securely anchored to ducts and to either the forms or reinforcing steel to
prevent displacement during concrete placing. The ends of ducts shall be
effectively protected to prevent entry of water, concrete, grout or debris.
Wires of parallel-wire assemblies shall not be spliced. Steel bar tendons
may be joined by couplers where shown or approved, provided they are
capable of developing the guaranteed minimum ultimate strength of the bars.
Strands to be spliced shall have the same lay or direction of twist and
the ends shall be cut by shears or abrasive grinders. No more than one
strand shall be spliced in any one member where single strand tensioning is
employed. Strand splices shall be capable of developing the full ultimate
strength of the strand. Slippage of the splice shall be checked and
correction made for differential slippage. Where multiple strand
tensioning is used, not more than 10 percent of the strands in any member
shall be spliced.
3.1.5
Tensioning Tendons
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NOTE: Determination of the initial prestress force
must consider prestress losses in accordance with
ACI 318, Section 18.6.
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Tensioning of stressing tendons shall be as specified and shown. The
stress induced in the tendons by any method of tensioning shall be
determined independently by both (1) measurement of tendon elongation and
(2) direct measurement of force using a pressure gauge or load cell. If
the results of these two measurements do not check each other and the
theoretical values within 5 percent, the operation shall be carefully
checked and the source of error determined and corrected before proceeding
further. Concrete cylinder tests shall indicate a breaking strength of at
least [_____] MPa psi before transfer of stress to ensure that the concrete
strength is adequate for the requirements of the anchorages or for transfer
through bond as well as meet camber or deflection requirements. The final
prestress load in each unit after seating shall be as shown. Safety
measures shall be taken by the Contractor to prevent accidental injury
caused by failure of a stressing tendon or tendon component. The exposed
ends of stressing tendons and anchorages shall be protected from damage
during stressing operations to prevent failure.
3.1.5.1
Pretensioning
Strand tendons may be tensioned by jacking of groups of strands or may be
tensioned individually by means of a single-strand jack. Before final
tensioning, all tendons shall be brought to a uniform initial tension of
approximately 4.5 kN 1,000 pounds per strand per 60 m 200 feet of bed, with
a minimum of 4.5 kN 1,000 pounds and a maximum of 13.5 kN 3,000 pounds per
strand. The force corresponding to the initial tension shall be measured
by a dynamometer or other approved method to aid in determining the final
elongation. After this initial tensioning, the tendons shall be stressed
to the total tension indicated on the drawings using hydraulic or
mechanical equipment with gauges or dynamometers graduated and calibrated
to accurately determine the load applied. Draped pretensioned strands
shall be tensioned partially by jacking at the end of the bed and partially
by uplifting or depressing strands, or they shall be held in their draped
positions by means of rollers, pins or other approved methods and tensioned
entirely by jacking. Approved low-friction devices shall be used at all
points of change in slope of draped strands while tensioning draped
strands, regardless of the tensioning method used. Cable stress shall be
SECTION 03230
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