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Slab supporting precast veneer panels has cracked.
2

Slab supporting precast veneer panels has cracked.

Slab supporting precast veneer panels has cracked.

(OP)
I have a situation where a slab supporting precast concrete panel veneer has cracked and I am having trouble determining the cause for the cracks.  The foundation system is grade beam on auger cast pile.  The structural system is steel framed with brick veneer.  However, in this particular side of the building, precast panels with thin brick were used due to limited space.  At this location, the slab is structural and spans from grade beam to grade beam, so the slab is sitting directly on the grade beam.  The precast panels are then supported on bearing pads sitting directly on the slab.  The cracks do not appear to be bearing related, but appear to be bending and shear related.  Finally, I have also noticed that bearing pad locations do not match those indicated on the shop drawings.  The vertical supports were supposed to be in the same location as the lateral supports, but they are not.   

RE: Slab supporting precast veneer panels has cracked.

can you do a wide shot so we can see the whole situation?

RE: Slab supporting precast veneer panels has cracked.

Can you provide a cross section?
Is there any relationship between the cracks and the vertical support points or between the cracks and the lateral support points?

RE: Slab supporting precast veneer panels has cracked.

Have you checked the grade beam design?  Does it have negative steel?  Have the cracks propagated into the grade beams or are they only in slab?  It looks like the cracks are at locations of negative moment and deflections in the grade beam may have caused the slab to crack.

RE: Slab supporting precast veneer panels has cracked.

(OP)
The cracks do not propagate into the grade beam and the grade beam does have negative moment steel.  Also note that the grade beam is cantilevered.  However, the calculated maximum deflection of the grade beam was only 1/16".  Attached is a section detail and I will also provide a provide a partial foundation plan in the next post.  

RE: Slab supporting precast veneer panels has cracked.

do you know how the grade beam rebar was detailed?  the detail is sort of generic. Were the top bars continuous over the the supports?  without direction I could see all the bars being cut to the same length with the top bars being stooped and started at the same place as the bottom.  Are the top bars on the cantilever grade beams hooked?

the column, pile, cantilever grade beam is curious to me as well.  The columns and not the precast could be the  source of the larger loads? Getting the beam in line with the column like C line makes sense, but the roundabout load path at B and D seems to be made a little more difficult than it needed to be.  

is there a reason why there are 4 piles at b and D and only 2 at C, the trib areas seem to be the same?


not sure i am helping any, but you have got me curious

RE: Slab supporting precast veneer panels has cracked.

Could these cracks somehow be related to the column blockouts?  Perhaps they were not formed and poured as cleanly as shown on plan, or the infill itself is cracking?

RE: Slab supporting precast veneer panels has cracked.

^^^ that is a good point, I like it,  can you see the extent of the cracks inside the building?

RE: Slab supporting precast veneer panels has cracked.

And I'm not sure if related to slab cracking, but your ties 16" on center do not appear to be spaced at least d/2?

RE: Slab supporting precast veneer panels has cracked.

My best guess is that the slab cracks are restraint shrinkage cracks.  The grade beam was cast first, thus restrained the lightly reinforced slab (no reinforcing at the edge), and the slab cracked in direct tension.  Do the cracks continue inside?

RE: Slab supporting precast veneer panels has cracked.

(OP)
The grade beams were detailed to be continuous.  I will pull the shop drawings tomorrow to verify that they are indeed detailed as continuous and that the bars are hooked.  The additional piles at lines B and D are due to heavier loads from the columns being interior as well as being part of the LFRS.  The cracks do not continue to the inside and do not appear to be linked to the column blockouts.  Previously, there was a crack noticed along the length of the exterior grade beam in the slab shortly after the precast was placed.  The location of this crack is approximately near the location of where the vertical hook bar comes out of the grade beam.  At the time it was judged that the exterior portion of the slab was still adequate as the bearing stresses were still within allowables.  

RE: Slab supporting precast veneer panels has cracked.

Nothing to do with moment or shear or how the bars are placed in the grade beam.  The restrained slab had to shrink and crack, and because there was no reinforcement close to the outer edge, the cracks were not controlled.

RE: Slab supporting precast veneer panels has cracked.

Did anyone check the elevations of the various bearing points?

Dik

RE: Slab supporting precast veneer panels has cracked.

