where Cpv,k = present cost of the kth maintenance action performed at time t; C=cost of the maintenance at the application time; and r = discount rate of money. As can be seen from Eqs. (1)and (3), the indirect maintenance cost depends, to a great extent,on the ADT. Because the ADT may be subjected to an annual ncrease rate, the maintenance cost will also be time dependent.Assuming a constant rate of increase, the ADT at time t can be calculated as