Compression vs Compressive Strength 11

[khinz]

The change in length of a compression member formula has the formula PL/AE where P is load, L is length, A is area and E is modulus of elasticity.

How does it relate to compression strength like 5000 psi?

Is it when maximum compression is reached that the 5000 psi would manifest? Is the pound per square inch that of compressed or uncompressed square inch of any material? How do you interpret it?

 

[BAretired]

A unit strain of 0.003 could be stated as 0.003mm/mm or 0.003inch/inch. A cylinder 8 inches long, when compressed to a unit strain of 0.003 will shorten 0.024" or about 0.6mm.

Concrete does not crush at a unit strain of 0.003. Maximum concrete strain for design purposes in CSA A23.3 is 0.0035 which means that concrete can reach at least that level without crushing.

BA

 

[khinz]

My structural engineer has only 2 practical solutions. Epoxy injection or replacing a portion of the concrete. But if one foot of concrete is replaced, there would be 2 cold joints near ground with strong base shear. Also if even 1mm or 0.5mm gap is left, he said the repair would be more risky than just injecting epoxy as the gap may get unnoticed (or seismic activity may hit building while shores in place and move the bars).

So we are left in a difficult position. He is not familiar about retrofits. And because he believes epoxy strength is enough (ignoring strain compatibility). He is not keen about retrofits. I'd like to know if anyone has tried putting a metal plate and attaching it to above the hole and below the hole (hole already filled with epoxy) so in case epoxy can't take the whole load. It can be taken up by the metal plates. If this is feasible. I'd convince him to put metal plates for peace of mind after he computes the contribution of the metal plates to the moment and shear of the column. But would the idea work? So as not to damage the concrete above and below the epoxy fill. The metal plate would be epoxied to the front of the column above and below the epoxied fill hole. Thanks.

 

[PUEngineer]

Is this ONE column in a series or group of columns that contribute to the seismic resisting frame? If so, would it be possible that the remaining columns could support the lateral loading? This ONE column would only have to be retrofitted to support the vertical loads and detailed in a manner that it only 'go along for the ride' in a seismic event.

 

[khinz]

It must be retrofitted in such a way that it not only resist gravity loads and seismic load too so the retrofitted materials must have seismic sense. So how do you suggest to retrofit it. Please show reference of how this occurs.

The best fix is still removing the bad concrete in one foot portion and replacing with a good one.. from ready mix truck.. but it can settles and produce moment magnfication factor issues with the gap.

Has anyone actually tried this repair those who have actually done this? non-shring grouts have hi modulus and may have good fire resistance to replace concrete.

To those who have actually done such procedure yourself. Please share how successfull it is. Thank.

 

[khinz]

http://imageshack.us/photo/my-images/46/stressstraincurve.jpg/

I talked to other engineers in my country. They all are unaware of what Ron is saying about strain incompatibility. So I spent many days reading many references and computing stuff in order to prove to them there is load reduction using epoxy filling (especially my structural engineer who forgot about the concept).

I read about the above graph:

"Elastic Behavior. At low stresses, up to about fc'/2, the concrete is seen to behave nearly elastically, i.e. stresses and strains are quite proportional; the straight line d represents this range of behavior with little error for both rates of loading. For the given concrete, the range extends to a strain of about 0.0005. The steel, on the other hand, is seen to be elsatic nearly to its yield point of 60 ksi, or to the much greater strain of about 0.002... Because the compression strain in the concrete, at any given load, is equal to the compression strain in the steel.... <snip>"


Now to compute for the load reduction carried by the epoxy filling. I'll use strain of 0.0005 or load in the elastic range.

from steel strain=0.0005, Modulus 29,000 ksi
stress = strain*modulus = 14500 psi or 99973.98 pascal

from concrete strain 0.0005, Modulus 3604.996 ksi
stress = strain*modulus = 1802.498 psi or 12427.79 pascal

from epoxy strain 0.0005, Modulus 450 ksi
stress = strain*modulus = 225 psi or 1551.32 pascal

