Achieving project scope on time at minimum cost is important for 
contractors to ensure profitability. Existing models attempt to arrive 
at optimal schedules, but without considering the delays that occur 
during construction, which have been statistically proven to occur on 
most projects. Since these schedules cannot guarantee delays recovery, 
it cannot be claimed optimal. Hence, this paper attempts to incorporate 
delay recovery during construction into the optimal schedule generation. 
Recovering delays that occur over the life of the project construction 
requires a set of project crashes for the remaining and incomplete 
activities in the project. Ensuring that the incomplete activities can 
be crashed at low cost in conjunction with the progress of the project 
construction represents a proposed solution to such a problem. And it is 
called in this paper the schedule's residual flexibility. In developing an 
optimal schedule, a trade-off between the project time, cost, and residual 
flexibility is considered.