It depends on the use of the tile. In the US, for standard floor tile without drains, it is common to put only a crack mitigation membrane followed by a layer of thinset tile adhesive. There is usually no mortar bed unless there is a need for leveling the floor.
In other areas of the world it is done differently....my understanding is that in Australia, latex adhesives are used for floor tile work, similar to wall tile. While I see nothing wrong with that practice, it is not commonly done in the US.
If the topping 50mm (2") load is not part of the SD (superimposed deadweight), do you guys consider 1.177 Kn/m^2 (23.54x0.05) heavy? The floor is 1/4 done and I may need to have it scrapped it asap later.
And what is your other solution if the floor is not leveled and mortal bed won't be used? Do curved tiles sound great, it's just on a small portion anyway.
How much leveling do you need? If this is an internal application, a leveling compound can be used. Done all the time here in Australia, and as Ron said, then the tiling is done using a flexible adhesive.
About 8 square meters is higher than the 150 square meters office floor. About 2 inches or 50 mm of leveling is needed. We don't have any leveling compound nor flexible adhesive. What we commonly do is add adhesive, sand, cement and use them as tile mortal bed. If I'd just ignore the higher 8 square meters and go ahead to use merely half inch of mortal to support the tiles in the slabs without any crack mitigation membrane, would the half inch mortal/adhesive crack and what would happen if the it cracks below the tiles. What would happen to the tiles? The floor doesn't get wet because it is NOT a roof deck.
Why is your floor slab 50mm higher in ANY location as compared to the surrounding area? That's miserable quality control.
Have you considered grinding the high spot rather than filling all the surrounding low spot?
...and it is a mortar bed not a mortal bed.
I would not consider putting tile on a floor slab with such variation.
50mm is very common in my place. what is commonly done is just to add mortar bed because experience shows tile on very thin bed can crack and the tiles can just move upwards in time. anyway, the slab is only 4 inches thick, chipping it can make it 3 inches and i dont think 3 inches slabs are allowable. have you ever designed 3 inches slabs?
It sounds like you have no quality assurance or quality control in construction. That's unfortunate. There is no reason to have that much variation in a slab on grade.
No, I would not design a 3" slab...the thinner the slab, the greater the need for tight quality control. For a 100mm slab, you should have a thickness variation no greater than about 6 to 8mm. To have to fill 50mm on a 100mm slab is absurd.
The reason you experience cracking in thinner adhesive layers for you tile is because you don't have a stable slab on which to install the tile. Make sure the slab is properly placed with consistent thickness and proper control joints, then you can use thin layers of adhesive for the tile and you won't have so many problems!
in my place, the technique is to put 2" of dry pack as bedding, dry pack has so few water and its 6 parts sand to 1 part water, anyway, anyone can compute the weight of dry pack versus normal concrete? if concrete is 23.54 kn per cubic meter, what is that of dry pack?
If this is a slab on grade the weight of the topping is irrelevant. If this is an elevated slab with such shallow thickness and poor quality control, it is likely dangerous. Get a local structural engineer involved...now.
The local structural engineer designed it. I'm just an apprentice. And you know what. 100% of all buildings in the country even 50 storeys high rise have same method.. uneven thin slabs supporting even up to 90mm toppings, and in each floor of 50 storey and all concrete and special moment frame.. we don't use steel frame because the welding quality is so poor they either overweld it and the connections got fragile and fall down.. that is why special moment frames of all buildings even up to 60 storeys and near major faultlines is normal.. because we are surrounded by faultlines.. over a million buildings are built this way.. and so far none fall down so the practice continues....
This thread took an unexpected turn into scary-land.
Ron- it seems like the OP is describing the old-world European system (not that it is bad) of an uncoupled sand bed system. I copied and pasted a good description from Schluter's website. I have seen a version of this system on engineered buildings in Spain, though I do not recall if the used a thin bed of sand to prevent bonding between the structural slab and the topping slab.
That is to say nothing about the scary structural descriptions provided by the OP.
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Ancient Wisdom
Thousands of years ago, European builders developed a successful means of installing tile that eliminated the transfer of stresses within the layers of the assembly, thereby eliminating failures. Evidence of their success can be seen today in the elaborate yet flawless tile installations found in many of the ancient European cathedrals. These builders understood that a forgiving shear interface between the building structure and the tiled surface was necessary in order to allow the two elements to move independently.
The installation method that the European builders utilized was a "sandwich" comprised of a structural base and a layer of sand, upon which the mortar bed (a mixture of sand, cement, or other binder, and water) and tile was adhered. The critical component in this sandwich was the layer of sand, which uncoupled the tile covering from the structure, allowing the two to move independently. The layer of sand could support the tile covering under heavy loads and, at the same time, provide a forgiving shear interface that effectively prevented movement forces (which emanated from the structural base) from transmitting into the tiled surface.
For many reasons, the use of the sand strata method is virtually extinct today. To begin, this type of assembly requires installer expertise, and these trained experts are few in number. Additionally, this type of assembly doesn't fit our current construction environment where there are considerable height and weight limitations. And yet, the "ancients" understood the physical dynamics of a composite floor covering assembly.