Column is 0.5x0.5m, the 0.2x0.5 section was replaced with epoxy, remaining 0.3x0.5 section with concrete. In other words, 33% of section replaced by epoxy.

steel area of 12 20mm bars (for concrete section) = 0.003769 mm^2
steel aread of 8 20mm bars (for epoxy section) = 0.002513 mm^2

For load carried by concrete section (0.3x0.5 of column) with 12 bars of 20m steel
P = Fc(Ag-As)+Fs(As) = 12427.29(0.146231) + 99973.98 (0.003769)
=2194.13 KN

For load carried by the epoxy section (0.2x0.5 of column) with 8 bars of 20mm steel.
P = Fc(Ag-As)+Fs(As) = 1551.32 (0.097487) + 99973.98 (0.002513)
= 402.4681 Kn

For load carried by entirely concrete(0.5x0.5 of column) with 20 bars of 20mm steel
P = Fc(Ag-As)+Fs(As) = 12427.79(0.243718) + 99973.98(0.006282)
= 3656.913 Kn

Loss of axial load due of the epoxy is
P(all concrete) - (P(concrete)+P(epoxy)) = 3656.913 - (2194.13+402.4681) = 1060.3149 KN

For P(nominal), fc of concrete is 28,000 Pascal and steel is 414,000 Pascal.
P(nominal) = 0.85 Fc (Ag-As) + Fs(As)
=0.85 (28000)(0.243718) + 414000 (00.6282)
=8401.236 KN
P(factored) = 0.65 * 0.8 * 8401.236 KN = 4368.643

Questions: the reduction factor of 0.65 and 0.8 is only used when the fc and fs used is the designed compressive strengths (maximum of concrete 28mpa and steel 414 mpa respectively, correct? For partial, it's P=Fc(Ag-As)+Fs(As)? I saw this in the book example when solving for the load carried by the concrete and steel at 0.0005 strain or elastic load.
Is my calculations so far correct? I need to present this to my structural engineer to prove there is loss of load carried by epoxy. He thought strain is not important and it is the epoxy compressive strength that counts only.

Next I'll show him possible moment magnification factor effect (or 2nd order effects during cyclic loading) of the epoxy material due to the bars taking majority of the load. I'm doing this because I need him to authorize to replace the epoxy. Without him. I can't do anything so hope you can comment on the calculations above. I don't want to confuse him (and dozen of other engineers who are not aware of this in my country) further by sharing wrong calculations whose concept and formula he already forgot back in his school days two decades ago so need your comments. Remember almost all structures in my country are repaired with epoxy injection and the engineers assume they carry same load as concrete and ignore strain incompatibility and load reductions so this is a national emergency and I may share this in structural newsletter to bring attention to this problem which many in the industry is unaware. Many thanks.

 

[khinz]

What I also learnt are:

1. the formula modulus=stress/strain is only valid in the elastic range.. meaning up to 0.0005 only. For above 0.0005, the stress-curve in the figure has to be used since it is based on actual data.

2. I also learnt that for fast loading vs slow loading, the steel carries more load in slow loading because the fc is smaller in value.

3. This is all assuming epoxy has similar stress-strain curve in the elastic range of the concrete and steel at 0.0005. What if it is different from them then the above calculations are not valid.

4. My structural engineer told me the reason why he believes the epoxy compressive strength at 11,000 psi is what counts is because of the formula E=57,000*sqrt(fc). I told him this is only valid for concrete and not for epoxy. What is the corresponding formula for epoxy then?

I need to convince him to get authorization for removing the epoxy and replacing the section of the column which needs major work. His issues is gap may formed if grout or replacement concrete used except epoxy injection whose shear bond is so strong that is why it's his only choice in hundreds of building repairs he authorized.

5. Has anyone of you actually replaced a section of concrete in middle of column? Or has no one amongst you ever did this? Please tell me. Thanks.

