Any confinement to the concrete could increase the strength and ductility within the member. The actual concrete compressive  strength  is independent mechanical property for concrete as a material. 

about increase this value it seems your path have some problem. this amount of increase not logical

if you cover whole the concrete with steel jacket maybe maybe show this increase

and as @Kamal Alogla said it depends on concrete property not jacket influence...

I agree with prof.arash

fc' is material property of concrete. Confinement with steel jacket may not be very effective  however i think FRP fabrics will be more effective as it is being used for rehabilitation of structures. 

confinement with steel jacket increases the fc' but not as much as you mentioned. A high confining pressure if possible practically will provide you such as increase. 

i agree with others. the increase expected is very high . moreover anchorage of the steel plate with the base concrete(8 MPa strength) may be impractical. 

You search for an increase of compressive resistance of factor 3. This can only be attained by confinement, be it steel or FRP. Most codes limit confinement action to a factor of 5 or 6. You find numerous literature on confinment action to concrete, e.g. fib bulletin (no. 14, I think) with which I found very accurate results in recalculating experimental behavior of concrete cylinders, confined with flax fiber sheets.

Good luck

I am in agreement with the other comments ant take it you have decided on steel but would ask a couple of questions, most of which I'm sure you would have considered.

How did the f'c get to 8Mpa, and from what design Mpa?

What caused this degradation? Age, chemical influence, others...?

What method would be used to place , tension and secure the steel?

Why was FRP eliminated?

Has complete column replacement been considered? 

Is the building of historic significance?

Given the very bad state, how much of the building is effected and was demolition considered?

And how good are the contractors that would be doing the work?

In my experience FRP's or CRP or epoxies and such is the easiest and generates the best results in corrosive environments. Replacement in failure state cases.

All the best for you project.

By the way Daia Zwicky it is No 14. We have followed it for about 15 years

Thank you Mr. Mick Sedunary

-The designed strength should be 24 MPa but due to freezing effects it is 8 Mpa now for some  columns.

- We do not want to use active methods so no tension is needed

- We calculated that with FRP  we needed 9 layers due to CODEs requirement. So it was so expensive

- We also considered concrete replacement but it is also 3 times more expensive than steel jacket and also have lots of risks

- The building is not historic

-only one percent of columns have same problem

We have considered all the methods of rehabilitation and we concluded that the steel jacket is the most economic one. but we have some doubts if it works practically.

Ok. three of my people all went to FRP when I asked them, and I would agree but different places differing costs.

A point that was raised  in our lunch time chat was; "What would you use between the steel and the concrete...?" Jamal said epoxy, Hans and Chen said zero shrink grout and I suggested epoxy grout. 

Anyway, we would be curious to see how your project proceeds.

Dear

the area of steel in the steel jacket should be at least 50% of the existing reinforcing bars, and the area between the jacket and the concrete should be filled with epoxy grout. the jacket should also be covered  with epoxy grout as suggested by Mick.

When steel is placed around concrete without gap, the vertical stress/strain can create lateral strain.If this lateral strain is constrained it can create radial compression in concrete due to the poisson effect..But the actual stress in concrete will increase which is not a desirable effect.Concrete strength may increase due to lateral confinement.But it is difficult to predict as per actual site conditions.I have doubt about the practical use. Better use some other alternative like transferring the load on steel jacket etc.

I thin 3 times is too much for confinement effect.

I would say that theoretically, this is achievable but very tricky to apply in practice. The effectiveness of confinement relies on the interface interaction between steel and concrete--and that alone is very complex.  Some research works available in the literature suggest that higher axial capacity can be achieved when concrete and steel are unbonded (Orito et al.), but such increase in axial capacity will ensue a localized damage in concrete which promotes softening of post-peak behavior. There are numerous analytical equations available in the literature to predict the strength of concrete under confinement, but given the very low compressive strength of your concrete, consideration for cracking and crushing should also be checked.

Dear Dr Saad, The increase in the load capacity of composite is come from 2 points:

first from additional area of steel pipe.

second due to enhancement in the value of fc due to confinement effect provided by steel tube.

Dear Hesamaldin Moeindarbari, theoretically yes

This can be done if the rigidity of the steel jackets is so high, which impossible practically.

See the figure, if you can put your concrete stress state at the hydrostatic axis, you will increase the strength.

Dear Hesamaldin,

Please remember to take into account in Your analysis the fact, that columns in the existing building are already loaded (preloading) and therefore:

1. there is some axial strain already imposed into the column, due to dead loads (substantial in RC buildings)

2. large portion of creep of concrete probably already took place.

These two conditions may have large influence on the overal strength of strenghtened members.

Best Regards,

Bartosz Grzeszykowski