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Concrete Creep

CloudHidden | Posted in General Discussion on December 5, 2006 04:38am

Anyone know anything about “concrete creep”? Something to do with sustained loads on the concrete as it cures, I think. The term came up in conversation, and even though I read some explanations, I still don’t have a great feel for the phenomenon and its impact.

Anyone have a 6th grade level explanation?

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Replies

  1. User avater
    PaulBinCT | Dec 05, 2006 04:49pm | #1

    I thought Concrete Creep was Spiderman's newest nemesis...

  2. User avater
    txlandlord | Dec 05, 2006 07:04pm | #2

    I know a Concrete Creep, he does not show up to work and when he does he does not have enough guys to do the work,  and half the ones he has are druck or recvoering from a hangover.

  3. alexpesta | Dec 05, 2006 09:10pm | #3

    I am not exactly sure, but I would render this guess. Since concrete is a liquid (like glass is a liquid), it is constantly moving as it cures, which may or may not ever completely happen. I would imagine the material would 'creep' under a constant load; as if it were squishing....very, very slowly.

     

    1. User avater
      artacoma | Dec 06, 2006 03:39am | #14

      Can you explain how glass and concrete are liquids? Every description I find calls them solid materials. Someone told me that once before but they were not able to prove it. They both are mainly sand and or aggregates as far as I can tell.Rik

      1. Omah | Dec 06, 2006 04:06am | #16

        When concrete cures it goes through a process of hydration ie it takes on a water molecule. i guess it does'nt go through a process of dehydration,ie. losing a water molecule. It's a very, very slow moving liquid. very slow. If youv'e seen really old glass on an old farmhouse or similar, you will notice that its thicker on the bottom than the top, and sometimes you can see a gap at the top of the glass where it used to meet the sash.

        1. User avater
          artacoma | Dec 06, 2006 04:34am | #17

          http://www.cmog.org/index.asp?pageId=745

          1. Omah | Dec 06, 2006 05:54am | #21

            I stand corrected and educated. Thanks

        2. JohnSprung | Dec 06, 2006 11:10pm | #26

          > If youv'e seen really old glass on an old farmhouse or similar, you will notice that its thicker on the bottom than the top, and sometimes you can see a gap at the top of the glass where it used to meet the sash. 

          That glass "flows" and gets thicker on the bottom is a myth.  It was checked by a professor back east someplace, IIRC, and he found that the thickness difference is randomly distributed both ways.  So, old glass was made that way and stayed that way.

          As for not meeting the sash on top, shrinkage, putty failure, and gravity would account for that.  

           

          -- J.S.

           

          1. caseyr | Dec 07, 2006 04:50am | #32

            "There is no clear answer to the question "Is glass solid or liquid?". In terms of molecular dynamics and thermodynamics it is possible to justify various different views that it is a highly viscous liquid, an amorphous solid, or simply that glass is another state of matter which is neither liquid nor solid. The difference is semantic. In terms of its material properties we can do little better. There is no clear definition of the distinction between solids and highly viscous liquids. All such phases or states of matter are idealisations of real material properties. Nevertheless, from a more common sense point of view, glass should be considered a solid since it is rigid according to every day experience. The use of the term "supercooled liquid" to describe glass still persists, but is considered by many to be an unfortunate misnomer that should be avoided. In any case, claims that glass panes in old windows have deformed due to glass flow have never been substantiated. Examples of Roman glassware and calculations based on measurements of glass visco-properties indicate that these claims cannot be true. The observed features are more easily explained as a result of the imperfect methods used to make glass window panes before the float glass process was invented."
            http://math.ucr.edu/home/baez/physics/General/Glass/glass.htmlJohn - IIRC a specialist in the subject indicated that it was in fact possible for glass to deform under natural conditions to give the wavyness of old glass, but he calculated that it would take longer than the age of the Universe for it to happen...

          2. dovetail97128 | Dec 07, 2006 05:05am | #33

            ... "age of the Universe..." well that explains why my glasses keep sliding down. ;-)

      2. woodturner9 | Dec 06, 2006 05:18pm | #24

        Can you explain how glass and concrete are liquids?

        Nope - because they are not liquids.  That's "urban legend".

        However, most solid materials will plasticly deform.  Consider a brick wall that has bowed in the middle.  If you quickly push it back into place, the mortar joints will crack.  If you SLOWLY push it back, nothing cracks.  So the practice is to set up a screw jack or similar device to push it back, turn it 1/4 turn a week, and put the wall back.

        Concrete, like most hard materials, has a plastic region, where it will plastically deform.  The engineering textbook example is a concrete beam supported on two vertical beams (posts) with a constant load applied continuously.  Over a period of years, the beam will plastically deform, causing the middle of the beam to "droop", but it does not affect the load capacity of the beam. That deflection is the concrete creep.

        Put another way, Seaquist* says "The material characteristics whereby an otherwise brittle material will act like a ductile or plastic material during long-term loading si called "creep"".

