#### Topic: Stiffness values for GAP material properties

Hi,

First: What I'm trying to do

I am modeling shear bond strength between old and new concrete. This means i have a contact surface between the substrate (old) and overlay (new) concrete. to test this i am basically creating a cube (150mm3) with two halves of different strength concrete and shearing them at the interface.

now...

in Atena, i am modeling this with a GAP material between the two surfaces. at this stage i am still playing around with the cohesion and friction factors to see their direct influence while keeping tensile constant. Am I correct in assuming that:

i) the Gap material is not a "filler" or "physical" material but  rather description parameters for the behavior and surface properties of the contact between the surfaces ?

This would imply that:
- Cohesion is actually adhesion between the contact planes and
- friction would indicate a mechanical friction coefficient between the planes
- the tensile strength is an indication of the pull-off bond strength capacity of the two surfaces. would cohesion and tensile strength not be the same then or do is the assumption for cohesion wrong?

ii) Would the Tangential and Normal Stiffness coefficients be the Stress / displacement slope up to failure ? you would calculate this by normal (or tangential) force / area / displacement to failure ?

iii) the min Ktt and Knn values used for computing be the failure slope (i.e. -> surface failed in normal or tangent when this value is reached and Ktt min is always less than Ktt (same for Knn)) ?

also, where can i get more info on the theory behind the gap material parameters ?

Thanks

#### Re: Stiffness values for GAP material properties

i) yes, the gap/interface material parameters are related to surface (not volume)

The influence of cohesion, friction, and tensile strength is best cleared in chapter 2.5 "Interface Material Model" of the Atena Theory Manual (includes  figures showing the failure surface and displacement-stress diagrams). You can download the latest version of the manual from our web (Products - Atena - Manuals - ATENA Theory) http://www.cervenka.cz/assets/files/ate … Theory.pdf

ii) yes

iii) the Ktt_min and Knn_min should be greater than 0 and smaller than Ktt or Knn, the recommended values are {Ktt,Knn}_min=Ktt{Knn}/1000. They define the tangential and normal stiffness AFTER failure (this is needed to prevent numerical problems).

#### Re: Stiffness values for GAP material properties

thank you  ! i was still looking at 3.3.0 Theory manual. its making more sense now.