Concrete experts needed.

[ELMO67]

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) Should I worry 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.

 

[rox]

Can't answer your question but as a basement fan, I am impressed with a basement under a garage...

Before we sold our last home we did a very very basic basement improvement. One thing we did was put in carpet. The realtor told us that people would be concerned with the carpeting in the basement becuase they would wonder if it was there because we were hiding cracks int he floor. And actually the floor did have a few cracks but we never got any water in them at all. So what I did was get the name of the best basement consultant in the area. Every realtor used this guy, he was a basement inspector extrodinaire. His word was law. I hired him, he came in and inspected the basement, wrote up a real nice report, and then we carpeted.

One of the things the basement inspector did was he used a moisture meter to measure if any fo the cracks had moisture. They didn't. However where the walls joined the floor he did pick up moisture, I never even knew it was there. The float on the sump pump was wrong (and had been for 12 years) so he adjusted the float. I went to the local hardware store bought a moisture meter for like $15 and within a week after he changed the float my moisture meter showed no moisture, had him come back, his moisture meter showed the same thing so we got the final report form him as a great basement.

If you are concerned about the cracks then you might want to get one of those moisture meters and test yourself. That is what the expert I hired used. We did have a few cracks in the wall, like you are saying hairline cracks but they weren't judged to be anything at all. Don't know if they were horizontal or verticle though...

 

[drm]

Ok, here is a question for the concrete experts. ... My questions are, (1) Should they be there, (2) Why are they there, (3) Should I worry about them now?

A quick reply to your questions:
1. Maybe, all concrete cracks.

2. As the concrete dries it will shrink. Shrinkage of the concrete can cause visible cracks. But, more than likely, based on the description and locations, the cracking is due to tensile forces in the exposed face of the wall at the location of the cracks. Concrete has very little tensile strength and will crack if it bends on the tensile face. Hence the reason for using reinforcing bars... to take the tensile stresses (and to limit the width of the cracks).

3. If they get wider then definitely yes. Immediately, get a Structural Engineer to inspect them. If they do not change then you SHOULD not have a problem but I can not say since I have not seen the wall, structure and backfilled areas. Is there a bow (inward) to the wall along its length? Use a string line to determine. If yes then worry; if not then who knows?

Some cracking, settling, bowing may not be a cause of concern but without being there I could not say one way or the other. Tensile failures in concrete are sudden and often catastrophic. I am not saying that this will happen just don't know.

...Derek

 

[CurlyDave]

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.

This is not a sufficient description of the reinforcing to tell you if the walls will crack. How high are they? How far apart are the vertical bars? Are they just straight bars, or are they L-shaped pieces?

As has been mentioned, concrete cracks easily if it is ever placed in tension. Sufficient rebar will prevent this, but, what is "sufficient" for me, is "overkill" to most contractors.

The only time I have ever had a slab or wall I poured crack was when I used the wire mesh reinforcement in a slab. This was an important lesson to me, and since then have only used #4 bar.

I always design by treating the earth behind a wall as having a density of 105 lb/cubic foot, then make certain it drains properly and apply a safety factor of 2 to the design (double the bar the above calculation requires).

I use a double curtain of bar, with the bar on the compressive side of the wall spaced about double that of the bar on the tensile side of the wall.

Slabs are 6" thick, not 4", and have a grid of #4 bar on 18" centers. 6" of drain rock as a base, vapor barrier, 2" of sand and then pour the concrete.

Sure this is more expensive and contractors tell me that I don't need to do it, but my concrete work does not crack.

Once you have developed cracks, all potential fixes are costly, much more costly than overkill at the start.

If the cracks have really stabilized (get an engineer to look at it) you may be able to improve the cosmentics by applying a coating. I have used something called Tamoseal which is not too expensive. Mix it up & apply with a very coarse brush. It will cover & conceal many small cracks.

 

[bugstruck]

What you're describing is not that unusual. Derek and Dave said it well. They both are cautious, conscientious, and able constructors. I would keep an eye on it from time to time but I don't see your description as a problem unless the cracks continue to open up. They shouldn't. If the wall is pushed as Derek describes (as a possibility) I still doubt it will go much more. That's a rather strong residential wall. If you build it like Dave it will be here when your grandchildren's grandchildren retire. I'm good with that though. Err strong, never weak.

