Slab cracking around penetrations
deerefan
| Posted in General Questions on
I was hoping to see what others with experience would suggest.
My foundation is slab on improved grade. The mat in the slab is #3 rebar 24″ OCEW. I was wondering if additional reinforcement is recommended around pipe penetrations within the slab to help prevent cracking and if so what is preferable: 4×4″ steel mesh, rebar on diagonal, etc?
I have posed this question to the structural engineer but he won’t answer…I guess because its not a structural question?
Thanks. I tried to attach a photo for clarification.
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Replies
I wouldn't worry too much about it. Your slab is going to crack. All slabs crack. You can use control joints to try and control where it cracks. Or you can mix in fibers and use as little water as possible, and that will limit the amount of cracking. But it will crack.
Is the concrete slab going to be exposed and visible in that immediate area? If so, use a control joint under the baseplate of the wall those pipes are presumably going to be in. Otherwise don't worry about.
Edit:
Some additional resources on crack control from the JLC:
https://www.jlconline.com/how-to/foundations/controlling-cracks-in-concrete-slabs_o
https://www.jlconline.com/how-to/foundations/joints-in-concrete-slabs_o
And some basics on ensuring the best concrete mix:
https://www.jlconline.com/how-to/foundations/concrete-basics_o
You might need to make an account, but it is free and it is good information.
Deerefan,
Sleeve all pipes coming through the slab, for a whole variety of reasons.
+1 for what sleeving penetrations.
I do mostly commercial work so I run into this a lot. I like to take sticks of PVC electrical conduit and cut them into short pieces to make sleeves. I place sleeves around any penetration that might move or has a non-circular profile (like a group of small pipes). You then need to seal the inside of the sleeve. I’ve used canned foam and elastomeric roof path. Both work.
Another option is to wrap the penetrations with the expansion joint material, but that only works for large penetrations.
That said, most of the time nothing special is done around penetrations. Make sure the ground doesn’t heave up around the pipes which will result in a too-thin slab in those areas. Putting control joints in a sort of box around large groups of penetrations (like under an electrical panel) can help contain any cracking to the immediate area of the penetrations.
You’ll almost always get some cracking, you just want to plan for it with control joints and sleeves or expansion space around anything you think might move or shift with time.
Bill
I didn't see any cracking around penetrations on the slab for my house. I'm not sure I agree with the assertion that all slabs will crack. We've had no cracks appear so far, 2 years post pour and we have an almost 60' span with no control joints. Having the slab completely within the conditioned envelope probably makes a big difference. Our crazy builder claimed to never had a crack in any of his buildings, but he is crazy so I do take that with a grain of salt.
Putting in sleeves would have been a good idea, but it wasn't done on our slab.
Deerefan,
For my penetrations, I just wrapped a few turns of left over sill sealer foam around each pipe and taped it off with spare Tyvek tape. This was done mostly to simplify my sealing efforts to secure against potential radon entrance. The Tyvek tape sticks well to the polyfoam and survived the pour quite well. I do hope they plan to put chairs under your rebar.
For what it is worth, I did have the contractor add polyester fiber to my 5" garage slab pour, which has not cracked anywhere. I did not have it added to my basement 4" slab pour which has cracked in several places. The basement level is stepped to accommodate the ledge I built over, so the cracks appearing at the step level intersections are pretty predictable. Both pours are over 3" recycled foam with pex embedded in case I get crazy in the future and heat them.
A few things to note: Steel whiskers or steel helixes may show up near the surface when working out the finish float. I was advised to go with the polyester ones for this reason. Truck loading aprons are not finished smooth, so there is not much of an issue with using them there. If you are planning to dye or acid stain what appears to be a residential slab, then maybe polyester fibers would be less likely to pose problems. Something to ask anyway.
Last thing, if you are intending to use the slab finish as flooring, do ask that the pour be done with enough good weather ahead to allow very hard setting of the floated surface. My basement floor got caught by a late evening sprinkle which created a permanent pattern. It is just shop space, so no biggie. Had I been planning for staining and sealing it might have added a texture that wasn't desirable.
Roger,
Thank you for your comments. Yes this is a residential slab with a large amount of exposed polished (unstained) concrete. The rebar chairs will be placed. Thanks for your note about the wires, I will make sure to have all of them flush. I will also look into the polyester fibers, I wonder if this affects the strength of the concrete - this is an engineered slab so everything has to be cleared by the engineer.
How does wrapping or sleeving pipes help with cracking anyway? Also, why hasn't anyone mentioned adding any reinforcement around pipe penetrations - seems like a logical way of trying to decrease this issue?
Thank you again.
>”How does wrapping or sleeving pipes help with cracking anyway? ”
The wrap or sleeve provides a sort of buffer between the penetrating pipe and the slab. Controlling cracks is all about controlling stresses, so you want to prevent the transfer of force from things that move differently than the concrete does. Common things to isolate are footings/piers for columns (they have more tendency to settle), and penetrations like pipes that may expand/contract differently than the concrete.
