Hi Alex

Quite interesting you talk about A2 rated tiles - what is the specified performance of the installed system (supplier test report) as the fit and materials of the grid may not necessarily deliver what you think.

This is not the forum for the question you are asking - there is insufficient data for a judgement e.g. do services / pipes / cables penetrate the concrete? Have these been adequately sealed with suitable fire stopping?

You really need a qualified fire assessor who can look at the as built specification, compare this to what is actually present and then provide recommendation.

Aesthetically concrete under floor is generally unattractive unless specifically prepared.

You may find there is then a need to apply white paint to brighten the illumination of the area or black paint to "hide" what is present.

Hi Alex - the answer to your basic question is that the concrete should be sufficient to provide effective compartmentation (all dependent on the standard of protection at penetrations as Roundtuit says) ASSUMING that the building is designed and constructed to modern Building Standards.

The suspended ceiling is there to look pretty and to hide all the services between suspended ceiling and concrete.

I would be very concerned if compartmentation was dependent on the integrity of a suspended ceiling, and more so, when you say it is so frequently compromised!

Now the materials of that suspended ceiling need to be difficult to burn - otherwise you have a repeat of the issues when it was popular to line ceilings with polystyrene - which even if notionally fire retardant burnt very well, whilst producing lots of toxic fumes once ignited. But not nearly as difficult to burn as solid concrete.