have mine also, but was hoping for some one with some proof of what goes on, followed by a spec. that is based upon some solid facts.

Yes, observations are good, but with conflicting opinions (not just these , I differ in some respects, but others I have heard), the full answer eludes me.

In general, my experience says shrinkage is related to W/C ratio when curing. The higher the w/c ratio, the more it shrinks.

But, I can't prove how it happens with slab curling, since so many factors affect a slab (subgrade condtions, vapor barriers, air temp, wind, slump, vibration, initial w/c, etc.).

Maybe the answer is the relative affect of these and other factors, all put together.

Still looking.

 

[Ron]

oldestguy...not just theory. I have seen and measured in practice.

Yes, the w/c has an effect on shrinkage and your premise is correct; however, if the w/c is the only variable, curling will not occur. Again, the most prominent cause is differential drying shrinkage.

Subgrade prep alone will not stop curling. The best way to reduce the initial amount of curling (which is the most important, since the effect is not fully recoverable) is to wet cure the slab so that drying shrinkage is mitigated during the important strength gain period.

 

[jheidt2543]

There has been a lot written on slab curling. Check with the Portland Cement Association (PCA) and the American Concrete Institute (ACI) and you will see that Ron's comments, as usual, are right on.

 

[Ron]

Thanks, jheidt2543. Yes, there has been a lot written about it and it still commonly occurs.

 

[reddog99o]

There is some solid engineering and experience in replies to date.
Here's my take after building 26 km of concrete canal in the Middle East.
1. W/C ratio is critical in all concrete work,finishers like the mix a bit looser to reduce their labor and extend their work hours as the wetter a mix,the longer it takes to take initial set hence their overtime increases.
2.Temperature of the sub-grade and the mix as placed should be as close as posibble. We sprinkled sub-grades continuosly from finishing up the iron work to placement time. It's murderous to place 85F concrete on 120F subgrade.
3. I am a massive proponent of some type of waterproof material under all slabs as it prevents the subgrade material from robbing moisture required for hydration from the concrete mass.
4. Curing, and WET curing is the only way to go. It must be maintained.Successful water curing and an impervious barrier below conjures up a delicious Concrete Submarine Sandwich. Nothing can replace that combination.
5. Take a hard look at the rebar drawings and consult with your Structural of choice. I've had the problem in light slabs and we bumped up the iron a bit plus really stepped up the inspection where we found troubled areas such as this. If the slab is being placed below the water table and is in a temporarily dewatered condition of some kind, make sure the pumping is not disturbed until you get significantly past initial set. If the subgrade is piping or boiling, whether you can see it or not during or right after placement, what looks like a deformation, may be an "as-built".
6. Any issues with the earthwork below would probably manifest it self in other ways such as un-normal crackung, breaking or whatever.
Good Luck,

R.A. Hassett, P.E.
rah1616@hotmail.com

 

[LCruiser]

Curling typically has nothing to do with subgrade, except that it slows the curing below. A big problem is curing compound - if you use white pigmented anyone can tell if it's on thick enough. If you use red it's more difficult to tell. We only use white anymore. Some contractors will give you all kinds of excuses as to why white is harder to use, but they're just trying to save money because they don't have to put as much red on.

 

[oldestguy]

Hello all:

In general, I'd say the question was quite nicely answered by all.

In summary, I'd say this: Do all that you can to not allow any change to the plastic concrete and during the early curing stages, mainly as water loss. Thus, placing it on a dry subgrade is likely to cause an immediate change below, and the permitting of early drying on top will affect that part. Then later on slow the drying phases.

A good response guys, thanks.

Next getting the specs to follow through and then enforce it is another matter.

 

[tlzannino]

Ron - I am evaluating joint movement of a 5.5" thick, un-reinforced slab that gets hard wheel forklift traffic every night. The base is crushed concrete over medium dense to dense sand with some silt. The contractor appears to have done what he was contracted to do as far as joint spacings (~12') saw cuts and curing compound. One explaination given was slab curl, but I feel there is not enough load transfer at the joints. How can I make my case.

 

[Ron]

tlzannino...there are several factors that can cause the joint movement. The spacing of the joints helps to mitigate the movement only in the respect that closely spaced joints have reduced shrinkage width, thus the potential for aggregate interlock (the primary means of load transfer in non-doweled slabs) is greater. Having said that, the joint spacing must then be coordinated with the coarse aggregate size. If the coarse aggregate is small, then the joints must be closer together.

I would look at the mix design, check the water cement ratio, the coarse aggregate size, and measure the width of the crack in the control joints. If the crack width is greater than about 50 percent of the diameter of the coarse aggregate, you'll likely have load transfer problems.

You can measure the amount of load transfer by measuring the differential slab elevations as a wheel goes across it. Even simple measurements using dial indicators will tell you what you need to know.

You can measure slab curl with a profiler or with a long level and some shims.

 

[oldestguy]

One more observation. The above mentioned long straight edge method of measuring works very well and proves or disproves it to any one interested. I also find that short straght edges, say 4 footer level, placed on the joint will show it, by rocking it on the high joint.

A 5 pound hammer bounced on the slab will also show voids underneath, by the differing sound created between no support versus concrete contact with subgrade. It is asking a lot of a joint(and any transfer mechanism) with no support there to have fork lifts drive over without problems developing.

I frequently see an easy way to "document" it on highway slabs. Just look at the road after a light rain. Then the dish shapes are obvious. Try some water in the building to see if there is curling. This also works if you wait for the higher elevation wet surfaces to dry some.

 

[moogbro]

Construction techniques, mix designs, curing rates, weather, etc. are all factors that will affect how a slab cures. If you are not already aware of this site/program, check it out.

Hyperpav.com