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what options do I have?

what options do I have?

what options do I have?

I m dealing with an existing RC slab.
I have a data about existing reinforcement.
After reconstruction (different purpose of structure - larger load on existing slab)I figured that existing reinforcement is not gonna be sufficient - in both zones (on both sides of a slab).

So I was thinking about adding another layer of concrete - let say 50 - 100 mm on an existing slab and reinforce it so I have sufficient reinforcement there and since slab is now thicker the existing reinforcement (tension reinforcement in bottom zone of a slab) is now gonna be sufficient. I have to properly connect new layer of concrete to existing (with dowels that can transfer horizontal shear flow) so slab will act as a composite - one element.

What do you guys think? Any other options?

If existing reinforcement in top zone of a slab is sufficient, then i would normally use/add FRP - fiber reinforced polymer at the bottom of a slab, but since reinforcement is not sufficient in both zones of a slab i think thats not the best idea. BTW can you glue FRP materials on both sides of a slab? I have seen only on the bottom side...

RE: what options do I have?

The tricky part with that idea is to bond the new concrete to the existing slab such that you get composite behavior.


RE: what options do I have?

Agree with BAretired about the difficulty of getting good bond, but it can be done. Takes detailed, enforced preparation of the existing concrete - don't need, or want the rebar dowels you mentioned. To get an idea of the steps needed see "Guide to Concrete Overlays, Third Edition" by the American Concrete Pavement Association.

Note: This document covers concrete on-grade, but the principles work exactly the same for elevated slabs. An existing elevated slab may have to be "unloaded" (perhaps by jacking) to allow the overlay to fully take it's share of the load.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: what options do I have?

whats the best way to connect it? classic 'U' shape rebars?

RE: what options do I have?

No, you don't use reinforcement, as SRE said. Preparation is the key. Shotblast for amplitude, and make sure it is clean. Don't use any type of chemical bonding agent, as they can easily act to debond.

This is similar to a structural topping on a precast slab.

RE: what options do I have?

thanks for help. just curious - why no rebars/dowels?

RE: what options do I have?

You don't have enough thickness to make shear reinforcement effective. And you can get the bond you require for composite action by a clean concrete surface, with some roughness for good measure.

RE: what options do I have?

Interesting... I got a question too:

What if you want to do that only on a part of existing slab? Is there a problem?

Lets say that load is larger only on half of existing slab. Does that mean you still have to add a new layer of concrete above entire slab or is it ok to do it just where you need it?

red - columns
blue - beams
gray - slab

RE: what options do I have?

n3jc - Unfortunately, there won't be a simple answer; too many related changes:

1. The topping increases the dead load of it's portion of the (elevated) slab.

2. But at the same time, the topping increases the structural capacity of it's portion of the slab.

3. How much the topping increases the structural capacity depends on the details (reinforcing, edge conditions, etc.) of the existing slab.

4. Nothing was done to a portion of the slab, but the changes where the topping is applied will affect the unchanged part.

5. Revised live load rating, will affect each part (topped and untopped) of the slab differently.

Will have to make assumptions and see what the calculations say.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

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