Concrete experts needed.

[Herb]

Ok, here is a question for the concrete experts. I
don't mean some one who "heard from his brother- in-laws neighbors father", either. Will all poured concrete foundation walls crack? The walls in question are 10" thick, 3500 psi, two rows of rebar (not sure now if it is #4 or #5) top and bottom, with some verticles tied in also. The house walls are all buried, with no big rocks against the foundation, with about 10''- 12'' showing above grade. The garage has a full basement with an overhead door under it and two walls are completely exposed (12'). All of these walls, house and garage, are 29' long. All of the walls have a hairline crack in the corners, and a couple walls have cracks in the middle of the span. These all run pretty much from top to bottom, the widest one ( only 1) is maybe 1/16'' wide, the rest are as I mentioned before, just hair width. None of the cracks are horizontal. These 2 foundations have been in for 4- 6 years each, and though the cracks started after 1-1/2 years they seem to have stopped appearing. My questions are, (1) Should they be there, (2) Why are they there, (3) What can I do about them now? Thanks in advance for your replies! Oh yeah, the footings are all on undisturbed hard pan, the excavator really did an excellent job by not digging too deep so nothing had to be back- filled and compacted.

 

[sayemthree]

all concrete cracks......

dont worry at all about hairline cracks being structural. control and expansion joints can be sued to limit and control cracking.

engineers design concrete on the theory that it will crack. The steel is there to resist any tension and the concrete resists the compression. If the cracks are larger than 1/4 inch then you should worry.

BTW - structural engineering degree/background

 

[PAToyota]

Background - registered architect, ten years working in structural engineering firms.

As above - structural concrete will crack to some extent. Rebar top and bottom "with some verticals" doesn't say much. How tall are the walls? I can design something that won't have more than a few hairline cracks (which you will nearly always have due to curing), but you'll be talking about rebar cages at that point - ie: horizontals and verticals at likely 16" o.c. both faces, expansion joints, controlled curing, and such.

It doesn't sound like what you have is anything to concern yourself about. The wide one (1/16") is likely due to not having any expansion joints - so it created one. You could grind it out and seal it with a caulk if there is any water penetration.

 

[boiler7904]

Background: Construction Management Degree, 8 years working for an architectural firm

Like the other two guys said, small hairline cracks in concrete are normal and expected. There isn't really a need to be alarmed unless one of the following is happening:

- The cracks start to visually open up. You can buy two-piece crack measurement devices that get glued to the wall on each side of the crack to monitor movement.
- Water starts to penetrate through the crack.
- You see movement (horizontal or vertical) somewhere in the foundation wall itself or between the concrete wall and the attached framing.

A steel reinforced concrete wall will actually bend somewhat when forced to. Quick example: I'm working on a project now where part of the foundation (60' long wall) was placed over buried topsoil (1' deep vein about 5 below the bottom of the footing - 10' deep overall). There is one tiny hairline crack in the foundation but we have almost 2" of vertical settlement and a 3/4" bow in the middle of that wall. The only reason that we knew we had a settlement issue is that the wall started to pull away from the interior slab.

 

[Herb]

I'm not panicking about it yet as the cracks started around 3 1/2 years ago and I haven't seen any new ones in the last year. The existing cracks don't seem as if they are any wider either. There is probably more rebar in place than I described, but I can't find the pictures of the form work to absolutely verify that. I guess what I really wanted reassurance of was that most concrete basement walls will do this, not just mine. There isn't any water seepage through the cracks since the outside was so well spray sealed. Thank- you for your educated input, that's exactly what I was looking for.

 

[xsponge]

Been a contractor for about 25 yrs now, the old boy that taught me about concrete told me there is one thing to remember about concrete, there is concrete that is cracked and there is concrete that is gonna crack. Simple. The only thing that can be done is to control the cracks so they end up where ya want em.

 

[GearHead_1]

I'm not an expert but I know everything there is to know about concrete:

1 - It will crack
2 - It won't burn
3 - No one is going to steal it