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kebwi 10-05-2009 12:29 AM

A tad concerned about my foundation
 
4 Attachment(s)
With reference to a foundation I poured yesterday...

All right, I'm not exactly in a panic. It is concrete after all. It's chalk-full of rebar and mesh, I can walk around on it without the slightest hint of a depression or general weakness. I mean, it's solid stuff, no doubt.

...however...

(Sakrete high strength concrete btw) The top quarter inch seems extremely dry and crumbly -- like petrified feta cheese I suppose -- not remotely smooth by any standard of concrete I have ever encountered. The attached photos somewhat illustrate my point; please zoom in on them to see their full macro detail.

I have been misting it with the hose periodically and keeping it covered with plastic btw.

During mixing, I relied primarily on the ball-in-fist method, shooting for a mud-ball that didn't crumble apart, but didn't swim and ooze. I thought I was doing a pretty good job, but now I think I made the entire thing way too dry and I'm not sure how that happened.

I admit, I didn't float it at all, I only screeded it. I really don't care what the foundation looks like (I'm flag-stoning or bricking it eventually) but if anyone wants to tell me that the sole cause of the result I got is a lack of floating, then I'll buy it; I just never would have anticipated floating could make so much of a difference. It seemed so chunky right after the pour-and-screed that I just couldn't imagine what effect rubbing a float around in circles could have ever had on it...but maybe I should have just done it anyway, I dunno.

I'm not too worried about it...I guess...but I would truly like to understand what happened so I can do a better job on the hearth. Did I mix it too dry or is the mix right and I just needed to float it or is there a third explanation?

Any thoughts are greatly appreciated.

Thanks.

david s 10-05-2009 02:13 AM

Re: A tad concerned about my foundation
 
After 24 hrs your concrete is not particularly hard. But it should not be dry, keep it moist by hosing or cover it with sand or plastic for one week. When you use a float the cement is brought to the surface and your concrete will be really smooth. It sounds like your rough texture is simply lack of trowelling with the float and shouldn't effect its strength.

ThisOldGarageNJ 10-05-2009 02:44 AM

Re: A tad concerned about my foundation
 
Quote:

Or give us a ring on our Help Line at 1-800-282-5828 to inquire about specific QUIKRETE® products and where to buy them, order product literature or obtain expert advice on concrete
here is the Quikrete help line number,, Im sure they can tell you if it;s ok, or what you need to do..
Cheers
Mark

RTflorida 10-05-2009 06:21 AM

Re: A tad concerned about my foundation
 
It looks to be the result of your dry mix and then not floating the surface. I don't see a problem other than appearance.
I'm guessing your 'feta cheese' is aggregate. Provided you did a thourough job of mixing, you should not have any issues. By using less water, it allows the aggregate to stay more suspended and not settle; add that to the fact that you did not float the surface and you have what your pictures show.
A wetter mix will have more water and portland cement rise to the surface. Since portland is the finest ingredient in the mix, when troweled with the escaping water, a very smooth surface is achieved.
A dryer mix keeps all ingredients more suspended with vey little cream rising to the surface - has better strength, but does not look so good. Using just a bit more water in the mix OR moistening/covering the surface sooner would have given a better finish. Since you will be finishing the surface with other materials, I wouldn't worry and move on to the next step.

RT

papavino 10-05-2009 09:16 AM

Re: A tad concerned about my foundation
 
Just keep it moist throughout the week. Hose it down every morning. I looked at your pour on your build thread this morning and it did look a little dry. But I think it'll be alright. Like RT said, since you're covering it with another material, I wouldn't worry much.

kebwi 10-05-2009 09:36 AM

Re: A tad concerned about my foundation
 
Thanks guys. I'll keep misting it, keep it covered, and go a little wetter on the cores and hearth...and buy a float. Magnesium seems to be the recommended float material, wood and steel being other options. Would anyone care to offer an opinion on that topic?

On a related tangent, I bought an angle grinder which came with a metal-grinding blade and I've had great success chopping up rebar with it. Is that definitely the wrong blade to use to clean up my construction joint, which closed up a bit during the initial setting?

