Strengthening two-way slab with topping? 4

[hx200]

I need to strengthening a existing 9" thick 2-way slab with new 5" topping slab on it to increase the slab capacity (to increase the "d" for bending and shear). We are going to short blast the existing slab and add a bonding agent and probably add some mechanical dowels. Is there any concerns we need to pay attention, for example the composite action between the new and old concrete, the shrinkage and cracking of the new topping slab, etc.The existing slab is 30 years old.

Thanks for any inputs.

 

[a2mfk]

Have you considered carbon fiber? Just a thought, I am not a mfr or supplier or anything...

 

[TXStructural]

You definitely need dowels across the interface to assure composite action. Use a heavier texture. You need to clean the the old aggregate sufficiently to get aggregate interlock with the new concrete. The dowels will hold the two slabs together while the aggregate provides the shear coupling between the slabs.

Consider that the current slab will be deflected, and before the new slab will take load, any shrinkage will have to be offset by added deflection. Shrinkage control or compensation should be handled carefully. Consider shoring the old slab if you think you will get deflection under the load of wet concrete. Depending upon your bonding agent, you might need to make sure the old slab is wet (SSD would be wanted without a bonding agent to prevent migration of moisture from the wet concrete to the old.)

You will be adding a great deal of sustained dead load to columns and foundation, and I assume you have evaluated that. Bars continuous across column strips/columns will now be working differently, so evaluate the need for new negative moment reinforcement. Also, in middle-middle areas, consider incorporating voids within the concrete mass to reduce dead load (if doing so doesn't compromise the function.)

5 inches of slab will require T&S reinforcement. Minimum reinforcement (0.0018) would be #3 @ 12" o.c., e.w., or an equivalent welded wire reinforcement. A closely-spaced mesh near the surface will tend to restrain cracks best where there is no reason to expect corrosion. Support the reinforcement properly on bar supports or using hooked ends of the dowels as support, and do not allow it to be "pulled up" during concrete placement. Supports must be spaced close enough to maintain reinforcement location under construction loads. The reinforcement can deflect, but must rebound to it's specified location once workers move off of it.

 

[Qshake]

Know that composite or not, the deadload will be carried by the existing slab so you need to make sure it can carry the additional topping.

Once the new topping has set and the two slabs act compositely this composite action is only for load beyond the original dead load.

In order to have the new composite section carry all the load you'll have jack the slab up to the original neutral position and hold it until the new topping sets adn the composite action in ensured. Then the shoring is released and all the load is taken by the rehabbed slab.

Good Luck.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[BAretired]

Is there any concerns we need to pay attention, for example the composite action between the new and old concrete, the shrinkage and cracking of the new topping slab, etc.The existing slab is 30 years old.

Yes, you should pay attention to the bonding of the old and the new concrete. It is fundamental to composite action...but then, I'm sure you know that. So far as shrinkage and cracking of the new slab is concerned, it is no more of a concern than any new slab.

BA

 

[hawkaz]

If there are any interior columns, add top steel in the new slab over the columns.

Also- Make sure to check punching shear due to the increased load

 

[delagina]

question: how will this new slab reinforcement be developed to existing beam? new slab is on top of the beam right?

 

[hx200]

The current 9" slab has a 5" non-structural (non-reinforced) topping slab on it. we will remove it and replace with structural topping. We will add heavy reinforcement on the top, maybe #5 @ 12" or 8" for negative moment increase. the positive moment is increased by the increased "d" for the existing bottom reinforcement. Existing column, beams and footings are capable of taking the additional load.

I am thinking of how to quantify the shear requirement for the composite action. should I design the dowels by ignoring the bonding between new and existing concrete. this will require a lot of dowels. If I count on the bonding, does ICRI (or ACI) has any guideline for shear capacity numbers for different surface preparation? If it's just couple of beams, I would add dowels and ignore the bonding. But for a larger slab area, more detailed design backed by numbers are justified by cost reduction.

 

[dcarr82775]

ACI-318 Chapter 17 specifies the steel required across the interface to achieve composite action.

 

[Brad805]

I am not convinced this is a cost effective solution. All those steps mentioned have significant costs associated with them. You can spend a lot on high tech solutions and still wind up saving the owner a significnat sum of money. Carbon fiber, external steel plates, or even external stressing methods would be options I would consider.

