Chapter 4 Partial Loss of Prestress 4.1 Introduction Prestressing a concrete member effectively applies a substantial axial force to the member that is in place for its entire service life. In both pretensioned and post- tensioned methods of prestressing, this prestressing force begins to decline imme- diately upon its application and continues to decline throughout its service life. This reduction in force is referred to as partial prestress loss and is addressed as part of the design of a prestressed member. Partial prestress losses, typically referred to as “prestress losses,” are divided into two broad categories: initial and time-dependent effects. Initial losses occur during stressing operation and include anchor seating, elastic shortening, and friction between prestressing steel and post-tensioning ducts or tendon deviators and harped pretensioned strands. Long-term losses occur because of viscoelastic material effects and include concrete shrinkage, creep, and tendon relaxation. This chapter covers the causes of prestress losses as well as techniques for estimating those losses. Numerous methods are available for estimating losses. This is likely due, at least in part, to the difficulty in accurately predicting losses. ACI 423-10 (2016) devotes an entire chapter to the variability of loss calculations and the reader is referred there for more detail. In summary, the variability in concrete mechanical properties, curing conditions, and exposure to environment are the primary causes of this difficulty. While there are many different approaches for determining initial and time-dependent effects on prestressed concrete, the most effective method is to construct the element and measure the losses in the field. Corrections are done in the field for friction losses on occasion; other corrections are not common except for research. In many cases, a high level of accuracy is not needed to ensure suitable strength and serviceability. Consequently, the design engineer estimates the losses and bases the design on this estimate. © Springer Nature Switzerland AG 2019 C. W. Dolan, H. R. Hamilton, Prestressed Concrete, https://doi.org/10.1007/978-3-319-97882-6_4 85