Post-tension concrete may be a mythical method for some contractors.
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However, some projects can greatly benefit from this prestressing method.
Additionally, concrete contractors should implement post-tensioning in a variety of situations.
This article will cover post-tension concrete&#;s what, when, how, and why.
Post-tensioning is a method of prestressing concrete. Prestressing concrete is when concrete has added compression internally. Doing so counteracts the external loads that will be placed on it.
Post-tensioning adds reinforcement and strength to the concrete with tensioning steel rods.
As the name implies, this prestressing happens on-site after the concrete has fully dried.
Now, many people understandably mistake it for pre-tensioning.
Pre-tensioning is when the steel strands are tensioned before placing them into the concrete. This step usually occurs during precast concrete construction.
A French man, Eugene Freyssinet, often receives credit for being the first to use post-tension concrete in for a marine terminal.
It wasn&#;t until that the construction of the Walnut Lane Bridge in Philadelphia relied on post-tensioning.
These days, this method is so popular there&#;s an entire institute dedicated to advancing the industry &#; The Post-Tensioning Institute.
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To understand how post-tensioning works, you need to learn about the behaviors of the two materials involved:
Concrete and steel.
Concrete is strongest when it is under compression. Meanwhile, steel is strongest under tension.
Post-tensioning combines both materials in their strongest states.
The result?
A concrete slab that can resist much higher loads than traditional concrete structures.
Who would have thought reinforcing steel was the best way to create reinforced concrete?
Adding rebar alone can improve the durability of concrete under tensile stress. Yet, post-tensioning improves that while adding strength to the concrete through compression.
To install a post-tensioning system, you need specific tools while following a specific series of steps.
A hydraulic jack is the only piece of working equipment you&#;ll need on your job site to implement post-tensioning.
The size and strength of the concrete members are being prestressed. Post-tensioning involves the elongation of very high-strength steel.
A powerful hydraulic stressing jack will pull on the prestressing steel without causing a malfunction.
There are several materials involved in post-tensioning structural concrete:
Post-tension cables &#; also known as tendons &#; are made from a seven-wire braided steel cable. These tendons are very strong and can yield up to 243,000 psi.
There are two different types of tendons.
A bonded tendon uses grout to permanently bond the tendon to the sheathing.
With an unbonded tendon, grease is used, and the tendon is free to move within the sheathing.
To protect the tendons from corrosion from the water in the poured concrete, it must be placed inside a tube.
These can be made from thin sheet metal pipes, plastic ducts, or tubing. The seams should overlap to prevent any seepage from occurring.
Anchors are vital in applying tensile forces to the tendons while keeping the tensile forces in place.
These devices attach to the tendons and anchor them into concrete on one end while the anchor on the other side attaches to the jack.
There are a few steps to follow to create a post-tensioned concrete slab.
Here&#;s a look at the process:
For example, a ½-inch 270 strand should be stressed to 33,000 pounds, according to PTI.
Lastly, the steel tendons are anchored into place, the ends are trimmed, and grout is placed into the anchor pocket to secure them.
You may wonder if all this extra work is worth the effort. Let&#;s discuss the benefits you can reap using this method on your concrete floors.
Post-tensioning gives architects more design freedom. The method allows for fewer columns and thinner slabs to support the rest of the structure. After all, the concrete slabs are now beast-mode strong.
Post-tensioning also allows architects and engineers to create structures with dynamic designs.
Post-tensioning also saves floor-to-floor height in commercial buildings. This allows for more floors to rent out without changing the building height. Not only does this save money, but it also provides the opportunity to make more money as well.
This reduced floor-to-floor height also translates into cost savings for:
Think about all the savings that could occur when you cut the height of a building. Elevator shafts are shorter, requiring less material. The building facade will also require less material.
Maintenance costs will be much more efficient, too. Heating and cooling a smaller building is cheaper. Insurance will be cheaper.
In fact, there are many often forgotten savings that come with a reduced floor-to-floor height.
Post-tensioning also lowers the chances of cracking due to shrinkage and improves the durability of the concrete. So you&#;ll see far fewer deflections and experience increased service load capability.
There&#;s also another way that post-tension concrete saves money:
It reduces the cost of reinforcement by lowering the amount of rebar needed. Post-tension steel tendons are cheaper than rebar. The reduced concrete and reinforcement weight also reduces the dead load on subsequent levels.
Lastly, you&#;ll see a much quicker construction process. This is because post-tensioning increases the strength of the concrete prematurely. So the formwork removal happens earlier.
This benefit also results in the follow-on trades and project completion happening faster.
Most projects would benefit from the use of post-tensioning. Some would especially benefit. But, of course, there are also some rare occasions when post-tensioning may not be the best course of action.
Let&#;s talk about the pros and cons of this method now.
