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?

 

[khinz]

http://imageshack.us/photo/my-images/843/stressanalysis.jpg/

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.

 

[khinz]

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.

 

[dik]

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

 

[khinz]

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.

 

[BAretired]

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

 

[BAretired]

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

 

[Robbiee]

BA,
Since I agree with Ron on the sainthood nomination, here is an article that might help.

 

[BAretired]

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:

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

 

[dik]

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

 

[BAretired]

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

 

[BAretired]

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

 

[khinz]

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.

 

[BAretired]

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

 

[khinz]

I got the figure from the following page:

http://www.epcserver.com/Structural/analysis/concrete/info_concrete_repair.asp

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.

 

[khinz]

Ba wrote:

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.

 

[hokie66]

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.

 

[BAretired]

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.

E[sub]epoxy[/sub] = 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 E[sub]epoxy[/sub] is greater than you have assumed.




BA

 

[BAretired]

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

BA

 

[dik]

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

Dik

 

[khinz]

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

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.

But there is one problem about creep. At sustained loading of strain 0.005, the capacity or curve may change to lower psi. Earlier. I mentioned that "I also learnt that for fast loading vs slow loading, the steel carries more load in slow loading because the fc is smaller in value." in which you answered "I don't know about that. I am not arguing, I simply don't know.". The following makes it clear. This epoxy thing can make one remember lessons learnt over 50 years ago. Again refer to the above figure. Quoting briefly from the book "Design of Concrete Structurs" where I learnt all this (this concept is important in this epoxy analysis):

"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.