concrete footing on soil, help please

[reza378salami]

dear sir,
i want to model a concrete foundation on soil, i apply the soil by PressureIndependent material with tag 1 and it works good (i use GID to build 861 nodes and 800 quad elements).
i add the concrete foundation like this:

model BasicBuilder -ndm 2 -ndf 2
#nodes for foundation
node 862 -1 20
node 863 1 20
node 864 1 21
node 865 -1 21

#material for foundation
uniaxialMaterial Concrete01 2 -4.0 -0.002 0.0 -0.005

#foundation element
element quad 801 862 863 864 865 1 "PlaneStrain" 2 0.0 0.0 0.0 -22
and the error is:
WARNING material not found
Material : 2
ForNodeQuad element :801
it seems that its not true to use uniaximaterial concrete01 with element quad ?
how can i define concrete slab on soil ?
thankx.