There are times when post-tensioning can bolster your concrete structure and is a no-brainer, and times when it&#;s a must.
Below is a list of scenarios that should implement post-tensioning in their concrete construction process to ensure a successful project, as mentioned in this publication from PTI.
We know that concrete can withstand an extreme load through direct compression. But it&#;s also very susceptible when undergoing lateral forces.
Wind and seismic forces can be catastrophic for some concrete structures.
Using post-tensioned slabs provides the reinforcement necessary to resist these powerful lateral forces.
The taller a building gets, the greater the need for lighter construction.
Post-tensioning makes taller skyscrapers possible by allowing for less material needed for each level. This reduces the dead load on each of the lower levels.
Conventional reinforced concrete is much heavier. That said, it also limits the building&#;s height before it&#;s too heavy to support its weight.
At times, the vision inside an architect&#;s creative brain won&#;t translate inside an engineer&#;s logical brain.
Curvilinear geometries create stunning structures that were once a severely complicated process. In the past, these required many engineering solutions with high costs and extensive time investment.
The ability to use post-tensioned concrete slabs made this process much more feasible. To this day, it&#;s the most widely accepted method to create these architectural works of art.
Creating a long span inside a concrete building once called for many pillars, columns, and thicker concrete slabs.
Post-tensioned concrete slabs are stronger and lighter in weight. As a result, they can create longer spans without the need for pillars and columns for support.
Post-tensioning distributes the weight of the concrete to help prevent any sagging in elevated slabs.
Post-tensioning will always increase the strength of a concrete slab. Yet, the methodology behind this innovation can make it disadvantageous on rare occasions.
These include:
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Once post-tensioned slabs are in place, they&#;re permanent. There&#;s no cutting or rearranging these slabs after this point.
Therefore, if you&#;re constructing a commercial building with hopes of a redesign later on, prepare for disappointment.
Of course, you can work around this if you plan for this redesign in the original design, leaving room for future knockouts or openings.
Post-tensioning concrete is a complicated and precise process that requires skilled labor.
If you don&#;t have professionals with experience using this method, it&#;s better not to use it at all.
For post-tensioning to work, it must meet very precise specifications. The forces applied and the machinery used can also be very dangerous in the hands of an amateur.
When done properly, post-tensioning is a fool-proof engineering technique. But some mistakes can occur along the way.
To ensure that you get the most out of your post-tensioning efforts, avoid these common mistakes:
In many structures, you&#;ll have restraints to the compression of the concrete. These include walls, columns, and other structures in place.
You&#;ll need to consider these when positioning your post-tension tendons.
After you&#;ve pulled the tendons and anchored them, it&#;ll leave behind some strand tails that require cutting.
However, these ends require a bit more work to finish the process. The end caps must be installed, cleaned from debris, and filled with mortar to seal the tendons.
While load balancing can extend 100% dead load, pushing this too far can result in a defective slab.
Using post-tensioning in place of proper design will cause overbalancing. It&#;s not like rebar, where you can just add more and more. It requires precise amounts.
Finite element software is helpful in the design process with post-tensioning. Yet, a PT professional must check all designs for correct calculations before construction begins.
As this article pointed out, post-tensioning is an engineering innovation that can and is applicable in various projects to improve the integrity of the construction further.
This method can improve the quality of your project. But it can also save you money, time, and maintenance and provide more freedom in your design process.
Here at FMP, we&#;ve successfully implemented post-tensioning in many of our client&#;s projects, and we&#;ve seen first-hand the benefits of that choice.
Curious about this process?
We&#;d love to discuss how post-tensioning can improve your construction project. Contact us today!INTELLIGENT WORK FORUMS
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when to use PT slab-on-grade?
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(Structural)
(OP)
11 Feb 08 20:01How exactly do you determine when it is best to use a post-tensioned slab for residential construction? Is it simply when the soils report indicates a high shrink/swell potential?
(Structural)
11 Feb 08 20:21I have never used a post-tensioned slab on grade for small residential construction. Post-tensioned slabs on grade are usually used in industrial structures with the primary objective of eliminating most of the joints.
There would be nothing wrong with using post-tensioned slabs on grade in houses, but the cost would be greater.
Why would post-tensioning help with shrink/swell problems? Increased stiffness of the footing system and articulation of the structure above are the main approaches in limiting damage due to moisture sensitive soils.
(Structural)
(OP)
11 Feb 08 20:25I have heard it being done a lot in houses in SW Louisiana where they have highly expansive soil.
(Structural)
11 Feb 08 20:33I would be interested to hear more about the LA houses. Do you know if they are stiffened raft slabs? A post-tensioned flat plate may work, but I would think it would have to be at least 8 to 10 inches thick to provide the required stiffness.
