Best way to examine cracks in soffit

[oneintheeye]

I have a suspended slab with cracks running through the slab in one direction. (two way slab). How is the best way to ensure cracks are not structurally significant. We will be strengthening the structure as the load is to increase. However what concerns me is the effect of the cracks especially determination if the cracks run deep enough to pass the N/A and effect the bending capacity of the slab.

 

[cntw1953]

Top or bottom slab? Location (in relation to stress zones)?
Width?

 

[oneintheeye]

in bottom, running one way through centre, continous through beams (which sit on columns) and into next span. Not sure on width yet may be <0.3 may be bigger.

 

[cntw1953]

This sounds like tension crack. Suggest to perform a level check to determine deflection, and carefully exam/evaluate previous design and loading history to find the culprit for such crack. If there is excessive deflection, you may need to shore it back to proper position, then perform strengthening.

 

[oneintheeye]

sorry this I already know. The new load is going to be over the design load. I am pretty sure they have overloaded in the past. I have done a analysis and recomended a load reduction. They will now go to strengthening. I am pretty sure the crack is as you suggest a tension crack from overload. The significance of this crack in relation to the strengthening work is what I am trying to find the best way of determining. There is no noticable deflection.
I believe that if the crack is shallow i.e. only in tension zone then this is ok, but how do I know the crack depth.

 

[cntw1953]

Get rebar detector, and take a few core samples at strategic locations.

 

[oneintheeye]

how will a rebar detector prove the crack depth? Cores have been done. Concrete and steel is being tested.

 

[cntw1953]

It was intended to place the core squarely in between rebars. If the core has been done, what depth is the crack?

 

[BAretired]

How do you intend to strengthen the slab?

BA

 

[oneintheeye]

it is not decided yet. I am awaiting results on concrete and rebar tests. It is an old structure, so the material strengths are less than current. It was designed to permissible stress codes to give you an idea. Using the minimal permissible stresses at the time the slabs, beams , columns and foundations are overstressed. I have thought of two options for the slab. FRP or cast a slab over top, dowelled in U bars to tie together the slabs and thereby increase the effective depth. Columns frp or increase size. Foundations may be trickier. May even need to remove areas of basement slab and add more piles and stich in. Any ideas welcome!

 

[cntw1953]

Talk BA out of retirement

 

[BAretired]

FRP is probably a good method. I have no experience with it, so I can't offer any help there.

Adding a slab above is problematic. You are adding the dead load of the new slab and additional live load to raise the floor capacity. If you tie the slabs together thoroughly, you benefit from an increased effective depth but you are unable to increase the cross sectional area of the bottom reinforcement (unless you add FRP).

A system of steel beams under the existing slab is another possibility, but it would not be fireproof, may encroach on headroom and may run into existing obstructions below the slab. I don't know if it is a practical suggestion, but you could place one beam at or near the crack location. This may alleviate some of your concern about depth of flexural cracks.

BA

 

[JAE]

Where is the crack located relative to the span of the slab? Midspan? If so, then this type of flexural crack isn't a concern in terms of loss of capacity as the concrete is assumed to crack in flexure anyway and the crack will remain closed, or non-existent, in the top portions where there is compression.

The concern would be if the crack is located closer to supports, in areas of higher shear. The loss of Vc capacity on the slab would be a significant concern and FRP on the bottom wouldn't really help then.

 

[oneintheeye]

to answer questions. crack is centre span. Headroom underneath is not a concern. (within reason) so steel beams may be idea. One problem then is the beams which support the slab. These are overstressed as well.