Cracks in concrete wall?? 3

[PostFrameSE]

I have a yet-to-be occupied 100' wide mono-sloped cattle building with an 8" steel reinforced concrete end wall cantilevered above grade 48". Anchored to the top of this wall is the endwall wood framing. This endwall carries 450plf under a full snow load, which the building has yet to go through a winter. The concrete wall is reinforced with a #5 vertical at 12" o.c. and #4 horizontal at 12" o.c. minimum. I'm told that the customer paid for even more reinforcement than this! The concrete wall extends 30" below ground where it attaches to the footing. To date, there has been very little actual gravity load on this wall. The metal-clad wood framing on top of the concrete is 17' at the low side going up to 29' at the high side. Any lateral or vertical load should be uniformly distributed, as there really is no way that loads can be concentrated on this wall.

At approximately the middle third of the 100' wall length there are several vertical cracks, largest at the top (1/16") and tight at the ground level. We have two buildings, and each of these has the same concrete wall section, and each of the four endwalls hav 2-3 cracks as described above in the middle third of the length of the wall.

Neither end of the wall, or anywhere in between is the wall supported any differently than the rest of the wall. It's a pure cantilevered wall.

Any ideas on what might be happening, or what has caused each of these four walls to crack in the manner they have?

Thanks.

 

[JD2]

You don't say anything about location(s) of control joints. Since the cracks are vertical and not horizontal, my initial guess would be shrinkage cracks. They would be tighter at the bottom due to restraint from the footing.

 

[JedClampett]

A concrete wall that long and thin is likely to crack. What we like to do is to outguess mother nature and put in construction joints at 30 feet or so and hope the concrete cracks there so it's not too noticeable.
If the cracks are perfectly vertical (pretty unlikely), you could open them up to 2 inches deep about a 1/4 inch with a concrete saw and then caulk them.
You could epoxy inject them, but come winter they might open up more.

 

[jike]

It sounds like what you are describing is shrinkage cracks.

Did you specify a limit on the length of pour or did you allow them to pour 100 feet?

Did you specify control joints? At what spacing?

 

[PostFrameSE]

Thanks to each of you for your responses. 3 out of 3 saying that it's shrinkage cracks. I wondered if that would be the case.

JD2, your response makes sense. Thanks.

JedClampett, thanks. I think we'll want to fill it in due to the corrosive environment that the concrete will be exposed to.

jike, I did not specify a limit of length that the crews could pour. What would you suggest? I assume that you think differently about exposed wall surfaces than trench foundations correct? Lastly, there were no control joints specified. I'm gathering that that is the lesson to be learned on this project.

So now I'm in the learning mode rather than detective. I looked in ACI-318, and unless I missed it, I don't see anything about construction joint spacing limitations. Is 30' a good number? As I look back in my concrete design book, I see that at a minimum, if a wall is 100' long, I could expect a 1/4" of shrinkage. I suppose as well that if water was added on site that that too would contribute to greater shrinkage. Anything else I can learn here??

Thank you

 

[JAE]

PostFrameSE,

I agree with the posters above.

For retaining walls, etc. similar to what you have, we usually express expansion joints every 100 feet (a 3/4" to 1" gap with backer rod and sealant) and control joints every 25 feet. The control joint is usually a formed "V" groove in the wall which is later sealed similar to the expansion joint - just not as wide.

To minimize shrinkage in walls, or in any concrete for that matter, there are numerous things to do:

1. Use larger aggregate (1" to 1 1/2" would be ideal)
2. Minimize cement content (4000 psi concrete will develop more shrinkage than 3000 psi).
3. Begin curing as soon as possible after concrete placement.
4. Use of shrinkage compensating cement is sometimes used.
5. Keep water/cement ratio low. Too much water develops higher shrinkage.

But even with all that - a 100 ft. wall will most likely crack with the base restraint that JD2 mentioned.

 

[msucog]

lack of c.j. my guess...maybe in conjunction with higher slump concrete. do the cracks happen to form where the horizontal rebar laps (say every 20-30')? i've seen cracks propogate in walls where the laps were deficient and all the laps were located vertically to each other instead of staggered.

 

[jike]

In a 100 foot wall, limit your pour length to 50 feet and provide control joints at 25 feet. At control joints, discontinue every other bar thru the joint.

 

[HvZ]

What you are looking at is shrinkage cracks.

1st thing to remember that you will allways have cracks. Whether the size of the cracks is a problem depends on the service of the structure (e.g. water retaining, aethetics, etc.)

I design water retaining structures in Australia, and the limits on cracks is understandebly stringent.

The idee is to accept that you will have cracks, but to keep the pattern constant (i.e. same length and even spacings) and to keep the width of the crack below the servicability limits.

In a cantilever wall you will find vertical cracks with length and spacing is dependant on the panel length to height ratio. The reason you get vertical cracks is because you cast the base first, which then restraints the wall concrete from shrinking no matter the vertical joint spacing.

The primary design factor that will control the cracks is the ratio of horizontal reinforcement above the base. For the geometriy of your wall, it is over the whole height of the wall. The vertical reinforcement only controls horizontal cracks at vertical CJ's.

For a required reinforcement ratio, smaller bars at closer spacing is preferred. For water retaining structures, i always try to use a 150mm spacing but never more that 200mm.

Other factor to keep in mind is the concrete mix design. The higher the cement content, the higher the concrete tempratures and the more shrinkage.

The most important construction activity is Curing. If the shutters are removed early, use curing agent until at least 7 days after the pour. Be very strict with the curing. Do not cast concrete on hot days.



Cheers

HvZ

 

[dik]

The only thing that I can see that is missing is that the horizontal and vertical reinf is switched and slump should be 3" max... the #5 should have been horizontal... There's enough combined info in this to create a faq...

Dik

 

[civilperson]

dik,
Why do you want major reinforcement in the horizontal when the wall is acting as a cantilever in the vertical?

 

[PostFrameSE]

civilperson,

I agree with you, but I attributed Dik's response to not knowing all of the details perhaps. It is acting like a cantilever, so the larger reinforcement should be vertical, however, there's nothing to say that we couldn't have increased the size of the horizontals.

This site, and all of your responses have been very helpful! Thank you very much.

 

[HvZ]

The design for strength and shrinkage crack control is two independent designs.

The reinforcement ratio required to limit shrinkage cracks is essentially your minimum reinforcement.

Where the strenght design requires more than the minimum reinforcement this need to be provided.

Cheers

HvZ

 

[dik]

True that I don't know what the design criteria is, but #5@12 just seemed a tad heavy... but the point was that horizontal wall reinforcing is often heavier than vertical.

Dik