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tendons can be selected. Spacings of two to four times the prestressed concrete pavement thickness
are recommended for the longitudinal tendons, and spacings of three to six times the prestressed
concrete pavement thickness are recommended for the transverse tendons.
c. Prestressing Steel Tendons. The tendons used for prestressed concrete pavement will consist of
either high-strength wires, strands, or bars.
(1) Wires will conform to the requirements of ASTM A 421.
(2) Seven-wire strands will conform to the requirements of ASTM A 416.
(3) High-strength bars will conform to the requirements of section 405(f) of ACI 318.
d. Prestressing Conduits. Conduits used for enclosing the steel tendons should be either rigid or
flexible metal tubing. However, the tendons may be plastic-encased.
(1) Metal conduits must be strong enough to resist damage in transit or during handling. The
metal may be bright or galvanized.
(2) When tendons are plastic-encased, the tendons should be permanently protected from rust
e. Placement of Tendons and Conduits. The transverse conduits will be placed on metal chairs at
the desired depth and used to support the longitudinal conduits or tendons. Conduits and tendons will
be tied firmly in place to maintain proper alignment during placement of the concrete. A preliminary
stress applied to the tendons may help maintain the alignment. The inside diameter of metal conduits
will be at least 6 millimeters (0.25 inch) larger than the diameter of the stressing tendons. The minimum
cover of the conduits will be 75 millimeters (3 inches) at the pavement surface and 50 millimeters
(2 inches) at the bottom of the pavement.
f. Tendon Stressing. The prestressed tendons must be stressed to provide a stress in the concrete
equal to 1.2 times the design prestress ds plus sufficient stress to overcome the frictional resistance
between the tendon and conduit. After concrete placement and prior to beginning the prestressing
operation, any preliminary tension in the tendons must be released. If the tendons are conduit-encased,
they should be pulled back and forth several times to reduce and to measure the tendon stress due to
friction. This need not be done for plastic-encased tendons. The measured tendon-friction stress must
be added to the tendon stress required to produce 1.2ds in the concrete. If the tendons were sized as
described in b above, the required tendon stress will be the selected anchorage stress (0.7fF or other
value if used to size the tendon), plus the stress required to overcome friction. After the maximum
tendon stress is reached, it will be held for several minutes and then released to the selected anchorage
stress. The longitudinal tendon stressing will be applied in three stages with the amount of prestress at
each successive stage being 25, 50, and 100 percent of the anchorage stress. The prestressing will be
applied as soon as possible to prevent or minimize the occurrence of contraction cracking in the
g. Grouting. When the stressing tendons are placed in conduits, the space between the tendons
and conduits will be grouted after the final prestressing load is reached. The grout will be made from
either cement and water or cement, fine sand, and water. Admixtures to obtain high early strength or to
increase workability may be used if they will have no injurious effects on the stressing tendons or
conduits. Grouting vents will be provided at each end of the conduits and along the conduits at intervals