Lightweight Concrete for High Strength - Expanded Shale & Clay

losses in the 10,000-psi girders totaled 29.6 ksi which was lower than that of the 8,000-psi
girders by 7.6 ksi. AASHTO-LRFD refined and lump sum methods overestimated total loses by
22.3 and 3.7 ksi, respectively.
In Figure 6.2, the predicted-to-measured ratio is shown. Losses are grouped in elastic
shortening, creep and shrinkage, total time dependent, and total losses. Overestimates appear as a
predicted-to-measured ratio greater than one, and the underestimates as lower than one. The four
methods overestimated the total time-dependent prestress losses, but they underestimated elastic
shortening losses regardless the type of HPLC. The AASHTO-LRFD refined, PCI and ACI-209
overestimated creep and shrinkage losses (CR + SH) by at least 100%. The underestimate in
steel relaxation losses given by the PCI and ACI-209 methods was probably due to the much
higher creep and shrinkage losses that they predicted which decreased predicted relaxation in the
steel.
Figures 6.3 and 6.4 present measured and predicted strain data found from cylinder
specimens. The fact that all methods underestimated elastic shortening was probably a
consequence of the procedures for measuring elastic shortening. The strain measurement was
taken after prestress transfer, which took approximately one hour. Therefore, the first reading
after transfer included not only instantaneous elastic strain, but also early creep and shrinkage.
The same argument can be used to explain why all methods overestimated time dependent
losses.
Within time-dependant losses (TD), differences between estimates primarily were due to
shrinkage losses. For the 8,000 psi girders, the PCI method estimated shrinkage losses as 5.1 ksi
(2.5%) while ACI-209 method estimated it to be 11.3 ksi (5.6%).
As shown in Figures 6.3 and 6.4, the largest relative differences were obtained on the
shrinkage portion where PCI and AASHTO refined methods significantly underestimated
shrinkage losses. For the 8,000 psi HPLC, the PCI and AASHTO refined methods
underestimated creep; but for the 10,000 psi HPLC those estimates were close to the measured
creep strains. The ACI-209 method gave the best creep estimate for the 8,000 psi HPLC, but it
overestimated creep for the 10,000 psi girders.
6-3