Commonly asked questions about unbonded tendons and their
installation.
What are the properties of the
prestressing strand that you specify for tendon fabrication?
Prestressing strand is generally made up
of 6 wires wrapped around a center wire. It's properties are defined
in the Specification ASTM A-416 and for slab foundations we normally
specify 0.5" diameter, grade 270 (270 ksi).
What are unbonded tendons?
Unbonded tendons are fabricated to easily
elongate within the cured concrete by being isolated along their length
with a lubricated sheath (typically plastic) similar to that of an
insulated electrical wire. As such they do not bond to the concrete.
What force is present in a tendon after
it has been stressed and locked off?
Typically and with a 0.5" dia. tendon,
the force should be no less than 31,000 lbs.
How can you tell if the tendons have
been stressed properly?
By measuring their elongation as
related to the face of the jacking-end anchor and comparing that value
with the calculated value (Δ) listed on the engineered plan.
What is the recommended procedure for
field inspection?
This is most easily accomplished by
placing a mark on the exposed tendon tail at a fixed length from the face
of the concrete prior to jacking and then measuring and noting the
difference after the stressing has been completed.
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