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# Compression vs Compressive Strength11

## Compression vs Compressive Strength

(OP)
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?

### RE: Compression vs Compressive Strength

They don't directly relate to each other.

Δ = PL/AE is a relationship between load and deflection.

Strength of the member is a totally different question.

### RE: Compression vs Compressive Strength

(OP)

Supposed you have a 3000 psi concrete vs as 1000 psi concrete.. how would it indirectly affect PL/AE or the deflection?

### RE: Compression vs Compressive Strength

σ = P/A gives the stress (psi) for a member of a certain area loaded axially.
Δ = PL/AE gives change in length of a member when loaded axially.
For a given cross section, A, the failure load is equal to σ*A
Also, by algebra: Δ = σL/E

Set σ = 3,000 psi and 1,000 psi in the above equations to see its effect.

Kirk G Hall, PE

### RE: Compression vs Compressive Strength

(OP)

Ok. Thanks.

Diamond has high modulus (stiffness) and high compressive strength. Can you give a natural object that has low modulus and high compressive strength? Or does this not occur naturally and have to be synthetically produced?

### RE: Compression vs Compressive Strength

khinz....when you have a 5000 psi concrete, that means that it should have an ultimate strength in compression of at least 5000 psi. If properly designed and controlled, the actual strength will be somewhat higher than the target strength.

When a structural member such as a column is placed in compression with a load placed on the column (as transferred from a slab or beam system), the column will shorten elastically by the relationship of Δ = PL/AE. This elastic shortening will continue until the column reaches its ultimate strength (failure). This is simplified and works for the concrete alone. When the concrete is reinforced as with typical columns, the elastic shortening of the concrete is resisted by the steel because they have two significantly different moduli of elasticity.

The actual stress interaction is a little more complicated than I've posed; however, not greatly so and the premise as noted here is sufficient for basic understanding.

### RE: Compression vs Compressive Strength

(OP)

According to Wikipedia, Concrete has young's modulus of 30 GPa while steel has young's modulus of 200 GPa. So I guess what you do is to calculate the Δ = PL/AE of the steel bars and the deformation of the column is a combination of the two. Is the stress interaction linear? Like there is contribution by both concrete and steel? What topic does this fall under, strain compatibility? What exact books do you know that has this details? I have read the book "Design of Reinforced Concrete" and others but the detail is not included.

### RE: Compression vs Compressive Strength

The Canadian concrete code has provisions which relate the modulus of elasticity to the concrete strength, but they are approximations. Concrete is not a material which possesses finely tuned properties.

BA

### RE: Compression vs Compressive Strength

khinz - sorry about my first post above - I misunderstood your question so my answer didn't make sense I'm sure.

I thought when you referred to "strength" that you were talking about the capacity (in strength terms) of a section.

Per BAretired - the value of E is typically related to f'c (the maximum compressive strength of concrete)
Here in the US it is E = 57,000 x sqrt(f'c) where f'c is the 28 day compressive strength.

It is approximate as BAretired states.

### RE: Compression vs Compressive Strength

(OP)

What's the unit of this other strength which you described as "the capacity (in strength terms) of a section.".

With regards to this compressive strength and modulus. How about steel. There is a corresponding formula for all materials that relate the compressive strength and elastic modulus?

### RE: Compression vs Compressive Strength

No, there is not. For steel, the ratio between compressive strength to modulus of elasticity varies considerably depending on the chemical composition of the steel and the process used in making the steel.

BA

### RE: Compression vs Compressive Strength

(OP)
BAretired, do you believe there is a natural formed object in the world that has low modulus of elasticity yet has high compressive strength?

### RE: Compression vs Compressive Strength

2
khinz...your question is not so simple as you might imagine. Yes, there are natural materials with relatively high strength but lower moduli. Wood is one of those. To determine this you have to look at the strength to modulus ratio. The reason you don't see this term much in engineering literature is that it has little or no relevance outside an academic pursuit. The modulus of elasticity has as much relationship to other properties of the material it does its strength.

In practice we simply look at a stress-strain curve to get a quick graphic of the material. Natural or manufactured, a "stretchy" material will have a low tensile modulus but can have a relatively high strength. It will exhibit a "flatter" stress-strain curve. A "hard material" will have a relatively high modulus as compared to other materials; however, you must understand that the modulus of materials as compared to their own hardness is irrelevant. As an example, steel generally has a modulus of elasticity of 29 x 10^6 psi without regard to its hardness or its yield strength. There are some slight variations of course, but generally that holds. Compare that to aluminum. Structural aluminum can have a yield strength comparable to common structural steel, yet it has a modulus of elasticity that is only about 1/3 of steel.

Suppose you have a bar made of steel and a bar made of aluminum, both exactly the same size, and you apply the same stress to each of them. The aluminum will stretch (or compress) more than the steel under the same loading.

I applaud your thirst for knowledge; however, you must understand that a little bit if information such as this, used in an improper manner, can be dangerous. For that reason, we have engineering education and engineering laws to protect the public from improper use of disjointed and partial information. You have been given lots of information on your recent issues and I know you have tried to absorb that information and apparently apply some of it to solving your problems in the field. Without an adequate understanding of the materials and their interactions (yes, strain compatibility is one of those interactions), you can improperly apply such pieces of information with disastrous results. That is what all of us who have given responses to your questions try every day to prevent in our own practices.

If you are an engineer...please keep learning from others and go slowly in your application of what you learn. If you are not an engineer, please do not use such snippets of information to solve field construction problems. Get an experienced engineer involved.

### RE: Compression vs Compressive Strength

Going back to the original question: the concrete E is proportional to the compressive strength of the material. As f'c increases, E increases and axial deformation would decrease. I'm not really sure what the O.P. is trying to solve though.

