Poor concrete installation?
Poor concrete installation is among the top causes of residential building collapse in the United States. Based on a study of more than 225 residential building collapses over a period of eleven years, poor concrete workmanship and construction deficiencies were the primary causes of residential building collapse. The study was conducted by the National Association of Home Builders, a non-profit association of real estate builders. This study looked into the causes of three typical residential collapse cases that took place over a three-year period; these cases were specifically chosen because they had relatively similar construction defects. 
In the case of one particular case, the homeowner poured asphalt shingles for a garage before pouring the concrete for the house. Asphalt shingles expand when wet while concrete tends to shrink, which makes it difficult to get them wet enough to form a thick coating that is waterproof when poured properly. Another case involved a builder trying to put in concrete on a surface that was too hard to make using manual tools. The builder used submersible spools that pushed concrete into the required holes. Unfortunately, these spools drained into a nearby lake, causing the concrete to erode significantly as it sat there for the next eight months without adequate water or repair.
A third case involved a homeowner who preferred a corner slab to a straight wall for his home. Although the corner slab was built using identical techniques as a regular slab, the homeowner overlooked the possibility of shrinkage and cracking due to soil conditions. After only two years, cracks started to appear in the concrete as it started to crack. After contacting a local contractor, the homeowner was able to have the corners of his house reinforced with rebound concrete boards.