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?
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horizontal cracks in tilt up panels 2
- Thread starter calculor1
- Start date
msquared48
Structural
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
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
msquared48
Structural
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
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
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2
- #25
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.
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.
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.
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.
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.
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.
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?
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?
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.
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