### RE: Compression vs Compressive Strength

I am intrigued that anyone answered this. The question wasn't from anyone trained in structural engineering.

Michael.
Timing has a lot to do with the outcome of a rain dance.

### RE: Compression vs Compressive Strength

Ron... a star... for your perseverance

Dik

### RE: Compression vs Compressive Strength

ACI 8.5.1: Ec = w(c)^1.5*33*sqrt(f'c).  For normal weight concrete, Ec = 57,000*sqrt(f'c).  So yeah, the modulus of elasticity is tied to the compressive strength of the concrete.

### RE: Compression vs Compressive Strength

(OP)

Actually I have discussed this with about 7 structural engineers in my place. It's very strange some are ignorant about this or they said they just ignored the stress-strain curve or don't study it because bars and concrete are automatically compatible and they don't work with other materials and strain compatibility is not an issue and they only work with ETABS and get the bars As and that's most they do. They should not have forgotten the conceptual. In fact I have to share with them what you guys mentioned and some of them don't even get the points yet about strain compatibility. On monday I'll discuss this with the country's top structural engineer and I need to get some facts so I can ask him or my questions even make sense.

Steelion. The data I need to know now is what if the test cylinder is composed of half of one material and half of another material. For example. One half of the cylinder is concrete and one half is iron. When you are doing compression tests of it. Would the concrete become invisible (since it has lower modulus)? Meaning only the stress to compress the iron would appear or would concrete contribute to a portion of the strain resistance when doing the stress compression test of this hybrid cylinder and how do you calculate the resistance offered by the concrete which has modulus lower than iron? I'd like to understand how they interact exactly. At the same strain than steel (because they belong to the same cylinder). Concrete would need less stress force and this translates to most load being transfered to the iron part of the cylinder, right?

### RE: Compression vs Compressive Strength

3
khinz -

You indicated familiarity with Δ = PL/AE.
For concrete - E is defined in the ACI 318 code as 57,000(sqrt(f'c)). If you have 5000 psi concrete then f'c = 5,000 psi and E = 57,000(sqrt(5000)) = 4,030,508 psi or 4,031 ksi.

The compressive strength f'c = 5,000 psi and the resulting E = 4,031 ksi are properties of your fully cured concrete.
The 5,000 psi strength (at 28 days) is verified by taking concrete cylinders and testing them to failure in a machine per an ASTM specification.

The tested cylinder strength, in actuality, might end up being a bit higher or lower than the target 5,000 psi strength. Say the test reveals that your concrete is 5,100 psi.
If so your E = 4,071 ksi.

So with that concrete your Δ would equal your PL/AE based on the E = 4,071 ksi.

If you had 3,000 psi specified concrete then E = 57,000(sqrt(3000)) and so on.
With weaker concrete (strength-wise) you would have smaller values of E and more deflection under load.

The value of E for steel is 29,000 ksi. No matter what maximum yield strength you have in the steel, the E is basically constant at 29,000 ksi.

I'm assuming you are talking about a cylinder where looking down on it you would see a half-circle of steel and a half-circle of concrete.
With that condition - if you tried to apply load to the cylinder, the two would compress equally under the testing machine - (same Δ - since the machine would probably be two heavy plates applying uniform load to the circular surface.

With a common delta you can then back calculate the load the steel takes and the load the concrete takes. Rearrange the Δ=PL/AE equation to get:

P(concrete) = (Δ x A(concrete) x E(concrete)) / L
P(steel) = (Δ x A(steel) x E(steel)) / L

The concrete force would be much smaller since it is "softer" than the steel. Think of compressing a cylinder with half of it steel and the other half of sponge or foam. The foam would take some load - but very very little....just enough to compress it by Δ.

### RE: Compression vs Compressive Strength

(OP)

Thanks Jae. It confirms what I visualized all night yesterday. On monday I'll discuss this with structural engineers in my country who knows the difference between compressive strength and modulus of elasticity. Most other structural engineers just ignore the latter because it doesn't affect the calculations of their As and Ag as they reasoned they only work with bars and concrete and this is automatically taken care of and need not understand modulus of elasticity or shear modulus or how they differ because they said they are not writing books. Very incompetent I know. Thanks to all here who are good in conceptual foundations.

### RE: Compression vs Compressive Strength

Interesting all the way around - snaps to Ron and JAE. The only very minor point regarding JAE's excellent post (the last one) is that he reported the E value as 4031 ksi from the relationship 57,000x(sqrt 5000) - rather than 4000 which is a bit more in tune with any purported accuracy of the relationship.

### RE: Compression vs Compressive Strength

BigH...I'm fairly sure that JAE did that only to make a point to khinz....that being the "calculated" difference in E, when using that relationship and the variations that you will normally see in the compressive strength of concrete. The point being that a 2% change in compressive strength transposes to a 1% change in E for his example....both being insignificant in the overall scheme.

Sorry to jump in JAE...just on here at various times and usually forget something if I don't do it while that single thought is rolling around in the vacuum!

BigH...I know you're slammed with work at the moment, but we miss your input here!!

### RE: Compression vs Compressive Strength

(OP)

The point of this thread is that any repair material must be compatible with the concrete in modulus. Here's the problem.

http://www.unisorb.com/pdf/v-1%20grout.pdf

The above non-shrink grout has modulus of elasticity of 6000 ksi. Now a 4000 psi concrete has modulus of 4030 ksi. This means if you put the grout and bond agent into part of the concrete compression section, the repair grout would be stiffer than the concrete and would take more load. Depending on the dimensions, this can result in the weaker concrete cracking during movement of the structure. So I guess it's safer to use grout that is a little bit less the concrete in modulus. At least it is the grout that have less load so the concrete won't crack or separate.

