Cracking of concrete suspended slab

[tclat]

Hi,

I had a quick look at a two way suspended roof slab supported on r/c beams. There are cracks on the roof surface which run parallel to the beams and about 18" away on each side of the centerline of the beams. The cracks pretty much run the full length of the beams. The cracks do not go through the entire depth of the slab. There are significant deflections in the slab.

My initial thoughts are that there is insufficient or no negative moment steel which has resulted in the crack as well as stiffness resulting in the excess deflection.

Could this type of cracking be as a result of plastic shrinking as well?

Any other thoughts?

Thanks

 

[frv]

I would have assumed the same thing you did.

I'm fairly sure shrinkage cracks would have been much more random.

Do you have access to the original drawings? Does the L/d ratio look excessive? Is there sufficient positive moment reinforcement in the slab? If so, is there adequate transfer to the girders?

This may be one of those situations where the slab is shot from a serviceability standpoint, but it may be perfectly safe from a capacity standpoint.

 

[JAE]

Keep in mind slabs don't crack under shrinkage unless there is significant restraint against that shrinkage. For many suspended slabs, the columns that support them usually don't have enough of that lateral restraint to really engage a lot of cracking.

Having cracks that seem to always parallel the beams suggests negative moment cracking and possibly a lack of enough rebar (negative top steel perpendicular to the cracks) to keep the cracks closed.

You could chip away at some of the slab along the crack to reveal what bars there are, their size, spacing and depth...and then evaluate the design. Original design drawings (if available) might help.

 

[rscassar]

How wide are the beams is they are 18" from the centre line. I think the negative moment steel running perpendicular to the beams is possibly curtailed too soon (or not even provided). What judgment are you using to say that the deflections are excessive? Is it visibly sagging when viewed from the soffit? Does excessive water pond on the surface? Has a survey been completed?

My opinion, the best way to find what reinforcement is provided is to use GPR (ground penetrating radar). This should be able to tell you what reinforcement is in the slab, at what depth and at what centres.

Good luck in making the decision on whether the structure is safe. I am sure you will make an ethical decision.

 

[hokie66]

I agree that this is flexural cracking. Evaluating whether it is serious requires a lot more investigation. Even if the strength is adequate and deflection serviceability can be tolerated, since this is a roof slab, a membrane is probably required to prevent water infiltration.

 

[rowingengineer]

It is no fun if we all agree, so I disagree! I don’t think the cracking is cause by flexure.
I think it is too early to diagnose, I want some more information.
What is the beam size? What is the slab thickness? What are the spans both beams and slabs? Are these cracks on the edge beams as well? What is the weather cycle (all year) like in the place of this building? When was the slab poured? How wide are the cracks? Do you have a photo? What is the temp of the inside v’s outside, is it heated?

My diagnosis is without further information is:
1. Settlement cracking; following a large bar placed outside the beam approximately .5m from the edge due to congestion in the beam. This bar being so large reduced the cover over this bar until it cracked all the way along this line of bar.

2. Thermal cracks; the beams being huge compared to the thin slab have a very small change in temperature v’s time, however the slab being a thin slab changes it temp relatively quickly causing a crack to occur at the point of change in temp.

3. Flexural cracking; just because it could be.


An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[IDS]

How wide is the crack?

Do the deflections exceed the code allowance?

Are they more than predicted by the designer?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[csd72]

Have you considered bonding Grp/FRP to the top of the slab?

Doesnt help shear though.

 

[tclat]

Hi,

Thanks for the comments.

The bay size is 20'x23' (center of supports). The slab thickness is 6" and the beams are 12x18.

Without doing any calcs, for this bay size I would have likely used larger structural sizes particularly since it is an exposed concrete roof slab.

I can see the signs of ponding but at the time was not able to measure the deflection accurately. I would guess between 1 and 2inches.

I can't tell the size of the cracks because they have been sealed with what appears to be some sort of expoxy compound to help with the leakage.

The building is a warehouse and is not air conditioned. Outside temps do not vary significantly. There are no drawings but the building is certainly over 5 years.

This is a single storey structure and the client would like to add an additional floor. While there are several things to check, I need to assess whether the slab in its current state is safe and will need to find out how much reinforcement is present.

I am planning on chipping the soffit and top surface of the slab somewhere near the supports to determine the amount and size of steel. Can one work backwards from a deflection calculation to get a rough idea on the reinforcing steel provided we determine what the concrete strengths are? I have never felt detailed deflection calcs occur in real life as calculated but I use them as a limit and worse case scenario.

Thanks

 

[VoyageofDiscovery]

Why don't you have stamped drawings for this age of structure?

The second you start tickering with this structure, are you willing to take the liability for it all?

Of course the Client wants to add another floor, but you need to know all the details. If you know everything else, rip out the floor and design it.

VoD

 

[csd72]

One option is a light weight seld spanning floor over the top. cost can be offset by the fact that they would not need to remove the waterproofing or rescreed it level.

 

[rscassar]

Drawings should be available for a structure 5 years old. I think you should use non-destructive means for investigating the reinforcement such as GPR. Externally bonded reinforcement can be used to strengthen the structure if you find that it is insufficiently reinforced. I would recommend using a Carbon FRP (instead of a Grp/FRP which csd72 suggested) because it is more durable.