Q.: The concrete floor in my brother's basement has cracks along both sides and both ends, about 3 feet out from the walls. There are also some diagonal cracks running from these cracks to the corners. There aren't any other cracks in the floor. The floor also seems to bulge toward the center. There would be room to put a topping over the whole surface to level it up and cover the cracks. Is this a good solution, and if so what would be the minimum thickness needed at the highest point?
A.: What you describe sounds like a floor being heaved by an expansive soil. To confirm this you might look for other signs or clues: outside ground sloping toward basement walls, hardly any cracks in the foundation walls, opening in the living area out of level, floor in the living area out of level, doors binding and hard to open or close, and cracks developing around door and window areas in the living area. If your brother didn't notice any trouble at all for the first few years, if he has the troubles just enumerated and if these troubles are getting worse, his main problem is probably an expansive soil.
If so, you should consult an expert on repair of concrete on expansive soils. Heaving of the soil is caused by buildup of water around the base of the foundation, so the first thing to do is to stop this process. The grade of the yard should be corrected to slope at least ½ inch per foot away from the foundation to promote drainage of rainwater. It will also be necessary to prevent formation of channels through the soil to the foundation. These form when the soil dries and cracks. Hence there is a paradox: to prevent water from reaching the foundation, keep the soil from cracking by watering it consistently during the dry periods of the year. (In a soil that is already cracked, water a little at a time until the cracks are finally closed.) These measures will allow abatement of heaving and prevent further damage to the foundation walls and the house framing.
The next step is to cut the floor free of any attachment to foundation walls. This job must be done by someone with experience with such floors because there is a possible danger from a sudden release of potential energy. The floor should also be freed from attachment to plumbing stacks and though you didn't mention them any columns or fireplace masonry.
After the cause of heaving has been removed, a good way to repair the floor is to cut it along lines about 12 inches on each side of the cracks and remove the resulting 24-inch-wide strips of concrete. Then fresh concrete can be placed in the same area and sloped from the high side to the low side, thus getting rid of the abrupt change in level. If this kind of repair is not to your liking, then a completely level surface can be obtained by placing a concrete topping, at least 2 inches thick at the highest point, over the whole floor.
Before undertaking any repair of the basement floor it is a good idea to trowel a thin layer of plaster of paris over a number of short lengths of the crack to see if the floor is moving. If the plaster of paris finally cracks, add another series of patches. When a set of patches do not crack during a period of 3 or 4 months, or just barely crack, it can be assumed that heaving has been arrested and it is safe to repair the floor.