Now 'wind up going through the floor' might be too dramatical (and comical too
) but yes there is always a chance of the floor cracking up. So the things you must ensure is that the right kind of items are used for making your gym flooring . An also try to find the actual weight of all the gym equipment in your room. If you know what is gonna be the actual weight of all the gym items combined, where they will likely placed then you can always inform your contractor about the same and he/she will take the necessary precautions. Overall, a sufficiently thick strong concrete will be great for you gym room but if you are not happy there is a blog called everything-about-concrete.com that provides all the necessary information on concrete. But I believe you won't be facing any serious problem (certainly not fallin through the floor ) so chill.