Slab supporting precast veneer panels has cracked. 2

[chrisrosebud2001]

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.

 

[ztengguy]

Why didnt you have them grout under the precast panels?

 

[PMR06]

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?

 

[chrisrosebud2001]

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.

 

[hokie66]

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.

 

[ron9876]

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.

 

[hokie66]

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.

 

[dhengr]

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.

 

[hokie66]

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

 

[ron9876]

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.

 

[hokie66]

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.

 

[ron9876]

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.