        *Diagnosing and repairing house structure problems, Edgar O. Seaquist, McGraw-Hill, 1980 0-07-056013-7

         

         

         

        Edited 12/6/2006 9:24 am ET by woodturner9

  4. JohnSprung | Dec 05, 2006 09:25pm | #4

    Do a Google search for Concrete Creep.  There are over 800,000 hits, the first few are academic papers.  One of them talks about measuring deflection in a test beam with a dial indicator down to 0.001 mm.   

     

     

    -- J.S.

     

    1. User avater
      CloudHidden | Dec 05, 2006 09:32pm | #5

      The ones I saw left me without a clear understanding of it, or a clear grasp of the practical implications. The first I heard of it was in a negative context, but some of what I read didn't come across the same way.Looking for an accurate, but simple, explanation of the phenomenon from someone with a solid grasp of the issue.

      1. BillBrennen | Dec 05, 2006 09:41pm | #6

        Brownbagg will know.

      2. JohnSprung | Dec 05, 2006 09:44pm | #7

        Yeah, those academic papers are pretty deep.  Here's the closest thing in Wikipedia:

        http://en.wikipedia.org/wiki/Creep_%28deformation%29

        If you get a better, more complete understanding of it, perhaps submit that to Wikipedia?   

         

        -- J.S.

         

      3. User avater
        SamT | Dec 06, 2006 03:01am | #13

        The practical implications for your work are none.

        However, if you design any free span concrete beams and dead load them near the tensile design strength, ie about 30% and up, they will deflect without significantly losing strength. IOW, they'll bend if you overload them. If you really overload them, they'll break.

        The experiment that comes up on Google measured creep, that is, permanent deflection, in 0.001 milimeters per meter of length. That would be a floor stiffness rating of n/10,000. Obviously, it would  be a concern if you are designing concrete frameworks for very large telescope mirrors.

        Compressive creep was about half that of tensile creep, so if you took a 1sqft block of 3000psi 'crete and put 130,000 pounds of load on it it will creep half as much as a similar free span beam top loaded to 30% of it's design tensile failure load.

        So don't go stacking 2 Abrahm tanks on 1 square foot of concrete. SamT

        Now if I could just remember that I am a businessman with a hammer and not a craftsman with a business.....segundo <!----><!----> 

        1. DanH | Dec 06, 2006 03:42am | #15

          There's a slightly different form of creep that's significant to the highway folks -- as traffic rolls over a concrete highway it slowly gets longer. This is probably technically due to microcracks rather than fluid creep, but whatever causes it, it becomes a significant problem when you have miles of straight road.
          People never lie so much as before an election, during a war, or after a hunt. --Otto von Bismarck

        2. segundo | Dec 07, 2006 12:30am | #27

          i am flattered that you are quoting me, but the truth of the matter is that i did not come up with that idea, that phrase was used by someone else before me.

          i am feeling guilty for not giving credit to the person i stole it from, clearly sam you are a better person than me.

           

          1. User avater
            SamT | Dec 07, 2006 06:10am | #34

            you are a better person than me.

            More likely, just more afraid of getting beat up on.SamT

            Now if I could just remember that I am a businessman with a hammer and not a craftsman with a business.....segundo <!----><!----> 

  5. User avater
    Gene_Davis | Dec 05, 2006 10:23pm | #8

    Put simply, creep in concrete is permanent deflection of flexural members, such as slabs and beams.

    I don't believe it would come into play to any noticeable degree in formed shotcrete domed structures.

    Think parking garages and structures like that.

    Search "creep" on YouTube and you get lots of hits, some of them videos of an old rock song.  Who is the original band?  The song has an unmistakable intro, and has been heard in numerous movie soundtracks.  I heard it recently in a Vietnamese-language movie called "Cyclo."

    1. User avater
      CloudHidden | Dec 05, 2006 11:03pm | #9

      >Put simply, creep in concrete is permanent deflection of flexural members, such as slabs and beams.From what cause?>I don't believe it would come into play to any noticeable degree in formed shotcrete domed structures.The context is that supposedly Dante Bini has observed creep in his poured/inflated domes--I heard this from someone who spoke directly with him--while we have not in the shotcrete ones. I'm looking for a better understanding of that.

      1. dovetail97128 | Dec 06, 2006 12:46am | #12

        Genes explanation is what I got from a structural engineer and a very , very knowledgeable concrete sub on a job awhile back. Why it wouldn't affect shotcreet I wouldn't know, never worked around that material. Perhaps different mix quantities? Water content may be a key perhaps curing time?. Intriguing question.
        Interestingly enough concrete also "curls" or at least that is the term I heard to explain why a slab that was poured lazer flat will turn up at the perimeter over time. I was told (same people explaining it to me ) is because of the difference in the tensions created at the top and bottom as the slab cures. This is a phenomena I have witnessed .

      2. User avater
        Gene_Davis | Dec 06, 2006 05:26am | #19

        I should have said "permanent and continuing deflection."  The deflection can increase over a period of time measured in months.

        Reinforced concrete is not fully elastic, as is steel, for example.

        I have a reinforced concrete floor structure in my garage, which is over a heated shop space below.  From full cure until about 20 months later, my floor exhibited creep, until reaching its "permanent" deflection.

