ACI says: “It is necessary to use large-diameter (0.7 and 0.85 in. [17.8 and 21.6 mm]) strands in unbonded post-tensioned structures to address the question on flexural response, particularly regarding its strength.”
The paper, written by Wei Zhou and Yahui Hu, says experiments were conducted to investigate the ultimate stress of the large-diameter strands as unbonded tendons in simply supported members.
According to ACI: “Two crucial factors—prestressing reinforcement index and global reinforcement index—are obtained from the cross-sectional equilibrium equation and test data of the present and already-existing experiments.”
Both factors are used to derive equations to predict the increase in stress and ultimate stress of large and normal diameter strands.
“Evidences support the behaviour and demonstrate the feasibility of large diameter strands for PT structure in terms of static loading,” ACI concludes.