Reinforced Concrete Slab Repair

[GalileoG]

Hello all,

I am working on the rehabilitation of a flat slab concrete floor that is cracked to hell with deflections significantly exceeding that of code, my experience in design has been only with structural steel, so I really have no idea what possible repair measures should be on the table for consideration. Also, we have taken cores from the slab to test for the compressive strength and we discovered that it is 10 MPa less than the compressive strength specified in the original design! Also, quite a bit of the cracks are pretty wide (1 mm plus)

I am thinking perhaps bi-directional FRP sheets would be one solution. Though I am not sure I want to proceed with a non-conventional repair method. Any other ideas?

Would appreciate guidance, thanks!

 

[csd72]

As far as I am aware, FRP only helps with the flexural strength, with concrete strength that low you may have shear issues.

The combination of effects that you describe suggests that you may have deeper more complex issues than lack of strength. Is there any indication that there may be corrosion in the reinforcement?

I would suggest you talk to a concrete repair grout supplier and see if you can get some free advice from them.

You may want to do further tests to check if there is sulphide or carbonation penetration.

 

[GalileoG]

csd, thanks for the reply.

There are no visible indications of steel corrosion (orange stains.)

Sulphide and carbonation penetration? What is the effect of such penetration and if there is such penetration, what can be done?

 

[cvg]

slab design is highly dependant on the foundation soil conditions. You should verify those conditions as well as the existing condition of the concrete and steel and determine the failure mode - before you start proposing a rehab method.

 

[GalileoG]

You mean differential settlement could be the culprit? I forgot to mention that the occupancy load on the floor has changed (became higher) and that is why I am confident that it is not a differential settlement issue.

 

[hokie66]

Clansman,

Have you determined that the flat slab is understrength for the new loading? Or is the rehabilitation to be done because of deflection issues?

What was the design loading condition and what is the new/proposed loading?

What are the spans and slab thickness? Drop panels? Column size? Specified concrete strength?

Is the cracking flexural or direct tension?

Sorry for all the questions, but knowing more about the slab may help in giving you advice.

 

[ThePenguin]

I would recomend checking "Structural Renovations of Buildings" by Newman for some ideas. I believe he may have presented a solution for a similar slab issue. I will check my copy when I get to work tomorrow.

 

[asixth]

I have excessive deflections in my building (1960's), I sometimes trip over because the curvature is so great near the columns. I can't see whether there is excessive cracking because there is a carpet lining.

Change of building use is always a problem, particulary when the designer uses live load reduction to obtain an efficient design.

I remember talking to an old school engineer once who was actually involved in a court case defending the design of a building because the owner had limits to who he could sell the building to. Defiantly something we should consider today, a building with a design life of 25 to 50 years may have two or three occupants over that time frame.

 

[kslee1000]

Observations from your posts:

1. The floor slab has cracked with significant deflection.
2. No obvious foundation settlement has observed.
3. The strength of the floor concrete is lower than the specified (rather than increase with age as concrete normally does).
4. The occupancy has changed, resulting in higher floor loading.
5. Unknown (actual) reinforcing configurations/condations.

Before planning any fix, a thorough investigation and evaluation should be in the order, the slab and its supports may need to be reconfigurred. At the maintime, if there is no obvious defects due to corrosion, you may want to talk to a concrete grout contractor to inject some grout, and see how long it will hold. Please note you might have to shore/jack-up the floor prior to grout injection.

 

[cvg]

I don't believe the posts indicate that foundation settlement or weak subgrade has been ruled out. the poster seems confident that the deflection is a result of change in occupancy load, without apparently checking the subgrade. Agree that load may have increased, but this should still be checked.

 

[csd72]

Suphide penetration causes corrosion of the reinforcement so by the sounds of things this may not be an issue, carbonation reduces the strength of the concrete so this could be a possibility.

It has been a while since I looked into this type of thing so I recommend you do some reading on the chemical processes of deterioration so that you can request the most appropriate tests.

There was an article on this in structure magazine I believe.

 

[LittleWheels]

csd72, are you sure that carbonation decreases concrete strength? I read a while ago that carbonated concrete usually has a higher compressive strength but I haven't tested cores of fully carbonated concrete myself.

 

[csd72]

Dave,

You could be right. As I said it has been a while since I was really involved in this sort of thing. I dont do much concrete at the moment.

 

[apsix]

Sulphate attack weakens the concrete and the sulphate usually comes from the use of inappropriate aggregate or from the soil/groundwater.
Carbonation destroys the protective alkaline property of the concrete leaving the rebar vulnerable to corrosion.
Neither will be a problem for an internal dry slab.

Has the slab been checked for strength with the reduced concrete strength? If the problem is only deflection there's probably not much that will help, apart from limiting loading. Grout injection will improve appearance.

Maybe external post-tensioning to the underside could be done.

 

[csd72]

Thanks apsix for the vital injection of informed comment.

 

[Teguci]

This might be a candidate for exterior cable tensioning, with the option of a thin grout overlay.

I am interested in hearing Clansman's response to the questions asked.