No. I won't do any repair. It is the structural engineer who will have the final say. But I need to have some basic background because there are structural engineers I met who are not familiar about strain compatibility because they don't do the column strength interaction diagram manually but from ETABS and STAAD so they forgot the concept.

### RE: Compression vs Compressive Strength

khinz...there is a saying that "the devil's in the details". 4000psi concrete has an E of about 3600 ksi, not 4000 ksi. You have computed for 5000 psi concrete...be careful with each of your relationships.

Next, LOOK AT SIKA!!! They have more experience in this type of repair than any group I know.

Further, you can "dilute" some of these packaged grouts with local aggregate...Do some testing to see the effect. You can probably match the concrete reasonably closely.

Also, what's wrong with using the original concrete with a change in coarse aggregate size? This can be done if you provide appropriate clearance and forming.

### RE: Compression vs Compressive Strength

(OP)

I know many home contractors using just site mix for concrete cement, the compressive strength as low as 2000 Psi because of light load as designed by structural engineer. 2000 psi translates to modulus of about 57,000*sqrt (2000) = 2550 ksi. Now the grout above has modulus of 6000 ksi and compressive strength of 11,000 psi. Now if one puts the grout to a compression section as replacement for concrete repair. There is significant strain incompatibility where the grout can take more load. Is this right? I'm not building one or doing this repair, just asking before this thread closes for future reference. I haven't heard about grout modulus incompatibility. Has anyone? Is this a valid concern or overreaction?

### RE: Compression vs Compressive Strength

#### Quote (khinz)

The above non-shrink grout has modulus of elasticity of 6000 ksi. Now a 4000 psi concrete has modulus of 4030 ksi. This means if you put the grout and bond agent into part of the concrete compression section, the repair grout would be stiffer than the concrete and would take more load. Depending on the dimensions, this can result in the weaker concrete cracking during movement of the structure. So I guess it's safer to use grout that is a little bit less the concrete in modulus. At least it is the grout that have less load so the concrete won't crack or separate.

I don't agree with the above conclusion. It is preferable to use grout with a slightly higher modulus in order to reduce the chances of cracking the concrete.

BA

### RE: Compression vs Compressive Strength

Khinz...I don't know about others, but this is very frustrating to me. It is as if you don't fully read and comprehend a prior response before you take off with some irrelevant tangential comment leading to another question. Why do you care about 2000 psi residential concrete that has nothing to do with your issue? Stay focused.

Apparently there is a testing laboratory available to you. Develop several mix designs for repair concrete or grout. Have them tested for compatible strength and bond. Prepare the existing columns to receive the repair materials. Shore everything to remove load from the columns. Make the repairs. Don't make the same mistakes again!

### RE: Compression vs Compressive Strength

(OP)

BARetired. It's for column or beam with one quarter of the section replaced with grout.

Ron. Just wanted to see it from all sides. Now it all finally makes sense. Many thanks.

### RE: Compression vs Compressive Strength

khinz,

For a column with one quarter of the section replaced with grout, it is preferable if the grout has a slightly higher modulus than the concrete, not a lower one.

BA

### RE: Compression vs Compressive Strength

(OP)

BA, how big in percentage is "slightly higher modulus"? like 5%? 20% I'm talking of modulus twice to that of concrete.. for example.. grout 6000 ksi vs concrete modulus of 3000 ksi. Here do you admit it will also crack?

Do you know how to do non-demolition testing of old structure concrete.. one where the compressive strength is unknown and you can't further damage it by extracting a cylinder out of it? I wonder if Mars Curiosity has this equipment that emits laser that can gauge the compressive strength and even modulus of rocks.. can it?

### RE: Compression vs Compressive Strength

There is more to structural design than strength alone, fire is another item that comes to mind. Has this been considered in the repair details?

http://www.nceng.com.au/
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

### RE: Compression vs Compressive Strength

khinz,

It is desirable to match Ec as closely as possible, but if you are going to err, it is better to err on the high side rather than on the low side as you suggested earlier.

BA

### RE: Compression vs Compressive Strength

Agree with BAretired...a slightly higher modulus is better.

khinz....the questions you ask do not always have "black and white" answers. You asked for a percentage of modulus value for the grout over that of concrete. This is an engineering judgment call. In some cases we might want something slightly higher, in other cases we might allow a much higher modulus. For your repairs, I would go slightly higher because of the shape, location and extent of the repair. I would not go lower in any case, which is what happened with the epoxy repairs. This entire issue is not as difficult as you are trying to make it. Important, yes. Difficult, no.

### RE: Compression vs Compressive Strength

Ron,
For the first time in a while I am going to disagree with you. If we are replacing 1/4 of a concrete column with grout than I think this needs to be considered well past the strength and modulus. If this column need to have some residual strength during a fire than this should be considered and how you consider this will be difficult and will require expertise in this area (this site cannot substitute for an expert). The tests that have been carried out around the world that have been used to work out the concrete rating are generally on concrete alone without grout patches. for small repairs this isn't a concern as the heat sink of the concrete around will ensure that the grouting isn't determinant to the project.

However with 1/4 of the column being grout than we have a different story. It is the same problem that occurs with high strength concrete (above 40 mPa), all of a sudden we have fail modes that hadn't been considered at the lower strengths of concrete.

