What are the common causes of crumbly concrete and how can it be prevented or repaired?

Common causes of crumbly concrete include improper mixing, excessive water content, and freeze-thaw cycles. To prevent crumbly concrete, ensure proper mixing, use the correct water-to-cement ratio, and protect the concrete from freezing and thawing. To repair crumbly concrete, remove the damaged area, clean the surface, and apply a concrete patching compound.