In this article, I’ll be giving homeowners some necessary information about the characteristics, advantages, disadvantages, and common questions/concerns that I receive regarding post-tensioned foundations.
Most of my experience with post-tensioned foundations stem from performing pre-pour foundation inspections in the Greater Houston Area, as this is the most common type of foundation production builders utilize.
Post-Tensioned Foundation Characteristics
The Post-tensioning Institute designed Post-tensioned foundations. The first edition of the Post-Tensioning Manual was published in 1972, and as of the time of this writing (2018), they are up to their 6th edition.
A post-tensioned foundation is a concrete slab foundation that is reinforced with stranded steel cables that are tensioned after the concrete hardens. The stranded cables are slid inside of plastic-sheathing, which prevents the stranded steel tendons from touching the concrete.
When the foundation/concrete has sufficiently hardened/cured, the plastic-sheathing is stretched/elongated. Stretching or elongating the post-tensioned cables applies significant force to the concrete system, lifting the slab into a compressed state, which reduces the negative effects from tensile forces (cracking).
The most common slab configuration I encounter has the following characteristics
- 24-30 inch deep perimeter footer/beams with interior beams of the same depth (they look like trenches). Each of the beams has an upper an lower tendon (commonly referred to as a draped tendon).
- 12-14 inch wide beams around the perimeter and interior of the slab
- The sheathed tendons are generally supported on plastic chairs spaced at 4 feet on center
- The interior pads are typically constructed to be 4-5 inches thick
Advantages of a Post-tensioned foundation
One of the reasons builders choose post-tensioned slabs stems from the relatively poor soil conditions found in the gulf coast region.
The problem soils often referred to as “expansive soils” are susceptible to periodic shrinking and expanding based on the moisture content around the property, which is why home inspectors and structural engineers recommend improving lot drainage and adding guttering systems.
There are many additional advantages post-tensioned foundations provide aside from their structural performance as well.
- Relatively simple installation (compared to a conventional rebar foundation) and easier for the concrete crew to pour the concrete due to the spacing and arrangement of the tendons, minimizing displacement from foot traffic
- Lighter overall structure compared to that of all steel reinforcing bar foundation and can be more economical
- Easier to inspect and repair
Post-tensioned Foundation Disadvantages
There are also a few disadvantages associated with a post-tensioned foundation. The most common complaint/concern that I hear is in regards to its propensity to develop long unsightly curing cracks, which will be forever visible in the garage and rear patios.
The cables are generally not tensioned until at least 7-10 days after the concrete is poured. Until the cables are stretched/elongated, they cannot provide any crack control, which is the primary reason for the development of the cracks. I’m told that the cables can be partially stretched to help off-set the cracking in the foundation, but I’ve never seen that happen.
Frequently Asked Questions & Product Recommendations
Q. I have long cracks developing a few days after my foundation was poured? Are these cracks structurally significant? What can I do about this?
A. There is always a chance that the cracks are structurally significant, but in my experience, this is rare. As mentioned in the article above, a common disadvantage of a post-tensioned foundation is the common occurrence of cracking. In many cases, the cracks will span the full length of the foundation. As far as what to do? Generally, there is no need for concern. But, if the cracks are located across the garage or rear patio, you may wish to treat the condition cosmetically.
If you’d like to treat the cracking in the garage/rear patio and you want a high-end touch; reach out to a local company and have them apply a professional grade epoxy floor system or concrete stain. Hiring a pro can save you the headaches us do-it-yourself types seem to be a glutton for, but it does come at a cost.
However, another great option, that can add a lot of resale value to your home is applying a garage floor epoxy yourself. There are a few great products out there, but I’ve used this product specifically, and I’m very happy with the results. I’ve also had other clients use a similar product, which solved the initial cracking concern and added the benefits provided by the garage floor kit.
If you choose to go this route, I do recommend that you clean, and then re-clean your garage & rear patio floors. These guys made an excellent video some years back; you should check that out as well.
Q. I have steel tendons exposed on the perimeter of my slab, is this a problem? What can I do to repair the issue?
A. Yes, this is an issue that needs to be corrected. Fortunately, it is treatable. If the stressing end pockets are not filled with a high-strength, non-shrink grout, it will continue to rust. Rusted steel is prone to expansion (thermal stresses), which can cause the surrounding concrete to spall (break). Over time, this can cause a fair amount of damage to the foundation and the cable.
Well, there you have it; I hope I was able to answer some of your questions regarding your foundation. I’ll try and keep this article updated with better photos and information as I acquire it. Thank you for your time.
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