how to set the absolute contact stiffness when considering cohesive zone material (CZM)property

I'm simulating the interface between a steel plate and the fire proof coatings covering the steel plate. The elements i use in the simulation is contact 173 with a material property of debonding,which is defined as cohesive zone model.(CZM)

 CZM in ansys provides  TBDATA such as maximum contact stress,contact at the completion of debonding, but what's the problem is that I failed to set the initial contact stiffness. it's true that ansys provides FKN to define the contact stiffness, but it's just a stiffness factor.

It's also true that we can set the FKN with a negative value which can be regarded as the absolute stiffness in ANSYS. But when I did this, another problem came to me, the stiffness would not change any more even if the contact status changed(debonding),it means the debonding material property failed to be gained.

I just wanna make it clear whether  some real constants or keyopts should be modified. is it possible that i can set the initial stiffness(an absolute one,not a factor) in the simulation considering CZM material property(debonding)?

thank u