# Cracked Lightweight Concrete Floors



## mr.lemna (Jun 5, 2008)

We just bought a small two-story office building built in the 1970's. Eventually we are going to replace all of the carpet upstairs. I suspected we had some cracking of the lightweight concrete under the existing carpet and pad. After we closed the sale I pulled up the carpet in the hallway and it looks like "flagstone", no pieces more than 18" square. One piece is loose and rocking. (See photo below left) I pulled up some more carpet in a vacant office and the cracking is not as bad, about half the floor but the pieces are larger and none appear loose. (Please see photo below right.)

I sure I'll run into this elsewhere on the second floor and really don't want to break up and repour 5000 sq.ft. of lightweight concrete. 

Can we overlay this with something that will stabilize the floor, after securing the loose pieces? We plan to just go back with more carpet and pad. If so what should we use to overlay it and to first secure or fill in the loose pieces.

Thanks in advance for your help.


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## Bud Cline (Feb 12, 2006)

Ya know what I would do with that mess?

I would use a high powered modified thinset mortar to hard-skim the entire floor. The thinset will get into the cracks and lock the fragments in place. Where the thinset shrinks and settles into the cracks, skim it again.

After that, run a floor machine over the surface to smooth it out a little and install the carpet.

Not a permanent fix but should last for a long long time.


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## firemike (Dec 11, 2005)

Darn near every building I work in that has LW concrete floors the floor looks like that. I am doing a condo right now that much of the floor looks just like that. Not much you can do. It cracks because of the flexing in the subflooring. 

Below is a photo of the floor in the condo I am working on now. When we removed the kitchen cabinets pieces of LW concrete were tipped down and actually resting on the drywall ceiling for the unit below. We took out about a 4 foot square area of plywood, as much of it was delaminating. You can see in the upper left the plywood is completely disintrigated, and you can see the yellow insulation.


Most of the time I find the subflooring is only 1/2" plywood, all that was required back then, which explains why they look like they do today. A few occasions I have taken some self-leveling cement like Custom's LevelQuik and mixed it up a little on the thin side and pushed it around with a flat squeegee in the bad areas, it locked the loose pieces in nicely. 

If you are going to carpet it again, I wouldn't worry about most of it, just take care if the smaller pieces that are loose and popping out. If they rock it is usually because a smaller piece has gotten under a bigger piece, causing the rocking, or the plywood has delaminated. You can use a vinyl-modified patching compound to fill in the holes, the rest should be just fine under carpet.


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## PrecisionFloors (Jan 24, 2006)

Is this gypsum or portland based?


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## Floordude (Aug 30, 2007)

Don't use portland based cement!!! It will spall/blow off the gypcrete.

Check out ARDEX GS-4


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## genecarp (Mar 16, 2008)

Bud Cline said:


> Ya know what I would do with that mess?
> 
> I would use a high powered modified thinset mortar to hard-skim the entire floor. The thinset will get into the cracks and lock the fragments in place. Where the thinset shrinks and settles into the cracks, skim it again.
> 
> ...


thats a good idea, although i would stain it.


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## Taranis (Feb 23, 2007)

Portland cement /water based screed will be ok provided preparation is done properly, in this case by using a neoprene primer with the screed. This should be supplied with the screed. Such as Ardex K15 or similar.

I would also try to get some bonding primer under the original screed which wil do two things; 
1 it may bond some of the f***ed screed to the substrate
2 it will help the s/l screed go down into the cracks deeper as water will not be drawn out of the screed you are laying on its way down, allowing it to get as far down as posssible without stiffening. (Too long a sentence?)


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## yes.itsconcrete (Apr 24, 2008)

i don't know how to repair it &, if i don't, you can't,,, on what data do you base the conclusion ' its lightweight concrete ??? ' & how's it been cast - stay in place pan forms ???


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## mdshunk (Mar 13, 2005)

yes.itsconcrete said:


> i don't know how to repair it &, if i don't, you can't,,, on what data do you base the conclusion ' its lightweight concrete ??? ' & how's it been cast - stay in place pan forms ???


(is a concrete guy really saying this? :whistling )


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## Taranis (Feb 23, 2007)

Doesn't matter what it is or how it was laid, all he wants to do is stabalise it for carpet. If he were installing any other topcovering the lot would have to be lifted. 
Looking at it it may be a granite or a sand/cement screed.

