Post Tensioned Slabs

In the San Antonio area since the 60's, we pretty much have  had all slab foundations in new home construction with mostly re-bar steel reinforcement slabs but also with many post tension slabs. In the last twenty five years the Post Tensioned Slab has become more and more popular.
Post-tensioning is a method of reinforcing (strengthening) a concrete  slab on grade with high-strength steel strands. These strands or cables are also referred to as tendons. High-tension steel tendons enable slab foundations to withstand the stresses of expansive and compressive soil conditions that are prevalent in Texas.
San Antonio and most of Texas is well known for their shifting and expansive clay soils. During the hot, dry summer of the San Antonio area, clay soils shrink, and during extended periods of wet weather soil naturally tends to expand.  Since the slab foundation itself protects the soil directly beneath from wetting and drying, most swelling and shrinking takes place at the edges of the foundation.

Precut post tension cables 1a) Pre cut tendons 
Plastic sheathed tendons (cables ) arrive at the site cut to length, with one anchor plate  already wedged in place . This pre attached dead end anchor is nailed to the form board half the slab thickness from its top edge, and a U shaped steel backup bar is wired to the anchor to help distribute the eventual tensioning pressure over a wider area of the slab edge. The thick sleeve at the end of the tendon is designed to protect the cable and fitting from corrosive soils that may be present. 
 2a) Pictured is the stressing/ tension end of a tendon

The tendon is then unrolled across the  foundation  to the stressing or tension end anchor on the opposite slab edge. Tendons don't have to run in completely straight lines.  The tendons can be diverted a foot or two to one side to avoid plumbing , forms or other items causing an obstruction, as long as the 1/2-inch tendons follow a gradual curve.

 Post Tension Slab before concrete pour
                 Slab Pour
3a)  Post Tension slab shown at pre pour inspection                                             3b)  Same slab shown after concrete pour
The exact spacing of the tendons will vary depending on the thickness of the slab and other variables as determined by the foundation design engineer. A spacing of 3 to 4 feet on center in a criss cross grid pattern is typically used on most residential post tensioned slabs in the San Antonio area.
The reinforcing strands/ tendons and their
specialized end fittings are cast into the center of the slab thickness  before the concrete pour.  After the slab is poured the forms are wrecked (removed) and the slab is allowed to cure.  
 Post tension slab after pour
4a)  Post Tension Slab shown curing after the slab pour wih cables waiting to be stretched to engineers specifications 

4-7 days after a post tension slab is poured and before it is totally cured, the contractor comes and tightens the strands/cable to within 7% of the engineers designed strength of between around 27,000 and 35,000 psi. After the concrete has cured for a few days, each cable/ tendon, is stretched with a hydraulic post tensioning and stressing jack, placing the
slab under
  5a) Post tensioning and stressing jacks

Post-tensioned slabs, put the slab in tension after the concrete has cured. This can help to minimize cracking and differential settlement in adverse conditions.
Tendons are sheathed with a tough plastic sleeve that prevents them from bonding to the concrete. A coating of a grease lubricant between the plastic sleeve and the cable inside allows the cable to stretch freely during the tensioning process.
 6a)  Post Tension cables shown, also known as strands or tendons. Cables are lubricated between plastic sleeve and strands
Post Tension Slab Blowout
 7a)  Apperent tensioned cable blow out       From the picture (7a) this appears to be a blowout in a post tension slab. The anchor shown should be embedded in the slab. Looks like there's concrete on the anchor here (in the pic) so the cable/ tendon was probably stretched as needed then had a blow out. Appears that cable and anchor have been exposed for a while due to the rusting. The size of the hole in the slab looks about the size of the anchor, another indication of a blowout. The effects of the blowout inside could have been just a loud bang with no damage or it could have tore up thru the concrete and flooring as it snapped. If so it was probably patched on the slab with an epoxy and  the flooring would have been redone. This could have been engineer approved or maybe not. The builder should have addressed the issue and resolved the problem with the original licensed/registered engineer that was paid to design and certify the slab in the first place.
But I also don't see why if they did resolve inside issues, if there was (damage) issues then why they didn't address the outside situation, the exposed, rusting outside slab reinforcement cable, wedge and anchor along with a good sized hole in the slab, it's a smoking gun indicating that there has been a structural issue with the foundation, even though a slab can still be sound with the blowout of just one tendon it should still be repaired on the outside as far as cutting and sealing the affected area.
Also, as seen in the picture there is not the required 4" slab exposure below the stucco, which of course is even more of an issue since there is a hole in the slab. A minimum of a four inch gap is required at the base of the wall between masonry and finished grade for proper drainage and to avoid water intrusion into the exterior wall assembly.

Post tensioned slabs have as noted before gained in popularity over the years especially with the volume builders, mostly due to cost factors, but remain more of a risk then the more expensive rebar slab foundations that still seem to be preferred by local custom builders. The main risk is due to the possible problems of improper stressing of the post tension cables. These concerns can be over come by good engineering and the contractors proper execution of engineers specifications. 

by Joseph W.  Keresztury, CPI - JWK Inspections, San Antonio Home Inspector

Pictures by JWK Inspections, Power Team Products, Home Inspector Pro/ David Nasser

Joe Keresztury of JWK Inspections is available for Home Inspections Monday thru Saturday for San Antonio, Schertz, Garden Ridge, New Braunfels, Boerne and all South Texas surrounding areas. Commercial Inspections, Consulting and Construction Management services  available.
Build your own Home services also available.







Very interesting Joe.

Very interesting Joe. Construction practices in my area are certainly different from those in San Antonio. I very seldom see a house on a slab-on-grade foundation and I have never seen one on a post tensioned slab. Large commercial buildings and parking garages have post tensioned slabs and beams, but I don't do commercial inspections.
Not everything in San Antonio is different from here in Kitchener Waterloo. In your photo gallery I saw that some of the defects are exactly what I would find here. Some honest mistakes but also several defects that show an I-don't-care attitude.
Keep the public informed with good post like this one. Thank you.
Bert de Haan.

Joe KereszturyJWK

Joe Keresztury
JWK Inspections
San Antonio, Texas
(210) 559-3236 Thanx Bert, I'm sure there are many differences in construction and home inspections in Kitchener Waterloo, Canada as compared to San Antonio. I'm assuming you have basements in your area. That might be a good idea for a blog on your site. Here the post tensioned slabs are gaining in popularity as compared to the rebar slab on grade.
For any one who wants to visit Berts site and view some of his interesting home inspection blogs, here is his info:
Bert de Haan
Benchmark Home Inspection Services

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