First off, one could argue that post-tensioning is superior to conventionally reinforced concrete in a pure engineering sense. The reason is because post-tensioning is a method of pre-compression, which means the concrete is put into a state of compression, which allows the concrete to take on flexure without cracking. To visualize this, think of a stack of square blocks lined up together.
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If you just grabbed the &#;A&#; and &#;D&#; and lifted up, only those two blocks will be lifted. However, if you place your palms on the outside of &#; G&#; and &#;J&#; then press in, lifting up will bring the whole group of blocks together. One could press hard enough and even be able to carry some load on top of the blocks.
This concept is called &#;pre-compression&#;, or also called &#;pre-stressing&#;. The &#;pre&#; simply means stresses are present in the concrete prior to any load that has to be carried has been applied. The &#;post&#; comes from the fact that the stressing happen after the concrete is placed and has reached the required strength. Pre-stressing is the important distinction between post tension concrete and conventionally reinforced concrete.
You see, conventionally reinforced concrete simply means that there are no active stresses going on within the concrete until after the load that it is designed to resist is applied. Because of this, conventionally reinforced concrete must enter a &#;cracked&#; state before the load carrying capacity is utilized.
DRIVING FACTORS
There are 3 driving factors concerning the usage of post tension reinforcement:
Notice engineering is not necessarily a driving factor!
In other words, since post -tensioning is simply a method of reinforcing concrete, the decision is only a matter of familiarity by the engineer designing it. Many engineers are not experienced in pre-stressed design and simply avoid it. Also, contractors not familiar with the installation requirements may simply avoid it as well.
Availability is also an important factor simply because if it is not readily available in a certain area, it may not be the best choice system. Material supply may be costlier due to transportation and travel costs. Again, engineers and contractors who are not available in a certain market or geographical area to service a post tension project may prohibit its use.
And finally, perception (or reception). In some applications, the prospect of having active compression forces existing within the concrete worries owners of buildings that may require relatively frequent modifications such as retail floor slabs. The fact that every time a tenant improvement requires saw-cutting the concrete slab it will also require an engineer and contractor with expertise in avoiding or purposefully cutting post tension cables usually deters the use. Then there are perceptions by the public that post tension is a cheap alternative to rebar and utilized at the cost of quality. However, as noted above, this is simply not true and in fact the opposite is true. As an engineering technique, it is superior.
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COST: The perception by the public that post tension is a cheap alternative to rebar is 100% true! Everywhere in the US where post tension is familiar, available, and used without negative perception/reception, it results is a lower cost to construct the foundations.
Without these 3 factors in place, post-tensioning usage as a reinforcement system may encounter resistance. Now you know why!
THE MYTHS
The following are some commonly held beliefs by the public:
TECHNICAL INFO
The above discussion is concerning slab-on-grade foundation construction. This type of foundation is classified as a &#;shallow&#; foundation system and subject to the movements of the near surface soils. Both post tension and conventionally reinforced systems have code accepted design methodologies associated with them that structural engineers use to determine the correct design for the applicable soil movement conditions. However, there are other foundation systems that reduce the risk of near surface soil movements. These systems involve &#;deep&#; foundation elements such as piers that rely on deep soils for stability. These deep elements add substantially to the cost of the foundation. So, when debating foundation &#;quality&#; one must be talking apples-to-apples in terms of system.
Another advantage of post-tension foundation design is construction efficiency. The use of post-tensioning can expedite the construction process as it requires fewer concrete materials and simplifies the installation of reinforcing tendons. This can lead to faster project completion times and potential cost savings.
Post-tension foundation design does come with its own set of challenges that need to be considered by the contractor and end user.
- Cost: Post-tension foundations typically require specialized design, materials, and installation techniques, which can lead to higher upfront costs compared to traditional foundations. However, the long-term benefits may outweigh the initial investment.
- Expertise Required: Designing and constructing post-tension foundations require specialized engineering expertise and experienced contractors. Hiring professionals with knowledge in post-tensioning is crucial to ensure proper installation and performance.
- Maintenance Challenges: While post-tension foundations are generally durable, they may require periodic inspections to monitor the integrity of the tendons and address any potential issues. Maintenance and repairs, if needed, can be more complex and costly compared to conventional slab foundations.
- Limited DIY Options: Post-tensioning is a specialized technique that is typically not suitable for do-it-yourself (DIY) projects. Professional expertise and equipment are necessary to ensure the proper installation and tensioning of the tendons.
- Risk of Failure: Although rare, if post-tensioned tendons are not properly installed or maintained, there is a risk of failure. This could lead to structural issues, including cracks or even collapses. However, with proper design, installation, and ongoing maintenance, these risks can be minimized.
An additional limitation of post-tension slab design is that it can&#;t always be used. If there is not enough space for the equipment or a grade beam does not have a defined exterior face to place the dead and live sides of the wire for the tensioning operating, those instances will require conventional rebar in a slab-on-grade design.
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