OT - Reinforcing concrete with CFRP - effective or ripoff?
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    Default OT - Reinforcing concrete with CFRP - effective or ripoff?

    Just posting this to see if there are any engineering opinions on the merits (or lack thereof?) of epoxying carbon fiber reinforced polymer sheeting to the underside of concrete structural floors in order to increase their load-bearing capacity.



    Does this stuff even actually do anything in the context of an old 15-16" thick pour with tons of plumbing, surface-mounted electrical, sprinkler, conveyor, etc. systems getting in the way? Is it just a feel-good upsell that customers with old buildings sometimes get suckered into? Or does it actually afford a reasonable amount of extra PSF for the incredible labor cost of ripping everything off the cieling, gluing the stuff up and re-installing everything previously ripped down? All without shutting down or disrupting the facility?

    Thoughts?

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    I'm sure it does add some capacity and safety factor, IF it's installed right. That means a truly clean and mechanically sound prepped concrete surface, proper epoxy mixing and distribution, and correct lamination technique. Easy to go suboptimal on any of those unless you have a good contractor doing the work.

    I've also seen such stuff for improving blast resistance in areas where terrorism is a risk, as well as seismic activity, especially wrapped support columns.

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    but the question is - how will it stand up to the test of time? and/if/when it fails - how are you going to repair it for the building to remain up to its design standards?

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    I am a structural engineer. I have very limited experience with FRP reinforcement. In commercial applications the repair or strengthening is designed by an engineer and the installation is monitored by special inspectors. In situations where it is appropriate it can be very effective and should typically hold up very well.

    In residential installations engineers and inspectors are typically not involved and the workers are often not well trained. I'm not a fan of using it under those conditions.

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    In tests in a lab it supposedly performed well, but as mentioned that’s under perfect conditions, the epoxy process controlled, the application clean on a test slab, not in a grubby car park, but it does make scabby concrete look pretty, just like the cladding on certain buildings, looks all new and good, the cladding burned really well, wonder how this stuff will do?
    Mark

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    Quote Originally Posted by boslab View Post
    In tests in a lab it supposedly performed well, but as mentioned that’s under perfect conditions, the epoxy process controlled, the application clean on a test slab, not in a grubby car park, but it does make scabby concrete look pretty, just like the cladding on certain buildings, looks all new and good, the cladding burned really well, wonder how this stuff will do?
    Mark
    Grenfell Tower? That was a tragedy. Or was it criminal?

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    I’d be inclined to say that at the very least the council were negligent, the stuff burned quite well, how if that is the case could it be given a fire or spread of flame test pass, I saw the testing, it was dripping down in flaming blobs, doesn’t seem to be safe to me, once again no one is responsible, and the inquiry will drag out till after the guilty parties have retired on ill health, seems to be a pattern imho
    I’m concerned that the reinforcing may do the same myself, helped by the glue
    Mark

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    Quote Originally Posted by boslab View Post
    I’m concerned that the reinforcing may do the same myself, helped by the glue
    Mark
    If they're using an epoxy, it's usually not terribly flammable after setting. Also, the predominantly "shielded" and horizontal layout means fresh oxygen doesn't get to the material as readily as the external, vertical use of exterior cladding.

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    Quote Originally Posted by Just a Sparky View Post
    Just posting this to see if there are any engineering opinions on the merits (or lack thereof?) of epoxying carbon fiber reinforced polymer sheeting to the underside of concrete structural floors in order to increase their load-bearing capacity.



    Does this stuff even actually do anything in the context of an old 15-16" thick pour with tons of plumbing, surface-mounted electrical, sprinkler, conveyor, etc. systems getting in the way? Is it just a feel-good upsell that customers with old buildings sometimes get suckered into? Or does it actually afford a reasonable amount of extra PSF for the incredible labor cost of ripping everything off the cieling, gluing the stuff up and re-installing everything previously ripped down? All without shutting down or disrupting the facility?

    Thoughts?
    CAVEAT: "military" Engineer, not RPE. So "proverbial 'grain of salt", but:

    I'd call it wiser to:

    A) Respect the 'surviving" load-bearing value, even if degraded, useti within sane limits until the economics justify full demo and from-cold-start / naked / bare ground, renewal.

    B) ELSE, incrementally shut-down sections of the facility and do 'natrally' severable sections OVER "proper" with ignorant steel-reinforced concrete.

    Iron (alloy) re-bar and common aggregates IN concrete have a right decent, economical, and VERY long PROVEN compatibility, predictability, known degradation modes, hence reliable expectations as to life span.

    Glass, plastics, carbon fibers, and polymers in general the exact OPPOSITE.
    Low or no inherent "compatibility", poor bonding and load-sharing integration accordingly.

    Why mess with Mother concrete's natural core chemistry?

    It only makes the Banksters richer up-front to PAY for it, then feeds too damned many Lawsters, going forward when it goes even PARTLY "wrong".

    Can you recover the cost "in between" off the back of value for money?

    Not with any sort of "high level of confidence", you can not.

    TANSTAAFL.

