Even when precautions are taken during the building of a house, time and weather can still take a toll on the soil beneath it. Rainwater and other moisture will cause soil to settle which in turn can eventually create hollow areas underneath a concrete floor. You'll know that you have a gap below your floor because it sounds hollow, rather than flat, when struck by a hard object. It isn't necessary to remove and replace the concrete, provided the damage isn't too extensive. You can perform the maintenance to fill in the hollows, making your floor sturdy and solid again.
Tap the area of the floor that you suspect to be hollow underneath. Use the hammer for this and as you go, mark around the area with the chalk so that you have an idea how large the gap is.
Drill a hole in the middle of the chalked-out area.
Mix the concrete in the bucket according to the directions on the packaging. The consistency will be very thin so that you can easily pour it into the hole.
Pour the concrete into the drilled hole slowly so that it can run into the entire hollow area and fill it up. Continue pouring the concrete until the hole fills to the top and is level with the existing surrounding area.
Just as with any concrete surface, it is a good idea to allow the area where you poured in concrete to set up for a few days to assure that it has cured sufficiently before you drive on it or place any significant weight on it. Concrete purchased at a home improvement store is the typical substance used for this type of repair. However, some contractors shoot dense foam into holes drilled in concrete to fill in hollows with similar results. If the floor is already sinking and is significantly lower in the spots where there are hollow areas beneath, you may need to contact a concrete company that has the equipment and capabilities to jack up the floor from below before concrete is pumped in to fill in the gaps and support the structure.