Lightweight Concrete for High Strength - Expanded Shale & Clay

Chapter 6. Prestress Losses in HPLC
6.1 Introduction
One goal of this research was to determine how the use of HPLC would affect the loss of
prestressing force in bridge girders. The creep and shrinkage data found from HPLC cylinders
and from AASHTO Type II girders made with HPLC (Chapter 5) were used to estimate prestress
losses in bridge girders. These experimental losses are compared with four models: AASHTO
refined and AASHTO lump sum (AASHTO-LRFD, 1998), ACI Committee 209 (ACI-209,
1997), and the PCI method (PCI, 1998), which are presented in Appendix D. For comparison
purposes, ACI-209 estimates were computed for 40 years after prestressing assuming that time as
the final loss state. Actual losses were computed from measured, experimental strains of
AASHTO Type II girders. The experimental data did not include steel relaxation losses.
Experimental strains were projected to the 40-year condition for comparison with the estimates
from the standards.
6.2 Strain Measurements and Comparison to Predicted Losses
Strain measurements of those girders as shown in Figure 6.1 provided experimental data
for actual prestress computations. Table 6.1 presents the comparison between measured and
estimated prestress losses for the 8,000-psi and 10,000 psi HPLC girders.
-1000
Microstrains (in/in x 10 -6 )
-800
-600
-400
Deck pouring
-200
8,000-psi Individual Girder Result
10,000-psi Individual Girder Result
0
0 20 40 60 80 100 120 140
Age (Days)
Figure 6.1. Experimental strains for the 8,000-psi and 10,000-psi HPLC 39-foot girders.
6-1