Transcript for:
Understanding Pre-tensioned vs. Post-tensioned Concrete

Do you know the difference between pre-tensioned and post-tensioned concrete? It's okay if you don't. I get that question all the time and that's what this video is all about today. My name is Tyler Lay and I'm so excited to be your guide through concrete land. Did you know that concrete is 10 times stronger in compression than tension?

Well guess what? The Romans did and that's why they built concrete arches because they were mainly in compression. And they helped avoid cracking. Ah, everybody hates cracks, right?

I know I do. But could we do this with our modern concrete? Well, sure we could.

We could make concrete arches, but we usually don't do that. Why? I've already covered that. Check this video if you haven't seen it yet. There's another solution.

We could use something called prestressed concrete and get some of the powers of the concrete arch in our modern structures. In pre-stretched concrete, there are two types, pre-tensioned and post-tensioned. We'll be covering both of those today. And I've already talked about some of these concepts in a video called Fighting Cracks, where I talk specifically about active reinforcement.

How do pre-tensioned and post-tensioned structures work? It's pretty simple. You stretch steel, you release the steel, and it transfers the load to the concrete, putting it in compression.

Pre-tensioned is pretty cool stuff. If you see big, long girders that are used over and over and over again, like this double T for a garage or an I shaped used in a beam, then you are seeing pretension concrete. This is where it thrives. This is where it's awesome. Using the same shapes over and over and over again.

Here's how you make a pretension beam. The type of reinforcement we're going to use is not. rebar, it's seven wire strand, much higher strength material.

You lay your material out. You actually anchor one side of it. You put a load on the other side, about thirty thousand pounds per strand.

You may have 70 or 80 strands. That's high amounts of strands, but you may have that much in a given girder. Then you cast concrete around it.

You wait for the concrete to get strength. Then you release the loads and it tries to shorten. Now there is something called losses that happens.

That's shortening of the girder, shortening of the strand, and something called the transfer length. So the load you put on the strand won't necessarily be the load that is inside of the concrete. And the difference is called the losses.

If we make these structures, we usually do them on very, very, very long lines. sometimes up to 300 feet in length so we can make a whole lot of girders at the same time there are a lot of benefits they are extremely economical if we're using repeat members over and over again with known loads there is less concern about protecting the reinforcement in these systems there's less cracking and improved durability but there are some challenges you're going to need large stressing equipment oftentimes these are done in specialized massive factories And you can only apply the pre-stressing once. And the cracking can sometimes happen at the point of release, right where that strand goes into the girder. There's sometimes some problems there at the end.

Let's talk about our friends post-tensioning now. Some examples are slabs or segmental box girder structures, which are made up of a complicated set of post-tension tendons and bars. You can also use post tensioning on the outside of girders to increase strength, but that is not done very often because we have to protect that steel heavily.

How you make these systems is you cast concrete with something called a duct in it or a hollow region. That duct can be filled with this again high strength pre stressing strand or a large number of strands. Then we have one end that is the dead end or the anchor.

Sometimes the single strands, they look like this. With multiple strands, they may look something like this. And there's a lot of load being transferred at that one spot. The other end is the live end or the free end. And this is what it looks like when you're looking at using single strands.

And this is what it looks like when using gang strands or a lot of strands in one spot. We're going to pull on that with a certain amount of load. And then we're going to put something to hold it. Something called a wedge. Here's an example of a wedge, two pieces of metal that fit around the strand and hold it in place.

That load transfer has losses as well but it's not near as high as pre-tension structure. The load's about 0.8, maybe even 0.9 of what the load that you actually placed on it. And there are two types of post-tensioning, unbonded and bonded. In our unbonded friends, the ducts, they contain oil or grease, and the anchors are covered up with grout.

If you have to, you can get back into those structures and you can retighten those strands. Although I don't think it's done that often. It is a possibility.

And they look like this after they've been patched on the outside. And most house slabs in where you have expanded soils, they are typically post-tension. The bonded, though, are the ducts that are filled with grout after stressing. These anchors are then covered and then with a grout. And this is what it looks like where the grout goes into that hole.

And that's supposed to fill up the entirety of the line. And sometimes we have some issues with that. Here is what a grouted duct looks like.

I'm showing the grout there in white and the strand there. This has been cut and if we're going to cut it down its length and it's going to look something like this. These systems sometimes have regions of poor grouting and sometimes areas of empty space.

Those regions are inviting water to get in and causing corrosion products to form. There are lots of benefits with these systems. There's typically reduced cracking, and sometimes there's only one way to build a structure, and it's post-tension concrete.

These ducts can take almost any shape, and that's extremely powerful in structural design. And the stressing can be done in the field, where it can be adjusted to what is needed for the structure. There is also a challenge with that stressing down in the field, because sometimes it can be kind of cumbersome and a little bit risky.

you need special equipment to do this with. It can be complicated to design. Protecting that strand is extremely challenging and important because if you lose it, you're going to lose a lot of capacity in your structure. And cracking at that anchorage region is a big deal. In summary, both pre-tension and post-tension concrete can help reduce cracking and enable us to build amazing and long-lasting structures.

If you want to learn more, you can see a certain application just for slabs about post tensioning with this video. Make sure you check it out. I hope you liked this video.

If you did, make sure you give me a thumbs up. Think about subscribing to my channel and leave me a comment below about another mystery of concrete you want me to unfurl. Hey, check me out on Instagram at concrete.tyler and of course, bye.