Topping on existing slab 1

I would be surprised if the existing slab were reinforced enough to handle the substantial new dead load of an 800mm thick topping slab, plus the live load. Why not leave the existing slab in place, and consider it as "stay in place formwork", and then design the new topping slab by neglecting any structural action from the existing slab? The new slab supports all of the imposed dead and live loads.

Thaidavid

Struggle66 - I have concerns about two of your three potential solutions. The topping slab, whether bonded on unbonded, adds potentially significant dead load to the structure's foundation. If you decide to pursue either of those approaches, suggest you evaluate if the existing foundation is adequate before going forward with design.

IMHO, Thaidavid's concept is well thought out and has potential to be suitable solution, but adds dead load to the foundation, also.

That leaves Sketch 2, demolition and replacement with a similar size structural slab, as the leading candidate. Based on info available to us, that is what I recommend.

To add just a bit to SlideRuleEra's comment - besides the foundations you also may have other beams carrying these slabs that might need consideration as well.

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Thanks thaidavid40, SlideRuleEra, JAE

Foundation is adequate according to project structural consultant.

And I am working for PT specialist.

FYI, this project got a history and is a bit complicated. During construction we can not demolish the existing ramp one way PT slab which is connecting to the existing warehouse. We can't disturb the existing operation. So if I choose option 2, we have to construct half of the new proposed PT slab as conventional RC slab with empty PT duct inside in the meantime the vehicles will use the other half of the existing ramp with proper traffic control. After that demolish that half of the existing ramp. Vehicles will start using half of the proposed new ramp and then construct the remaining half of the new ramp. So option 2 is not as simple as option 1. Sorry if my explanation is not clear enough.

Also the topping is originally around 400mm thick. Option 1 is what we proposed and how we got this project. So my boss want to stick to option 1 (even though I prefer option 2). But later due to architect revised proposed ramp level become 800mm.

At this moment what I am trying to study is how can I utilize the capacity(L.L=15 kN/m2) of the existing ramp.

You can connect the new slab to the old to achieve composite behaviour. At this "topping" thickness, however, I'd have dowels drilled into the existing slab to transfer horizontal shear rather than just roughening and bonding compound.

Could you use void form or rigid insulation locally to effectively turn the new slab into a waffle slab or pan joist system that would be much lighter than 800 mm of solid slab? You'd need to give careful consideration to corrosion protection.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

KootK,

Thanks

So without the roughening and dowels drilled into the existing slab, would the topping slab be like slab on grade?

You're quite welcome Struggle66.

With the weight of an 800 mm topping in play, you're bound to get considerable composite behaviour even without the dowels. I'd just want the dowels to make me confident that reliable composite action were available.

Slab on grade? I suppose there would be some commonality. In this case, your slab on grade is extremely stiff, highly restrained, and supported by a substrate of variable stiffness in plan.

An 800 topping will require a lot of temperature and shrinkage reinforcing. With that installed, the strength and stiffness of the topping may dwarf that of the existing slab.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

I think your Option 1, as modified by thaidavid40's suggestion above, would be the best way to go. But 800 mm of concrete will be about 19 kPa, so your 15 kPa live load comes up a bit short as lost formwork. Would probably be fine, but I would put in some props. Your construction sequence is not clear to me without a plan, but why are your prestressing it?

Dear All,

Thanks it was great to have your replies.

I would like to bring up this thread again just to share my proposal to client which is under construction now. You may not recall . Please see the attached step by step construction sequence.

For the topping on the existing slab, we have decided to use Politerm Blu 300kg/m3 with a top layer of 100mm conventional concrete with reinforcement mesh.