Vaccum the joints but try not to lift any pieces, that will only loosen the rest of it.


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## Demonseed (Jun 22, 2007)

Knowing you plan on replacing the old carpet and pad with new carpet and pad, the sub floor does not have to be perfect. If there are loose pieces of the subfloor coming up I would secure them or remove them, clean the sub floor as best as you can, Prime the heck out of it and Patch as needed. Maybe not the "correct" approach, all you are trying to do is avoid any telegraphing through the new materials.

Could you visually detect the cracking under the existing carpet or pad, could you feel it?


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## BKFranks (Feb 19, 2008)

Use Planipatch http://www.mapei.us/
I'm working on an 800 unit apartment building and that is what they are using to patch the lightweigh concrete floor.


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## Floordude (Aug 30, 2007)

Portland over a gypsum based substrate = failure down the road.

You can use gypsum patch over portland but not portland over gypsum.


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## PrecisionFloors (Jan 24, 2006)

PrecisionFloors said:


> Is this gypsum or portland based?





Floordude said:


> Don't use portland based cement!!! It will spall/blow off the gypcrete.
> 
> Check out ARDEX GS-4





Floordude said:


> Portland over a gypsum based substrate = failure down the road.
> 
> You can use gypsum patch over portland but not portland over gypsum.


See any connections here? Floordude is trying to help you here :whistling


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## Demonseed (Jun 22, 2007)

Seriously, you must be joking precision. He is putting a tack and pad install over the floor, and you guys are telling him, to level the floor like it is VCT. It is not needed. The original floor cracked and failed and the only way he knew it was by ripping it up.

Why not just do it right, tell him to chip out all of the failing concrete and repour everything. Isn't that the CORRECT way to do it? Put a few extra K into un needed floor prep? Why?. So the world knows you do quality work?

This is a weird thread, with too many overblown ego's. None of the solutions anyone offerred are permanent, so you are basically saying pay someone to do something that will never be right, but you are covering it with Carpet and Pad, so ummm...why spend the money on floor prep?

Please explain, I gave my opinion, explain yours?


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## PrecisionFloors (Jan 24, 2006)

Demonseed said:


> Seriously, you must be joking precision. He is putting a tack and pad install over the floor, and you guys are telling him, to level the floor like it is VCT. It is not needed. The original floor cracked and failed and the only way he knew it was by ripping it up.
> 
> Why not just do it right, tell him to chip out all of the failing concrete and repour everything. Isn't that the CORRECT way to do it? Put a few extra K into un needed floor prep? Why?. So the world knows you do quality work?
> 
> ...


Somebody piss in your oatmeal toughguy?

I only give advice to do things one way.....the right way. I could care less if the rest of the world knows I do quality work or not, but they now know you don't, or at least its not a priority.

As far as egos go , I'd suggest you settle the f#@* down, okay cupcake. I didn't piss on any advice you gave (oh wait you didnt, you gave an opinion) Sounds to me like your ego came out to prove how unnecessary our's was. I suggest counseling to explore your inferiority complex....life's way to short to have those demons, I hope you get 'em exercised.

Oh yeah....what happens when a power stretcher gets put on that new carpet install and it pulls the tack strip & patch right off of the slab. There's more to a quality install than just telegraphing. Let me know if you need some educational resources.:thumbsup:


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## Taranis (Feb 23, 2007)

I have to say, although Precision the best way is to remove and redo, it is not really necessary here.
I agree with Demonseed, there is no need to go full scale on prep. The client doesn't require it and doesn't want to pay for it. Do what is necessary for the job in hand. Make good the substrate then give it a finish thats suitable for sheet carpet. Gripfill the gripperrods if necessary instead of using nails. 
It will last until the next time the flooring needs replaced and maybe until the one after that. 

Cement based screed CAN be used on top of and anhydrite/gypsum with the correct primer. A lot of the time we use Ardex or Balls screeds. Full prep details can be seen here;

http://www.f-ball.co.uk/subfloor_preparation.asp

click on calcium sulphate tab.
I'm sure Ardex will have similar information on their website.

