When should one use a concrete plasticizer? Should it be used for internal floor slab?
Concrete – When to use concrete plasticizer
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From your images it appears you're looking for a polished concrete floor. The process isn't very complicated, but it can be messy and time consuming. It's not unlike refinishing hardwood floors.
Ensure floor is suitable for polishing
Sources I've seen state that nearly any concrete floor can be polished, so long as it is structurally sound. Floors that are very porous, need extensive patching, or are wavy are probably not good candidates. New slabs need to cure for 28 days before polishing.
Grind floor flat
The actual polishing consists of a series of progressively finer grinding and polishing steps. They can possibly all be performed using the same piece of rented equipment. The first, coarsest grinds (typically performed with metal-bonded cutters) flatten the floor and open up the concrete's pores.
Densify, and maybe stain
Then a chemical densifer is applied, which seeps into the concrete, hardening its surface and filling voids, while preparing for the final polish. Around this time is when a colored stain could be applied if desired.
Polish
Next a series of progressively finer polishing grits (typically abrasive grit in a resin binder) is used until the desired level of shine is reached.
I can't recommend adding a plastic moisture barrier between the pads. Really if you have moisture in the soil then it needs to work its way out. Moisture comes out of concrete all the time - we only see the worst case scenarios.
If you leave the plastic in there then the water will get trapped on the bottom slab. Very good for the short-term. Long-term though this will cause the bottom slab to decay and break down. And since you are building on this, that's not a good thing.
I usually advise in drilling down into existing concrete and installing rebar or a mesh unit to join the floors. To be honest with you I have never installed concrete on top of a floor like the one in your picture. Common sense would tell me there wouldn't be much movement... but if I was giving out a 10 year warranty with my work I would rebar.
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Best Answer
You use plasticiser when you need to improve the workability of concrete.
Concrete's workability improves as you add increasing amounts of water. Increasing the amount of water in the mix beyond the optimum leads to weaker concrete. Insufficient water also makes concrete weaker, as it is harder to compact it to remove air voids.
For a normal floor slab, I wouldn't expect to need plasticiser to get the concrete sufficiently workable.