Concrete Slab With Two Layers of Different Materials 1

[zrabmehs]

Hi. I was assigned with this problem at my new company. We have a 600mm basement slab (on ground) initially to be mixed with integral waterproofing to make it impermeable. But, it was too expensive for the client so it was decided to make it 300mm thick impermeable concrete placed on 300mm thick normal concrete (I've attached a figure). The concern here is the on-site pouring procedure. We need the first layer (normal concrete) to be still wet when pouring the second layer (impermeable concrete) to ensure bond between two mixtures. But, the contractor worry that the upper layer (impermeable concrete) might sink during vibration. Are there any reference you can recommend to help me with this? Or If you have any ideas/ experience you could possibly share with a similar situation?

Thank you in advance.


P.S. I already proposed two time pouring but needs shear connectors to ensure bond between two layers. Which was denied due to cost effect of the shear connectors. ;(

 

[jayrod12]

With proper finishing and preparation it can be done with 2 pours and no shear connectors. I would bet with proper roughness, cleaning, cement paste application you would essentially have the same strength as a monolithic pour.

Why so thick if it's on grade?

 

[zrabmehs]

Thanks jayrod12. Foundation is on piles. It's a 50 storey building. Anything lower that 600mm, slab fails on punching shear from pile caps.

 

[robyengIT]

it is a bit odd to me that the cost of a foundation is so economically predominant vs a 50 stories building : am I missing something ?

 

[Ron]

You must have a good concrete salesman calling on you. First of all, your layers are backward. Waterproofing should be on the bottom, not the top.

Integral "waterproofing" from an additive or admixture only serves to repel some of the water, not make the slab waterproof.

I would place a 50mm blinding layer, waterproof that with a high quality waterproofing membrane (coal tar with roofing felt if you can get it, asphalt if you can't. Then place your structural layer on top of that. Don't worry about bond.

 

[zrabmehs]

Thanks Ron. I guess I forgot to include this >> we do not intend to waterproof the whole area. Just some parts for the raw water tank. And a huge one.

 

[jayrod12]

Considering a 50 story building. This is not the place the be saving money. Have the owner house a different flooring type, or a different paint. There has to be a better way to find the cost savings then attempting something so unorthodox.

 

[BridgeSmith]

It's hard to predict how a slab with a construction joint through the middle would react to punching shear loads, even if it is very rough. I agree with the previous posts - don't take chances with the foundation of a multi-million dollar building to save a few pennies.

Edit: I also agree with Jayrod that applying some kind of waterproofing membrane on top of the slab would seem the be a better solution.

 

[bones206]

zrabmehs,

In your sketch, what does the dashed line below the mud mat represent?