How to Keep a Slab Flat After Curing

How many inches of sand/gravel were placed as an underlayment before the slab was poured? And was the underlayment properly leveled and compacted? The only time I have seen what you are describing is when concrete has been poured on uneven native soil.

At 4" slump, excess water wouldn't be a factor in your case, another factor that would be involved.

Food for thought

I have to assume, since you're making multiple pours, that the previous pour has cured somewhat and the forms removed to allow the new pour to butt the old---the plastic underlayment (I'm asuming you used) on the new pour must overlap the old pour to avoid the old pour from "wicking" excessive water from the new pour at the joint--assuming the multiple pour material are the same they should cure to the same height with all other conditions the same.

Donzi

Hello CFECO,

As Charlie_r has said, the only time I have seen this is where the foundation was really non existent, and the concrete was laid directly onto un-compacted soil. Even with the form still on the edges could have moved if the edge was on soil and all building site traffic, including the very heavy trucks delivering and, pouring pumps, could push the soil down which in turn pushes or, can push the edges of the concrete up. If the edges were not anywhere near any traffic, say, on the inside of the slab it is almost certainly the three separate pours are to blame. I have never heard of a pour done in stages on a slab without steel strengthening, and certainly never in layers. I have seen teams working 24/7 to keep the edge going on large concrete pouring sites. The steel stops any movement. It sounds like the fibre allowed enough movement to cause problems?

Even if the foundations were properly done and all was solid, everything should be ready and, any forms finished to allow a constant pour throughout. If, as seems here, the pour was done in layers with fibre strengthening, putting the fibre on the part finished pour must be done with haste. It is not the time to be having a rest or for a round of coffee and biscuits. So it could be that the first pour started to cure slightly, depending on the time between pouring the first and consecutive, and final layer.

You do not give much info' on the strengthening mesh fibre used, but it could be too much fibre was used and possibly too much fibre was put around the edges? This could allow slip? If this is the case, the concrete will have slipped against the fibre on the second and third, layer. The bottom part of the second and third pour will be wider and will have expanded more than the top portion of the second and third pour have. Is there fibre extending over the edge of the slab?

I have read your post again and you say it was done in three pours? Why?

I would suggest if you continue to do things in several different pours, you should stagger the joints of each layer perhaps a couple of metres.

But I cannot understand why you had to do it in three separate pours? The full thickness of the three layers should have been complete, and staggered overlapping allowed. When the second pour started, again, the full thickness of the pour should be laid. And the same with the next pour.

The way it was done with all one layer done, then a break, I presume until next day? It does not allow any amalgamation through the fibres.

The setting concrete is like a chemical factory and, if the first layer was finished and virtually touch dry before the subsequent layers were done, there could not have been the complete unified heat and bonding process. And, I am assuming you did build expansion joints into the slab? Though it would have been almost useless if you did three layers at different times. I have only seen expansion joints on the full depth of a pour. That is then filled with tar or something similar.

There seems room for a new technique to be tried on any other work done.

Good luck. Keep us in touch with whatever may be happening.

Take care......................

The slab was poured in three sections due to the large area,over 100,000 square feet, not in three layers, sorry for the confusion. The slab was poured on top of 4 in. of spec ABC compacted to greater than 95%, the fiber-mesh is engineer approved and put in the mix at the concrete plant in an accurate quantity and mixed (turns of the mixer drum counted and must be within allowable or load is rejected) properly. This building is a tilt up warehouse so there is no "foundation" under the main slab as the edges are held back from the final wall location by 4 feet, which is then poured back after the walls are raised. The edges and where the pours met have a 1' x 1' toe-down with, 2 #4 horizontal re-bars continuous.

The products of hydration of portland cement occupy less volume than the parts, hence we must be aware of the potential of shrinkage cracking, that is one reason we cut joints in concrete. I have read that in slabs, differentail curing (faster curing/moisture loss) from the top of the slab can result in something like this, specifically from the text "Concrete" by Mindess, Young, and Darwin the statement is:

"Even with good jointing, uneven shrinkage caused by moisture loss only from the top of the slab can cause curling at the edges."

I have never seen this personally, but it does seem to fit here. I would doubt that the subgrade would be the cause of this, especially where the deflections are only at the edges, occur at most edges, and no cracking of note is observed. Also, no load has been applied to the slab as yet! This explanation would also explain why you see more curling near formed edges, and less at cut edges. This is because some shrinkage had occurred before the cut was made, so less curling would occur there.

As for preventing this, the top of the slab should be cured more slowly (was it covered with plastic coated burlap and kept wet for 7-14 days?), and shrinkage compensating cements are available. Also, additional joints and steel reinforcing can help.

