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?

 

[BigH]

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.

 

[Ron]

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!!

 

[khinz]

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 grout.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.

 

[Ron]

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.

 

[khinz]

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?

 

[BAretired]

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

 

[Ron]

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!

 

[khinz]

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.

 

[BAretired]

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

 

[khinz]

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?

 

[rowingengineer]

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."

 

[BAretired]

khinz,

It is desirable to match E_c 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

 

[Ron]

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.

 

[rowingengineer]

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."

 

[Ron]

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"!

 

[BAretired]

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

 

[khinz]

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.

 

[khinz]

http://imageshack.us/photo/my-images/842/columnhole.jpg/

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?

 

[rowingengineer]

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."

 

[khinz]

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?