6. In your structural design. Is your service load in the elastic range of the strain in the concrete or does it go above 0.0005 strain? At 0.001 strain, the concrete is permanently deformed for the external forces that acted on it.

 

[BAretired]

What I also learnt are:

1. the formula modulus=stress/strain is only valid in the elastic range.. meaning up to 0.0005 only. For above 0.0005, the stress-curve in the figure has to be used since it is based on actual data.

I agree that the stress/strain curve for concrete is not linear beyond a certain point...perhaps 0.0005 unit strain.

2. I also learnt that for fast loading vs slow loading, the steel carries more load in slow loading because the fc is smaller in value.

I don't know about that. I am not arguing, I simply don't know.


3. This is all assuming epoxy has similar stress-strain curve in the elastic range of the concrete and steel at 0.0005. What if it is different from them then the above calculations are not valid.

I cannot provide a stress-strain curve for epoxy, so I would agree with your statement.

4. My structural engineer told me the reason why he believes the epoxy compressive strength at 11,000 psi is what counts is because of the formula E=57,000*sqrt(fc). I told him this is only valid for concrete and not for epoxy. What is the corresponding formula for epoxy then?

Don't know.

I need to convince him to get authorization for removing the epoxy and replacing the section of the column which needs major work. His issues is gap may formed if grout or replacement concrete used except epoxy injection whose shear bond is so strong that is why it's his only choice in hundreds of building repairs he authorized.

I don't believe that Eng-Tips should be unduly influencing the structural engineer through you. If he wants to discuss the matter with us, we are available.


5. Has anyone of you actually replaced a section of concrete in middle of column? Or has no one amongst you ever did this? Please tell me. Thanks.

I have never done such a thing in fifty five years of practice.

6. In your structural design. Is your service load in the elastic range of the strain in the concrete or does it go above 0.0005 strain? At 0.001 strain, the concrete is permanently deformed for the external forces that acted on it.

Columns designed by Canadian Code CSA A23.3 are not designed on the basis of service loads. The maximum factored axial load resistance is the sum of the concrete resistance and the steel resistance. I believe that the service load is likely within the elastic range of strain, but I have not researched the matter and cannot guarantee it.

BA

 

[khinz]

BaRetired, you said you have never replaced a section of column in middle of column in 55 years of practrice. You have many structural engineer friends (dozens) in your hometown. Do you think they have ever done this? If you meet them, please ask. I wonder what material they insert it with, original concrete or non-shrink grout or otheers and is there really a machine where you can pump concrete from bottom of formworks? My structural engineer has never done this too. He only injected epoxy. So he wants me to ask others if they have done any procedure and how to do it.

If others amongst you have actually done this.. replacing a whole say 1 foot section of column in middle of column. Please share it asap what happened. Thanks.

 

[BAretired]

khinz,

My best answer at the present time is that, if I were presented with the problem you have, I believe I would do the following:

1. Shore the structure so that the column is carrying no load.
2. Remove all concrete within the depth of the "bad" concrete.
3. Pour new high strength concrete from floor to about 1.0" below the top of the removed concrete.
4. Finally, I would pack high stength grout into the gap in the same way I would pack grout under a steel column baseplate.

I do not claim any special knowledge in this area, but that is what I would do. Others may disagree.


BA

 

[khinz]

BA, according to many engineers locally, the reason they avoid it is the connections between the concrete inserted to the column would have minimal shear bond strength. Whereas they said if one puts epoxy, the shear bond strength is around 3000 to 5000 psi. They reason that if the new concrete is not bonded to old one. Gaps may eventually form during regular dynamic movements of the column increasing moment magnification factor. So the new column has to be bonded to old one with high shear bond. I'm still analyzing their claim. What do you think.

 

[BAretired]

I don't know what magnitude of shear your columns need to resist, but if the surfaces are intentionally roughened, I believe the shearing resistance would be pretty good, particularly with all that column reinforcement passing through and thoroughly anchored above and below the repair.