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    My concern is that this stuff has been tested in a laboratory in which it has been applied to a concrete member whilst NOT under load, THEN subjected to stresses.

    In a real installation like this, the concrete has already deflected due to load. Add CFRP in this state and it will take up zero load because it will never be under tension unless MORE load is added - ostensibly more than the concrete can handle on it's own before the CFRP actually begins to take up a useful percentage of it.

    Or am I off the mark?

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    If the reinforcement is applied to gain more load capacity, it's probably of marginal value. But if it's for increasing safety margins when in a seismic or "war" zone, it's likely of some benefit.

    Here's a pretty bogus video of the benefits (bogus because of how simple the first beam is, and the loading of the second isn't nearly what they claim):

    https://www.youtube.com/watch?v=KDT63ymOQvY


    This is a much more thoughtful vid, with some interesting information about increasing the benefits of a CF overlay on existing structures (in this case, bridge beams)

    https://www.youtube.com/watch?v=Ommd8_JOEV0

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    if pre-stretched it makes perfect sence, imo.

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    Quote Originally Posted by dian View Post
    if pre-stretched it makes perfect sence, imo.
    That's going to require the applicator to either lift the underside of the floor up temporarily, or have special fixturing to apply tension to the CF and lift/press it in place against the underside of the floor until the epoxy sets.

    It would also be very difficult to do in situations like they show in the second video I linked to.

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    Quote Originally Posted by dian View Post
    if pre-stretched it makes perfect sence, imo.
    "Pre-Stressed" is the term used in the concrete industry.

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    Quote Originally Posted by Just a Sparky View Post
    My concern is that this stuff has been tested in a laboratory in which it has been applied to a concrete member whilst NOT under load, THEN subjected to stresses.

    In a real installation like this, the concrete has already deflected due to load. Add CFRP in this state and it will take up zero load because it will never be under tension unless MORE load is added - ostensibly more than the concrete can handle on it's own before the CFRP actually begins to take up a useful percentage of it.

    Or am I off the mark?
    S**t.

    Welll.... you weren't REALLY gonna buy that bridge in Brooklyn, nor the summer-cottage beach house on Alaska's North Slope, either, were you?


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    Quote Originally Posted by Milland View Post
    If the reinforcement is applied to gain more load capacity, it's probably of marginal value. But if it's for increasing safety margins when in a seismic or "war" zone, it's likely of some benefit.

    Here's a pretty bogus video of the benefits (bogus because of how simple the first beam is, and the loading of the second isn't nearly what they claim):

    https://www.youtube.com/watch?v=KDT63ymOQvY


    This is a much more thoughtful vid, with some interesting information about increasing the benefits of a CF overlay on existing structures (in this case, bridge beams)

    https://www.youtube.com/watch?v=Ommd8_JOEV0
    that 1st vid is a complete joke, not worth talking about it

    about the 2nd one, I can't understand what they expected to achieve with the vertical bands, the loads on those beams don't go together with the vertical strips at all, and the horizontal ones seem to be too short to show any reasonable effect, and they would be far more effective glued to the underside, it looks like they simply wanted to waste some money experimenting with this

    then there is the issue of application, methods they showed are the worst when it comes to laminates, they just don't deliver the performance the materials could provide (and what one usually expects when using epoxy resins and carbon fiber materials), there will be excess resin acting like a rubber compared to the CF strands allowing them to move around when loaded thus reducing the performance of the laminate, I also remember something about strands compressed together (autoclave) "rubbing" on each other thus providing a lot of stiffness to the end product, which is impossible with the wet layup method

    now if they made new structures with these materials, I'd say - great, but the efforts shown there seem like bandaids on a gangrene infested limb, the only possible benefit I see from what they did is when that beam would start cracking and falling apart, smaller pieces would be prevented from falling on to the traffic below...

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    Quote Originally Posted by jz79 View Post
    that 1st vid is a complete joke, not worth talking about it

    about the 2nd one, I can't understand what they expected to achieve with the vertical bands, the loads on those beams don't go together with the vertical strips at all, and the horizontal ones seem to be too short to show any reasonable effect, and they would be far more effective glued to the underside, it looks like they simply wanted to waste some money experimenting with this

    then there is the issue of application, methods they showed are the worst when it comes to laminates, they just don't deliver the performance the materials could provide (and what one usually expects when using epoxy resins and carbon fiber materials), there will be excess resin acting like a rubber compared to the CF strands allowing them to move around when loaded thus reducing the performance of the laminate, I also remember something about strands compressed together (autoclave) "rubbing" on each other thus providing a lot of stiffness to the end product, which is impossible with the wet layup method

    now if they made new structures with these materials, I'd say - great, but the efforts shown there seem like bandaids on a gangrene infested limb, the only possible benefit I see from what they did is when that beam would start cracking and falling apart, smaller pieces would be prevented from falling on to the traffic below...
    Yahbut.. for those who can SELL the concept?

    "It's a living".

    Much akin to the fellow who was VERY well-paid in a traveling circus side-show, former East Bloc.

    .. wherein he would offer-up two slices of fresh Russian Black bread, a "volunteer" would pony up his price, then crap a fresh turd onto the bread... the performer would eat it... Grin. Lick his chops.

