existing RC slab - insufficient top reinforcement 3

[greznik91]

Hello

I have existing RC slab 140 mm thick.
There is little to none top reinforcement (above inner wall/middle support).
Investor would like to add some extra load on this slab so Im afraid that slab will crack on top.

What is your opinion on this idea:
I would like to add a new concrete layer on top of existing slab (lets say 80 mm)
This new layer will have sufficient top reinforcement above middle support (calculation based on new thickness of the slab). The surface of the existing slab will be prepared for better adhesion (existing + new concrete).
I will calculate the required number of dowells based on horizontal shear flow.
I will check if existing bottom reinforcement is still sufficient (larger self weight + new load)

Do solutions like this works in practice? Can old + new layer of concrete act as composite element? Any advice or suggestion? THX

 

[r13]

In my opinion, it is doable, but I'll pay extra attention to two areas - adequate concrete cover around the tension bars to develop required strength, and adequate shear reinforcement to prevent the premature crack of the new slab.

Two areas you may want to explore - 1) if the existing positive reinforcement is more than adequate (over designed), then moment redistribution is possible and allowed by code; 2) can you add steel beams to assist in supporting the added load?

 

[SlideRuleEra]

1) Do solutions like this works in practice?
2) Can old + new layer of concrete act as composite element?
3) Any advice or suggestion?

1) Yes, but not per your proposal.

2) Yes. Dowels not used if designed & constructed correctly.

3) Look into "bonded concrete overlays" used to rehabilitate existing pavement, bridge decks,floors, parking garages, etc.:

"Bonded Concrete Overlays" - ACI

"Bridge Deck Concrete Overlays" - Texas DOT

http://www.SlideRuleEra.net

 

[r13]

Sorry, I didn't notice your slab has cracked in the negative moment region, therefore, my mentioning of moment redistribution is a non-sense.

 

[JAE]

This is a very applicable project for composite strengthening methods - either by adhesion of FRP strips to the top of the slab over the negative moment region, or I've seen where they saw cut long grooves in the slab and insert bars (as a form of top negative momeent rebar) in epoxy to create a negative moment capacity.

Check with this firm:

https://www.vector-construction.com/

(I'm not associated with them in any way)

Or some examples here:

https://seblog.strongtie.com/2017/01/qa-best-practices-frp-strengthening-design-2/

http://aslanfrp.com/resources/Aslan-200-CFRP-Bar.pdf

 

[BridgeSmith]

If the slab is adequate functioning as 2 simple spans, there are concrete healer/sealer compounds that will fill and seal the cracks (Methyl methacrylate), or epoxy resin overlays.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[dik]

Is the bottom reinforcement adequate for simple span? If so, can you sawcut the top of the slab over the support to 'force' a crack and provide caulking? The bonded topping and the FRP solutions are great and would be my second choice.

Dik

 

[STrctPono]

If it were me, I would consider the following options in this order:

1. Can you check the stresses in the slab for an unreinforced section and see if they are relatively low? You didn't mention that the slab has cracked but rather that you are worried that it will crack. Therefore analyzing it as an unreinforced section should still be applicable.
2. Does ACI allow moment redistribution? If so, I would check to see if you can make the slab work out with that rationalization.
3. If it still doesn't come out then BridgeSmith's and dik's recommendation of redistributing all the moment to the bottom steel is a good option if you have the capacity.
4. Next, I would look into Near Surface Mounted FRP laminates that you can install into grooved cutlines over the negative moment region. FYFE used to make these but my understanding is that recently they got out of that side of the business as it wasn't profitable. There are other companies who make similar systems.
5. Lastly, I would consider an overlay but don't think you can just pour any old concrete on top. This will take some research and knowledge on your part to get a successful product and you need a competent Contractor. There is plenty of information out there regarding bonded overlays available for reading. Nowadays, the theory is not to even use dowels but rather put the time and effort into surface prep and using a high performance polymer overlay that gets tremendous bond values (~250 psi direct pull-off). If you run the numbers on your horizontal shear between the two materials, I think you will find that they are actually not quite as high as you think they will be. If you do it correctly, then yes you can get a composite section.

 

[greznik91]

Thank you for suggestions.

Slab is not cracked yet...

 

[r13]

greznik91,

What do you think after reading all the suggestions, what is your decision? How much additional load the investor asks, to entire floor, or just locally? What is the slab system, slab-beam, flat slab, one way, two way...? Just curios.

 

[JoshPlumSE]

My thoughts:
1) You'd have to shore the slab significantly during construction. Because, if there is little to no top reinforcing, the slab will have little strength to absorb the extra dead weight before it is fully cured.
2) Have you looked into FRP? This sounds like an ideal situation for them. Not sure about relative cost, but it's definitely something to think about.

 

[KootK]

If you wind up deciding that the slab needs to be reinforced, most of your options will be pretty costly. You might consider broaching the option of load testing the slab with the client and/or owner. These systems often have a lot of reserve capacity lurking about in places that our traditional strength evaluation methods do not look. This article is a pretty good overview of the slab load testing process: Link

 

[greznik91]

Thank you all for your help.

After additional loads I figured that existing bottom reinforcment is still sufficient even if cracks appears above the middle support.
So i was thinking about FRP but I would need a lot of it and its pretty expensive.
What about the suggestion dik was talking about - sawcut the top of the slab over the support to 'force' a crack and provide caulking? Any experience with this? Is this common?

 

[KootK]

It sounds as though your negative bending cracks would be little more than an aesthetic issue. Is your client very concerned about that? If not, this might be a good "do nothing" situation.

 

[r13]

If no crack right now, it means the bars have not stressed to the extent that causing flexural crack to occur. Agree with comment above, I'll do nothing as nothing is broken.

 

[haynewp]

What affect would there be on the slab's shear capacity if the cracks initiated to the right or left of the support instead of directly over it? That is something I would think about if there are little to no top bars in this area.

 

[mats12]

I am wondering - if you go with new RC layer on top of existing RC slab - should new layer be supported on a bearing walls at the end/edges? If you already have existing walls that's an issue?! Because in this case the existing slab takes all the shear? New RC layer acting like a load (like a screed on top of a slab)?