Q. How much shrinkage and cracking are acceptable in a poured concrete basement wall? Our basement is below grade on three sides, with the fourth side about 14 feet above the ground. The walls have both vertical cracks and cracks on the diagonal. There is also efflorescence and mold on the walls. Our humidity is high, and we’re wondering how much of this is due to the cracks in the basement walls. Also, how much shrinkage should be expected in the floor slab? There are gaps ranging from 1/4 to 1/2 inch between the floor and the wall around the entire perimeter of the basement.
A. The short answer to both of your questions is “not much.” Beyond that, it’s hard to address the concerns you raise without additional information. What are the height and length of the walls? How soon after placing the foundations were they backfilled? What climate zone are you in? How wide are the cracks in the walls? Is there any damage or cracking in the walls above the foundation? Are there signs that water is coming through the cracks into the basement? What size is the floor? But even without those answers, we can offer a few general remarks.
In an average sized basement you can expect one hairline crack in the length of a wall, typically somewhere near the middle. When window openings are cast in a foundation, a crack will frequently appear off one of the corners of the window.
Cracks can also result from backfilling foundations before they have sufficient strength to resist the forces generated by the backfill. Some of those cracks can be diagonal. Floor joists should be in place before backfilling to support the top portion of the foundation walls.
If there is water coming through the cracks in the wall, it’s a sign that the waterproofing on the outside of the wall has failed. That is a situation that must be corrected. If there are many cracks in a wall, you might want to have them epoxy injected. However that will not take the place of the need to repair the waterproofing. The fact that there is continuing efflorescence could indicate the continued movement of moisture, too. Now let’s address the floor slab. From what you describe, it sounds as if the shrinkage in the floor is excessive. With a good mix design you should expect about 3/4 inch of shrinkage in 100 lineal feet of concrete. The floor you describe has total shrinkage of as much as 1 inch, considering the gap on both sides, and for a standard size home that is excessive. It may indicate there was too much water in the concrete when it was placed.
With concerns about saving energy, homes are being built tighter now, so moisture control is becoming more of an issue. One control for this involves placing vapor barriers below concrete floors on grade. This isn’t done on many residential floors yet, but the day is coming when it will be routine. You might consider an “air-to-air” handling system to replace air in your home at proper intervals and to maintain proper humidity levels. This will also help to reduce mold growth.
If you have continuing concerns about your concrete, you should get professional testing and a consultant’s help.