Yesterday I had a 50 year old house on a basement. There were a few clues outside that peaked my interest: 1st there was a significant negative grade at the northwest corner of the foundation. 2nd there was a dimpled drainage mat attached to the foundation wall in this same location. 3rd there were a few vertical cracks and most importantly some horizontal cracks above the ground level. 4th there was a small section of foundation wall visible in the furnace room with a horizontal crack patched. I recommend the buyer ask to have the interior paneling removed so I could see the extent of the cracking. Turns out the foundation had horizontal cracks 2-foor apart at the elevation of the form ties. The form ties were never removed on the outside, which allowed water to seep in and rust all the horizontal reinforcing steel that were supported on the form ties. As the rust expands on the reinforcing steel it can apply up to 12000 psi. The concrete typically can only resist about 300 psi in tension, which created the wall cracks. As the reinforcement rusted it also debonded from the concrete. Basically it acted like a zipper and opened the wall on 2-foot spacing. Small repairs can be make by chipping out the concrete exposing the rusted reinforcing steel. Once exposed the rust has to be cleaned by sand blasting or replaced along with sealing the cracks both inside and outside. In this case due to the amount of damage I recommended replacing the foundation walls. Seller is not going to be happy. The current owner has only lived there 2 years and was unaware of the problem.
The lesson here is horizontal foundation cracks in concrete walls is rare (not the case with masonry blocks). Horizontal cracks along the form tie level is a sign of rusting reinforcing steel from water intrusion at the form ties. The moisture intrusion is from the outside on a basement walls and moisture from the exterior or interior of a crawlspace. Crawlspaces with high humidity that sweats on the inside walls is the only moisture needed. Very common to see the form ties not removed on the inside of crawlspaces.
