I don't know what it's called but concrete ceiling has a layer at the bottom like skim or something? my ceiling that layer an area is loose, when you knock with hand can hear hollow, and 1 area after knock actually cracked and a small piece fell down revealing the concrete ceiling inside.

there will be plaster ceiling so it's not an issue.

problem is the area around it also loose, i figure the more i try to remove it the effected area will just get bigger & bigger.

but if i leave it as it is, i'm thinking due to gravity sooner or later they will come off or drag the surrounding pieces off the ceiling also. or start falling in small pieces.

is there anyway to work around this? i think it's important to remove the already loose areas but how to prevent it from spreading further?

for walls it's easier because gravity doesn't act on walls as much as ceiling.

joint compound

how to fix the area already coming loose? do we remove those area or just straight put joint compound into the gaps? if remove how to safely remove those area without causing the areas surrounding it from coming loose.

Photos of it? What you meant is the plaster ceiling that crack and coming loose is it?

If yes, you need to add a new support on top then screw the plaster ceiling back then repair the crack using joint compound.

it's not plaster, but something like skim you see on walls that is added onto the original concrete ceiling. that piece an area is loose from the concrete, so when you knock it sounded hollow and if you use abit force you can peel off that layer. problem is when you peel off the loose area you drag the surrounding area too making the problem area bigger.

Okay, I think I get it now. What you meant is the white colour (plaster like) layer above the grey colour concrete. It has turned bloated and hollow inside.

It happen when you have moisture in your concrete. Maybe you need to remove it then do waterproofing. Same goes to my wall, they're peeling like oranges skin.

i don't think it's humid, maybe they did not apply properly or when they work on it they pushed a spot out or use too much water till it doesn't stick.

not sure if temperature causing expand & shrink will cause this also but when i try to remove a small spot the surrounding gets affected and comes loose too just like orange skin. anyway to prevent surrounding being affected?

if i leave it like that i figure gravity will continue to pull it away from the ceiling. if remove is alot of area. that area will be plastered with ceiling but i don't want them to start flaking off inside the plaster ceiling.

If the surrounding will come loose, it'll flake off soon or later also. Why not do it altogether.

I'm not sure about yours. For mine, the wall is literally 'cold' when touch. When I peel it off, it comes out like rubber sheet instead of crisps even my paint has left for 2 weeks. Proven it's not totally dried, water based paint should comes out in flakes or pieces but not like orange peel.

so how did you fix yours?

mine is 1 spot the rest is stick on ceiling, but if i try to peel off the loose one i drag the surrounding with it too.

Peel off the affected areas, grind off some of the cement surface (make it rough) then apply joint compound. It feels firmer now, I'm not sure whether this is a permanent solution.

when you peel off the area won't you spread the area to surrounding?

It will, I peeled it in a round direction back to the hole to minimize the size. But I believe if the surroundings will follow the peel means the surrounding wall is affected as well. It's just the matter of time to crack.

You should remove it and skim and paint it again.

remove as in the loose patch or the whole piece?

if remove loose patch how to ensure the surrounding don't become loose when removing?

also after skim, if no paint will be problem? since there will be plaster ceiling. i just don't want it to flake off later on.

If you plan to install plaster ceiling then just scrap off the loosen and half loosen part will do. No need to paint too.

yea but my question is what is safe way to remove them without causing the adjacent areas to become loose?

Use a scraper. And don't worry about making them loose, because if it is loosen so easily. It shall be removed now rather than later.

for walls, if i knock sound hollow, have to do the same remove the area?

because some area (near hacking area) will sound hollow and when knock can feel vibration however i try to pull the piece or shake it's quite solid but means inside already loose to cement wall?

some areas i knock, sound not as hollow but still hollow compare to other areas, yet there is no vibration, so these areas, need to remove and redo or they are ok? the thickness is thicker than the ceiling, maybe around 1"-2".

and some areas, when knock it's solid and won't move when you apply force, however visually speaking you can see the line between the skim & cement. those area just skim back ok?

Ideally all hollow areas shall be scrap off. Hair line crack areas you can just leave with it.

Wow, you treat your wall as an art piece.
The wall laid by foreign worker and not even before plastering. Please do consider that there are some hack of the brick for piping/wiring purposes.
From Enginnering aspect, concrete wall is not load carrying wall.