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horizontal cracks in tilt up panels
2

horizontal cracks in tilt up panels

horizontal cracks in tilt up panels

(OP)
Just recently completed a tiltup project with 10 inch panels (7" + 3" architectural face). The architect called me to say the panels had developed long horizontal cracks that penetrated the full depth of the architectural face.  These cracks are not a result of deflection as I have rechecked my calcs and that seems okay.  Just wondering if anybody has had a similar experience with sandwich panels?

RE: horizontal cracks in tilt up panels

So far you say that you have rechecked your calcs for deflection problems.  However, your problem may originate at the site not the engineering desk.  

If the concrete was not at strength when lifted, that's a problem.  Can you determine this condition?

If the concrete is still not at strength or gaining strength as it should, this is a problem.  Can you determine this condition?

If the wrong reinforcing was placed or placed incorrectly, this can be a problem....can you determine this condition?

There are likely other conditions to check for also.

Good luck

Regards,
Qshake
pipe
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
 

RE: horizontal cracks in tilt up panels

(OP)
Yes I have reviewed field reports for reinforcement inspection they appear to be inorder and I reviewed all concrete reports and yes they all meet project specs.  I'm thinking the cracking is more related to creep  and the fact this is a sandwich type panel with the composite connectors of which I'm not a big fan.

RE: horizontal cracks in tilt up panels

Lifting cracks probably.   

Mike McCann
MMC Engineering

RE: horizontal cracks in tilt up panels

When you say this is a "sandwich panel", does that mean the two layers were cast at different times?  An explanation of how the panels were cast would help.  My guess is the cracking is due to differential shrinkage of the two parts, and the structural layer restrained the architectural layer.

RE: horizontal cracks in tilt up panels

(OP)
The panel consist of a 7" thick structural face, 2" rigid insulation with composite pins at 12" oc and 3" thick architectural face.  Structural face placed first cured, insulation and pins installed then architectural face placed and cured. I've designed miles of this type of wall and have never encountered this issue before.I can't see differential shrinkage due to the composite pins and insulation between layers. Also have a hard time believing it's due to lifting.

RE: horizontal cracks in tilt up panels

If these were cast architectural face up, then lifting is probably not the issue, as the architectural face would be on the upside of the lift.  

Better verify the water concrete of the mix with the concrete company.  Seen a lot of water added onsite, whichis usually the case, that may have affected the shrinkage.

Mike McCann
MMC Engineering

RE: horizontal cracks in tilt up panels

I agree with Mike....check the water added.  Restraint condition could contribute, but like water addition issue.

RE: horizontal cracks in tilt up panels

Likely cast with architectural face down, I would think.  Also, there could be some adhesion/suction in lifting the panel from the mold. This would cause cracking of the panel face.

Have you checked the cover to the rebar?  How long are the panels? What are the properties of the mix used?

Dik

RE: horizontal cracks in tilt up panels

Is there reinforcing in the 3" skin?  By architectural face, what do you mean?  Profiled face?  Coloured concrete?  When you say composite face, do you mean there is composite action between the two, or just that the face is a veneer tied to the backup?  If it is truly composite, the pins would be very stiff.

RE: horizontal cracks in tilt up panels

I don't know how it would be cast A face down when the A face was poured last.  That makes no sense.

Mike McCann
MMC Engineering

RE: horizontal cracks in tilt up panels

Although opposite of what was stated, it is most logical to me to pour the A section first, place the foam, then pour the structural section.  If this were the case, then lifting cracks could be seen on the A face.

Mike McCann
MMC Engineering

RE: horizontal cracks in tilt up panels

I always thought that the architectural face was placed face down first.  Then the insulation, then the structural.  The lifting inserts to lift and place the panels would be in the back side structural face, right?

How did they lift the thing - through the architectural face?

 

RE: horizontal cracks in tilt up panels

how many panels done in the pour? how many panels cracked? if only a few... could a retempered cold joint(s) be at play?  we include a concrete placement log with truck times in our testing so downtime is documented.

RE: horizontal cracks in tilt up panels

JAE... my understanding, too...

Dik

RE: horizontal cracks in tilt up panels

Architectural levitation

Mike McCann
MMC Engineering

RE: horizontal cracks in tilt up panels

If the structural face was cast, has subsequent plastic shrinkage and the the architectural face was cast then the structural face would act as a restraint to the architectural face shrinkage.
i.e. the structural face would have done most of its shrinkage when the architectural face shrinkage was at its maximum.
Just a thought.
 

RE: horizontal cracks in tilt up panels

csd72,

That was my thought also, as expressed above.  But the OP believes the 2" layer of insulation between the two layers of concrete makes that unlikely.  I don't know much about what makes this type panel act compositely, but still think our differential shrinkage theory is a possibility.    

RE: horizontal cracks in tilt up panels

The 2" insulation has no relevance if the ends of the panels are closed off with concrete - this is where the restraint is.

RE: horizontal cracks in tilt up panels

(OP)
The Arch face was cast first, the only way the arch face is attached to the structural face is via the composite pins, essentially when the panel is erected it hangs off the face of the structural panel and the ends of the panel are not closed off with concrete therefore there is no restraint provided by the structural panel. BTW these panels were cast almost a year ago and the cracks are only noticed because they starting sand blasting the panels.