Wiley 10-05-2009 10:19 AM

Re: A tad concerned about my foundation
 
Kebwi,
I agree with your conclusion of your mix being too dry. That being said I have found that often Sakcrete while rated at 5000lbs is by appearance shy of what I want in concrete. To bring it up to something I like both in appearance and workability I add about a quart of straight portland cement to every 80 lb sack of the stuff. As to water in the mix, a certain amount is necessary for the cement to turn into what is often referred to as a paste and there is a certain amount of mixing required to cover each particle of the sand and aggregate with this paste. That is why on the ticket you get from a truck delivery states that the barrel has made a minimum of so many revolutions. This is to insure that deliveries close to the batch plant are adequately mixed.

Yes, a unmixed unopened bag of Sakrete and Quikrete will eventually harden if just left, however, if you open such a ruined bag and smack the "concrete" with a sledge hammer you will find it is weak and crumbly.

The addition of water beyond the minumum amount of water increases the workability but causes the completed concrete to be more porous and prone to shrink cracks. To account for the excess water needed in pumping concrete the mix is often increased to 7 or 8 or more bags per yard. The extra cement makes up for the excess water and results in a concrete with similar strengths to a lesser bag mix but with the proper amount of water. So keeping the water to what is needed and not much more for the workability you want is what you are striving for.

Screeding serves several purposes: it settles the mix and brings trapped air bubbles to the surface resulting in a denser final product. The agitation brings "fines" and cement up and works the heavier aggregate below the surface. This makes the surface easier to finish. The usual routine is to mix and place the concrete, then screed and then rough or "bull" float the surface. Now one waits, the water in the concrete comes to the surface twice: once after and during the bull float and then it seemingly dissapears, back into the concrete. In a couple of hours the concrete has started to set and the water by appearance comes to the surface again and that is when one works to achieve the final finish. That's why one sees concrete workers standing about between floating the concrete and final finish, they are waiting on the concrete, not being lazy.

Properly done the surface will be finished before that "second water" once again recedes into the concrete. It is not uncommon to seen workmen sprinkle water upon the surface to extend the work time. However, that is a questionable practice as it often results in small surface cracks and in areas of frost and freeze the surface will separate and spall.

I would suggest that you keep your newly poured slab quite wet for at least a week. It will most likely be plenty strong as the pounds per square inch of these WFOs is relatively small. After the week and if you didn't place a vapor barrier under the slab the moisture in the ground should be sufficient to complete the cure and the slab shuld be strong enough to carry on construction.

Concrete achieves a high percentage of its final strength in the first 28 days after pouring. After that time it will continue to harden for many years. All that is fine save that you can continue construction before the 28 days if you are careful as the surface will be easily marred and chipped. There is is also an arguement to be made that the bond between the next layers and the slab will be better if the slab is green when adding the blocks.

Sorry to go on like a lecture, all of the above is info available in books on working concrete and thru observation and working with those who place concrete for a living. It certainly is more info than one needs to know and most likely much of it you already know. I have run on and added it as many who join this Forum do not run a search of the archives to see what is there.

Hope this helps,
Wiley

Seeing your post before I had finished editing mine: for normal home use a well made wood float is quite sufficient. I have several sizes and they are easily made. Unless you want to pend the money on a mag float I wouldn't bother. Regarding blades for your angle grinder: in my experience a dry cutting diamond blade is worth the small expense. Be sure to wear eye protection as well as respirator and some sort of ear protection. If you are planning on doing alot of concrete work I have found a concrete vibrator a most useful item. Harbor Freight sells one that I have and that I have found adequate.

kebwi 10-05-2009 10:30 AM

Re: A tad concerned about my foundation
 
Thanks Wiley (that's my last name, btw). The note I would add is that, unlike you suggested, I actually did add a vapor barrier. There are two layers of 4ml plastic (b/c I couldn't find any 6ml) between the concrete and the gravel.

Whatever.

Thanks.

Wiley 10-05-2009 12:25 PM

Re: A tad concerned about my foundation
 
Kebwi,
Sorry, I didn't mean to suggest that you or anyone shouldn't place a plastic vapor barrier beneath their slab. I have a barrier under mine and under all the foundations I have on my property. It is just that in some areas such a barrier on such a foundation as this might be considered unnecessary. I live in an area of fairly high acidity in the soil, besides that we have high ground water in the wintertime. Keeping extra moisture out of my WFO is a concern to me.

Since you have such a barrier you might consider simply watering the slab well and covering it with a plastic sheet. This will keep evaporative loss to a minimum. I would still check it daily and add water if it seems dry.

Bests,
Wiley

RTflorida 10-05-2009 12:58 PM

Re: A tad concerned about my foundation
 
Wiley, nice post. Great information.......thanks for the schooling.

RT


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