    2. User avater
      zak | Dec 06, 2006 05:51am | #20

      I think Radiohead had a song named "Creep"zak

      "When we build, let us think that we build forever.  Let it not be for present delight nor for present use alone." --John Ruskin

      "so it goes"

       

    3. woodguy99 | Dec 07, 2006 02:16am | #28

      Search "creep" on YouTube and you get lots of hits, some of them videos of an old rock song.  Who is the original band?  The song has an unmistakable intro, and has been heard in numerous movie soundtracks.  I heard it recently in a Vietnamese-language movie called "Cyclo."

      Stone Temple Pilots

      1. User avater
        Gene_Davis | Dec 07, 2006 02:49am | #29

        Thanks.

        I don't belong now.

  6. kate | Dec 05, 2006 11:12pm | #10

    TXlandlord must know the same concrete creep I do...

  7. Omah | Dec 05, 2006 11:55pm | #11

    Everything is in motiom all the time  although at different rates due to different conditions or forces being applied or withdrawn. concrete can flex to a certain extent depending on as many factors  such as reinforcement, mix, temp, stress, ect. ect. ect. But  my Oh my I've think I've seen about everything till I've seen an elephant fly, I mean a concrete creep. sorry couldn'nt help myself.

  8. User avater
    CloudHidden | Dec 06, 2006 05:22am | #18

    Let's see if I understand.

    Two methods each use relatively thin (2.5" - 5") thickness of concrete covering fairly large areas.

    Method 1 exposes the concrete to the elements--sun, wind, etc--fairly fast.

    Method 2 never exposes the concrete to the sun and wind, and allows the curing to take place in a sealed environ that has high humidity.

    Method 1 has experienced some creep, while Method 2 has not. Does it stand to reason that the large surface area of method one, curing rapidly from both sides of the shell (since it's fully exposed), leads to excessive, or premature, or uneven curing, leading to creep? Whereas the other method promotes a slow, even cure?

    Am I getting warm? cold?

    Thanks.

  9. STAINLESS | Dec 06, 2006 07:04am | #22

    Felicitations CH.

    What I know about creep in concrete has been gleaned from a few old engineering books I found some place.

    From "Reinforced Concrete Fundamentals" Third edition  Phil M. Ferguson; it seems that creep is the term given to an increasing deformation in concrete over time, even though the load applied remains constant. ie.The concrete continues to change dimension as time goes by even though no more forces are being added to make that happen.

    It can be compressive creep, where the concrete gets reduced in dimension or tensile creep where the concrete increases in dimension as it gets pulled apart. These deformations due to creep are permanent and don't recover if the loads applied are removed ie. once creep has done its thing the concrete never goes back to its original size even if the forces are removed.

    The final deflection of concrete is usually from 2-1/2 to 3 times the initial deflection and occurs over a lengthy period. Ultimate creep is usually reached in about 1500 days (just over 4 yrs.) ie. a deflection calculated from just the strength of concrete and the loads on it will likely undershoot the final deflection by as much as 3 times and can take up to 4 yrs to happen.

    Things that tend to increase creep are things such as loading the concrete while it is still "green", cement with a high water-cement ratio,and exposing the concrete to drying conditions. Concrete completely wet or completely dry creeps only a little and old concrete has a decreased creep compared with new concrete.

    Creep deformation is not always harmful, as it can release stresses. It is not an indication of a bad batch unless low creep concrete was required.

    Other forces are at work at the same time as creep is doing its thing, from what I've read, concrete is like wood to some extent; it increases in volume (swells) when it gets wet and decreases in volume (shrinks) when it dries out. It also changes size due to temperature, gets larger when hotter & smaller when colder, just like some other things I know of!.

    Hope this helps.

    STAINLESS

     

    1. User avater
      CloudHidden | Dec 06, 2006 07:44am | #23

      Now THAT'S a thorough explanation in language that makes it easy to assimilate. Well done. Thanks. I get it.

  10. woodturner9 | Dec 06, 2006 05:23pm | #25

    still low cracks. you control creep by controlling the temp and make no sudden changes. let it stabilize slowly.

    You may be using "creep" in a different way than the engineer is.  The curing process has little to do with creep, as engineers define the term.  To an engineer, creep is the long term deformation of a loaded structure due to the plasticity of the material.

    1. brownbagg | Dec 07, 2006 03:27am | #30

      I,m sorry, I will delete all my post and never answer again

      1. rez | Dec 07, 2006 04:29am | #31

        Roar!

         

         damn, am I fat!

      2. woodturner9 | Dec 07, 2006 05:19pm | #35

        I,m sorry, I will delete all my post and never answer again

         

        Why?  Really no reason to get upset about a friendly discussion.

        Terms are often used differently in different trades and professions.  One of the big problems in engineering is terminology - even different fields of engineering sometimes define the same term differently, causing a lot of confusion.

        No reason to be concerned about different usage of the term in your trade.

        Edited 12/7/2006 3:36 pm ET by woodturner9

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