(Structural)
11 Feb 08 21:52Hokie66,
You are showing your Australian heritage. Haven't you seen all of the posts on different sites on the large cracks in residential PT slabs in the USA. They are designed using a PTI method that would not be accepted here.
(Structural)
11 Feb 08 22:27Rapt,
No, I haven't been keeping up with that type information because I don't get involved in houses. But I can imagine. By the way, my heritage is not Australian, I just live here. My wife is the Aussie. I started in the US, and I don't agree with a lot of the things they do either.
(Structural)
(OP)
11 Feb 08 22:52rapt,
You say that there have been reports of large cracks for PT slabs in the USA?? Can you elaborate on what the cause is?
(Structural)
12 Feb 08 21:52Hokie66,
Some of your replies have indicated Australian practice so I assumed you were one. Lucky for you where your wife comes from!!
abusementpark,
I am not into house SOG design either so have not investigated the design method myself but have been told by others that it is lacking.
Personally, I do not think this type of slab is a logical PT option. PT gives good crack control until the concrete cracks, then, especially unbonded PT, gives no crack control at all if the strain is still applied, as would happen with large soil movements. I would be using RC stiffened raft slabs with a lot of ground preparation if it was my house (and I am a PT specialist).
From what I have been told, the P/A levels being used in these slabs is very low and there is not much reinforcement so they tend to crack at fairly low loading and stay cracked and the cracks are big. If the subgrade preparation is not much good, as would often happen with this type of structure, or if the movement is large, which is what you are expecting, then you have no chance of crack control, whereas, with an RC solution, the crack widths will be controlled as long as there is enough reinforcement so that the steel does not yield.
(Structural)
12 Feb 08 23:54rapt,
Yes, have lived and worked in Australia for last 25 years, so that is why I approach things from the Aussie perspective.
abusementpark,
My concerns with the PTI slab approach mirror those of rapt. I had a look at their site. As I understand it, they are recommending ribbed slabs, with a 4" slab cast integrally with the thickenings. They lightly tension the slabs, and NEARLY ALL OF THIS TENSION MUST BE RESISTED BY THE EDGE THICKENINGS AND PASSIVE SOIL PRESSURE. So there is hardly any prestress in the 4" slab, the tendons which do exist are not bonded, there is little or no deformed bar reinforcement, and when a shrinkage crack forms, the slab is free to slide toward the edge, or at least toward the next rib. One discussion paper I found on the site actually discussed how this happens, but they still recommend the method. Go figure.
Apparently 50% of these type house slabs are in Texas, 25% in California, and the rest scattered across the South, including Louisiana. So they have convinced the cowboys.
(Structural)
13 Feb 08 09:13PT is not my thing, but one thing I have learnt is that it is very expensive to do small PT jobs. The set-up cost is too high and can be more than the actual work cost.
This alone would make it a poor choice for domestic houses.
hokie/Rapt,
I am the other way around (kind of), Australian,english wife, and living in the US.
(Structural)
13 Feb 08 17:27csd72,
You are forgiven. I thought from some of your posts that you have that sort of broad based experience.
Where posttensioning is done routinely, the set up cost would be reduced. One source I read said that about 40% of the posttensioning tendons in the US go into house slabs. Hard to believe, but obviously the sales people have prevailed.
(Structural)
14 Feb 08 12:0625% of the Post Tensioned slab on grade houses built in US are in California, (Texas is second). Very cost effective and desirable for lack of visible cracks.
(Structural)
14 Feb 08 20:06civilperson,
Do you have personal experience with these slabs?
(Structural)
15 Feb 08 13:00My experience was only observation. The subdivision that I worked on in Orange County California had lots and I recall that all used PT slab on grade. Approximately 4' spacing of the tendons in a rib under a 5" slab. The tensioning occured five to eight days after the placement of slab and took a three man crew about two hours per lot.
(Geotechnical)
15 Feb 08 13:50One reason we use post tensioned slabs on grade here in California is because the code requires that a special foundation be designed if the EI is over 20. The CBC requires that the foundation slab be designed with either the Welded Wire Reinforcing Istitute (WRI) specification procedure for expansive material or the Post Tensioning Istitue (PTI) procedure. See section .3.2, and .8 of the CBC, and Most large home builders have learned over the years that it is best to just specify the post tensioned slab and be done with it. The lawsuits otherwise cost to much. The cost per unit is not that much higher than a conventional design. What civilperson describes is fairly typical. We even have clients who want pt slabs even if the soils are not expansive, it just saves them from any other problems down the road.
(Structural)
15 Feb 08 16:07Well, good luck. I still don't see how effective crack control can be provided with the system as I understand it.
(Structural)
17 Feb 08 21:52Muuddfun,
Can we assume from your comments above that because CBC requires it and it is a PTI method then the designer and builder are not responsible for problems as long as it is designed and built in accordance with the PTI procedures.
