Lightweight Concrete for High Strength - Expanded Shale & Clay

Appendix D. Prestress Losses in HPLC
D.1 General
Prestress losses in special concretes such as structural lightweight concrete (SLC), high
performance concrete (HPC), and high performance lightweight concrete (HPLC) follow the
same principles and are affected by the same factors that affect normal weight normal strength
concrete (NWNSC), but they are influenced by the particular properties of each.
Prestress losses in HPC: HPC usually has higher modulus of elasticity, a lower creep and
a similar or lower shrinkage than a NSC. Based on that, it is expected to obtain fewer losses due
to elastic shortening, fewer losses due to creep losses, similar or fewer losses due to shrinkage,
and more losses due to steel relaxation. The expected increase in steel relaxation losses is a
consequence of a higher stress level in the prestressing steel due to a decrease on concrete losses.
Total losses are expected to be less than NSC. According Roller et al. (1995), measured longterm
prestress losses in HPC prestressed girder were approximately 50% less than the expected
value.
Prestress losses in SLC: Properties of SLC may vary in a wide range, so the prestress
losses may also widely vary. In general SLC presents a lower modulus of elasticity than a NWC
of the same strength. It also has a higher ultimate creep and ultimate shrinkage than the NWC
counterparts. Therefore, elastic shortening, and final creep and shrinkage losses are expected to
be greater in SLC. Steel relaxation losses, however, are going to decrease due to the increase in
the others. ACI-213 (1999) concluded that combined loss of prestress in a SLC member is about
110 to 115% of the total losses for NWC when both are cured normally. If they are steam-cured,
prestress losses in SLC are expected to be 124% of the losses in NWC. PCI (1998) gave a range
for total prestress losses of “sand-lightweight” members of 30,000 to 55,000 psi which is about
15% higher than the range given for NWC.
Prestress losses in HPLC: To the authors’ knowledge, there is no previous research on
prestress losses of HPLC; however, from the material properties some conclusions can be drawn.
Elastic shortening losses are expected to be similar or less than NWNSC but more than HPC.
Creep and shrinkage losses would be similar to the one of HPC. Steel relaxation losses would
tend to be higher than losses in NWNSC because the previous losses are lower.
D-1