Regarding reinforcing around penetrations, sometimes a steel ring is set into the slab around areas of a lot of penetrations (like under electric panels), but it serves the same purpose as sleeves.
Bill
To my understanding, extra rebar around the penetration won't really help with crack mitigation. Rebar's purpose is to add tensile strength to the concrete, not stop it from shrinking (why cracks form). From all the rebar reinforced concrete I've seen, it still forms cracks. Also, from that photo, it looks like the rebar job wasn't completed, so you could very well have rebar around the penetrations.
You haven't mentioned what your concrete design is, but I'm going to assume its a typical rebar reinforced slab with control joints. As Aedi mentioned, review the control joints around it and maybe add more.
Otherwise, I'm no concrete professional, but with how I work, my suggestion is to use sleeves, have the whole slab Rebar and glass fiber reinforced, control joints if you want (I don't like them aesthetically), very dry mix during the pour, and use burlap during the curing and water daily for 2 weeks. Its the evaporation of water from the slab as it chemically creates heat that causes it to shrink and crack. having less moisture to begin with and controlling the moisture loss during curing will let it shrink much more slowly and uniformly. The Glass fiber also helps keep everything together, and even if it does crack, they're hairlines and not too noticeable.
Jamie,
What kind of sleeves do you recommend to use? Would these be only around pipe penetrations through the slab or as they cross internal beams as well. Thank you.
Deerefan,
Talk to your concrete finisher before deciding to add fibres. They can make polishing a lot harder.
> polyester fibers, I wonder if this affects the strength of the concrete
Yes - in a positive way.
If you want to further reduce risk of cracks, consider shrinkage-compensating admixture. And additional rebar (2" below concrete surface).
You can add #4 rebar on a diagonal around pipe penetrations and at corners, the places where concrete is most likely to crack as it shrinks. If you use control joints, make sure they align with penetrations and corners. (I don't like how they look so I rarely spec them.) Most importantly, keep the concrete damp for at least a week, so it has enough moisture to complete the chemical reaction that gives concrete its strength. Or use a high early strength concrete, but even that should stay damp for a few days.
You may have to fight with your builder on this, as everyone likes to get onto the slab ASAP, but by doing so you greatly reduce the strength.
Another way to minimize cracking is to use more cement, either Portland cement or Pozzolan admixture, which can replace up to 50% of the Portland cement without affecting strength. The IRC calls for 2500lb mix for slabs, but you can bump that up to 3000lb or higher.
Anyone who says "concrete always cracks" is not correct. Concrete with poor details that dries before it can cure usually cracks. And occasionally it will crack even if you do everything right. But I've rarely seen cracks on my jobs, when these specs are followed.
Michael,
Exposed polished slabs are all the rage around here. People generally include control joints, but leave them open - and I agree I'm not wild about them. I always grout the joints. My own house has grouted control joints every three feet. No cracks anywhere, and I think they add something to the look of the floor.
A related question. Some of my internal beams ended up being deeper than the 24" required. The rebar crew didn't really pay much attention and put the standard size stirrup so now the space between the mat and top bars in the beams is 8-10 " in some beams. My question is: should I take apart the smaller stirrups and replace them with appropriately sized ones or should I add more stirrups to the already present ones and add another set of top bars. I think that taking things apart may be quite more time consuming but I will do that if that is what is needed.
The joys of building a house. Thanks again.
I take it this is slab-on-grade with integral poured concrete beams? Did they overexcavate for the beams?
Yes, it is and yes, they did. I see some people "correct" this by using heavier rebar for the mat and then suspend the beam steel to the mat, which is not correct either as it leaver a greater space below the bottom steel. My thought is to add stirrups and another pair of bars 6-8" above the prior top pair rather than taking all apart. Furthermore, it would be impossible to find all different stirrup sizes. Thanks again.
It all seems to be a bit of a joke.
Deefefan, it depends on the load the beam is carrying. There is a minimum required distance from the rebar to the face of concrete (in any direction) to minimize water infiltration that will lead to corrosion. 2" is typically enough when using standard 3/4" aggregate. There is a minimum distance to develop enough strength in the rebar-to-concrete connection, so the concrete stays put under load. 2" may be enough, or may not. Finally, for the beam to develop strength in bending, you want the rebar as far from the center of the beam as possible. Your engineer would be best equipped to give you a good answer.
If the beam was designed correctly and the stirrups and rebar are as spec'd, I'd leave it as is. The extra concrete is just taking the place of soil at the overexcavated beam.
I really don't think that's how it works. I thought the rebar needed to be close to the bottom to help with tensile stresses in that location. If its not there what is there to stop the crack from starting due to those forces? I am not saying it would definitely happen but I do believe in constructing things as designed.