Debonding Strain of Externally Bonded FRP

[asixth]

ACI440.2R-08 Says in 10.1.1 that to prevent a crack induced debonding failure mode, the effective strain in the FRP should be limited to:

?fd=0.083*sqrt(f’c/(n*Ef*tf))

where ?fd is the limiting strain of the FRP, f'c is the strength of concrete, n is the number of plies of FRP, Ef is the E-modulus of the FRP and tf is the thickness of the FRP.

Using 4,640psi concrete, FRP with a E-modulus of 23,925ksi and 0.055" (1.4mm) FRP laminates will give a limiting strain of 0.005 where the FRP has an ultimate strain of 0.017. This is a reduction factor of 0.3. Later in the clause it goes on to say that the committee recommends a limiting strain of 0.7*?fu where ?fu is the ultimate strain.

What porportion of ultimate strain do people use for the designing of externally bonded FRP applications.

 

[ishvaaag]

All that comes to my mind is that member design has a reliabiliy index (I don't remember is this label is the correct one technically) against falure of about 3 and connections should have over 4 to keep the general reliability index of the structure homogeneous with that of required from the members. In this light the implied safety factor respect ultimate strain set by the code seems in accord with what normally being done.

 

[rapt]

asixth,

The way I read this is that the ultimate strain is limited to .7 for ductility purposes, but you also have to consider delamination which in this case is the controlling criterion and anchorage bond at the ends. .3 is low compared to some other recommendations I have seen which went as high as .4 - .45.

 

[asixth]

Thanks, I have been given another document from RMIT which limits the strain to 1500/(n*Ef*tf)*?fu which comes out closer to 0.45*?fu. This document was referencing a 2002 revision of ACI440.

We have decided against the FRP strengthening because of the adverse nature of the design and the fact that all the literature that we have recommends strengthening the existing with externally bonded FRP and ensuring that the structure will still have the ability to resist collapse in the event of FRP rupture or debonding.

This would not be satisfied with what we were trying to do, we were relying solely on externally bonded FRP as reinforcement, if the FRP failed because of high strains, fire or vandalism than the structure had no redundancy to prevent collapse.