Civilperson
If it is stressed 5 to eight days after pouring, it will be cracked before they stress it and the cracks will be wide unless there is a lot of normal reinforcement, which I understand is not the case, or it is very low shrinkage concrete (in a house slab!!!) so there goes the crack control!
I am with you on this Hokie66!!
(Geotechnical)
18 Feb 08 01:00rapt
No matter what rules you followed there will still be lawsuits if there are problems. The lawyers don't care what you did as long as they have someone with some money to blame. My understanding is that the use of pt slabs has caught on because they are reducing the amount of cracking in the slabs and are performing better than the alternative methods, and thereby reducing the amounts of lawsuits. If it was not reducing the amount and severity of cracking then the lawsuits would have continued and the builders would not be as accepting of the method as they are. My experience is with supplying the geotechnical input parameters for the structurals to do the design, so I am not completely familiar with exactly how the detailing of the tendons and ridge beams are being caried out. From what my boss has told me,he has done some forensic work on situations where pt slabs were used, he has seen them in a situation where the soils were slid out from under the side of a house and the foundation was ok with the house cantileved out 12' over the edge do to the slide. So in some situations it was done well. I guess it really comes down to how they are being designed and built in the field, if shody work is done by either the engineer or the contractors then there will be problems just like any other type of construction
(Structural)
18 Feb 08 05:56To summarize, we have three US engineers in this thread who report that a lot of these houses in the US are being built with post-tensioned slabs, but do not personally know much about how they are designed or built. And we have two engineers in Australia who are sceptical based on our understanding of how the slabs are built. It would certainly be interesting to hear from someone who knows the details. With that many houses, surely there is someone out there---, but maybe it is a touchy subject.
(Structural)
18 Feb 08 20:25I did the post tension design for tract homes in California, Nevada and Arizona. I use the PTI software to design and all the building departments from three states have accepted the design. Basically, you can design 5" slab with rib beams spacing about 12'-15' apart or you can design 10" mat slab without footing. We need the soil parameters from the Geotechnical Engineer for the design inputs. The software can do both expansive and compressible soils. The cost is a little bit more than the conventional foundation but the feed back from the builders are great.Less craking, less customer service.
(Geotechnical)
18 Feb 08 20:51So lets see how each of you would design differently for the following conditions, and maybe well get a little closer to what the real differences are in practice.
These parameters are from a recent job for medium expansive materials.
Soil Information
Liquid Limit (LL) 43
Plastic Limit (PL) 18
Plastic Index (PI) 25
Percent Fine Clay 40
Clay Type Montmorillonite
Expansion Index 69 (MEDIUM)
Summary of Design Parameters
Approximate Depth of Constant Suction:
Center Lift 7 Feet
Edge Lift 7 Feet
Approximate Soil Suction, pF: 3.6
Approximate Moisture Velocity: 0.7 inches/month
Thornthwaite Index:
Center Lift -20
Edge Lift -10
Average Edge Moisture Variation Distance, em:
Center Lift 5.5 Feet
Edge Lift 2.9 Feet
Anticipated Swell, ym:
Center Lift 2.8 Inches
Edge Lift 0.7 Inches
Say for a slab of 30'by 90', what stress would you use on the tendons, and what spacing and layout of ridge beams and tendons would you use? And a bearing capacity of psf.
(Structural)
18 Feb 08 21:14I have not run the calcuation yet. From the numbers you listed, my estimate would be 5" ribbed slab with 12" wide x 18" deep (below the finished grade) exterior footing and 12" wide x15" deep interior footing spacing 12'-0"+/- both ways. 1/2" dia. Cables should be 4'-0" on center on the 30' direction and 3'-0" on center on the 90' direction.
(Structural)
18 Feb 08 22:15For a post-tensioned solution, I would avoid the ribbed option and choose the 10" constant thickness slab, placed on a plastic sheet over a sand bed. This would tend to deal with the restraint issues. If a deep beam is required at the edge, it should be separate from the slab and not tied or bonded together.
I would want about 1 MPa stress, so 2-12.7mm (1/2") strands in tendons at (say 3'-6") centres in each direction. So about twice the PT of HN's solution. And all the tendons would be bonded by fully grouting the ducts.
The solution proposed by HN for a ribbed slab sounds about right, but I would not use this with post-tensioned reinforcement. I would use all deformed bars.
Most of the slabs designed with the PTI method will probably result in no complaints, but I would not risk it.
(Structural)
19 Feb 08 11:39I agreed with hokie. Yes, you can do the 10" mat slab with 2" thickened edge to cover 2" of sand between the slab and the visqueen. This practice is done in Northern California and Reno areas. Southern Cal does the 5" ribbed mostly.
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