(OP)
I did verify that the grade beams were detailed as continuous.  I do not think the actual elevations of the bearing pads were checked.  However, the heights of the shims/bearing pads to vary over the length of the slab as there was some variation in the levelness of the slab.  This is mainly due to the tolerance in the cast in place concrete.  The precast panels were checked and they are plumb.  Hokie, do you think the shrinkage could also explain a slight gap between panel bearing points between the slab and grade beam?  In moving forward, I plan to recommend that the cracks, panel elevation, and possible panel rotation at the base be monitored.  And for now, sealing all cracks to prevent water infiltration.  Does anyone have further suggestions?

RE: Slab supporting precast veneer panels has cracked.

I think hokie66 is correct that the cracks don't have anything to do with the grade beam reinforcement. Normally the slab shrinkage cracks wouldn't be visible because the slab couldn't move far enough to show a crack of the size in the photos. In some of the photos it looks like there is an open joint between the beam and slab. Shrinkage across these open areas is likely causing the problem.

RE: Slab supporting precast veneer panels has cracked.

Why didnt you have them grout under the precast panels?

RE: Slab supporting precast veneer panels has cracked.

I agree, the slab cracks are probably shrinkage related.

I'm still curious though, how does a beam with 28 #8 not need ties spaced at least d/2?  Or are there other details that account for tighter spacing?

RE: Slab supporting precast veneer panels has cracked.

(OP)
Grout was not placed under the precast as it was not necessary to meet bearing capacity requirements and to allow space for flashing and drainage under the wall.  PMR06, you are correct, the tie spacing shown in this detail is not correct.  The correct spacing of 6" was shown on the foundation schedule and the grade beams were installed properly.   

RE: Slab supporting precast veneer panels has cracked.

I can't shed any light as to why there is an apparent gap between the slab and beam.  A gap or separation is not shown on the section.  Maybe the builder decided there needed to be a control joint of sorts, or else there was some debris on the beam which is now washed out.  But I disagree that the cracks would not have appeared without the gap...the cracks are due to tension in the slab, and lack of reinforcement near the edge means the cracks are uncontrolled, therefore wide.

RE: Slab supporting precast veneer panels has cracked.

Not so sure hokie. The cracks are too wide for that and they aren't wider at the top that bottom. The bond of the slab to the beam (if it is bonded) wouldn't allow that large a crack without a similar crack in the beam.

That is why it looks to me like the slab may not be bonded to the beam in some locations so there can be wide gaps.

RE: Slab supporting precast veneer panels has cracked.

If it were not bonded, it wouldn't crack, it would just shorten.  Maybe the bond is not great, but the reinforcement across the joint restrains the slab.  The slab is in direct tension, so the cracks are expected to be the same width bottom and top.  The beam is heavily reinforced, so cracks in the beam, if any, will be miniscule.

RE: Slab supporting precast veneer panels has cracked.

Isn't it likely that the slab and the beam are bonded, but not uniformly bonded, due to the gaps, etc.  And, the slab edge might have cured faster than the interior of the slab, thus starting the cracks.  At the gaps or low bond areas the tension and cracking will accumulate, while at well bonded areas the beam will tend to constrain slab movement and distribute any shrinkage cracking more uniformly.  The crack at the bottom of the slab will be larger where to beam offers areas of concentrated  constraint, while the top of the slab was not constrained initially, and not much more constraint is offered by the bearing pads under the precast panels, and at a later time also.  I do grant that the beam constrains the slab shrinkage, but changes in the bond btwn. the two can concentrate the movement and cracking, and finally that the missing slab edge reinforcing was certainly a design and detailing error.

RE: Slab supporting precast veneer panels has cracked.

No argument from me there, dhengr.  You are just thinking like the slab, which is what we all should do.

RE: Slab supporting precast veneer panels has cracked.

dhenger that is what I meant. You can't have a significant crack in a slab where it meets a beam without a corresponding crack in the beam unless they aren't bonded. I agree that the slab reinforcement will try at act as top steel for the beam in negative moment regions but the reinforcement in the beam will control the width of the cracks.

RE: Slab supporting precast veneer panels has cracked.

Bond will not overcome the direct shrinkage tension force.  When you cast a wall on a footing, shrinkage cracks will always occur at 3 to 4 metre centres.  The width of these cracks will depend on the amount of reinforcement in the bottom of the wall, and without the wall reinforcement, the cracks will be wide.  This is similar to what has occurred here.

RE: Slab supporting precast veneer panels has cracked.

You may be right hokie but I haven't ever seen anything like that. I know that shrinkage is such a large force that it is very difficult to resist. In a condition like this I think that there are multiple micro cracks along the length of the beam/slab interface that relieve the stress. To get what you are describing there has to be movement/debonding along a length of a slab and I don't see how that could happen if the slab is bonded to the beam. Especially with reinforcement between beam and slab.

 

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