Unless you are prepared to get an expert on board (ie SIKA's in house team or similar) than I suggest you replace the column.

http://www.nceng.com.au/
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

### RE: Compression vs Compressive Strength

RE...you are correct, thus my comment about the engineering judgment. That needs to be interjected into this situation and done locally. You are also correct that many considerations need to be made. My point was simply that once such considerations are made, the implementation of such a repair is not difficult. They can't seem to get past the appropriate decision phase. Maybe that will come when the elusive structural engineering consultant finally visits the site.

There also seems to be a prevailing contractor mentality of "why spend $10 to replace the column when we can spend$100 repairing it"!

### RE: Compression vs Compressive Strength

I agree that replacing a quarter of the area of an existing column with grout in the lower one or two feet is a process which fails to inspire much confidence.

Perhaps removal of the entire concrete area for a height equal to the "bad" concrete would be a better option and less costly than removing the entire column. It does result in a cold joint close to the floor, but if that is deemed acceptable, it might be worth considering.

Whether the entire column is removed or just the lower portion, shoring would be need to be designed to carry the full column load.

BA

### RE: Compression vs Compressive Strength

(OP)

About "cold joint" close to the floor (I assume the meaning of cold joint is where the concrete is not monolithic). Concrete can't take tension and during lateral movement, it is the bars in the columns that take tension, not the concrete. This is why in construction one first construct the foundation then ground column, then 2nd floor beams,slabs then another column, then another beams/slabs, then column. The building is not casted monolithically so what is wrong with cold joint close to the floor as each pour is a cold join between each column and beam/slab in a building? What is written in your codes? This is for both ordinary and special moment frames.. but I haven't heard of entire buildings of special moment frames being cast monolithically. Please elaborate. Thanks a lot.

### RE: Compression vs Compressive Strength

(OP)

http://imageshack.us/photo/my-images/842/columnhol...

rowinghengineer and BARetired, when I mentioned 1/4 of column replaced with grout. I meant the above illustration (where the hole is in the same horizontal plane as the concrete.
The elusive structural engineer finally visited the place. He said he hasn't tried grout because he is not sure about bonding of old concrete to new concrete, and during movement the new grout inserted may get detached. But I said inside are edge bars that hold it. He said he hasn't tried it and afraid it may detach. Now I'd like to know what failure mode can be created if the bonding agent or bonding between old concrete and grout is indeed detached. I won't do it but need to know what will theoretically happen as he doesn't know know too as he hasn't tried the procedure.

The structural engineer also let me do tests where one half of a cylinder is composed of concrete and one half composed of epoxy, the cylinder to be used is 4" diameter by 8" height. This is to compare with a cylinder of pure concrete and check the difference or compressive strength. He just wants to see the results. Do you think this small scale test will be representative of actual macro behavior?

### RE: Compression vs Compressive Strength

I don't think your test will produce any worthwhile results. As for your first question I think the structural engineer is correct,there is no way you can guarantee an outcome from this large repair that will ensure the repair has a better structural solution than the original column.

http://www.nceng.com.au/
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

### RE: Compression vs Compressive Strength

(OP)

Why won't the test produce worthwhile result? The structural engineer doesn't believe me when I said the concrete can take greater load than the epoxy as Ron explained. In fact, he said it is about compressive strength like 4000-8000 psi that matters. Not only this. I found out most contractors and structural engineers in my country just use epoxy injection for voids. Even the SIKA branch and technical group in my country said epoxy can be made of large volume and injected into column voids even inside steel just like my intended use when I called up their head engineer.

Even after explaning to him the formula involved. He said actual work better than theory. So he told me to do tests of concrete cylinder and another one with one half concrete and one half cylinder. I don't know exactly what the test would result. He doesn't make it clear since he believes it's just the same since it is the compressive strength that matters. But then if the theory is right. The half cylinder tests would have the concrete crush when the load is just over half of the other whole concrete cylinder test. Why won't this work when you cut the hybrid cylinder at the same length so if concrete takes 0.003 strain and epoxy takes 0.18 strain, the concrete would crush at 0.003 strain at just 20% of the epoxy strain. Unless you are saying there is no way to cut the cylinder to accuracy of 0.001 strain. Btw.. strain has no units.. what is the equivalent milli or micrometer of the concrete strain of 0.003? Is it in percentage of the length of the object felt by the compressive force?

### RE: Compression vs Compressive Strength

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

### RE: Compression vs Compressive Strength

(OP)
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.

### RE: Compression vs Compressive Strength

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.

### RE: Compression vs Compressive Strength

(OP)

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.

### RE: Compression vs Compressive Strength

(OP)

http://imageshack.us/photo/my-images/46/stressstra...

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).

"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.

### RE: Compression vs Compressive Strength

(OP)

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.

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.

### RE: Compression vs Compressive Strength

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.

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

### RE: Compression vs Compressive Strength

(OP)
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.

### RE: Compression vs Compressive Strength

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

### RE: Compression vs Compressive Strength

(OP)

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.

### RE: Compression vs Compressive Strength

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

### RE: Compression vs Compressive Strength

(OP)

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.

### RE: Compression vs Compressive Strength

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*106 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 3600mm2...in Canada, 20M bars have an area of 300mm2, 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

### RE: Compression vs Compressive Strength

#### Quote (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.

The shear strength of a concrete beam or column is made up of two parts, Vc and Vs, 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 Vs would not change. For 4000 psi concrete, the factored concrete shear stress vc is in the order of 90 psi so Vc 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

### RE: Compression vs Compressive Strength

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.

### RE: Compression vs Compressive Strength

(OP)
BA wrote:

#### Quote:

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.

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.

### RE: Compression vs Compressive Strength

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...

### RE: Compression vs Compressive Strength

2
I nominate BAretired for sainthood!

### RE: Compression vs Compressive Strength

(OP)

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.

