Effect of shrinkage cracks on the intergrity of a suspended slab

[asixth]

Hi

I have been asked to review/investigate a reinforced concrete slab which displays sparse shrinkage cracks which are randomly patterned throughout the slab. Some of these cracks exceed 2mm opening (1/12") and are not visible from the soffit of the slab, which leads me to believe they are shrinkage cracks.

How do these cracks effect the strength of the slab. Some of the cracks are locate transverly across midspan where the slab surface is in compression (flexural) however the cracks appear to be open so I can't see how the slab can transmit the forces (unless a reduced depth is being utilised).

Will this slab need to be repair and if so, what procedures and products would be the best to use?

The cracks are not external nor located in an industrial/coastal environement however should corrosion of reinforcement be a concern?

All comments and recommendations will be appreciated.

 

[Ron]

That's a pretty large/wide crack. Is this a steel frame building? It looks like a crack that occurs in steel deck supported slabs. Is that the case?

What is the slab thickness? What is the rebar pattern? Where are columns in relation to the cracks? Have you done a crack map?

 

[apsix]

Is there top steel which could be taking the compression forces?

 

[JAE]

I can't see how the slab can transmit the forces (unless a reduced depth is being utilised).

I think you have the idea right there. The top surface apparently shrunk and cracked and now the "d" is shorter. I would inject the cracks with epoxy.

With a reduced d you have reduced flexural strength (you could perhaps check that but without knowing the real depth of the crack it would be hard to be exact.

The other concern is that with a reduced "d" you have reduced shear strength. Again - epoxy might be the answer - also be particular about WHICH cracks get injected as some cracks (parallel to the span) may not be detrimental to strength and thus won't need repair.

 

[ToadJones]

What is the structure?

 

[asixth]

I am still sourcing structural drawings (built 1983) and have sketched a crack map which I will post later today. It is not a steel deck building (rc flat slab, I will post a framing plan after work).

 

[rowingengineer]

I think JAE has the right idea, however i would use a cementitious filler, not epoxy.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[asixth]

Attached is a crack map I have sketched up (didn't want to issue drawings for privacy). The columns are spaced at approximately 8000 (27'). Carpet has been partially laid which has prevented me from scoping out the full extent of the cracking.

I don't know structural details because I am still trying to obtain structural drawings.

If the cracks are repaired with epoxy or a cementitious filler, is it adequate to assume that the structure has been repaired and that the full depth can be used for ultimate design.

 

[JAE]

asixth - might be nice if you re-post the sketch with the associated support beams so we can see how the slab functions in terms of bending and shear.

 

[SkiisAndBikes]

asixth,

In the past we have tended to inject structural cracks with epoxy. I am not as familiar with cementitious fillers. Depending on quantity of injection, epoxy can get expensive. Where time permits, we try to monitor the crack width over time to determine if the crack continues to open/close, the results of which can affect your product selection.

In past cases I have investigated, we have used a covermeter to determine rebar layout on top and bottom mats. This can be compared to the original structural drawings or provide you basic layout information where no drawings are available. It would only take a 1/2 day or so to map out one bay, top and bottom. In one parking structure we discovered the TUL and TLL of rebar had been switched/reversed as well as the BUL and BLL.

 

[hokie66]

JAE, he said it was a flat slab. But it would be informative to know the depth of slab, depth and size of drop panels, and spandrel beams.

asixth, some of the cracks appear to be in the vicinity of top bar terminations. To me, they suggest heavy loading in some panels, with light/no loading in other panels, resulting in flexural cracking where the top bars stopped.

 

[msquared48]

From looking at the crack pattern diagram, the cracks seem to mainly be in the positive moment regions of the slab.

If this is a PT slab, I am wondering if these regions received too much PS force in these + moment regions, opening up cracks on the top. If this is the case, relaxing certain tendon PS forces could close the cracks.

If it is just reinforced concrete construction, then I have to defer to JAE's comments.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[asixth]

Apologies for my delay in getting back to this post but I have been pre-occupied with other things. I have re-posted with a floor layout to show the extent of the cracks, the beams are 690x900 wide (28"x36") and the slab is not stressed. The cracks don't appear to be located in negative moment regions but more likely where the top steel is terminating, would this be critical and would it be specified to repair the cracks with a cementitious grout as suggested above.

The structure was built in 1984 and I spoke with the original engineer of the building late last week who was unaware the slab had displayed such an extent of cracking, however it was their opinion that the cracks where from drying shrinkage.

 

[asixth]

with attachment.

 

[BAretired]

In addition to the cracks shown, it would be helpful if you could show the carpeted area. I am guessing it is the large area near the bottom of the plan where there are no cracks shown.

If you are going to fix the floor, you will probably have to remove the carpet so that you can do a thorough inspection.

The cracks appear to be shrinkage cracks which likely occurred during the curing of the slab. Epoxy injection seems to be a sensible measure. I don't have any experience with cementitious fillers. Perhaps rowengineer could clarify his comment on why this would be preferable.

After epoxy injection, I would consider the slab fully repaired.

BA

 

[rowingengineer]

The reason to spec cementitious fillers is that they have compatible thermal expansion properties to that of concrete. This probably isn’t as important in your slab (but i wouldn't know), but if your slab was subject to thermal movement the epoxy would cause new cracks. The epoxy tends to be a rigid material compared to concrete and doesn’t combine to act as a homogeneous material with the concrete after repair.

However there is a time and a place for epoxy do get me wrong, I have used epoxy injection in the past, but only in times of high compression situations.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[hokie66]

I agree with the original engineer's opinion that the slabs are drying shrinkage cracks. Epoxy injection would be my choice of repair method.