Why concrete floors crack:
1. The sub-base wasn't properly compacted and the concrete settled in one area causing the floor to sink and crack.
2. The concrete floor isn't heated in the winter and the dirt under the concrete freezes causing it to expand and lift, or heave, the concrete. This upward pressure will make the concrete crack.
3. Newly poured concrete shrinks when it dries. If no expansion joints are cut into the concrete floor then shrinkage cracks will appear at some point in the future. These are quite common.
There are some other reasons like; pouring the concrete too wet, or too much calcium chloride was added to the mix and it dried too fast, or the concrete wasn't properly cured. All these will lead to shrinkage cracking at some point.
If you determine the concrete floor cracks are due to settling or heaving, then that is a sub-base issue and the concrete may have to be removed to correct that problem. Repairing these cracks will only be temporary if the concrete continues to move up or down.
If you think the concrete floor is done moving, you can repair those cracks and the shrinkage cracks with a polyurethane/epoxy concrete repair material you inject into the crack and add a silica sand for a filler.
This will weld the crack back together creating an excellent concrete floor repair.
Step by step guide for repairing cracks in concrete floors:
STEP 1. Clean out the crack with a vacuum, remove any loose cement or aggregates.
STEP 2. Inject the concrete crack repair resin into the crack to wet it, it will soon get tacky.
STEP 3. Push the dry silica sand into the crack to fill it.
STEP 4. Thoroughly saturate the sand with the concrete floor crack repair resin and fill it to the surface.
STEP 5. In 10 - 15 minutes scrape the surface level with a putty knife.
STEP 6. Sand the surface smooth or lightly grind it with a hand grinder for a neat appearance.
TIP: If the cracks are very small, narrow or jagged, you may have to rout them out with a diamond blade and 4" hand grinder. Making the crack a little wider with smoother edges will make for a better and stronger repair.
If you've never used a 4" grinder with a diamond blade, find someone who has and let them do it for you. You could also try a file or screwdriver to help widen the crack.
*Injecting urethane in the crack*
*Crack after rubbed smooth*
Use the moist material you just scraped off with the putty knife to patch any holes, chips or scaled areas in the concrete floor.
Hairline cracks can be done without the sand. Wide cracks can first be filled partially with the sand then start with step 2.
This crack repair material sets up very quickly, only do 10 - 15 feet at one time.