### RE: Compression vs Compressive Strength

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/
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

### RE: Compression vs Compressive Strength

(OP)

http://imageshack.us/photo/my-images/843/stressana...

BA, you brought up a point about transformed section that makes it worse and the calculations may not be valid. In the illustration above, the centroid of the above section goes to the right side. Now do you think the concrete in the illustration will bend to the left or right (let's ignore for now the rebars contributions whose composite action makes it more complicated)? My reasoning for bending to the left is because the weight of the section above hole is pushing it down. My reasoning for bending to the right is because the strain in the neck is bigger (meaning more compressed) than the upper or lower part, so it bends to the right (because epoxy won't compressed as much because load is mostly in the right). Or maybe the interface between the hole filling (say epoxy and concrete) would have much stress from the different strain in the interface? But where do you think will it bend, to the left or right? Anyone? (btw the illustration above is just example and not exactly my column situation as the hole is not that big)

TXstructural.. there may be modulus changes in between different parts of the concrete but does it vary by 8 times? The modulus for 4000 psi concrete is about 3600 ksi. For epoxy the modulus is merely 360 ksi or 10 times less. This is the problem.

### RE: Compression vs Compressive Strength

(OP)

rowingengineer, in my country we don't have fire rating in columns... our ready mix concrete are only one kind. The column with hole filled with structural epoxy is 1 foot below the ground floor level slabs with tie beams connecting to it. In other word, it is a joint but the tie beams are not for balancing individual footings but to make the columns in the combined footings stiffer.

Today. I had a very heated arguments with the contractor head engineer and president. He can't understand my explanation about load reduction of the epoxy and said it's the standard repair in the country. He said I'm just weird as most in the industry use it as it's the only plausible solution. He is not confident about removing a section of the column and replacing with concrete and ask me how I intend to do it and he will just listen. I asked him how to remove the epoxy already strongly bonded to the rebars. He doesn't know. Neither am I.

I may no longer remove it and I talked to my structural engineer about retrofiting the column-tie beam joint. Although I may find other structural engineers locally who have actual experience in such column partial removal and repair. My contractor said he will just pay the repair company.

For my frustrations. I'll seek other structural engineers in my country who understands this and let him write articles about it to spread far and wide to the industry as majority don't know about strain relationship and composite behavior of concrete and epoxy.

### RE: Compression vs Compressive Strength

Ron... ditto... Saint BArt...

(My thoughts) I think it was lost on the OP that BA's calcs were to show that the epoxy, due to the small E... was largely ineffective... another manner of repair should be considered... the manner of repair by BA and Robbiee is pretty much in line... I've repaired some serious honeycombing by cutting away the poor concrete to behind the rebar and using a cementitious grout patch (Sika product sometimes)

I'm almost of the opinion that even the structural engineer on site is missing a couple of dots...

Dik

### RE: Compression vs Compressive Strength

(OP)

Both the contractor and structural engineer don't understand and refuse to analyze this epoxy load reduction stuff. They told me they build 20 storey buildings and use epoxy as repair all the time and it is standard as there is no other more effective repair replacements. That's right. In the Philippines we fill gaps in columns with epoxy, 99% do it.

With no support from either. I'd just have to decide to get down the load by not anymore adding another storey. In other words. Just 2-storey with light metal roof instead of roof slab. They both are not confident of removing a section of concrete even partial. And I don't know how to remove the epoxy which is very strongly bonded to the rebars if I'd remove the epoxy and replace with high modulus non-shrink grout (both the contractor and structural engineers never tried this yet). If one drills away the epoxy from the rebars.. there may be tiny microdamages in the more brittle grade 60 20mm bars.

The nominal axial load capacity of the column is 8400 Kn. My original service load is 1200 KN. By not adding another storey. My service load would be just 800 Kn. So with ten times less the nominal axial load. I think the epoxy load reduction can be taken up by the 10 times load reduction factor and my 2-storey building with metal roof will hold.

### RE: Compression vs Compressive Strength

khinz,

On your sketch with the cavity on the left, the centroid of section shifts to the right. If the load from above and the reaction from below are assumed centered, then the eccentricity is left of the centroid and deflection will be to the right. The stress on the epoxy filled section will not be uniform as assumed in earlier calculations. Under elastic conditions, stress will vary linearly across the section with maximum stress at the left and minimum stress at the right, but the sum of stress times area must be equal to the axial load.

BA

### RE: Compression vs Compressive Strength

khinz,

I really don't know what more we can say on this subject. If you are prepared to limit the building height to two stories with metal roof, that is up to you as owner, but it seems to me that you will have wasted a lot of money on labor and materials for a five story building and now have to accept much less.

If what you are telling us is correct, it seems to me that further research is needed in the Philippines to justify present practices.

BA

### RE: Compression vs Compressive Strength

2
Sorry to disappoint you guys but I don't meet the first two requirements and #3 seems unlikely. The following is a quote from Robbiee's article:

#### Quote (The Telegraph)

However, there are a few things that must happen before anyone can become a saint.

1. To become a saint you must first be a devoted Christian, ideally a Catholic.

2. You must lead a saintly life. This includes being selfless and benevolent and an exemplary role model and teacher. It also involves loving and serving God.

3. You must perform at least two miracles. These are seen by the Church as affirmations that you can in fact intervene on the part of humans, and verifiable miracles are required for canonisation.

BA

### RE: Compression vs Compressive Strength

Robbiee... I'm not so sure of his qualifications based on your posting... I gather from his earlier posts (on other threads, too) that his perspective has not been limited by his halo...

BA... at the end, the concrete will crush to the point that the epoxy may take greater loading <G>. Two stories or 20, the same thing happens...

