I had a call this morning from a prospective client who was concerned about cracks in the concrete foundation of the home they were building. Most often the cracks extend across the entire foundation and occur prior to tensioning of the tendon cables. This is a typical concern for those purchasing new construction who are not familiar with post tension cable reinfored foundation construction methods and soils conditions in our area.
The foundation in question was a post-tension cable reinforced foundation as opposed to a foundation which is reinforced with steel rebar (See the typical photograph in this article). This means the foundation is reinforced by cables laid in a grid arrangement and tensioned after the concrete cures sufficiently with the intent to create compression in the foundation concrete to resist cracking during bending. If movement occurs in the foundation prior to tensioning, cracks may occur due to the lack of any reinforcement at that time. Cracks more often occur during curing because the concrete generates heat and expands during the chemical process and shrinks as it cools. Because there is not yet tension on the tendon cables there is no reinforcement to hold the foundation together and cracks often occur during this period of time. This is often seen across a long narrow foundation which shrinks more in the long direction resulting in more visible cracks. Builders can partially tension tendons during the curing process to reduce this occurrence but it rare in production construction. The curing time before cables can be tensioned may vary, but most builders don’t tension for 10 days or more. During this time cracks often form, and once the crack has occurred, there little you can do to eliminate it. Small cracks do not normally create a significant concern other than for cosmetic reasons or those associated with pest control and are considered insignificant once the tendons are tensioned.
In looking for resources to explain this type of foundation to my client I found a good article on the internet I though was worth sharing. The author clearly explains the foundation and probably answers many questions my potential client has.
Concrete Construction Magazine: http://www.concreteconstruction.net/concrete-construction/post-tensioned-slab-on-ground-foundations_2.aspx