Epoxy Modulus and Creep 1

[ticas]

Something I don't quite get even after reading the following portion many times (see My questions below)

http://mmgrouting.com/foundation.htm

"Epoxy grouts cannot be reinforced with rebar like cementitious materials because of the differences in the thermal coefficient of expansion and modulus of elasticity of epoxy grout and steel. Epoxies, which can have high tensile strengths, do not transfer the tensile loads to the rebar like cementitious materials. This is because epoxies have such a low modulus of elasticity that they stretch rather than transfer the tensile load to the rebar when compared to concrete. Epoxy grouts have a modulus of elasticity about half that of concrete or when loaded equally epoxy grout will compress about twice as much as concrete. ....... Epoxy grouts can creep or cold flow over time, further contributing to alignment problems. This problem is exaggerated by both depth and temperature. The greater the depth and temperature the greater the creep."

My questions.
1. Supposed you have a foundation or beam or column with a portion with epoxy holding rebars. The above stated that when in tension, epoxy do not transfer the tensile loads to the rebars (I know some epoxy has tensile strenth of even 5000 psi). But tensile strengh of rebar is 40,000 psi. In tension, even if the epoxy do not transfer the load to the rebar, the rebar will automatically take the tensile strength, why what would happen to the epoxy? I'm aware that cement has tensile strengh of maybe 100 psi while epoxy has tensile strenght of 5000 psi. (How does pure epoxy behave versus epoxy grouts?)

2. If epoxy doesn't have the same compression as concrete.. what would happen if they go together in let's say a beam. I can't imagine how they would just separate. Pls. illustrate.

3. The above stated that creep occurs with depth and temperature. Is it due to the depth having more pressure? what would happen to pure epoxy when it is in under constant pressure? it will just break apart as in break to pieces?

Thank you.

 

[Ron]

ticas...you are missing their point about epoxies and the load transfer mechanism to rebar. The modulus of elasticity for epoxy is significantly different than for concrete. Even though structural epoxies have a much higher tensile strength than concrete, this is not the property that determines load transfer to rebar encapsulated in either material. Epoxies are "stretchier" than concrete. Concrete is quite brittle and fails in that manner (relatively). Epoxy has a very ductile failure mode, meaning that it stretches a lot before failing in tension.

Regardless, the load transfer mechanism to rebar is by shear strength, influenced by the shear modulus, G. The shear modulus of epoxy is typically less than 10 percent of the shear modulus of concrete. For that reason, concrete will transfer its load to the rebar much more efficiently than epoxy.

Additionally, if epoxies are subjected to sustained loading they will creep or stretch under that loading. Much more so than concrete. For that reason it is important to keep the thickness of epoxy interfacing with concrete and/or rebar to very thin sections.

 

[Ron]

Also, to give you a frame of reference, the following ranges are common....

Elastic modulus of concrete- 3500 ksi to 6000 ksi
Shear modulus of concrete - about 3000 ksi

Elastic modulus of common structural epoxy - 600 to 800 ksi
Shear modulus of epoxy- 200 to 300 ksi

 

[ticas]

So what if epoxy can't transfer its load to rebars more efficiently than concrete. Since epoxy has very ductile failure mode, it can take the load itself. What's the problem with it?

Also how thin is "thin section" interfacing with concrete and/or rebar, what if its 2 inches?

 

[Ron]

No, epoxy cannot take all the load itself. That's what the steel is for. It has a much higher tensile strength than the epoxy and is intended to take the tensile load within the concrete....the epoxy cannot do that.

As for the thickness of the epoxy, 2 inches would be considered very thick epoxy...not something you would use for structural applications. When I referred to thin sections of epoxy, I was referring to something on the order of 1/16 to 1/8 of an inch.

It is apparent that you have very little experience with construction and that you do not understand the necessities of materials science with regard to construction materials. Are you an engineer? Are you a contractor?

 

[ticas]

Ron. Of course I know epoxy can't take all the load. Of course. Let's say it's tensile load is only 5000 psi. When tension reaches above 5000 psi for that section, steel with much higher 60,000 psi would take over. What I meant was even if it's concrete or epoxy.. steel will be the final material to resist the load. So nothing wrong if concrete or epoxy.

Why, what's wrong with 2 inches of epoxy? Do you think it will just break apart even if it's rated at 5000 psi compression strengh if the load is less than that?

I'm a civil engineer.

 

[VoyageofDiscovery]

ticas, I think you have asked your question all wrong! Provide a sketch of your problem. Then, maybe we can see what you are trying to get at.

VoD

 

[wannabeSE]

ticas,
You may be missing some fundamental concepts such as the relationship between stress and stain (force versus deformation). The forces will distribute in proportion to the stiffness of the elements. Understanding this is only the very beginning of understanding structural engineering and reinforced concrete design.

I do not know what you are trying to do with the epoxy. If you need it only to protect the reinforcing steel from corrosion, it may be an expensive alternative. If you need it to replace concrete in a structural element or you need to remediate a deteriorated structure, you probably need an expert in epoxies to lend assistance. Epoxies are good for some things and bad for others. Also, the specific epoxy used can be crucial. Have you looked at any of the major (reputable) manufacture's websites or catalogs to see if they have an epoxy that is designed for your application?

