delprete
New Member
Posts:23
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| 17 Sep 2012 02:50 PM |
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Not sure if this is the right forum... I poured my slab-on-grade for my passive solar home some time ago and never cut in control joints. In fact, I was convinced against my better judgement by a builder friend of mine with many years of experience to not bother. The cracking that's occurred has been minimal. Basically, two very fine cracks each running perpendicular to NS and EW edges of the slab.
Do I bother cutting in any joints at this point?
Is it ever too late to cut them in?
The slab is 7" thick with 6x6 w2.9 mesh and overkilled with rebar at internal corners and footings.
Thanks
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Alton
Veteran Member
Posts:2164
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| 17 Sep 2012 03:34 PM |
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My opinion: Saw joints will not prevent shrinkage cracking. If you plan to have saw cuts, then they should be done within a few hours after the concrete sets. Basically, saw cuts just create a weakened plane to encourage the cracking to be hidden in the saw cut. Saw cuts do not always perform as expected. That is, cracks can develop in other areas, and if so, then will be noticeable. |
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Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
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delprete
New Member
Posts:23
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| 17 Sep 2012 03:55 PM |
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That's similar to what my friend had said. He explained that the saw cuts don't prevent cracking, but instead may encourage the crack that more than likely will occur to happen where you'd rather have it, and since I'd be flooring over the entire slab I didn't need neatly layed out and symmetrical "cracks".
Now my engineer said to place them (other than according to her schedule) wherever I had loads on the slab to better control and direct cracking. I don't know what to make of that since seems to me that the point of all the mesh was to control cracking, or rather to provide strength where movement occurs.
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jonr
Senior Member
Posts:5341
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| 17 Sep 2012 04:46 PM |
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If you were about to put in an interior wall or carpet, anywhere that it is a good place to hide a crack, then I would, with the goal of maybe preventing one in a visible area. |
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cmkavala
Veteran Member
Posts:4327
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| 17 Sep 2012 05:34 PM |
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Concrete will crack! you have chance of picking where you want it to crack if you saw cut, otherwise it will just crack on its own at the weakest point |
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| Chris Kavala<br>[email protected]<br>1-877-321-SIPS<br /> |
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delprete
New Member
Posts:23
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| 17 Sep 2012 06:04 PM |
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I guess the crux of my question is if the joint is structural or aesthetic.
Seems to me the responses suggest that it's an aesthetic concern.
If the living space portion of my slab will be entirely covered with a finished floor effectively hiding any cracks, then is it worth bothering?
My utility room will remain concrete, so if I want to control the cracking there would it be effective to still cut in the joints even though I poured last fall? In other words, will the concrete crack do to shrinkage (which has already occurred) and then again when my bearing wall stresses it? |
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Alton
Veteran Member
Posts:2164
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| 17 Sep 2012 07:19 PM |
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Posted By delprete on 17 Sep 2012 03:55 PM
. . .
Now my engineer said to place them (other than according to her schedule) wherever I had loads on the slab to better control and direct cracking. I don't know what to make of that since seems to me that the point of all the mesh was to control cracking, or rather to provide strength where movement occurs.
Wire mesh is not strong enough to prevent cracking. Wire mesh is supposed to hold the cracks tightly together. In real life, wire mesh does not do much of anything since it is almost impossible to install it correctly. To be effective, wire mesh should be installed within the top 1/4 of the slab but unless the mesh is under tension then the mesh ends up towards the bottom. In my area, the code requires either wire mesh or fiber. At least fiber in the mix can be throughout. |
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Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
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delprete
New Member
Posts:23
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| 17 Sep 2012 07:26 PM |
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Posted By Alton on 17 Sep 2012 07:19 PM
In real life, wire mesh does not do much of anything since it is almost impossible to install it correctly. To be effective, wire mesh should be installed within the top 1/4 of the slab but unless the mesh is under tension then the mesh ends up towards the bottom.
I can't tell you how much time I put into prep with the mesh and once that concrete pounded down on it I saw all the perfect engineer drawings being little more than something that looks great on paper! |
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ICFHybrid
Veteran Member
Posts:3039
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| 17 Sep 2012 08:30 PM |
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I find #3 bar to be a little more stable and predictable. I'll never do the wire mesh again unless it is for a quick utility slab or something like that.
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Alton
Veteran Member
Posts:2164
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| 17 Sep 2012 09:37 PM |
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I have no use for wire mesh in a typical slab! |
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Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
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jonr
Senior Member
Posts:5341
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| 17 Sep 2012 09:50 PM |
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wire mesh should be installed within the top 1/4 of the slab I've also heard bottom 1/3, middle and top 1/3. Is there good data to support any of these? |
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ICFHybrid
Veteran Member
Posts:3039
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| 17 Sep 2012 10:02 PM |
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If the living space portion of my slab will be entirely covered with a finished floor effectively hiding any cracks, then is it worth bothering? If you are going to lay tile, the magnitude of the cracks and whether or not they will continue may have some bearing on the membrane under the thinset. Otherwise, it's not an issue. I've even seen those hairline cracks in slabs that were left bare (polished) that were acceptable. The time to sawcut was after the pour, though. Do your bearing walls have thickened slab sections with heav(ier) bar under them? It's hard to predict if they will continue to crack after the initial shrinkage cracks. What fill is under the slab and how was it compacted? |
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Alton
Veteran Member
Posts:2164
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| 17 Sep 2012 10:41 PM |
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Wire mesh is too weak to be considered reinforcing steel, therefore, for crack control it should be in the top quarter of the slab as opposed to rebar that is placed on the tension side. |
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Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
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delprete
New Member
Posts:23
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| 17 Sep 2012 10:50 PM |
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Posted By ICFHybrid on 17 Sep 2012 10:02 PM
If the living space portion of my slab will be entirely covered with a finished floor effectively hiding any cracks, then is it worth bothering? If you are going to lay tile, the magnitude of the cracks and whether or not they will continue may have some bearing on the membrane under the thinset. Otherwise, it's not an issue. I've even seen those hairline cracks in slabs that were left bare (polished) that were acceptable. The time to sawcut was after the pour, though.
Do your bearing walls have thickened slab sections with heav(ier) bar under them? It's hard to predict if they will continue to crack after the initial shrinkage cracks. What fill is under the slab and how was it compacted?
Yes, the bearing walls rest on a thickened portion of the slab with bar in addition to the mesh. Under the slab is 14" of 1" crushed stone that I compacted in multiple lifts. Under that is a base of 2" minus that I replaced the woods soil with that I removed. That had also been compacted in lifts and left to sit over winter and spring. My soil type is gravel and coarse sand. I guess I'm maybe being dense, but trying to understand if the sawcut joints are only meant to direct cracking and not actually limit it, then is it necessary if the concrete surface won't be exposed. Or is directing the cracking in the load bearing wall location necessary to prevent a structural problem. Thanks |
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ICFHybrid
Veteran Member
Posts:3039
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| 18 Sep 2012 01:52 AM |
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then is it necessary if the concrete surface won't be exposed No, don't worry about it anymore. Or is directing the cracking in the load bearing wall location necessary to prevent a structural problem. Once you have the prescribed rebar in there, you're not going to have a structural problem from the hairline cracks. |
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whirnot
Basic Member
Posts:186
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| 02 Oct 2012 11:59 AM |
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Personally I would cut it. It just doesn't cost that much, and down the road if you want to change the flooring there is a better chance that the cut joints will have controlled the cracks that WILL occur. |
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