RE: horizontal cracks in tilt up panels

a mystery indeed.

RE: horizontal cracks in tilt up panels

So...  Is this the correct construction sequence then:

1.  The edge form is assembled and the Architectural forms placed in the casting bed.  Any rebar in the architectural condrete is placed.

2.  The pins are placed, form oiled and the Architectural concrete placed.  It is allowed to cure for 28 days, constantly being kept wet.

3.  The rigid insulation is placed and the reinforcing placed for the structural section of the panel.  The edge form is oiled.

4.  The concrete for the structural section is placed and allowed to cure for 28 days, constantly being kept wet.

5.  The panels are picked and either placed or stored flat until they can b e placed.

Mike McCann
MMC Engineering

RE: horizontal cracks in tilt up panels

calculor1-
In an earlier post, you stated the structural panel was cast first but your last post stated the arch panel was cast first.  

Can you clarify which sequence is correct?

RE: horizontal cracks in tilt up panels

Some points I see:

1.  The Structural panel is more rigid than the Architerctural section, being thicker and probably more heavily reinforced.

2.  The pins are embedded further in the structural panel than the Architectural panel, generating more moment in the structural panel.

3.  Any bending from lifting in the structural panel will be amplified to the Architectural panel through the pins.

4.  The cracks were discovered via sandblasting after the panels were erected.  

5.  Any shrinkage cracks in the Architectural panels could have been silted in during the pour of the structural panel to be washed out later by the sandblasting.

Thoughts?

Mike McCann
MMC Engineering

RE: horizontal cracks in tilt up panels

2
Calculor1 obviously mis-spoke when he said the structural face was cast first. The outer, architectural wythe is always cast first, normally reinforced with WWF. The insulation is applied over the fresh concrete while still plastic, and connectors are inserted in pre-drilled holes in the insulation. The connectors are, I believe, normally installed at 8" on center in each direction. They are stronger than heck, and support the full weight of the architectual wythe when the panel is lifted. The two wythes are not connected together in any way, under normal circumstances (there are axceptions), because it is important that the wythes move independently for thermal reasons, and so as to not create any thermal breaks in the wall.

The reinforcement, lift & brace hardware, embeds, etc. for the inner structural wythe can be placed, and concrete poured, immediately... supposedly. That is generally not the case. Normally the outer wythe is given at least 2 or 3 days to cure before proceeding with the Structural wythe.

If anybody would like more info, see thermomass.com.

It is my opinion that the cracks were caused by restraint between the architectural wythe and the casting bed, normally the slab-on-grade. Depending on when the slab was poured... differential shrinkage between the two... inadequately applied bondbreaker, etc. It gets worse the bigger the panel is. Also, tilt-up cast on a slab-on-grade may cross a control joint in the slab. The joints are normally taped so they don't "read" into the face of the panel, but the movement at the joint can literally pull the yet-to-harden concrete apart. I've seen it, on more than a few occasions.
 

RE: horizontal cracks in tilt up panels

Thanks, spats.  I think your explanations are the best so far.

RE: horizontal cracks in tilt up panels

Calculor1,

Did anyone suggest fibermesh?  I don't care what you do, it's gonna crack (especially a thin slab).  At least the fibermesh will help eliminate large noticeable cracks.

Is the panel very tall compared to its width?  That generally will cause horizontal shrinkage cracks.

 

RE: horizontal cracks in tilt up panels

NGW,

Concrete doesn't crack because of shrinkage.  It cracks because of restraint when the shrinkage occurs.

RE: horizontal cracks in tilt up panels

Hokie,
A slab on grade or on another slab has restraint, right?

It can crack from lots of things:  restrained shrinkage, deflection, restrained thermal deflection (like a slab in the hot sun that arches then gets pushed down with a heavier slab).  The point is fibermesh allows more tensile strength.  Rebar or wire seems to allow less but larger cracks.  

RE: horizontal cracks in tilt up panels

NGW,

Yes, but you can do lots to reduce the restraint.  And sure, cracking due to external loading occurs.

I am not as persuaded by fibermesh marketers as you are.

RE: horizontal cracks in tilt up panels

All fibermesh does is help reduce plastic shrinkage cracking. It is not structural, and should never be substituted for real reinforcing.  

RE: horizontal cracks in tilt up panels

spats,

I agree. I'd still used the steel, but add fibermesh.  

I guess you'll never know if you don't try it.  If the 7" slab isn't cracked, does it make sense that the 3" slab is cracked due to lifting?  The stiffer panel will be under more stress, correct?

 

RE: horizontal cracks in tilt up panels

The way I look at it, the 3" outer wythe is just along for the ride. It is connected at 8" o.c. each way to the structural wythe. Normally panels are designed to lift uncracked, so I don't think lifting would be the issue. I believe the cause of the cracking to be as I previously stated.

RE: horizontal cracks in tilt up panels

Spats,

I reviewed your earlier post.  I think your probably right.

Are you saying fibermesh won't help mitigate the shrinkage cracking?  If so, why?   

RE: horizontal cracks in tilt up panels

NGW,

Only when the concrete is still in a plastic state. After the concrete hardens??? ... I guess you could say that it just doesn't have the guts.

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