recently I was looking into ContactLoop and saw that Contact stiffness K_n are formulated for ConcreteMat differently compare to that for FrictMat.
while in FrictMat (here K_n)
K_n = 1/(E1A1/l1 +E2A2/l2)
Concrete Mat to my understanding is more or less obtained by averaging E1 and E2 of two particles (link to code here K_n):
Is there a reason behind it? to me the first formulation makes more sense, since the stiffness will depend on the softest material.