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priest lake rmk
Well-known member
So why add rebar if its going to cause cracks. No Rebar = no cracks. lmao.
Concrete guys?
- Thread starter Superjag
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So why add rebar if its going to cause cracks. No Rebar = no cracks. lmao.
OMG! You guys are killing me. I hope neither of you are in the concrete business. If you are, well, you should already know better. 
Rebar placed in flat work is there to stop vertical shifting and or a crack from widening. In theory concrete basicly has to crack before the rebar comes into play. Concrete doesn't know or care if it has rebar in it or not. If it's going to crack for whatever reason, it's gonna crack and that's when the rebar starts to come into play
Rebar can promote cracking because the rust or corrosion around the rebar swells causing the concrete around it to crack.
By the way I said CAN promote cracks. I didn't say rebar makes it crack.
I'm curious, in what magical way do you think rebar keeps concrete from cracking?
Rebar placed in flat work is there to stop vertical shifting and or a crack from widening. In theory concrete basicly has to crack before the rebar comes into play. Concrete doesn't know or care if it has rebar in it or not. If it's going to crack for whatever reason, it's gonna crack and that's when the rebar starts to come into play
Rebar can promote cracking because the rust or corrosion around the rebar swells causing the concrete around it to crack.
By the way I said CAN promote cracks. I didn't say rebar makes it crack.
I'm curious, in what magical way do you think rebar keeps concrete from cracking?
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OMG! You guys are killing me. I hope neither of you are in the concrete business. If you are, well, you should already know better.
I'm curious, in what magical way do you think rebar keeps concrete from cracking?
Disclaimer: I do not pour concrete, or place rebar. That being said...
"Assessment of structural behavior of corrosion affected structures is an important issue; which would help in making certain decisions pertaining to the inspection, repair, strengthening, replacement and demolition of such structures. Depending on the level of oxidation of metallic iron, corrosion products may have much greater volume than the original iron which gets consumed by the process of corrosion. This volume expansion is mainly responsible for the through thickness cracking of the cover concrete; which would indicate the loss of service life for the corrosion affected structures. The paper presents an analytical model to predict the time required for cover cracking and the weight loss of reinforcing bars in corrosion affected reinforced concrete structures. The proposed model also incorporates the modeling aspects of the residual strength of cracked concrete and the stiffness contribution from the combination of reinforcement and expansive corrosion products. An attempt has also been made to arrive at reasonable estimates of the various parameters in the model related to the composition and properties of expansive corrosion products based on the available published experimental data. It has been found that the model is quite capable of providing the estimates of predicted time to cover cracking and weight loss of reinforcing bars that are in reasonably good agreement with the experimentally observed values as well as the analytical predictions of other researchers. It has also been found that both predicted time to cover cracking and weight loss of reinforcing bars are significantly influenced by tensile strength of cover concrete, annual mean corrosion rate and modulus of elasticity of reinforcement plus corrosion products combined."
So you're both right, rebar (reinforcing bar) adds to the tensile strength of concrete preventing cracks. The oxidation of metallic iron (rebar) causes corrosion (rust) with leads to an increase in size of the reinforcing metal. This increases the tensile stress on the concrete and you get cracks.
Solution: Anti corrosive coatings for the rebar.
Ta-da!
Source: Engineering structures ISSN 0141-0296 CODEN ENSTDF
2006, vol. 28, no8, pp. 1093-1109 [17 page(s) (article)] (30 ref.)
He spends more time writing my name in it....
A
ak ryda
Well-known member
OMG! You guys are killing me. I hope neither of you are in the concrete business. If you are, well, you should already know better.
Rebar placed in flat work is there to stop vertical shifting and or a crack from widening. In theory concrete basicly has to crack before the rebar comes into play. Concrete doesn't know or care if it has rebar in it or not. If it's going to crack for whatever reason, it's gonna crack and that's when the rebar starts to come into play
Rebar can promote cracking because the rust or corrosion around the rebar swells causing the concrete around it to crack.
By the way I said CAN promote cracks. I didn't say rebar makes it crack.
I'm curious, in what magical way do you think rebar keeps concrete from cracking?
I am not necessarily in the concrete business, I am an engineer in the transportation constuction business where we do alot of concrete work for bridges, retaining walls, and structural foundations.
I will be the first one to admit that I didnt pay much attention in my concrete or mechanics of materials classes while in engineering school, but I do like to think I know the purpose and intent of rebarr.
I think where we are getting crossed up is you may be talking about "flat work" like you stated earlier and I am talking reinforced structural concrete members. In flat work like garage slabs, patios, driveways, side walk, etc then like I said before, you dont need rebarr and we totally agree.
You are in the concrete business, you know concrete fails primarily under tensile loading. Concrete primarily fails by cracking. Re-steel provides additional tensile strength which therefore helps keep concrete from cracking. I hope this is a good enough explanation of the "magical way" rebar keeps concrete from cracking.
I do agree that corroded rebar can cause cracking, especially in bridge decks (which is why we use alot of epoxy coated rebar in our bridge decks). But given how EVERY design for structural concrete has rebar reinforcement and the vast majority of that re-steel is has no coating, it is safe to say that the increased strength the rebar provides outweighs the chance of the rebar getting corroded and causing cracks. Also, "That is the purpose of rebar, too hold it all together after it cracks, it has nothing to do with concretes ability to NOT crack." simply is not correct. Again, we may be getting crossed up if you are talking about flat work, but I would still argue that nominal rebar does indeed prevent shrinkage cracking.
