Precast Concrete Sandwich Panel Cracking

[MindofBarca]

Hello All,

A precast manufacturer is seeing some longitudinal hairline cracks coming from the head (top) of the panel extending down about 12"-24". These hairline cracks appear on most of their panels for this specific project (maybe 60-75% of panels).

The panel composition is 3" wythe of concrete (BIF/exposed face) - 4" insulation - 5" structural wythe of concrete (TIF/not exposed face).
The 3" exposed face is the face with the hairline cracks.

f'c=6,000 psi
f'ci=3,500 psi
Prestressed with 3/8" to 70% in both wythes - centered in concrete wythe.
Both wythes have mesh 6x6 in both wythes.
Both wythes have transverse #3 bars at top and bottom.

Photo attached.

Any advice on cause of crack? Are these cracks related to the strand? Bad mix designs? Lower concrete release strength? Improper lifting/stripping from form? Improper cutting of strand sequence?

Is there any guide/paper/literature on cracking of these sandwich wall panels?

Thanks in advance!

 

[oldestguy]

Were these panels horizontal when cast? Could one edge be more fluid then (higher water content)?

 

[hokie66]

Those aren't what I call hairline cracks. They seem to be evenly spaced, and go right to the edge. Anchorage of the strands causing the splitting? Hard to tell from that picture, but perhaps the strands aren't actually centered in the wythe.

 

[Prestressed Guy]

I have a project that is getting similar splitting except that my panels are 3"+3"+8"x 12' wide panels with (10) 1/2"ø and (6)3/8"ø @ 0.75%. The cracks are only on the strand closest to the panel centerline. for facing transverse reinforcement, I have #3 @ 12".
For the pour this week, I have added a #3 (each side of strand) to T&B of panel. I also changed the destressing sequence so that they cut the outer strand, then the center strand then work from the outside inward.
I will let you know next week if it did any good.

 

[Tomfh]

What are the prestress details? What sort of bars? Do the bars align with the cracks?

 

[MindofBarca]

Thanks for all the responses so far.

@oldestguy: these panels are horizontal cast (on face), 3" wythe being BIF finished face.

@hokie66: these cracks are measuring at 0.25 gauge or 0.01 inch which is classified as "fine/hairline". They wet the panel down to extenuate the crack locations. QC went out and verified all strands are centered in wythe (within tolerance of course).

@Haydenwse: I have a similar situation and I proposed a similar solution. I have (10)3/8" cable each wythe, pulled to 70%. I had the precaster switch from 6x6 mesh to 4x4 mesh AND add #3 transverse bars @3" O.C. for first 12" top and bottom. I have also inquired the precaster about detensioning sequence, I will have to keep this thread informed of their response to this.

@Tomfh: It is very odd, some panels are cracked on just 1 end. Some panels are cracked on both ends. Some panels are have 2 cracks on one end. Some have upwards of 6 cracks. Some have 4 or 5 on one end and only 1 or 2 on other end. I was really hoping to find some kind of pattern so I can nail down the issue, but no such luck. As far as the prestressing details, I have (10) 3/8" cables pulled to 70% centered in both concrete wythes. Panels are roughly 12' wide with standard 6x6 mesh in top and bottom wythes. Now the tickets I am adding #3 transverse bars to the first 12" of the panel (on both ends) in hopes this helps.

 

[Tomfh]

Do you prestress in both directions?

Sorry for these questions. I’ve only done post tensioning of Slabs. Not pretensioned panels

 

[MindofBarca]

@Tomfh - no sir. Just longitudinal with the length/height of the panel. In my photo above the strands are running left/right (same direction as the cracks) as the panel is being lifted by the jambs/sides.

 

[Tomfh]

My guess is transverse splitting/bursting forces due to cable loads fanning out into concrete. What concrete strength do they cut the cables?

 

[MindofBarca]

@Tomfh - on the production tickets we are specifying 3,500psi for f'ci (with f'c=6,000psi). But the precaster informed me this morning their concrete breaks were showing 4,250 psi+.

I'm leaning toward an issue with the strands as well. Just not understanding why some panels are fine, while others are not. And some panels have issues on just top but not bottom (and visa versa).

 

[Tomfh]

Do they give you strand extension/stress values? What sort of device do they use to stress? How do they release the stress?

Note that the tensile force to form a 12" x 3" crack is roughly the same as the tensile force in a strand at 70%. Maybe it's border line and some fall this way and some fall that way.