Dik

### RE: Compression vs Compressive Strength

#### Quote (dik)

BA... at the end, the concrete will crush to the point that the epoxy may take greater loading <G>. Two stories or 20, the same thing happens...
That is true, dik but if concrete crushes, wouldn't that constitute failure, possibly an explosive failure?

BA

### RE: Compression vs Compressive Strength

khinz,

Has your structural engineer considered the possibility of:
(a) shoring the structure to remove all load from column
(b) removing concrete in the height of the "bad" concrete, but leaving the epoxy intact
(c) replacing the concrete with epoxy fill which would bond to the existing epoxy as well as concrete above and below.

The problem with materials having two different E values in the same layer has already been discussed, but with a uniform layer of epoxy across the column, there would be a 500x500 epoxy layer 300 thick sandwiched between concrete surfaces. That would save you the difficulty of removing epoxy from reinforcement.

BA

### RE: Compression vs Compressive Strength

(OP)
BA, you said the centroid of section shifts to the right so the eccentricity is to the left of the new centroid, but you said the section "deflection will be to the right". Did you mean the column will bend to the left or right? If to the right, then you are saying the midspan will sway to the right and the top to the left? In a column, if the eccentricity is on the left, it's like P is relocated to the left by the formula e=M/P, therefore the column would bend to the left, not right, but you said "deflection will be to the right", please clarify.

And why would the concrete crush. If the column bends to the left towards the epoxy, the right side would be in tension, the concrete may crack but not from compression but from tension. The epoxy would take greater load but it is the main steel that will resist it on the left side.

About the repair material plug being grout. Imagine the plug is only 4 inches or 100mm in height. At the elastic strain of 0.0005. The deformation change would only be 0.05mm. Now if you put any repair material, are you sure the upper part will be binded to the old concrete with an accuracy of 0.05mm?? if even that micro settlement occurs, or gap form would be say 0.1mm, then the repair material would be almost nonexistence. Whereas if you use epoxy, it will bind to the concrete molecular level. Epoxy works because the adhesive can flow to the micro holes of the material and that's how its adhesiveness works. I think this is the reason the structural engineer is so afraid of gap using grout or concrete as replacement. So repair either has to be by epoxy or the entire column to the upper part replaced as this is the only way to insure continuity. Or the non-shrink grout has to be pumped inside under pressure, but so far no one pump this material.

### RE: Compression vs Compressive Strength

If a column is hinged top and bottom and compressed with axial load P, the stress is uniform at every cross section, namely P/A.

If a rectangular notch is cut out of the left side of the column at mid-height, the centroid moves to the right at the notch. The axial load falls to the left of the centroid and the notched section will move right relative to the hinged ends.

If the notch had been filled with material with low E, the behavior will be similar, but the centroid will not shift as far so the filled notch will not move as far to the right as the unfilled notch because the fill is carrying some stress but not as much as the concrete.

You asked why the concrete would crush. You do not have a perfectly rectangular notch across the full depth of the column. What you have shown on your sketch looks like an irregular cavity. Some of the concrete extends to the left edge of the column and that is the part that would crush first.

If axial stress exceeds bending stress, there is no tension on any part of the cross section, simply variable compression with maximum value on the left and minimum on the right.

At a strain of 0.0005 and a depth of only 4" of epoxy, I would not expect enough strain in the column to cause the kind of problem described above. But if the column is carrying its full design load and the epoxy thickness is 12" instead of 4", I would have serious concerns about the safety of the structure.

BA

### RE: Compression vs Compressive Strength

(OP)

I got the figure from the following page:

http://www.epcserver.com/Structural/analysis/concr...

The illustration above about form extended above void was what my contractor told me how they would proceed if I demand it replacing it including the whole section removed. But the problem with this is, when you pour any concrete or grout, there is not sure way to uniformly fill up to 0.05mm (0.0005x100mm) accuracy, gap of 0.35mm (0.0035 x 100mm (crushing force of concrete)) would even be common. Therefore I won't do this unless the formworks are sealed and the concrete or grout injected under pressure. They know no one in the country who do it. And even if it can be done. How do you apply bonding agent and installed sealed formworks within 20 minutes and pour the concrete before the bonding agent no longer takes effects. Also bonding agent is rated mostly at 500psi to 1500psi slant shear. The structural engineer said if movement of the column debond it and the piece moves and even micro gap form, it may pose greater problem, that is why he always recommended epoxy as there is no other practical choice. So I'd only attempt replacing it with hi-modulus repair material if I can find a company that can pressure pump it inside sealed formworks with long decayed bonding agent. If there is such company in the U.S. or Europe, etc. please let me know as my contractor said he'd pay for repair so i'll invite international repair team and force him to pay as it's his fault. In my country, quality control is so poor, they don't test for integrity of each column before putting the beams above. They just do it fast, that is why they miss the hole. And when they find holes afterwards, they fix it by injecting epoxy even after 15-storey building finished. This explains the structural situation in the Philippines.

### RE: Compression vs Compressive Strength

(OP)
Ba wrote:

#### Quote:

Khinz,

Has your structural engineer considered the possibility of:
(a) shoring the structure to remove all load from column
(b) removing concrete in the height of the "bad" concrete, but leaving the epoxy intact
(c) replacing the concrete with epoxy fill which would bond to the existing epoxy as well as concrete above and below.

The problem with materials having two different E values in the same layer has already been discussed, but with a uniform layer of epoxy across the column, there would be a 500x500 epoxy layer 300 thick sandwiched between concrete surfaces. That would save you the difficulty of removing epoxy from reinforcement.