Here are a couple photos from one of the piers off the last bridge I did, I promise you all the rebar had more of a purpose than to just hold it together after it cracks. Been fun talking with you but I am feeling dirty for being such a nerd this morning so I am done unless we continue this over beers, first round on me, cheers.
This conversation was regarding rebar in concrete and if said rebar could, would, or should eliminate concrete from cracking. Let me say it slower this time. Rebar has little to nothing to do with concretes ability to crack or not!
People get soooo hung up on the idea that rebar will keep concrete from cracking. Being an Engineer I can't believe you are even arguing this fact. Now let me say again. We have been talking about concrete cracking with or without rebar. Nothing else.
Let's use your piers as an example. Why is there so much rebar in those piers? Does all that rebar mean the concrete can't or won't crack? NO! Do all those vertical pieces of rebar, most of wich are only wire tied or tacked together, have much vertical strength on their own? NO! Would just a straight, no rebar, pier still have alot of strength? YES! When that hypothetical no rebar pier showed cracks, cause you know it will, would you get concerned? H*ll Yes! When the pier full of rebar shows cracks, again cause you know it will, are you going to worry? Not really. And why is that? Because all that rebar in that pier creates a cohesive unit. It has little to nothing to do with the concrete in that piers ability to crack. It simple helps bind it all together and continue to hold it all together when it does crack.
I'm in no way saying you shouldn't use rebar but don't fool yourself into thinking rebar has anything to do with concrete and cracking.
People get soooo hung up on the idea that rebar will keep concrete from cracking. Being an Engineer I can't believe you are even arguing this fact. Now let me say again. We have been talking about concrete cracking with or without rebar. Nothing else.
Let's use your piers as an example. Why is there so much rebar in those piers? Does all that rebar mean the concrete can't or won't crack? NO! Do all those vertical pieces of rebar, most of wich are only wire tied or tacked together, have much vertical strength on their own? NO! Would just a straight, no rebar, pier still have alot of strength? YES! When that hypothetical no rebar pier showed cracks, cause you know it will, would you get concerned? H*ll Yes! When the pier full of rebar shows cracks, again cause you know it will, are you going to worry? Not really. And why is that? Because all that rebar in that pier creates a cohesive unit. It has little to nothing to do with the concrete in that piers ability to crack. It simple helps bind it all together and continue to hold it all together when it does crack.
I'm in no way saying you shouldn't use rebar but don't fool yourself into thinking rebar has anything to do with concrete and cracking.
U
Uller
Member
Sambo11,
Me thinks you need to take another look at a concrete design manual. The type of book that discusses mathmatical formulas used to determine the quantity of rebar and thickness of concrete needed to NOT have shrinkage cracking develop in a concrete slab.
OR
You could take a walk in a Costco and look down at the floors. Sir, those slabs have been designed to NOT crack by using rebar, mixture design, and control joints.
Rebar exhibits surface tension, which counteracts the shrinkage properties, which are a trait of the material concrete.
Properly designed, placed, and cured concrete will only crack in predetermined locations primarily through the use of rebar. That is also the reason for score joints in sidewalks. To allow for a perfectly placed crack every 4 feet.
Me thinks you need to take another look at a concrete design manual. The type of book that discusses mathmatical formulas used to determine the quantity of rebar and thickness of concrete needed to NOT have shrinkage cracking develop in a concrete slab.
OR
You could take a walk in a Costco and look down at the floors. Sir, those slabs have been designed to NOT crack by using rebar, mixture design, and control joints.
Rebar exhibits surface tension, which counteracts the shrinkage properties, which are a trait of the material concrete.
Properly designed, placed, and cured concrete will only crack in predetermined locations primarily through the use of rebar. That is also the reason for score joints in sidewalks. To allow for a perfectly placed crack every 4 feet.
Unless this is a recent change, Costco does not use rebar in their floors just like freeway paving does not use rebar.
Again, rebar's primary use is to aid in cohesion and control the crack. Again, using your example/s of shrinkage cracking. Shrinkage cracking is caused by rapid hydration. Rebar assists in minimizing shrinkage cracking by controlling the crack, you guessed it, after it has already happened. You can use all the rebar in the world but if you have dry hot windy conditions with alot of direct sunlight your going to get shrinkage cracks. As I would hope that you already know a certain amount of cracking, and i'm not talking the kind you will stub your toe on or you can put you finger in but small spider web cracks, is a good thing becasue it releases stored energy and that release helps eliminate big cracks.
Cracks in concrete come in a variety of shapes sizes and most certainly causes. Don't get hung up an thinking there is only one kind of crack pertaining to concrete.
I've probably read the same books you have. They give a great overview of theory but theory and real world are often two entirely different books.
This has been an entertaining debate. Tone and expression are difficult to gage on a computer. I hope I haven't offended anyone because that was never my intent.
Again, rebar's primary use is to aid in cohesion and control the crack. Again, using your example/s of shrinkage cracking. Shrinkage cracking is caused by rapid hydration. Rebar assists in minimizing shrinkage cracking by controlling the crack, you guessed it, after it has already happened. You can use all the rebar in the world but if you have dry hot windy conditions with alot of direct sunlight your going to get shrinkage cracks. As I would hope that you already know a certain amount of cracking, and i'm not talking the kind you will stub your toe on or you can put you finger in but small spider web cracks, is a good thing becasue it releases stored energy and that release helps eliminate big cracks.
Cracks in concrete come in a variety of shapes sizes and most certainly causes. Don't get hung up an thinking there is only one kind of crack pertaining to concrete.
I've probably read the same books you have. They give a great overview of theory but theory and real world are often two entirely different books.
This has been an entertaining debate. Tone and expression are difficult to gage on a computer. I hope I haven't offended anyone because that was never my intent.
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