I am trying to relate degree of corrosion with pressure exerted in surrounding concrete. My common sense tells me that higher the degree of corrosion, higher should be the pressure exerted, but my equation tells me it is constant. Obviously, i am making a calculation mistake, but could not figure it out. What mistake am i making (figure attached)?

Consider unit length along the rebar:

Area after corrosion is

A1=A0*(1-p) ; p is percentage of corrosion

the volume occupied by rust if there was sufficient space is given by converting the volume of steel to equivalent volume or rust

A2=A1+p*ros/roc*A0;

Initial volume

V1=A2-A1

final volume

V2=A0-A1

Change in volume

dV=V1-V2

strain=dV/V1

solving, strain=(-roc + ros)/ros

there is no P.

What am i doing wrong?