I wonder or curious how you could come out with such an idea of epoxy layer sandwidth between concrete spaces. Unless the advantage is more than the disadvantage. The advantage would be no eccentricity (at least internally as there is always external eccentricity), the disadvantage is lower axial load capacity. The whole layer of epoxy would have capacity of only

P = Fc(Ag-As)+Fs(As)
=1.55(500x500-6000) + 100 (6000) = 978 KN
This is compare to load carried by entire pure concrete and bars in the 0.5x0.5 of column
P = Fc(Ag-As)+Fs(As) = 12.4(500*500 - 6000) + 100(6000) = 3625 kN

There is loss of 3625 Kn - 978 Kn = 2647 Kn. However, if service load is above 978 Kn. The load or strain would be transferred to the bars. So instead of 0.0005 strain. It becomes 0.001 and the load capacity of the bars + epoxy becomes P = Fc(Ag-As)+Fs(As)
= 3102.641(0.243718-0.006282)+199948 (0.006282) = 2012.242 Kn.
given epoxy strain 0.001, modulus 450 ksi, stress 450 psi or 3102.641 pascal and concrete strain of 0.001,modulus of 3604 ksi, stress of 2400 psi or 16547 pascal. Note I use excel for the formula that is why it's not rounded off.

The elastic limit before steel yield is 0.008 based on ACI so 0.001 is within the elastic strain of the steel. But we don't know about epoxy because we don't have the stress-strain
curve of even SIKA structural epoxy. Below the epoxy section, the concrete section would have the same bearing capacity as above it. The reason why I won't remove the concrete part of the epoxy layer anymore is because so much concrete has to be removed and we would have difficulty reaching the internal area. Also the epoxy part in my actual column is just 30% and not 40% of the column. And the eccentricity of the load is on the 70% concrete part and the structural engineer said my epoxied column has capacity many times the service load with computations
P(normal) = Fc(Ag-As)+Fs(As) = 28000(0.243718) +_414000 (0.006282) = 9424.852 Kn
P(nominal) = 0.85 * P(normal) = 8401.236 Kn
P(factored) = 0.65 * 0.8 * P(nominal) = 4368.643
My service load is only 1200 Kn. There is a purpose why the ACI have the 0.65 and 0.8 reduction.. exactly for construction related problem. In your calculations, my original concrete + epoxy has capacity of 2175 + 391 = 2566 Kn or factored load of 999Kn + 197 Kn = 1196 Kn. This is the reason I won't add another floor for service load decrease to 800 Kn. I already conveyed this to the structural engineer so he can compute if the axial load reduction can increase the tension in the eccentric exterior columns of the 2nd floor beyond the yield strength of the bars. I am in constant contact with him don't worry.

I wrote this all down because in a few months I'll forgot I may forget all of this. Please comment more if you can on the properties of the epoxy sandwich between the column and if my computations are off. In fact, the structural engineer has done this repair to other building he said as it's the only way to molecularly bond with no gaps.

### RE: Compression vs Compressive Strength

I don't agree that you could depend on the epoxy filling a gap like that. You know little about the characteristics of a given epoxy under sustained loading, and you know nothing about the characteristics of the epoxy if exposed to fire. Ditch the whole epoxy idea, dig it out, and do a proper repair.

### RE: Compression vs Compressive Strength

khinz,

My idea of using a layer of epoxy between two concrete surfaces was that (a) there would be no eccentricity and (b) you could permit greater strain in the epoxy layer than the rest of the column.

Eepoxy = 450,000 psi (where did this figure come from and is it a linear relationship throughout the complete range of stress?)

If unit strain in the epoxy layer is 0.005, then epoxy stress = 0.005*450,000) = 2250 psi which results in a greater axial capacity than the concrete column, using CSA A23.3. If the epoxy layer is 12" thick, total strain in the epoxy layer = 0.06" or about 1.5mm which is a trivial settlement and of no great concern.

Even at a strain of 0.003, the epoxy stress would be 1350 psi, so the existing columns as built may be capable of carrying more than two stories with a light roof.

You really need to establish a reliable stress/strain curve for the epoxy you are using. Until you do, we are really just guessing. It may be that Eepoxy is greater than you have assumed.

BA

### RE: Compression vs Compressive Strength

I was writing before reading hokie's post. Fire resistance is an issue I hadn't considered. That would have to be evaluated.

BA

### RE: Compression vs Compressive Strength

BA... Not necessarily... might just redistribute loads a bit <G>... depends on what you define as failure...

Dik

### RE: Compression vs Compressive Strength

(OP)

http://imageshack.us/photo/my-images/46/stressstra...

Hokie, I've been concerned about the greater epoxy creep than concrete (details below). I'm still looking for the repair company that can do the job while trying to think how to introduce all this strain compatibility thing to my structural engineer and contractor who still literally ignore all this. Most use ETABS and STaad nowadays and almost none do it manually so they miss this epoxy thing (which they don't see at ETABs).

BA, You mean epoxy elastic strain reaches up to 0.005 while concrete crushes at 0.0035? But if you push concrete to 0.001 strain, it's axial load is much greater at 0.001x3600,000 = 3600 psi versus the epoxy 0.005x450,000 = 2250 psi. But right now. Even Sika doesn't produce stress strain curve and the spec of 400 ksi to 600 ksi come from all the epoxy manufacturer. Maybe calculated from the chemical properties of epoxy.

"Example 1.2 One may want to calculate the magnitude of the axial that will produce a strain of unit shortening strain(c)=strain(s)=0.001 in the column of Example 1.1. At this strain the steel is seen to be still elastic, so that the steel stress fs=strain(Es)=0.001x29,000,000= 29,000 psi. The concrete is in the inelastic range, so that its stress cannot be directly calculated, but it can be read from the stress-strain curve for the given value of strain.

