Lightweight Concrete for High Strength - Expanded Shale & Clay

D.2 Standard Prediction Methods
Prestress losses methods can be classified into two groups: (1) final prestress losses
estimate and (2) losses estimated at any time. There are three methods for estimating final
prestress losses: Precast Prestressed Concrete Institute Method (PCI, 1999), refined estimate and
approximate lump sum estimate, both proposed by the American Association of State Highway
and Transportation Officials Method (AASHTO-LRFD, 1998). For losses at any time, American
Concrete Institute Committee 209 (ACI-209, 1997) proposed a prestress loss estimate method
based on creep and shrinkage estimates (Equations B.1 and B.3 in Appendix B).
Even though anchorage seating losses and friction losses can be an important portion of the
total prestress losses, they are not considered here because such losses are related with the
manufacturing process rather than material properties.
D.2.1. PCI Method
The PCI method gives an estimate of the final prestress losses of a prestressed concrete member
based on four equations for each type of losses. They are applicable to NWC and SLC. Total
losses are given by Equation D.1
T . L.
= ES + CR + SH + RE
(D.1)
where
T.L.: total prestress losses (ksi)
ES: elastic shortening loss (ksi)
CR: creep of concrete loss (ksi)
SH: shrinkage of concrete loss (ksi)
RE: steel relaxation loss (ksi)
Elastic Shortening. Caused by concrete shortening around tendons as the prestressing force is
transferred, elastic shortening can be estimated by Equation D.2.
K
es
⋅ E
ps
⋅ f
cir
ES = (D.2)
E
ci
where
ES: elastic shortening loss (ksi)
K es : elastic shortening constant, 1.0 for pretensioned members
D-2