Penetration experiments on glass found in the literature show that the target fails ahead of the projectile (the "failure wave" is faster than the penetration velocity). Thus, during penetration, the projectile is in contact with failed glass ahead; consequently, the properties of failed material are required for accurate numerical simulations. This paper shows, using a confined compression experiment of a cylindrical specimen combined with an analytical model for interpretation of the data, how it is possible to determine the elastic and the Drucker-Prager constants for damaged borosilicate glass (or, in principle, any other brittle material). An important and unexpected result from these characterization tests is that the elastic properties of damaged glass do not change dramatically even if the glass is pulverized (in situ comminuted). This is probably because the tests are highly confined and the glass has no space to move, and thus the elastic properties remain essentially unchanged. Estimates for the Drucker-Prager constitutive constants for in situ damaged borosilicate glass are also obtained.