Also, just a note. The common fiber reinforcing (tiny plastic fibers, so small you can barely see them in the concrete), reduces shrinkage cracking. It does not impart significant structural strength to a slab. As soon as a crack forms, these fibers no longer provide any strength. Steel on the other hand, really only starts to work significantly after a crack has formed, movement is required! This is an issue, as people often confuse the two. The reason fiber is often substitued in slabs is that the steel in slabs that are supported by a properly prepared subgrade was really only there, or mostly there, to inhibit shrinkage cracking, so for slabs with a very well prepared subgrade you could do away with the steel and only put in fiber. (I usually spec both for the same reasons stated by another member.). There are fibers that are made to replace steel as a structural element in certain cases, these fibers are much larger, made of polymers or steel, and are a very good replacement for steel reinforcing in some instances, however they can be expensive. I would have thought that these larger fibers would have been used here, Fiber-Mesh does have many of these in their product line.

Do you have pictures that show this curling?

Personally, I am not a fan of fiber reinforced concrete. Call me old school, but on demolition sites I have worked, the FRC came apart too easily, where the older method of re-bar was more often harder to remove. I have seen FRC flex on stable foundation where re-bar did not.

Just my experience with it.

Largest pour I have been involved with has been 25K sq. ft. monolithic. Normally, the company I worked for would try to split the pour into smaller sections with fiber expansion joints between each. However, I do realize in your case that wasn't an option, if I am reading your posts correctly. The slab in question is where you are to pour your tilt-up sections? If it was me, I'd just hire a grinding co and be done with it.

Yes , grinding would solve the problem, but the finish ( being different than the non-ground areas) is unacceptable to the customer. They are accepting of the slab as is , I am just looking into the cause of the curling, and a way to make the finished product better. The raised edge's I believe, are a curing-shrinkage issue, as it occurs where the slab is saw cut for stress relief also, although to a lessor extent.

Thanks again for the feedback.

Gentlemen

Halt

Basic concrete pouring 101

Thou shalt not make a "cold" joint in a large concrete slab poured on grade without providing an adequate mechanical transfer of structural integrity for loads across the cold joint.

This is valid whether mechanical loads crossing the joint are static (uplift dehydration) or transit (heavy fork lift).

Keying and re bar dowels are normally used. I have built several million sq. ft. in the last 60 years and follow this same procedure whether building floors or vehicle concrete parking lots/driveways/streets.

To understand what is happening to your slab...lay a piece of 1/2" plywood on a lawn....observe "flatness early daylight...noon and the plywood late afternoon ( clear weather)...watch it flex and curl as moisture enters and leaves the plywood.

Same thing is happening the the individual slab sections making up your pour.

Incidentally FRC as I use it only permits steel "fiber" adding to it's structural integrety............never plastic fiber

MR. GUY

The cold joints were all keyed together. And there was no moisture barrier under the slab at all called for, however the tight compaction would preclude rapid moisture transfer downward. It is a distribution warehouse and subject to heavy traffic. We had a concrete pump and 10.5 yard mixers on it with no problem. The moisture differential between the base and the surface is probably an issue. The slab had a cure-bond breaker applied liberally after finishing and saw-cut control joints done soon after while the slab was still green.

Hello CFECO,

Just to say sorry for getting things totally wrong with the advice I gave. I have never used the plastic you mention and do not understand why it is used? I can see no use for it. Instead of the cement to cement bond, there is going to be a cement to plastic bond, or should that be 'no bond'?

What is actually happening, are you getting ridges where the joints are?

At first I was thinking it was a problem on the outer edge of the slab, not through the inner pat of the building as well. Is this the first time you have noticed this? Is it possible there was too much plastic/fibre added and before it could set the fibres have perhaps sprung up and so the concrete is not as dense as it should be?

I have been looking at various bits and there does not seem to be a good reason to add this fibre from I have read so far. I hope it is not going to turn out like the 'light' concrete of about thirty years ago, where after ten years building started to collapse because it did not have a solid build. I am not sure now but I think it was absorbing water and it rusted the re-bar?

Good luck. I think you should tell the BSRIA, as this cannot be normal. It could prevent proper fittings and fixtures actually being used.

• BSRIA - providing consultancy, instrumentation, test and research ... ... and research organisation providing services in construction, building ... A member association with over 50 years experience, we support industry ...
  www.bsria.co.uk/ - 13k - Cached - Similar pages
• Construction Industry Research and Information Association 15 Jan 2009 ... Research concerned with improving the performance of all involved in construction and the environment. Catalogue available for download.
  www.ciria.org/ - 8k - Cached - Similar pages Take care...................

