Prestressed Concrete
Talk to any concrete expert and they’lltell you the first rule of concrete is this: it’s highly a great deal assured to crack. But now not all cracking is viewed equal,and there is a way to strengthen concrete to reduce its terrible impacts. Today we’re speaking about prestressed concrete. This tutorial is subsidized through Dashlane, by no means neglect a password again. More on that later. Despite its first-rate traits as a structural material, concrete has some weaknesses, too. One that we’ve mentioned in preceding videosis that it has nearly no energy towards tension. Concrete can face up to a extraordinary amountof compressive stress, however when you try to pull it apart, it offers up easily. Concrete’s different weak point is that it’sbrittle. It doesn’t have any “give” or stretchor ductility. Combine these two weaknesses, and you getcracks. Concrete loves to crack. And if you’re designing or constructing somethingmade of concrete, appreciation how lots and the place it’s going to crack can be the differencebetween the success and failure of your structure. To recognize how engineer’s format reinforcedconcrete structures, we first have to apprehend diagram standards - or the desires of the structure. The apparent intention that we all recognize isthat it shouldn’t fall down. When a auto drives over a bridge and the bridgedoesn’t collapse, the shape is accomplishing its format criterion of last strength.
But, in many instances in structural engineering,avoiding fall down surely isn’t the limiting graph criteria. The different necessary aim is to keep away from deflection,or motion below load. Most structural participants deflect pretty a bitbefore they virtually fail, and this can be horrific news. The first motive why is perception. People don’t experience protected on a shape thatflexes and bends. We prefer our bridges and structures to feelsturdy and immovable. The different cause is that matters connected tothe shape like plaster or glass would possibly wreck if it deflects too much. In the case of bolstered concrete, deflectionhas any other impact: cracks. The reinforcement inside concrete is usuallymade from steel, and metal is a whole lot greater elastic than concrete. So, in order to mobilize the power of thesteel, first it has to stretch a little. But, not like steel, concrete is brittle - it’sdoesn’t stretch, it cracks. So that frequently skill that concrete has to crackbefore the rebar can take up any of the tensile stress of the member. This demonstration is from a check in a previousvideo displaying a conventionally strengthened concrete beam. Go lower back and take a look at that video out if you haven'tseen it yet.
Notice how this beam is resisting the load on it, even although it is cracked at the bottom. It’s assembly format criterion wide variety 1 - it’sholding the load (in this case 6 tons) barring failing. But it’s no longer assembly format criterion number 2 (serviceability) - it’s deflecting too a good deal and the concrete is cracked. Those cracks no longer solely seem bad, however in anactual structure, they may want to permit water and contaminants into contact with the reinforcement,eventually inflicting it to corrode, weaken, and even fail. One answer to this trouble of deflectionin concrete individuals is pre-stressing, or placing compressive stress into the structural memberbefore it’s put into service. This is generally achieved with the aid of tensioningthe reinforcement inside the concrete. This offers the member a compressive stressthat will balance the tensile stresses imposed in the member as soon as it is put into service. A conventionally bolstered concrete memberdoesn’t have any compression to begin with, so it will deflect too plenty nicely earlier than it’sin any threat of now not being sturdy ample to maintain the load. So with traditional reinforcement, you don’teven get to take full gain of the structural power of the member. When you prestress the reinforcement withinconcrete, you don’t always make bigger its strength, however you do decrease its deflection. This balances out the most load allowedunder every of the structural diagram criteria, permitting you to take fuller gain of thestrength of every material.
The first approach is pre-tensioning. And sure that terminology is a little confusing. It’s pre-stressed due to the fact the metal is stressedbefore the member is put into service, however pre-tensioned due to the fact the metal is stressedbefore the concrete cures. To make this work, I had to construct a littleframe to go round my concrete beam. This body will keep the metal in tensionwhile the concrete cures. I hooked up threaded rods via the moldand frame, and then tensioned these rods through tightening the nuts. I tried to use the pitch of the ringing toget them at round the identical tension, and you can see how a whole lot my body is flexing fromthe pressure in these metal rods. The different technique for pre-stressing metal ispost-tensioning. In post-tensioning, the metal is stressedafter the concrete cures, however nonetheless earlier than the member is put into service. In this beam I solid in easy plastic sleevesin the mold. The metal rods can slide without problems inside thesleeves. Once each molds have been prepared, I crammed themup with concrete.
And I ultimately acquired a development grade concretevibrator as well. This computer helps get all the air bubblesout of clean concrete earlier than it cures, a method known as consolidation. After the concrete’s has had some time tocure, it’s time to check the beams out. On the pretensioned beam, I can unscrew thenuts and take off this frame. Because the concrete hardened round the bolts,the metal rods are nonetheless beneath anxiety inner this beam. I put it underneath the hydraulic press for testing,and the effects are effortless to see. In a conventionally bolstered beam wherethe metal is actually forged into the concrete besides any tension, cracks begin formingat round four tons. In the pretensioned beam, the cracks didn’tappear till double that force at round 8 tons.
The anxiety already in the metal is capable totake up the pressure of the press besides requiring the beam to flex. Then I tightened the bolts on the rods topre-stress the steel. Under the hydraulic press, the consequences arenearly identical. The anxiety in the metal held beam in compressionfor tons longer than a conventionally bolstered member could. Of course, the cracks subsequently appear, butit takes plenty greater pressure earlier than they do. That’s because, including pressure to the beamis no longer growing tension, however simply lowering the compression that’s already been introducedthrough the anxiety in the metal rods. It’s vital to factor out that we didn’tnecessarily make these beams stronger. Both the metal and concrete have the samestrength as they would except prestressing the steel. But, we did amplify the serviceability ofmember via decreasing the quantity of deflection beneath load. Of course, none of these examples actuallyfailed due to the fact of the reinforcement, and that wasn’t the factor of the demo.
But, it’s nevertheless greater exciting to take a look at everythingto failure. Pre-stressed concrete is used in all kindsof constructions from bridges to constructions to silos and tanks. It’s a extraordinary way to decrease cracking andtake fuller benefit of the outstanding energy of bolstered concrete. Thank you for watching, and let me comprehend whatyou think! Thanks to Dashlane for sponsoring this tutorial. I’ve been the sufferer of at least ten majoronline statistics breaches in my life, consisting of Facebook, LinkedIn, and Equifax. Obviously all these passwords have been changed,but if I was once reusing the identical password for all my on-line accounts, any statistics breach couldgive humans get entry to to my financial institution accounts, fitness records, and the entirety in between.
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