Contrary to what has been said, I believe there would be value in comparing cylinders of concrete to cylinders with 50% concrete and 50% epoxy all tested to failure in pure compression. If strain measurements can be made, that would also be of interest.

BA

 

[khinz]

The shear bond strength is on the surfaces of old concrete and new concrete or mateial. It has to reach 5000 psi too. If not. He said to imagine a big stone and a tiny stone. You put the big stone on top of small stone, the small stone has tendency to break, the small concrete he refers to the concrete that would be inserted between the column. It is smaller and can easily be crack durig dynamic movement, and when this occurs, gap would form that would make it behavior bad.

 

[BAretired]

khinz,
Just a comment on your earlier calculations (shown in blue):

Now to compute for the load reduction carried by the epoxy filling. I'll use strain of 0.0005 or load in the elastic range.

from steel strain=0.0005, Modulus 29,000 ksi
stress = strain*modulus = 14500 psi or 99973.98 pascal 100MPa or 100*10[sup]6[/sup] pascals.

from concrete strain 0.0005, Modulus 3604.996 ksi (let's call it 3600 ksi)
stress = strain*modulus = 1802.498 1800 psi or 12427.79 pascal 12.4 MPa

from epoxy strain 0.0005, Modulus 450 ksi
stress = strain*modulus = 225 psi or 1551.32 pascal 1.55 MPa

Column is 0.5x0.5m, the 0.2x0.5 section was replaced with epoxy, remaining 0.3x0.5 section with concrete. In other words, 33% 40% of section replaced by epoxy.

steel area of 12 20mm bars (for concrete section) = 0.003769 mm^2 3600mm[sup]2[/sup]...in Canada, 20M bars have an area of 300mm[sup]2[/sup], could be different in Philippines
steel area of 8 20mm bars (for epoxy section) = 0.002513 2400 mm^2

For load carried by concrete section (0.3x0.5 of column) with 12 bars of 20m steel
P = Fc(Ag-As)+Fs(As) = 12427.29(0.146231) + 99973.98 (0.003769) = 2194.13 KN
12.4(300*500 - 3600) + 100(3600) = 2175 kN.

For load carried by the epoxy section (0.2x0.5 of column) with 8 bars of 20mm steel.
P = Fc(Ag-As)+Fs(As) = 1551.32 (0.097487) + 99973.98 (0.002513) = 402.4681 Kn
1.55(200*500 - 2400) + 100(2400) = 391 kN

For load carried by entirely concrete(0.5x0.5 of column) with 20 bars of 20mm steel
P = Fc(Ag-As)+Fs(As) = 12427.79(0.243718) + 99973.98(0.006282) = 3656.913 Kn
12.4(500*500 - 6000) + 100(6000) = 3625 kN

Loss of axial load due of the epoxy is
P(all concrete) - (P(concrete)+P(epoxy)) = 3656.913 - (2194.13+402.4681) = 1060.3149 KN
3625 - (2175 + 391) = 1059 kN

Note: The above calculation assumes uniform strain throughout the column. If a transformed section is used, the centroid of the combined section would shift toward the concrete portion. That would cause bending stress in addition to axial, so the condition is likely going to be worse than calculated.


BA

 

[BAretired]

The shear bond strength is on the surfaces of old concrete and new concrete or mateial. It has to reach 5000 psi too. If not. He said to imagine a big stone and a tiny stone. You put the big stone on top of small stone, the small stone has tendency to break, the small concrete he refers to the concrete that would be inserted between the column. It is smaller and can easily be crack durig dynamic movement, and when this occurs, gap would form that would make it behavior bad.

The shear strength of a concrete beam or column is made up of two parts, V[sub]c[/sub] and V[sub]s[/sub], the shear resistance of the concrete and shear reinforcement respectively. In no case can the shear strength come anywhere close to 5000 psi.

In your case, the ties would remain in place, so V[sub]s[/sub] would not change. For 4000 psi concrete, the factored concrete shear stress v[sub]c[/sub] is in the order of 90 psi so V[sub]c[/sub] is approximately 90*b*d. As for the big stone, little stone argument, I find it totally unconvincing.