Hogwash! Hogwash! Hogwash! Dowelled joints will not prevent the slab curling that you have experienced. Properly installed, which is a rare occurrence, they will help in surface load transfer across cold joints. In order for that to actually happen, the dowels must be intimately embedded on one side of the joint and inside a tight sleeve that is intimately embedded on the other side of the joint.

What is occurring is due to the moisture differential between the upper surface of the slab and its contact with the subgrade. Did you place a moisture barrier above the aggregate base or below it? Research into this phenomenon has led engineers to believe that placement of a moisture barrier so as to be in contact with the concrete slab will lead to more severe slab curling than if it is placed under the aggregate base. Unless the slab is to be used for heavy vehicle traffic, the aggregate base should be a free-draining material rather than a mixed-gradation product so as to not hold moisture against the slab.

Hello Doogleass:

Why do you not just say what you mean?

Why do you respond to an issue that you clearly know nothing about?

Hello Doogleass:

Do you not feel better now you have expressed yourself? I knew you would. Just needed a little encouragement?

I was in the building Industry half my life directly, and indirectly involved all my life. I have been invalid for ten-ish years and there was no such thing as concrete with plastic or nylon or whatever it is, in it. So I do talk with experience.

However I was not the only poster to think the OP had poured three different lots, one for each 'layer'.

The fact is I got it totally wrong in trying to see the use of fibre and or plastic used, rather than re-bar. Re bar I am used to, the other I was not and was trying to help the OP, knowing they would get back to me and others to confirm or not what I and or others had said.

In case you had not realised, that is how this question and answer forum works!

My Father and his Brother and, four uncles were and are in the building trade, so I do not come from a point of total ignorance!

I have apologised to the OP for my lack of incite in the use of plastic in concrete. And I still see no good reason why this should continue to be used. My computer keeps freezing, even though I have wrapped a scarf round it, and so I am not able to search at this moment for the concrete I mentioned in my second (I think ) post, about the so called 'light' concrete which was used for about 25 years before building started to fall down and IT was the cause. So I now do not take things at face value. I want to know the ins and outs of why, bugger up a perfectly good product ......concrete...... with admixtures that not just me but others have said they do not like! This is not a matter of my ignorance. (Though your posts to me may be) though they are kind of making me feel warm now. I just wish it would reach my computers freezing problem, the the warmth could be indigestion?

This kind of forum across countries can often times highlight a problem to do with the 'stuff' a product is make with, the finish, and or packaging etc. And the jury is out as to whether concrete with fibres is any improvement on a product which has been used as is for at least 2000 years.

On some posts it takes someone with your attitude to bring an otherwise unimportant thread alive. My long post was one of the first as a reply to the OP and there have been posts with only three or four respondents. I did not know you, as the new up and coming 'expert', with your arrogant style and bordering on the edge of rudeness, brash insolence was going to be here, or I could have found better things to do with my time!

This is one of the only things I can do now, that is to help others. I am far from being the only respondent to admit their dislike to fibres in cement or concrete. And, if you take a look at my profile you will see I have half a dozen building related GAs and one specifically with ref' to concrete. So, enough of the attitude and posts which amount to bullying and insults. Try to realise there are maybe over a thousand respondent on this site and all go to make up the whole, and give the site its skillful and at the same time serious and funny way of giving out advice, known as helping others. Most of whom are professionals, so any failing will be picked up pretty sharply. It is not for anyone to tell any other post on this site what they can and can't write in about, and time will tell who is the fool and who is the expert. And I mean true expert. Not just someone who can shout.

Help is what I endeavour to give, not criticism. That kind of bul-sh-t type of outlook belongs on the streets or other forums, not here, otherwise too much time is distracted from the top.

Take care......................

If after two months the edges are definitively curled up by 1/4 inch (I am guessing that this raised edge only occurs over a length of a few feet?) then it must be due to a relative tension of the top surface of the upper half of the slab to the lower compressed half of the slab. It does sound like a heat of hydration problem insomuch as the lower layer was kept cooler due to contact with the sub-base whilst setting. Then, when the whole slab gets to the average temperature, the upper part gets to a relatively lower temperature and so contracts slightly. But this happens over the entire surface of the slab and manifests itself at the edge where there is no longer a top and bottom equilibrium condition. Does this make sense? A solution might be to put a high strength polystyrene layer beneath the slabs so that the slab maintains the same temperature throughout its thickness.

This one makes the most sense, thanks. The control joints were at least 3/4 in deep, it was immediately cured with a cure-bond breaker, however the day time temps were in the 90's, Tucson,AZ.

If you would like some good articles on this subject let me know and I will e-mail them to you.