 

[ticas]

http://www.pbase.com/ticaslogos/image/149351058/original

See the above picture.. a corner of a column was hit by a truck. We removed the broken concrete with a rebar exposed at the corner (the rest is perfectly fine as numerous test by engineers have confirmed so this is not a concern in this message). An epoxy company put epoxy in the hole (see bottom of the picture above). It is about 2 inches hole vertical. Now what I want to figure out the past days is. What is the behavior of the epoxy part in relation to the concrete? During tension, the epoxy would stretch more, the concrete less as they eventually transfer the load to the rebars. What would happen in this case, would the epoxy and concrete break apart from each other? Similar in compression, the epoxy would compress more than the concrete, would they separate provided the column is moving
during lateral loading. The spec of the structural epoxy is 2 part resin and hardener with polyamide with the following characteristics:

Compressive Strength Minimum 8,000-11,000 psi
Maximum 11,000-120000 psi
Tensile Strength 3,000 - 5,000 psi
Flexural Strenth 6,000-9,000 psi
Bond Strength (Slant Shear Test) 3,000-5,000 psi

 

[Ron]

ticas...if this is the extent of the repair, the primary goal would have been to only protect the rebar. It would appear that you have done so. The repair could have been made with a polymer modified mortar to achieve the same result at a lower cost.

I would not be concerned with creep or any significant stress on the epoxy for this repair.

 

[ticas]

http://www.pbase.com/ticaslogos/image/149352082

column damage before epoxy

http://www.pbase.com/ticaslogos/image/149352114

column damage after epoxy

Ron. I didn't show the larger picture. It's actually worse. See above picture where 4 bars in front are exposed with about 2 inch space behind the bars so 4 gallons of epoxy with injection are used. So the epoxy is not just to cover the bars but are taking partial loads of the column. This is why I need to know exactly the behavior of the epoxy part as it interacts with the concrete.

Again. During tension, the epoxy would stretch more, the concrete less as they eventually transfer the load to the rebars. What would happen in this case, would the epoxy and concrete break apart from each other? Similar in compression, the epoxy would compress more than the concrete, would they separate provided the column is moving during lateral loading. The spec of the structural epoxy is 2 part resin and hardener with polyamide with the following characteristics:

Compressive Strength Minimum 8,000-11,000 psi
Maximum 11,000-120000 psi
Tensile Strength 3,000 - 5,000 psi
Flexural Strenth 6,000-9,000 psi
Bond Strength (Slant Shear Test) 3,000-5,000 psi

 

[Ron]

ticas...that has been one of the frustrations for all of us trying to assist you. You have given snippets of information expecting conclusory answers.

Yes it is worse. You might have a significant strain compatibility issue. This is what happens when insufficient consideration is given to the compatibility of stresses, strains and material reactions. This repair has potentially changed the dead load stress distribution in the column and will correspondingly affect the live load distribution. Is it detrimental? Who knows. We can't determine that from afar. The column should be monitored as more load is placed on it.

Good luck. I hope it works out for you and the structure.

 

[ticas]

I just need to know how epoxy interacts with concrete when portions of them added together. When I know how. Then I'll calculate based on the actual geometry and configuration.

If anyone has already thought of this.. how the tension in epoxy would affect the surrounding concrete... like would it break them apart? Let me know. I just want to know a theoretical framework of it for useful knowledge for future considerations.

The concrete is 4000 psi. Are you saying that non-shrink grout would be a better replacement as it is compatible with concrete?

In case the epoxy has to be removed. Know any compound that can just melt it as it is now even harder than the concrete? Can it be removed successfully especially around the rebars based on your guys actual experience?

 

[hokie66]

You won't get a "theoretical framework" for any calculations, as what you have done has no basis in the engineering literature. It is not enough to guess that it might work.

 

[ticas]

I want to compute the different percentages of axial load (factored and nomimal) (by 15% increment) and gauge the deformation length of the epoxy and concrete so as to estimate the crack potential in order to can decide on very expensive process of breaking the epoxy apart and the concrete and replacing the bars which can take significant hit from the removal process. So if anyone expert on strain compatibility and can know enough about composite behavior of epoxy and concrete can point to research paper about it. Let me know as soon as possible so I can decide. Thanks.

 

[VoyageofDiscovery]

You obviously still have a problem here! Get a structural engineer involved in this right away. It's not the first time and there are all kinds of possible solutions with associated costs and implications which cannot be easily sorted out in a forum. Not a small job by any means as there is alot to consider.

 

[Ron]

I hope that no one provides additional information that might lead you to "compute" the items you noted. I suspect that you will compute something and rationalize to yourself and others that it is correct when you obviously know nothing of what you've done. That's incredibly irresponsible and dangerous.

The advice you've been given is not from novice engineers. We have seen the result of poor field decisions and the impact they have on structures and their performance. I make my living investigating structural and construction defects and failures. Others in this forum have even more experience than I. You need to listen.

Engineering is not just computing things. It is a combination of many endeavors, all intended to promote and protect the health, safety and welfare of the public.

GET A STRUCTURAL ENGINEER WITH REPAIR EXPERIENCE INVOLVED NOW!

 

[bookowski]

star to Ron for patience

 

[ticas]

Ok. I will. But some structural engineers I asked forgot about strain compatibility as they only work
mainly with rebars and concrete and not composite and haven't come across what I experience. So
I need to look for expert in repair.

In my school. I remembered the purpose of rebar is to avoid compression crushing or brittle failure of
concrete. The bars at bottom of beam at midspan is a clear illustration and the yield and compression
side of column. But now I learnt that although both epoxy and concrete can have both 3000 psi
compression strength, their elastic and shear modulus can be different and this is important
in load transfer mechanism to rebar. Now I only need to ask something and won't calculate
anything. Is load transfer mechanism of rebar not only important in yielding of rebars but
in normal loading when compression strength is not exceed??? This is very important question
and all I need to know now because it can make me walk extra miles during holidays in search
of structural engineers who can do the repair. Because if the answer is yes. Then there is
danger. If not, there is still danger but I really need to know this. So I hope someone can
answer just this one question. Thanks.

 

[Ron]

Yes. Any loading creates strain, whether compressive or tensile. That strain becomes very important when the moduli are different in materials trying to respond compositely.