Hollowcore Plank splitting longitudally.

[bcpotter]

We're having issues with some hollowcore planks splitting longitudally between strand. Our hollowcore is 4'-0" wide, with (5) 1/2" strand placed in the bottom. These particular planks are ripped to be 3'-4" wide with (4) strand in them. The pieces are stripped with no problems, but when they get shipped to the jobsite there is significant splitting between the outermost strand (on the rip side) and the middle ones. It is as if the outermost strand is trying to peel away from the piece. Is anyone familiar with what might cause this?

Thanks
bcpotter

 

[JAE]

Shipping stress induced cracks by incorrectly supporting the HC on the truck?

The only other thing would be larger differentials in the tendon stresses side by side.

 

[hokie66]

Very perplexing. So when you say the planks are ripped, I take it that they are sawcut lengthwise. When is this done? Where does the cut occur relative to the voids and removed strand? I assume there are 4 voids. How thick are the planks? Just trying to get a better handle on the geometry.

Another thing, I assumed the planks were stressed with 5 strands in place, then one removed. If you left that one out and only pulled 4 strands, the lateral stress differential would be suspect.

 

[JAE]

hokie66 just got me thinking - his statement: "where does the cut occur relative to the voids and removed strand?".

Is the crack occurring on the cut side of the plank or on the opposite side to the cut?

And as you are cutting it, are there any induced stresses on the plank as the saw proceeds along the piece?

 

[bcpotter]

Some more information on these cracks:

the pieces are being sawcut lengthwise. the cracking is only occuring on the side that is cut.

Our planks are 4'-0" wide, 8" thick, with 6 voids at full width. these particular pieces are actually 3'-0" wide, so have 4 and a half voids (the cut is going through one of the voids).

Furthermore, some pieces also have notches cut out on either end of the piece. they are about 6" wide by 4'-0" long, but they vary in size. The cracking is primarily occuring in pieces with these notches.

As far as I know, there are no induced stresses by the saw. All pieces are saw cut while they are still on the form.

 

[hokie66]

Sounds like the notches could be acting as stress risers. Did you pull all 5 strands, including the one which was cut off, or just 4? Is the cracking through the first complete void in from the cut edge? Is it in both top and bottom?

 

[bcpotter]

All 5 strands were pulled. The cracking is occuring in the first complete void from the cut side, on both top and bottom.

 

[hokie66]

bcpotter,

Thanks for the info. I am afraid I don't know what is happening, but will continue to think about it.

 

[bcpotter]

Update on these slabs:

I was first able to see them yesterday (they had to come back from the jobsite) The cracking is primarily through the web of the first strand on the cut side. The culprit seems to be a combination of low voids (causing less concrete around the strand) and strand off-position slightly (about 1/8"-1/4"). Though they are within our tolerances, its my guess that the two in conjuction with the sawcutting is causing this cracking.

 

[TomasAron]

It´s a different code but EC2, or rather EN 1168:2005 recommends that

splitting stress = (P/(b_w*e))*(15*alpha^(2/3)+0,07)/(1+(l_pt1/e)^1,5*(1,3*alpha+0,1))

where alpha = (e-k)/h

P = initial prestress force
b_w = thickness of web
e= eccentricty
l_pt1=lower design value of transmission length
k = core radius = (section modulus of bottom fiber)/A_c

and the splitting stress should be less than the tensile strength of concrete at the time of release.

Tómas Aron