In the final analysis, it is your structural engineer who must assume responsibility for the column repair. I do not wish to unduly influence his decision.

BA

 

[Robbiee]

khinz,
I have done some repair similar to this but not exactly the same. Using BA's lines, the sequence would be:
1. Shore the structure so that the column is carrying no load.
2. Remove all concrete within the depth of the "bad" concrete.
3. Provide formwork shaped that allows one side to maintain 300mm of concrete head.
4. Cast self consolidating concrete with max. aggregate size of 10mm
5. remove formwork 3 days after casting and saw cut the extra concrete for the side of column.
6. Remove shoring when concrete reaches the design f'c.

Given the size of the infill and the amount of reinforcing, shrinkage will not cause the loss of aggregate interlock.
The interface shear, not beam shear, needs to be checked against the max. shear on the column. Shear keys can be cut to increase shear capacity.
This is how I would do it.

 

[khinz]

BA wrote:

Note: The above calculation assumes uniform strain throughout the column. If a transformed section is used, the centroid of the combined section would shift toward the concrete portion. That would cause bending stress in addition to axial, so the condition is likely going to be worse than calculated.

Yes. I thought of this yesterday while driving and imagining the stress distribution from the top to the bottom of the column... above the hole, it is 3625 Kn. In the hole filled with epoxy capacity of both concrete and epoxy is just 2566 Kn. So I wonder how that section could take the 3625 Kn above. The epoxy+concrete section have to compress more (or larger strain) than the above to take the extra load. During the straining, the centroid as you worded it would shift towards the concrete portion. But would the epoxied section have the same strain (or elongation) or would the concrete portion strain (or compresses more?) from the formula elongation = PL/AE? If so then it can have bending stress. But I thought about the longitudinal bars at the other side. Without longitudinal bars, the column would have more bending stress in that section. But the bars on the other side (8 to 10 pcs of 20mm grade 60 bars) can prevent more bending via tension. Now the loading of the building is such that in the concrete section there is more load than on the epoxied section, the column is eccentric. In a perfect building with perfect column. All loads would be concretric and all sections of the column would have equal strain and stress. But in real world, columns are eccentric. Now since the concrete of the epoxy+concrete part has more real load. I was thinking if it would cause moment magnification factor increase.

My structural engineer doesn't understand the above when I talked to him because he said he doesn't think about stress and strain behavior for a decade anymore. Also he said the total dead load + live load + sd load of the column is about 1200 KN. The column has capacity factored load of P = 0.85*0.6*((0.85Fc(Ag-As))+Fs(As)=0.85*0.6*((0.85(28000)(0.25-0.006282)+414000().006282)=4368 Kn with the nomimal load being 8401 Kn. Therefore the column capacity is 4 times the service load. I know load combination makes the column load capacity 4 times the actual load. This is also what you do isn't it? He said because of the huge column capacity, the repair section would be as strong as the old. Again he is ignoring the strain compability which I can only make him realize after I show him all formula and after mastering the concepts myself so I can debate him which I already did for one hour the last time.

Btw.. the portion with epoxy and concrete is right at the joint between ground tie beams and column and the joint is located below ground floor slabs by at least half foot. And the tie beams are 1 meter above strip foundation where the columns share one big footing. The footing is designed for 5 storey but we only build 2 storey. The tie beams are used to make the columns more stiff in the combined strip footing because if the column directly goes from strip foundation to second floor.. it reaches 4.5 meters whereas with tie beam supporting the columns the effective length is 3.5 floor to floor according to the structural enginner. Strip foundation is not higher because the soil below is much harder. If he doesn't want to have the epoxy removed and the joint recasted maybe because the tie beams can have cold joint at the joint, then I was thinking what if the column below the slabs would be pedestral (or concrete put around the columns below tie beams to make it even stiffer) and to make the joint stronger. What do you think of this idea. I need to convince him first by understanding the concept myself or he and I would be both ignorant and nothing to talk about in next meeting (as he still totally ignores the concept of strain compabitility).