For this design, have you considered the shoring requirements?

Brad

 

[hx200]

Brad,

We need to increase the live load capacity of the slab by 40%. the 2 way slab will have both positive and negative moment increase demands in two directions. The FRP requires sand blasting on both top and bottom and we probably still need to take out the existing 5" nonstructural toppting because the composite action is not there between structural slab and topping. Also FRP does not deal with the punching shear we will need add some other strengthening to help. I couldn't think of any easier way to strengthen 2 way slab with external steel plates or external stressing.

As for the shoring requirement, our typical bay is 30 ft x 30 ft with drop panels at column. We had not go through the numbers yet but I am thinking of maximum slab deflection during concrete puring be 1/4". To prevent unexpected slab crack during shoring, I don't want to put if any upward preloading on the shoring . the bottom reinforcement will have some prestress in it but should not effect the strength design as long as we can ensure enough ductility. Please let me know your thoughts. Thanks

 

[Brad805]

I agree the FRP or plates would do nothing but help to increase the bending strength. I suppose the localized topping demo might wind up costing almost the same as removing all of the topping.

The reason I asked about your solution is we recently got some quotes to remove an area of topping in a building, and I was quite surprised. The demo contractor did a test area to prove the time requirements. In our case there was even a bond break between the structural slab and the topping.

For the punching shear how about steel column capitals to move the critical shear zone? Any room for steel beams below? The middle strip problems should be easily fixable, but I agree that the column strip issues are the key problem.

I assume there are no architectural constraints created by the new slab?

By the way, my reply was not intended to be rude. I am just tossing out ideas for interest sake. I can tell you have done your homework.

Brad

 

[hx200]

Brad,

I appreciate you and everyones input here. This is a great forum of us engineers to bounce ideals and share opinions. Can you tell me the price you got from the contractor for demoing topping slabs and, if you don't mind, what was the location of the project you mentioned. I am in Kansas City and was expecting $30 per square foot for the topping slab demo. but my number could be way off. Thanks

 

[TXStructural]

Out of curiosity, why not remove the topping and use that to offset increased live load requirements - you will be removing 60 psf of dead load... how much LL increase is 40%? DL is 100% coverage while live loads allow loaded area reduction.

ICRI does have surface prep info, and ACI 318 17.5 gives an amplitude of 1/4 inch.

Whether you design the connections between the slabs as ties to connect them or dowels to transfer the full shear, then you have to develop them fully, which is difficult in a 5 inch thick slab. You probably need 135 or 180 degree hooks hooked around slab reinforcement, or u-bars (and getting holes properly spaced is always an issue.)

 

[JAE]

I'm not sure I'd buy into the idea of creating a composite layer and thus increasing depth. That is a very large amount of doweling necessary. And, as indicated above by others the punching shear at the columns isn't going to be improved by the extra layer imho.

 

[msquared48]

At 5" thick, would there be enough new depth to install new studrails at the columns if punching shear was a problem?

You could also get more capacity in the slab if you jacked up the center span of the slab a little, reinforced and poured the new slab, cured, and removed the shores.

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

 

[rscassar]

I think you have got the most of it.

1. Longitudinal/Horizontal shear across the joint to ensure composite action.
2. Punching shear- If composite action is achieved by providing sufficient dowels over the column then I think the full cross section could be used for punching shear.
3. Construction sequencing- I don't see no need for shoring. Once the topping is removed, the new topping will be placed level and you know the slab has sufficient capacity to support the wet weight of concrete.

 

[JAE]

Kikflip - I still have big worries about counting on extra punching shear. There would be little, or no mechanism for the shear to flow from the concrete surrounding the column into the column.

Remember, the topping slab is placed around the as-cast column and has no means of transfering vertical shear from the downward force into the column. It is a cold joint.

Therefore ALL the vertical shear flow goes through the original slab-column interface. The topping is just a topping with a hole in it for the column to slide through.

You could attempt to install dowels around the column, but you'd probably have concrete blow-out through the top of the topping at each dowel long before you fully engaged the capacity of the topping slab Vc.