The faom in a Zipp disk is structural, but it is fine if some of it is dented or broken down. It acts as a spacer between the two skins. The skins bear the torsional and gravitational loads. The foam just acts as a stabilizer to make the skins, rim, and hub act as a rigid box structure. By having a rigid spacer in between the carbon skins, one will be in tension and the other compression. This keeps both skins from being in compression as side loads areaput on it, which would make for a weak disc, as carbon is relatively weak in compression. Think of an I-beam. The central truss spaces out the top and bottom which are in tension (bottom) and compression (top) to support loads.