My firm was recently hired to look at cracks that had developed in a post-tensioned floor slab-on-grade. Not so unusual, except for the following:
The slab was decorative concrete - it had been stained a dark brown, scored and polished. This was present throughout the home, no floor covering anywhere.
The cracks were in a grid-like pattern. Most, but not all, cracks extended from edge to edge of the slab, about 7 main cracks total.
There was no reflective cracking in the superstructure - sheetrock, wall tile, fireplace brick, and exterior brick were all “clean.” No warped door/window openings.
The house is new - completed in Dec. 2011. The cracks are about 1mm or less in width, but are very visible due to the dark brown floor stain. Homeowner says a large, heavy-duty compactor was working next door and that’s when he noticed the cracks forming - actually had small pieces of slab “jumping” from the most pronounced crack. I was skeptical at first that the vibration could cause such wide-scale damage (although these are zero lot-line houses, 15 to 20 feet between structures), but after doing some research and very basic calculations I am at least less skeptical. There is nothing to suggest settlement or shrink/swell issues. To me it seems obvious that the cracks are related to the cable system of the slab, based on the pattern. Is this the type of thing that could result from overtensioning the cables, or tensioning too early? Is this something that is unusual for a PT slab? Cannot attach pictures (?), but will be happy to email. Has anyone seen anything similar? Appreciate any thoughts.
I have not seen this in PT slabs here in AZ. I did come across a home that the owner said that the foundation and interior walls cracked when another builder was using dynamite on the mountain behind his home for construction. I would have the owner check with the builder, as most homes have a ten year structural warranty, at least here in AZ they do.
mmaizel
(Marvin Maizel, CMI® Fl State Lic HI 681)
3
I am afraid I am at a complete loss here. If post-tensioned slab on grade I assume that would mean that it is stem wall foundation with the slab bearing on the perimeter block or concrete walls. I am still guessing here, but would that be because the soil was unsuitable for slab support?
Thanks all, I appreciate the comments. The homeowner has filed a claim on his homeowner’s policy, and we are actually working for the insurance company providing a professional opinion as to cause. Mary Ann, he actually does have a 10-year structural warranty so has some recourse there, but since he is not our client I’ve had very little interaction with him and am not aware of his “game plan.”
I have a geotechnical engineering background and had first been approached from that standpoint - until I actually got a look at the house, lot, slab etc. Didn’t know that it was a post-tensioned slab until I got there to look. I should mention that the house in located near Jackson, MS, in an area known for highly plastic soil. Before seeing the slab I had assumed there was some association with that. But I would not expect to see cracking in the middle of the house if that was the case, should have seen it primarily at corners, slab edges, etc. Probably in the superstructure. His driveway had virtually no cracks.
Marvin, there was no sign of subsidence adjacent to the house or in surrounding areas. I suppose the fill soil (which was likely clay although I don’t know) could have consolidated as a result of the compaction vibration, but I would have thought that would be evident around the structure? The cracks were well-distributed throughout the slab, no area of particular concentration. Again, nothing at all in the brick facade or interior sheetrock.
Barry, thanks for the link. Very informative. I had thought about restraint-to-shortening cracks, but those would have been evident early on, wouldn’t they? He claims not to have seen any cracks until he had been in the house for 6 months and the compactor was being used next door. Of course I cannot verify that.
I uploaded some pictures to flickr here: http://www.flickr.com/photos/89135779@N07/
You may have done this, but I fine it helps to have a scaled floor plan with the cracks drawn in. I would also draw in the cable locations. Is hard sometimes to see the big picture when you look at individual cracks. Was a floor level survey done?
Typically shrinkage cracks occur prior to post tensioning and erecting the walls, so these would likely have been noticed when the floor was stained. Cracking due to shrinkage after tensioning is not common and if they do occur they would be very small. One thing most post-tension slabs are not reinforced for is vertical movement. This summers drought may have caused some settlement around the perimeter or another possibility is a water leak under the interior of the slab causing some soil swell. You could drill a core hole through the slab to see, but I first would have a plumber do a pressure test of the lines. Situations like this is similar to putting a jig-saw puzzle together blindfolded.
Some of these cracks are pretty typical, and not too concerning, however, the long straight cracks and the wide crack that has crumbled are not common in PT slabs.
I believe they are the result of poor placement and/or displacement during the pour, as I indicated in my previous post.
Randy, excellent points and I agree with your blind-folded jigsaw puzzle comment! There was no floor level survey performed, but I did make a scaled sketch of observed crack locations and there was a definite grid pattern that stood out and a couple of long cracks extended from edge to edge of the slab. Jeff Pope’s observation that the cracking is most likely related to slab construction seems to fit well with the other factors. Maybe poor construction initiated the cracks internally, and external factors such as shrink/swell or vibration brought them to the surface. Do you have many areas of highly plastic clay in Missouri?
Jeff, thanks for taking the time to look at the pictures, post them, and provide your opinion. I can tell from the other posts that you have a lot of experience with post-tensioned slabs. What you’ve noted complements the conditions I’ve observed at the house site.
I posted two replies earlier this morning and for some reason only one has made it to the board so far. Just wanted to make sure that this one got through as well. I appreciate all the comments and will post again if there are any changes in status or updates.
