Suspended Slab Cracking

[ataman]

Hi

I have a situation where a recently poured suspended slab has some severe cracking in my opinion. The cracks are fairly long and straigt and generally intersect perpendicular to each other. There are however some cracks which are random is length and direction. Some of the larger cracks I can push a quater halfway down the crack and a bit of tying wire 2" into the 6" slab.

The conctractor has taken some cores for testing but believes that the cracks are plastic shrinkage cracks. I agreed the they could be since the concrete was poured in hot weather. I was thinking that there must have been too much water added to the concrete which caused the significant drying shrinkage cracks but the contractor insists that it isn't. The contractor is also a civil engineer.

My thoughts to the client is that the slab should be removed and redone especially since it is a suspended slab. The conctractor wants to wait for the test results and if the strength is ok suggests that we keep the slab. He is also willing to do a load test.

The slab is an indoor slab so it will not be exposed to the elements.

Any thoughts on this?

Thanks

 

[JAE]

Where are the cracks relative to the spans? Many times cracks in suspended slabs are not that big of a deal as the slabs are usually reinforced to such a degree that the cracking doesn't adversely affect the strength.

However, some cracks, if located in high slab shear areas can be an issue. Injecting with epoxy or other adhesive resin is one solution.

But I would carefully analyze each crack's position relative to shear.

 

[hokie66]

Do the cracks go completely through the slab? If so, they are slmost definitely plastic shrinkage cracks, as opposed to drying shrinkage cracks. As JAE says, you would need to assess each crack as to how it affects the strength of the slab.

When were the cracks first noticed? Plastic shrinkage cracks should have been seen and attended to by the finishers.

Can you give a description of the structure? Flat slab, flate plate, band beam, or whatever. And dimensions. And what supports the slab? Another reason for wide cracks can be restrain by walls or beams.

 

[ataman]

Hi

The cracks are everywhere! I'm sure they go right through the slab its just that they are bigger at the surface.

The slab is supported by r/c walls and beams and the biggest bay is about 30'x18'

Thanks

 

[hokie66]

Other than hot, what was the weather like the day the slab was placed? Wind is worst factor in causing plastic shrinkage cracks. Low humidity doesn't help. Was the temperature of the concrete measured as delivered to site?

If the larger cracks are perpendicular to walls and beams, that could indicate restraint cracking. Do the cracks go across the beams?

You say most of the cracks are "long and straight" and "perpencicular to each other". How thick is this slab? Not settlement cracking over the bars due to insufficient compaction?

 

[271828]

The type of slab matters greatly: if it's a composite slab or if it's a PT flat plate for example.

What type of slab system? I looked twice and apologize in advance if I missed it.

If it's a screeded level composite slab, cracks over beams and girders along column lines should not be much of a surprise. These are a little on the big side, though. If it's a cip concrete slab, then they sound much worse.

 

[ataman]

Hi,

It could get windy on the site as well. The temp of concrete was not measured.

The slab is 6" thick and is a structural cast in place slab

The reason I thought it had something to do with water being added maybe by the truck drivers, is that the contractor has poured several other slabs under similar conditions which have not cracked this much.

Thanks

 

[jike]

This might be a stupid question but is there two layers of rebar?

 

[hokie66]

OK, it sounds like it is a 6" conventially reinforced slab spanning one way and supported on beams and walls. I take it the slab span is 18' max. and the beam span is 30' max. If you can more completely describe the system, we may be able to help. Not trying to check your design, but cracks can mean vastly different things in different situations.

What is overall length and width of floor? What are the beam sizes? And again, do the cracks go across the beams?

 

[ataman]

Hi,

There is negative reinforcement over the walls and beams. The slab is predominantly supported on r/c walls so it is a slab on stiff supports. The beam spans are not large, probably 10 feet max. The cracks do go over the beams, walls etc.

I am not the designer but I haven't got that far since the formwork is still in place. Is there a reason why you need this info?

The slab is 9 days old. I was called in to see it after the slab was 3 days old. I don't know if the cracks formed while the concrete was still plastic. I'll have to do some more research.

Thanks

 

[hokie66]

Cracks often form transverse to stiff supports because the slab shrinks quicker than the beam, or when the support is a wall, the wall has already done most of its shrinking. That is the reason for some of my questions. But an adequate quantity of shrinkage reinforcement should control the width of this type of crack.

Your cracks, being the width of a quarter, have not been controlled and thus I think that the cracks must have formed in the plastic state.

Cracks which are parallel to the span should have no great effect on the capacity of the floor. Cracks that are across the spanning direction are another matter, and can severely affect the stiffness unless filled. Epoxy injection is the remedy you should consider in those cases. Talk to the people who do injecting in your area, but I would think it would be far easier to do the injecting before the forms are removed, as the forms will serve as your barrier on that side. After stripping, you can still inject from below in the case of incomplete filling.

 

[ataman]

Thanks Hokie66,

The building is a multi-storey building and the contractor has proceeded to do his formwork for the walls over the slab for the next floor. This has resulted in the cracks being filled up with dust etc. We have indicated that he is doing this additional work at his own risk which he has no problem with.

I doubt an air compressor will be able to get all the dust out. That can't be good for epoxy grouting the cracks in the slab.

Do you or anyone know more about this process or can direct me to an appropriate place to get info?

Thanks

 

[hokie66]

Ataman,

I am at a disadvantage because I am in Australia. Don't know where you are, but probably not here. Perhaps others on this site can help, or you could contact Sika or Fosroc, etc. for advice and/or referral to companies in your area who do injection work.

Sounds to me like you need to slow this builder down, make it his problem rather than yours. Reject the slab unless rectified, then he can propose a solution. And the cracks which affect the slab stiffness should be attended to while the slab is still propped, thus avoiding deflection caused by closing of the cracks.

 

[haynewp]

Are you able to post any pictures of the cracks?

 

[civilperson]

Cracks do not make the concrete product unusable for the designed use if the average strengths required are achieved. Cosmetic solutions may be what are required to sell the slab. You must find a specification paragraph that can be applied to this instance before rejecting the floor. Nebulous phrases like "workmanship" or "journeyman quality" will embroil you in a lawsuit with small chance of winning.