1. If the member has been loaded at a fast rate, curve b holds at the instant when the entire load is applied. The stress for strain = 0.001 can be read as fc=3200 psi. Consequently, the total load can be obtained from

P=fcAc + Fs(Ast)

which applies in the inelastic as well as in the elastic range. Hence, P=3200(320-6) + 29,000 x 6 = 1,005,000 + 174,000 = 1,179,000 lb. Of this total load, the steel is seen to carry 174,000 lb, or 14.7 percent.

2. For slowly applied or sustained loading, curve c represents the behavior of the concrete. Its stress at a strain of 0.001 can be read as fc=2400 psi. Then P=2400x 314 + 29,000 x 6 = 754,000 + 174,000 = 928,000 lb. Of this total load, the steel is seen to carry 18.8 percent.

Comparisons of the results for fast and slow loading shows the following. Owing to creept of concrete, a given shortening of the column is produced by a smaller load when slowly applied or sustained over some length of time than when quickly applied. More important, the farther the stress is beyond the proportional limit of the concrete, and the more slowly the load is applied or the longer it is sustained, the smaller the share of the total load carried by the concrete and the larger the share carried by the steel. In the sample column, the steel was seen to carry 13.3 percent of the load in the elastic range, 14.7 percent for a strain of 0.001 under fast loading, and 18.8 percent at the same strain under slow or sustained loading.

### RE: Compression vs Compressive Strength

#### Quote (dik)

BA... Not necessarily... might just redistribute loads a bit <G>... depends on what you define as failure...
I agree that it would not necessarily fail, but we need to know that it won't fail. We won't know if we don't have a stress vs. strain curve for the epoxy.

CSA A23.3 defines failure of concrete as a strain greater than 0.0035. I believe ACI 318 says a strain of 0.003. Other codes may have different definitions of failure.

BA

### RE: Compression vs Compressive Strength

#### Quote (khinz)

BA, You mean epoxy elastic strain reaches up to 0.005 while concrete crushes at 0.0035?
Nothing wrong with that. The epoxy can be at a greater strain than the concrete if they are layered.

#### Quote (khinz)

But if you push concrete to 0.001 strain, it's axial load is much greater at 0.001x3600,000 = 3600 psi versus the epoxy 0.005x450,000 = 2250 psi.
Not true. The most you can get from 4000 psi concrete by code is about 1950 psi no matter how large the strain is.

#### Quote (khinz)

But right now. Even Sika doesn't produce stress strain curve and the spec of 400 ksi to 600 ksi come from all the epoxy manufacturer. Maybe calculated from the chemical properties of epoxy.
I don't know how it is calculated, but it should be possible to measure it by test. In order to justify the procedure which you say is common practice in the Philippines, i.e. filling large voids in concrete columns with epoxy grout, you need to know the properties of the epoxy. That includes a stress/strain curve.

BA

### RE: Compression vs Compressive Strength

(OP)
BA or others,

The book says steel tensile strength peak can go up to 0.008. But it also said that "If the small knee prior to yielding of the steel is disregarded, i.e., if the steel is assume to be sharp-yielding, the strain at which it yields is

strain(y)=fy/Es=60,000/29,000,000=0.00207"

but earlier the book says "The steel reaches its tensile strength (peak of the curve) as strains on the order of 0.08..."

So is it 0.00207 or 0.08 before steel permanently deforms from the elastic limit and which does the ACI code follow??

BA, for strain of 0.005 for epoxy, since it is strongly bonded to the bars, then the bars would also have strain of 0.005 in since their strains are equal in axial compression, and if the above limit of 0.00207 is true, then the bars would already yield (permanently deform) at 0.005. Therefore we need to keep it at 0.00207.. which would produce 0.00207 (450,000) = 931.5 Kn only and not 0.005 (450,000) = 2250 Kn. What do you think?

### RE: Compression vs Compressive Strength

OK, I finally got a chance to look at the pre- and post-repair photos. The "before" photo seems to be a void left because of inadequate consolidation. My experience is that what is unseen is probably as bad or worse than what is visible. Be sure you don't have additional voids that are not visible. I agree with Hokie that your concerns are valid, and that you need to know the precise properties of the epoxy before making any further assumptions.

At this point, it is a practical impossibility to remove the epoxy, except possibly by hydrodemolition. I would not be terribly concerned about leaving small areas or large thin layers of epoxy on bars or concrete surfaces after removing the bulk of the epoxy for replacement.

BA, the photos seem to show solid epoxy, not epoxy-aggregate grout. If they used epoxy-aggregate grout, the modulus would be very nearly that of portland cement grout, since the epoxy would bind and confine the aggregate particles, but the load would be transmitted through direct (or very nearly direct) aggregate contact. If it was competent epoxy-aggregate grout, I would not hesitate leaving it in place and letting it carry load.

### RE: Compression vs Compressive Strength

Es = 200,000 MPa
Fy = 400 MPa
Yield strain of steel, ey = Fy/Es = 0.002

For strains greater than yield strain, stress in steel remains at Fy (not strictly true, but it is the assumption structural engineers make and is close enough for practical purposes).

The strain in the steel, going through a small layer of epoxy which is strained to 0.005 will not change that quickly but, even if it did, the steel will remain at yield stress for all practical purposes.

In any event, the whole thing is academic if the epoxy will not stand up to the elevated temperatures of a fire. I don't believe epoxies perform satisfactorily when temperatures approach 1000o C so if you are going to rely on them, you would need some kind of fire protection around them.

BA

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