Minimum thickness of a concrete slab?

[drummerdimitri]

After having removed the tiles in my workshop, I noticed that about 1/3 of the existing concrete slab is 5 cm higher than the rest of the area.

I would like to pour new concrete over the existing slab in order to close the 5 cm gap and have a flat and level floor.

For the 2/3 of the surface area being at a lower level than the rest of the workshop, I was told to insert expanded polystyrene blocks to fill in the gap and then pour a new slab on top of that.

Now my question is what is the minimum slab thickness I can get away with in this case? Cost is not an issue but weight is since the building is old and was ill maintained throughout its 60 year old life and so I would like to reduce the weight as much as possible.

The workshop will be used for light to medium duty metal fabrication and the main activities will be grinding, plasma cutting, welding, machining etc.

 

[FMB4]

Delete.

 

[ConCretin]

If my metric conversion is correct, you need to make up about 2". Assuming the existing floor is structurally sound, you could probably get by with 2" over the high areas, which would be 4" in the low spots. I wouldn't mess around with the foam, just place right over what's there.

It's not always necessary for a garage floor to slope to the door. You can check your local code if you want but it would probably be grand-fathered anyway if it wasn't originally constructed that way. For your uses, I'd think you'd want a flat floor.

 

[drummerdimitri]

If my metric conversion is correct, you need to make up about 2". Assuming the existing floor is structurally sound, you could probably get by with 2" over the high areas, which would be 4" in the low spots. I wouldn't mess around with the foam, just place right over what's there.

It's not always necessary for a garage floor to slope to the door. You can check your local code if you want but it would probably be grand-fathered anyway if it wasn't originally constructed that way. For your uses, I'd think you'd want a flat floor.

I definitely want a flat floor and not a slopping one.

The only reason I decided to go for the foam is because I would probably get away with 2 inches over the entire floor instead of having to do 4 inches to cover the existing gap which would weigh too much as the structure is not as sound as it should be that way I would save a huge amount of load bearing on the existing slab.

I was told that my slab should hold around 500 kg per sqm safely and according to my calculations, only 110 kg of that would be eaten up by the new concrete slab so I would still have around 390 kg/sqm for stock, machinery and furniture which should be more than enough for my needs.

 

[duneslider]

What do you mean the slab will only hold so much weight? Is this a suspended slab with living underneath? Or is this slab on the ground? So there is dirt under the concrete?

 

[billconner]

Are you going to cover leveled slab with a finished flooring material? If so, just use a self leveling underlayment and it can be very thin at high spots and adds very little weight.

 

[drummerdimitri]

What do you mean the slab will only hold so much weight? Is this a suspended slab with living underneath? Or is this slab on the ground? So there is dirt under the concrete?

Yes, it is a suspended slab (someone else's roof) so I have to be very mindful of what I decide to lay down.

 

[drummerdimitri]

Are you going to cover leveled slab with a finished flooring material? If so, just use a self leveling underlayment and it can be very thin at high spots and adds very little weight.

No, I would like to go for a polished concrete finish with a surface hardener.

 

[FMB4]

Delete.

Yes, it is a suspended slab (someone else's roof) so I have to be very mindful of what I decide to lay down.

Delete.

 

[billconner]

sorry I don't have this in metric, but an average of 3" of concrete (2 to 4) adds around 35 pounds per sq ft. Around here the live load design is usually around 50, so unless the original structure was very over designed or unless you reinforce the structure, collapse is a real possibility imho.

 

[ConCretin]

A suspended slab is an entirely different conversation. I wouldn't be doing anything without the involvement of an engineer.

How is it you have a suspended slab that is so far out of level to begin with?

 

[mcbane]

As LLWillys mentioned, you need to consult an engineer. The 500kg/m3 capacity you mentioned is not unusual in industrial buildings but is more than is typical for a residential garage. Even new you likely had less capacity than you have been told and after 60 years of neglect there may be further losses in capacity.