The outcome of collisions between pre-planetesimals is important in the theory of planetesimal formation by collisional growth and strongly depends on their internal structure. Since pre-planetesimals are highly porous, reaching 90% porosity, they could show the so called anomalous behaviour (decrease of density during shock compression, e.g. Bolkhovitinov & Khvostov 1978). Due to involved sizes (>dm), laboratory experiments are unfeasible therefore numerical simulations equipped with adequate porosity models are necessary.

Here we focus on the P-α model and its variations. We found that they are suitable for applications in the high porosity range only after a modification of the basic equations, that avoids an inconsistency and takes into account the effect of dissipated energy, is performed.