I have only this week to decide. The contractor would cast the ground slabs already on monday and can't wait further more. The civil engineers in the contractor company don't even understand the difference between axial load and bending moment. They don't even know what moment means. Hence they even find it hard to understand what I'm saying. And my structural doesn't understand strain compatibility. Ok. Here's the twist. I'm the owner of the building although I'm also an engineer being a Communications and Electronics engineer so understanding technical details is not hard for me. So please bear with me a couple of days more as I need ideas to convince them what to do or to have them entertain the idea what kind of repair to do and I need to decide fast. For now. They just don't want to do anything and want the project done as soon as possible and I can't replace the structural engineer because he designed the buildings and all data with him. So try to answer each paragraph above. Thanks very much.

 

[TXStructural]

From the sketch you provided on April 1, a plug of high-modulus material will act as a single piece of aggregate. You didn't provide dimensions, but judging from the proportions of the sketch, I would ignore the 4-digit computations of an estimate, based on an assumption. I would use a competent repair mortar, or would inject a pumpable, flowable concrete as Robbiee suggests. Since you will probably not get the chance to unload the column, just make sure you get the repair done before the remaining loads are imposed on the column. If the sides of the void do not provide a rough surface, chisel out the void to give it rough surfaces, and if you can, give it parallel sides or create a slight dovetail to hold the plug in place, but avoid making it larger than necessary, since it will be the vertical thickness of the repair that really affects the function in the column (PL/AE and all that).

There is wide variation in the "modulus" of formed and placed concrete, and the computations to reach an elegant, useful solution are based on code approximations, not first principles. You are trying to over think a problem that doesn't warrant this level of concern. The reality is that concrete is frequently imperfect, and yet it still functions remarkably well. Inside a column, there is an irregular distribution of forces, based on many things, such as aggregate distribution, paste fraction, degree of contact between rocks, how well the concrete is consolidated, and the way loads are imposed on the members. The method of design is based on notional models and loads which represent typical construction.

We frequently puddle very high strength concrete at columns, and this leads to to very irregular borders (varying by a foot of more in slabs in beams) between very different concrete strengths, and this is NEVER a problem. There is an organization called the International Concrete Repair Institute, and together with the American Concrete Institute, they are publishing a concrete repair code later this year.

In the mean time, ICRI has repair guides:
(

http://www.icri.org/publications/guidelines.asp)

and ACI has a few things:

http://www.concrete.org/BookstoreNet/SearchResults.aspx?CATEGORY=311290

 

[Ron]

I nominate BAretired for sainthood!

 

[khinz]

TXStructural. It could have been easier if the void is still not filled. But two weeks ago. The contractor told the epoxy company to check and the epoxy engineer said they can cover it up with epoxy and it's stronger than concrete. And the structural engineer complied.

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

(before repair)

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

(after repair)

Now the problem is the epoxy is very hard and binded to the bars and difficult to remove. It's also part of a joint that connects tie beam so replacing it with repair mortal or concrete can make the tie beam unconnected to it. My structural engineer said he never insert any repair material in his hundreds of buildings except epoxy because epoxy binds all, bars and concrete and has high shear bond strength. What do you think? He just ignored strain compatibility as he doesn't know what I'm talking about in this thread.

As the owner, I can tell the contractor to do anything as they have to shoulder any expenses. I have meeting with them later and they are closing up the ground floor with slabs this weekend. The void in column is 1 foot below floor slab level as I just measured it now.

Ron. Dont worry. Just a day or two more before I close this thread. I know how wearied we are already of this. Lol. But I wanna thanks you so much for all the basic concepts about strain compatibility that many miss or refuse to look at. As an engineer too. I realized this is important in predicting the present and future behavior of the building in dynamic motion.

 

[rowingengineer]

Khinz,
i will admit that i think your ticas and a lot of my replies are based on this, is this correct? What fire rating does the column need.

http://www.nceng.com.au/

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