Compression vs Compressive Strength 11

[khinz]

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

How does it relate to compression strength like 5000 psi?

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

 

[BAretired]

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

 

[BAretired]

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.

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.

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

 

[khinz]

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=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?

 

[TXStructural]

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.

 

[BAretired]

E[sub]s[/sub] = 200,000 MPa
F[sub]y[/sub] = 400 MPa
Yield strain of steel, e[sub]y[/sub] = F[sub]y[/sub]/E[sub]s[/sub] = 0.002

For strains greater than yield strain, stress in steel remains at F[sub]y[/sub] (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 1000[sup]o[/sup] C so if you are going to rely on them, you would need some kind of fire protection around them.

BA