We recently had a 5,696 square foot post tension slab poured here in North Texas on 2/14/2014. As of today, March 3 no one has come back out to stress the tendons in our slab. Today I noticed, what I (the homeowner) consider, severe cracking in the slab. Now I understand that there is some cracking that can happen due to shrinkage and the fact that the PT cables are unbonded to concrete. I accepted that. But what I am witnessing today is very long cracks (some over 50’ in length) that go across the house, through walls, to the porches to the edge of those porches, and then down vertically beyond the border between the soil and concrete. There are two of these cracks specifically that I am concerned about.
I have had to kick and scream to get my contractor to get someone out here to tension these cables. After stopping payment on the concrete subcontractor’s check for the slab, they seem to be snapping into action. But now, I am concerned we have a serious issue. I am bringing an engineer to the site to see the cracks before they tension because I am concerned they will not disappear upon stressing. I am also plotting every crack on a copy of my blueprints so I know where all of them are at.
At this point, I am not sure how to proceed. I have even told the contractor that, depending on what the engineer said, he may be required to start the home over from scratch with all the costs of demo and removal of debris on his dime. I’m sure it will improve upon tensioning but it should not have sat this long.
I don’t have sufficient privileges here on this forum to post photos but have photos. Any advice you guys can offer? I have seen posts here by Jeff Pope that seems very knowledgeable about PT foundations and would love his take on this.
Your engineer is your best bet. The slab can not be tensioned until the number of days have passed for the concrete to get to it’s minimum compression strength. There should be some documentation as to when this is.
Hey Chris, what you are describing seems normal for a post tension slab.
Cracking in post tensioned slabs prior to tensioning the strands/tendons is very common and this situation has a specific name called “Restrained to Shortening” or RTS cracks. As concrete shrinks during the drying process it would not crack if it were supported by a perfectly smooth frictionless surface, but in reality this is not the case. The ground surface the slab is poured on will restrain the concrete from sliding, which develops stresses in the concrete causing cracks to develop, i.e. RTS cracking. If reinforcing steel or wire mesh were added near the surface ( 1.5” to 2” below the top) it would strengthen the concrete and help resist the dry shrinkage forces that develop, however most post tensioned residential slabs typically do not have any reinforcement other than the post tension cables, which are not designed for dry shrinkage. The good news is these RTS cracks are typically harmless and may partially close up if not too much debris has fallen in these cracks before the cables are tensioned.
You can send me your photos at will@redfishinspections.com
Where about in North TX are you? If close to Dallas, you can contact Barry Adair http://www.adairinspection.com/
I never really understood the advantages of using the tensioning over the reinforcement but most of the concrete pouring I have ever seen or been a part of has been commerical/ industrial
Chris and I have been in contact, and I think he’s got a handle on his problem. I also reached out to John Onofrey to assist Chris on this.
Most of my experience was with commercial and industrial as well, and I’ve installed, stressed, grouted, pulled, removed and de-tensioned thousands of miles of cable in my career.
The advantage is in compression of the concrete. Concrete has great compression strength (which can vary depending on the mix), but relatively low tensile strength.
Once the concrete is compressed, it takes much more force to overcome the compression and bring the concrete into a tensile-type stress. If/when the compression is overcome, the rebar acts as the next line of defense to prevent the concrete from failing.
Many people do not understand that rebar is dormant within the concrete until the concrete cracks. Only then is force being applied to the reinforcing steel. Cables (both post-tension and pre-stress) make it “more difficult” to reach the point where the rebar is being used to support the concrete.