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Re: Precast Panel Cracking

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	Chad,
		You say, "there is no other mild steel in the panel."
		That could be part of the problem. It will not cause the
cracks, but the prestressing strand is not fully developed for about 7'
along its length. Therefore, restraint developed in a line 100' long will
likely create cracks at the end of the tail bars on the embeds, despite what
I said the other day about not needing expansion joints for about 300'. I
bet the tail bars on the embeds are 2' long or less. The end of those
tailbars is where the panel becomes not quite plain concrete as the strand
is partially developed, but perhaps underreinforced.
		I do not know what to make of the fact that all panels are
cracked there, my explanation only explains why one panel might be cracked
at one end. If one of the internal mesh trusses is at 2', then I know why
the face cracks there. That mesh is a discontinuity in the concrete. You
said the strand in the face was pulled taut, not pretensioned, and there is
no mesh in that wythe. A crack could start because the strand provided no
precompression and it would propagate as the concrete is not held tight by
mesh.
		I also do not know how the vertical trusses might affect
this. I think they would be much weaker about their minor axis than the
horizontal panels are axially, even if underreinforced.
		Panels that long showing 2" of differential bow are not
unusual, but there should have been a method provided for jacking and
holding them together. We usually provide clips to be used only for that,
and the connections do not need to be made otherwise (with a big note on the
detail to that effect for the inspector).
	Jim Getaz
	Precast Engineer
	V: 540-665-3204
	F: 540-665-3250


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