What is Prestressed Concrete?

Speaker 1: Despite its excellent qualities as a structural material, concrete has weaknesses to one that we’ve discussed in previous videos is that it has almost no strength against tension. Concrete can withstand a tremendous amount of compressive stress, but when you try to pull it apart, it gives up easily. Concrete’s other weakness is that it’s brittle. It doesn’t have any give or stretch or ductility. Combine these two weaknesses and you get cracks. Concrete loves to crack. And if you’re designing or building something made of concrete, understanding how much and where it’s going to crack can be the difference between the success and failure of your structure to understand how engineers design reinforced concrete structures. First, we have to understand design criteria or the goals of the structure.

The obvious goal that we all understand is that it shouldn’t fall down when a car drives over a bridge and the bridge doesn’t collapse. The structure is achieving its design criterion of ultimate strength. But in many cases in structural engineering, avoiding collapse actually isn’t the limiting design criteria. The other important goal is to avoid deflection or movement under load. Most structural members deflect quite a bit before they actually fail, and this can be bad news. The first reason why is perception. People don’t feel safe on a structure that flexes and bends. We want our bridges and buildings to feel sturdy and immovable. The other reason is that things attached to the structure, like plaster or glass, might break if it deflects too much. In the case of concrete, deflection has another impact cracks. The reinforcement within concrete is usually made from steel and steel is much more elastic than concrete. So in order to mobilize the strength of the steel first, it has to stretch out a little bit. But unlike steel, concrete is brittle. It doesn’t stretch, it cracks. So that often means that concrete has to crack before the rebar can take up any of the tensile strength. So the member. This demonstration is from a test in a previous video showing a conventionally reinforced concrete beam go back and check out that video if you haven’t seen it yet. Notice how the beam is resisting the load on it, even though it’s cracked at the bottom. It’s meeting design criteria.

Number one, it’s holding the load, in this case six tons without failing. But it’s not meeting design criteria. Number two, serviceability. It’s deflecting too much and the concrete is cracked. Those cracks not only look bad, but in an actual structure. They could allow water and contaminants into contact with the reinforcement, eventually causing it to corrode, weaken and even fail. One solution to this problem of deflection and concrete members is priest stressing or putting compressive stress into the structural member before it gets put into service. This is normally accomplished by tensioning the reinforcement within the concrete. This gives a member a compressive stress that will balance the tensile stress imposed in the member once it’s put in the service. A conventionally reinforced concrete member doesn’t have any compression to start with, so it will deflect too much well before it’s in any danger of not being strong enough to hold the load. So with conventional reinforcement, you don’t even get to take full advantage of the structural strength of the member. When you stress the reinforcement within concrete, you don’t necessarily increase the strength, but you do reduce its deflection. This balances out the maximum load allowed under each of the structural design criteria, allowing you to take full advantage of the strength of each material. There are two main ways to pre stress reinforcement within concrete. And of course I built a demo to show how this works. The first one is pre tensioning. And yes, that’s a little bit confusing. It’s pretty stressed because the steel is stressed before the members put into service.

But pretention, because the steel is stressed before the concrete cure’s. To make this work, I had to build a little frame to go around my concrete beam. This frame will hold the steel intention while the concrete curves. I installed threaded rods through the mold and frame and then tension these rods by tightening the nuts. I tried to use the pits of the ringing to get them at around the same tension. And you can see how much my frame is flexing from the force in these steel rods. The other method for pre stressing steel is posted in post environment. The steel is stressed after the concrete cure’s, but still before the member is put into service. In this beam I cast in smooth plastic sleeves in the mold, the steel rod can slide easily within the sleeves. Once both molds were prepared, I filled them up with concrete and I finally got a construction grade concrete vibrator as well. This machine helps get all the air bubbles out of fresh concrete before it cures a process called consolidation. After the concrete’s had some time to cure, it’s time to test the beams out on the pretention beam. I can unscrew the nuts and take off this frame because the concrete hardened around the bolts. The steel rods are still under tension inside the beam. I put it under the hydraulic press for testing and the results are easy to see in a conventionally reinforced beam where the steel is simply cast into the concrete without any tension, cracks start forming at around four tons in the pretention beam. The cracks didn’t appear until double that force at around eight tons. The tension already in the steel is able to take up the force of the press without requiring the beam to flex. For the post tension beam, I inserted the steel reinforcement after the concrete had cured, then I tightened the bolts on the rods to Cristeros the steel. Under the hydraulic press, the results are nearly identical. The tension and the steel held the beam in compression for much longer than a conventionally reinforced member could. Of course, the cracks eventually appear, but it takes much more force before they do. That’s because adding force to the beam isn’t necessarily creating tension at the bottom, but simply reducing the compression that’s already been introduced through the tension in the steel rods. It’s important to point out that we didn’t make these beams any stronger. Both the steel and concrete have the same strength they would without stress, but we did increase the serviceability of the member by reducing the amount of deflection under load.

Of course, none of these examples actually failed because of the reinforcement. And that wasn’t the point of this demo. But it’s still fun to test everything to failure. Precise concrete is used in all kinds of structures, from bridges to buildings to silos and tanks. It’s a great way to minimize cracking and take full advantage of the incredible strength of reinforced concrete. Thank you for watching and let me know what you think. Thanks to Dash Lane for sponsoring this video. I’ve been the victim of at least 10 major online data breaches in my life, including Facebook, LinkedIn and Equifax. Obviously, all those passwords have been changed. But if I was reusing the same password for all my online accounts, any data breach could give people access to my bank accounts, health records and everything in between Daschle and simplifies online security and password management by seamlessly integrating into your browser and singing between any kind of device automatically. And it’s more than just a password manager.

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