Limitation of concrete stress during tensioning in steps: the values of k4 and k5
Eurocode 2 part 1-1: Design of concrete structures 18.104.22.168 (4)
If prestress in an individual tendon is applied in steps, the required concrete strength may be reduced. The minimum strength fcm(t) at the time t should be k4 [%] of the required concrete strength for full prestressing given in the European Technical Approval. Between the minimum strength and the required concrete strength for full prestressing, the prestress may be interpolated between k5 [%] and 100% of the full prestressing.
The recommended value for k4 is 50% and that for k5 is 30% for defining the limitation of concrete stress.