    And bank the money.

    "It was a living".

    Until one day he got a slice of Russian bread.. with a HAIR in it!

    Promptly vomited. Quit the bizness.

    Took up an even shiddier line of work. But less risky as to unidentified defiling objects.

    As a Job Shop Machinist.

    A man has to have "standards", y'see. As with lousy concrete or"mystery metal", the fresh turd was "witnessed". A known input. All part of the job.

    But NO idea where that damned HAIR had been, from what critter, nor how it had gotten loose ... and into the bread.

    So goeth "plastics" and THEIR universe!

    Some days? A person could be better-off with a shit sandwich, Russian Black bread or otherwise, coochie hair optional.....


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    Actually I do see the value of the technique as demonstrated in the first video. And those two concrete beams acted exactly as I would have expected. Concrete is at it's best when under compressive loading. When the single person tried to step on the plain concrete beam, half of the beam was being compressed (the top half) and the other half (the bottom half) was placed in tension. Concrete sucks with a tension type load - BIG TIME. So it immediately broke. No surprise there.

    In the second beam, with the underside reinforced with the strip that was epoxied to it, ALL of the concrete was under a compressive load. No problems there. And all the tension was taken up by the added strip. Obviously the added strip was at it's best with tension. So the two materials worked together, each demonstrating it's best properties.

    This is much like putting re-bar or steel mesh in concrete. The concrete above the steel provides the compressive resistance and the steel takes care of the tension. The concrete below the steel is mostly there to protect the steel from the elements so it does not rust; at least not quickly. This is standard, concrete beam construction.

    Pre-stressed concrete beams take this a bit further by applying a bias to place all of the concrete (above and below the steel) under compression before it is installed. The loads on the beam in actual use increase that compressive load above the steel and just decrease it below the steel. In theory all the concrete, even that below the steel, remains loaded in compression so cracks can not develop even under the steel. The beam is both stronger and it will last longer because the concrete below the steel does not develop the small cracks that can allow moisture to reach the steel. Concrete members can also have their tension elements tensioned after installation to give much the same effect.

    The technique of applying the tension member under all of the concrete is much like the idea of pre-stressed concrete beams. Only the tension and compression are both applied only in actual use. I would not be surprised to see this technique expanded to add some initial tension to these added elements as they are attached.

    The thing that I fear is that concrete can deteriorate over time and the surface that the reinforcement strips were originally attached to can literally fall apart, leaving the reinforcement strip unattached. This construction would require constant and careful inspection to insure that this is not happening. Those Texas DoT engineers were worried about this: that is why they were concerned with attaching the reinforcement to the body of the beams with those anchors in drilled holes.

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    I wonder how long it will take for sunlight and ozone to degrade the plastic. Maybe they will paint it with special paint but that will either disolve the epoxy or have micro cracks that still allow chemicals in.
    Bill D

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    Quote Originally Posted by EPAIII View Post
    Actually I do see the value of the technique as demonstrated in the first video. And those two concrete beams acted exactly as I would have expected. Concrete is at it's best when under compressive loading. When the single person tried to step on the plain concrete beam, half of the beam was being compressed (the top half) and the other half (the bottom half) was placed in tension. Concrete sucks with a tension type load - BIG TIME. So it immediately broke. No surprise there.
    Hi EPA,
    Exactly - no surprises at all, which (to me) means it was too simplified. Among other things, the beam was too long and small a cross section for a non-reinforced concrete structural element, there was no information about strength, cure time, etc. It was designed to fail immediately (practically just sand).

    The CF beam had the guys standing on it very carefully, if there'd been any significant twist during loading the beam would have still failed quickly. It's why I called the video bogus, even though I still linked it.

    The technique of applying the tension member under all of the concrete is much like the idea of pre-stressed concrete beams. Only the tension and compression are both applied only in actual use. I would not be surprised to see this technique expanded to add some initial tension to these added elements as they are attached.
    If it can be done at the time of building construction, I think it would be helpful, no doubt. I could even imagine a continuous strip CF product that has anchoring features (like mushrooms or perforated ribs) that gets laid into the troughs of the corrugated sheeting that form the undersides of floors when the concrete is poured in. But you'd need a fiberglass scrim between the CF and the steel, otherwise galvanics rear their head again.

    The thing that I fear is that concrete can deteriorate over time and the surface that the reinforcement strips were originally attached to can literally fall apart, leaving the reinforcement strip unattached. This construction would require constant and careful inspection to insure that this is not happening. Those Texas DoT engineers were worried about this: that is why they were concerned with attaching the reinforcement to the body of the beams with those anchors in drilled holes.
    Yes, I mentioned the bonding issues in an earlier post. This is the weak-link in most "after the fact" composites - not having proper surface prep and properties for best adhesion when adding layers or new features.

    I'm not sure what the TX guys did to prep the surface of the bridge beams before they added their CF. And while I understand the use of the holes and "tie strips" between the sheets, I'd be nervous about adding new moisture paths and possibly damaging existing rebar.


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