Determining aging coefficient and time-dependent losses and deformations of prestressed members with due consideration of percentage of steel Original Research Article

Aging coefficient (T.K. Bandyopadhyay and B. Sengupta, ACI J. 83(2) (1986) 236–243; ACI J. 85(2) (1988) 190–205) is affected by the percentage of steel and its disposition in a member. Neglecting the effect of reinforcement in the calculation and consequently assuming stiffness coefficient α = 1 indicates a prestress loss less than the actual loss. This difference in prestress loss from the actual loss is appreciable for age at loading varying from 180 days to 3 days, with an increase in difference of about 8% for a lower age at loading of 3 days. If the time of first loading is 28 days and the creep coefficient is low, the aging coefficient may be even less than 0.25. As explained by T.K. Bandyopadhyay and B. Sengupta, (ACI J. 86(1) (1989) 107–108) and T.K. Bandyopadhyay (Ph.D. Thesis, Jadavpur University, 1989) for age at loading 7 days ⩽ t0 ⩽ 28 days, the aging coefficient is always positive for αφ values greater than 0.30, 0.20, 0.10 and 0.075 for t0 ⩾ 7 days, 10 days, 14 days and 28 days respectively. If the loads are deferred up to 90 and 180 days, then the losses may be reduced by up to 70% and 60% respectively. The percentage of loss completed is also greater for higher percentages of reinforcement with an optimum value of about 1%. Creep deformation (T.K. Bandyopadhyay, Ph.D. Thesis, Jadavpur University, 1989; T.K. Bandyopadhyay and B. Sengupta, Trans. SMiRT-12, Stuttgart, 1993, Paper H10/3) varies directly with αφ. For a singly reinforced section, the creep reduction coefficient generally varies from 1.0 to 0.6. Although the creep reduction coefficient increases for higher grades of concrete, its product with the creep coefficient decreases.