BTW Demonseed, not too many egos here, Bud Cline summed it up in a nutshell early doors.


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## Demonseed (Jun 22, 2007)

Precision (sounds like you had a chip on your shoulder), the point is no matter how much he spends on trying to fix the sub floor, it will never be "Right". If a customer called for an estimate to install Carpet and Pad over that floor, would you deem it neccessary to throw in 10k+ in floor prep? For what amounts to about a 600 yard Install? Think about it, what is the concern here? To have a subfloor suitable for a tack and pad install, that is all.


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## Floordude (Aug 30, 2007)

For carpet in old apartments with that junk, we busted it all out and installed plywood.


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## mrghm (Nov 19, 2006)

in the end the installer is responsible.
so what the installer does on each job is up to them,
a simple patch job might come back to bit them in the butt a few months down the track.

in all honesty who would want a sub floor like that under there carpet, even if patched up all some one needs to do is drop something heavy and it is going to crumble.


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## Mike Costello (Aug 1, 2004)

I run into this stuff all the time in hotels and condos.I have yet to meet one job super or homeowner that wanted to either re-pour the stuff or rip it out and put plywood.

So in a real world- I dont wanna go home-get the job done as best you can mannerI have primed the stuff (ok soaked it) with dependable latex and then skimmed or patched it the best i could with dependable mixed 100% with the latex.

Beats going home for the day as long as whoever is making the desicions on the job understands that this is a band aid


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## csheafer (May 28, 2008)

*A lot of good advice here*

The "right" answer is the one that will satisfy the customer and allow this next installation of carpet and pad last as long as it should.

At the end of the day, the floor needs to be stable enough that it is not makeing noises under the carpet and pad. To me, the best solution in this case is to secure the very loose pieces and do a quick skim coat of the gypsum based patching compound.

What will help this situation also is using a high density pad to minimize the movement of the carpet and help protect your subfloor.

Be prepared to also pay extra for the installer to put down the tack strip.
It is so cracked that special tack strip they make for this application nails are not going to hold and will need to be re nailed big time so that the nails go through the light weight concrete and into the plywood subfloor.

There is another option here that has not been mentioned.
There are company's that will come in with basically a self leveling compound that can cap the entire floor. A regular carpet installer can do this type of work, but it will end up costing you more because carpet installers do not have the specialized equipment to do this kind application.
THEY ALSO WARRANTY THEIR WORK!

Let us know what you decided and how the job came out!


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## PrecisionFloors (Jan 24, 2006)

Demonseed said:


> Precision (sounds like you had a chip on your shoulder), the point is no matter how much he spends on trying to fix the sub floor, it will never be "Right". If a customer called for an estimate to install Carpet and Pad over that floor, would you deem it necessary to throw in 10k+ in floor prep? For what amounts to about a 600 yard Install? Think about it, what is the concern here? To have a subfloor suitable for a tack and pad install, that is all.


READ my posts. NOWHERE did I state the need for 10k in floor prep did I? The whole intention of my posts was to make sure the op new to use the appropriate material to skim it with.....YOU took it upon yourself to put words in my mouth and jumped on your high horse, chip intact, and decided to try and make me look as though I were giving unnecessary advice. Prior to this, my posts consisted of TWO sentences. I guess you can add mind reader to your list of trades under your screen name......although you suck at it :laughing:

To those that followed you and failed to READ befor responding, I'd suggest quicker eyes and a slower mouse 

I stand by my two whole sentences. My posting of advice in this forum speaks for itself...I damn sure don't need your support to denounce or solidify it.

OP good luck with the job, patching is just fine in his application, I was just trying to point out that you want to make sure of a good bond between the patch and the substrate, otherwise its a waste of time. 

I'm done with this one.


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## comp (Jan 15, 2008)

i haven't read the whole thread yet ,,but in the early 80's i was laying carpet daily on lite weight crete,,,,had problems in some buildings and finally found out there was different contractors .. After that i got the OK to add a latex sealer (we rolled it on ) after i did the take up ,,, i could put tack strip down with nails (no contact cement ) and it was almost like working with wood


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## jimmers (Feb 20, 2012)

I have the same problem on my 3rd floor condo. i want to install laminate flooring but I'm worried that its too uneven in some areas. Any thought?


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