 

[Egon]

I'm not an expert but would say Dave has given some excellent points on construction design.

The next question would be " Did the pour and rebar spacing adhere to specs"
Not always is the concrete in the truck in the best of shape when it arrives. Vibrating may not be properly done.
Rebar may have not been placed correctly.

Next.
Was the concrete cured before the walls were backfilled?
Was there a change in soil moisture or sidewall pore pressure after the pour?

I think one has to look at Design, construction techniques, and changeing variables after construction.

 

[ELMO67]

The foundation wasn't backfilled for 30-60 days in both cases. The garage has two walls that don't have any fill against them, they are completely exposed as mentioned, approx. 12' tall. I back filled the other two walls with a lot of 1-1/2'' washed stone so I know there is no hydrostatic pressure build up. None of the walls in question have bowed at all. It's really just ugly to look at coming down the cellar stairs, and I haven't had a chance to inspect other recently poured walls in the area to see if they have cracked. The funny thing is, the basement floors in both house and garage have no cracks at all.

 

[drm]

The foundation wasn't backfilled for 30-60 days in both cases. The garage has two walls that don't have any fill against them, they are completely exposed as mentioned, approx. 12' tall. I back filled the other two walls with a lot of 1-1/2'' washed stone so I know there is no hydrostatic pressure build up. None of the walls in question have bowed at all. It's really just ugly to look at coming down the cellar stairs, and I haven't had a chance to inspect other recently poured walls in the area to see if they have cracked. The funny thing is, the basement floors in both house and garage have no cracks at all.

Based on not seeing any bowing of the wall and I am assuming that the walls are true and vertical and have not moved, then the cracks become less significant. Are the tops of the walls tied into the floor framing well? Not just a few J hooks. The basement floor slab should also be placed tight to the wall. Assuming that the top and bottom of the wall are properly supported I would tend to believe that the cracking is not structurally significant. BUT, this is an assumption that may be wrong without seeing it.

Sorry for all of the disclaimers and qualifiers but I do not want to mislead anyone because I do not have all of the information. I testified in a trial and when crossed the plaintiffs attorney tried to get me to say that any crack in any foundation wall is not structurally significant. I had said that the observed cracks in the subject wall were not structurally significant. (These cracks in another wall may have been a concern.) A trained expert will get a wealth of information from a visual observation of the wall that someone else may not see. Their expert did not appear to know the reason for the cracking and felt the whole thing would crumble into a pile of rubble.

If you are concerned get a local structural engineer that is a PE to look at it and discuss it with you. Get someone who is an expert in concrete construction and does this type of work on a regular basis. If you talk to them up front and describe what you want you may only need a site visit and a verbal discussion of the PE's observations. This should cost a couple of hundred and may be worth while for your peace of mind.

..Derek

 

[CurlyDave]

The foundation wasn't backfilled for 30-60 days in both cases. The garage has two walls that don't have any fill against them, they are completely exposed as mentioned, approx. 12' tall. I back filled the other two walls with a lot of 1-1/2'' washed stone so I know there is no hydrostatic pressure build up.

Do you really mean that these walls are 12 feet tall?

Were they designed by an engineer or was it just "by gosh & by golly" by the contractor?

A 12 foot retaining wall is very high. At that height you would need a key at the bottom, a flat pour and a lot of rebar, bent in L-shaped pieces embedded in both the flat concrete and in the wall.

I am building a house with a 14 foot retaining wall, and it starts out 2 feet thick at the bottom, goes to 18 inches several feet above its lowest level and the top section is 12 inches thick.

The amount of rebar you described is great for a 12 inch high wall, but a 12 foot one is going to require a lot more. Are there drawings for the wall?

You really need to talk to an engineer...

 

[MarkV]

Personally I don’t think what you describe is a problem or that unusual. If you are worried that the cracks are still moving here is a simple check to find out. You can take a piece of window glass about 6” square and use epoxy to glue the glass on both sides of the crack. Keep an eye on it and if the glass is not cracked the wall is not moving.

MarkV