huangyhg

cracks in flat plate slab
two-way (flat plate) slab poured back in middle of december is just now showing cracks in last 2-3 weeks (according to gc).
- 10" thick flat plate roof slab
- column bays are 24' x 26' (c-c)
- cracks have occured on top surface only at every concrete column location. top cracks follow outline of concrete column below, then radiate from corners, 45 degrees out. will try to upload photo.
- cracks are very small, appear to be shrinkage-cracks. basically no cracks on bottom surface, and no visible deflection/sagging
- we went back and checked our design and did not find any issues.
- cracking has occurred at top surface of roof slab (exposed parking) - we have the same slab design (thickness, span, reinforcing) at the fourth floor parking below with no cracking/issues.
- concrete contractor says they left forms on for 4-7 days, and they placed reinforcing and slab thickness correct
- design used 5000 psi concrete, 28 day breaks came in at 9000-10,000 psi, 7 day at 5000+ psi.
anyone have any ideas or seen something similar? thanks.
photo
another photo
i may be wrong but it looks like punching shear cracking. as to why this happens on the roof only (and not the other floor), maybe the exposed slab has suffered significant temperature shrinkage..?
i would gladly provide more information but haven't seen this a lot.
since the column stops at the roof, the slab moments are higher and this usually increases the unbalanced shear.
the steel may be low which would lead to both flexural problems and punching shear problems. try to check the depth and quantity of reinforcement. maybe pulled the shores too soon.
could be curing conditions were not ideal or the top mats were placed too low. hard to say. are the slab deflections larger than those of the floors below?
ba
i agree with qcjr. i have only seen this one other time, 37 years ago, and i think that was a punching shear problem.
the cracks do not suggest punching shear. they suggest tensile stresses in both directions causing radial and circumferential strain in the top fibers of the slab.
punching shear failures are rapid and do not show telltale cracking around the column before failure takes place. the column reaction is only 24*26*125 = 78,000#. what is the size of the column? punching shear stress must be pretty low.
ba
ba:
your reasoning seems sound....
load sounds low enough.
cracking indicates tensile stresses at right angles
punching failure is sudden with no warning
but the crack pattern sure looks to me like something that i would expect if the load (column) was trying to punch thru. could this occur, if the top layer of bars were set too low?
if it was flexural cracking only, i would expect somewhat concentric circles.

i agree with baretired. the cracks are negative radial lines created by tension in top of the slab. you have to check the area of the reinforcment in top of the plate.(columns)
do it by the yieldline-theory and check the tension in the reinforcement. if the stress is greater than the yieldstress for the reinforcement you have a bearing problem. if the stress is greater than 250 mpa you will have a crack-problem.
chrede

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