maintained between anchorages until the concrete has reached the specified
compressive strength.
3.1.5.2
Detensioning
Forces from pretensioned strands shall be transferred to the concrete by
either the multiple-strand release or the single-strand release method.
The stress transfer shall not be performed until concrete strength, as
indicated by test cylinders, has reached the specified transfer strength.
If concrete has been heat-cured, the detensioning shall be done immediately
following the curing period while the concrete is still warm and moist.
During detensioning, the prestressing forces shall be kept nearly
symmetrical about the vertical axis of the member and shall be applied in a
manner that will minimize sudden loading. Eccentricity about the vertical
axis shall be limited to one strand.
a. Multiple-Strand Release: In this method, all strands shall be
detensioned simultaneously and the load transferred gradually to the
concrete by hydraulic jacking.
b. Single-Strand Release: In this method, all strands shall be
detensioned by slow heat-cutting the strands in accordance with a
pattern and schedule as approved. The strands shall be heated using a
low-oxygen flame until the metal gradually loses its strength, causing
release of the strands to occur gradually. The low-oxygen flame shall
be played along the strand for a minimum of 125 mm. five inches.
Strands shall be so heated that the failure of the first wire in each
strand shall occur after the torch has been applied for a minimum of
five seconds.
3.1.5.3
Post-Tensioning
Tensioning shall not be performed until the concrete has reached the
required strength at transfer of stress. Before final tensioning of
tendons, all tendons shall be brought to a uniform initial tension of
approximately 10 percent of the full load. The force corresponding to the
initial tension shall be measured by a dynamometer or other approved method
as a starting point in determining final elongation. A temporary
overstress above the final prestress force as approved by the Contracting
Officer shall be used to overcome stress losses. The units shall be
tensioned until the proper elongations and jacking pressures are attained
and reconciled within the limits stated above. Straight tendons may be
tensioned from one end. Curved or draped tendons shall be stressed by
simultaneous jacking from both ends using a common pump with identical
hoses and jacks, unless otherwise shown.
3.1.6
Grouting Post-Tensioned Tendons
Grouting between each tendon and its enclosing duct shall be performed
within 5 days after completion of the tensioning operation. Grouting shall
not be performed if air temperature below 7.2 degrees C 45 degrees F is
anticipated within 48 hours after grouting unless an approved method of
temperature control is used. The grout shall be mixed in a mechanical
mixer of a type that will produce uniformly and thoroughly mixed grout.
First water shall be placed in the mixer followed by cement and admixture.
Grout shall be continuously agitated until it is pumped. Grout that has
begun to set shall be discarded. Just before grouting, the ducts shall be
flushed with clean water and then blown clear by compressed air to removed
excess water. With the grout vent open at one end of duct, grout shall be
SECTION 03230
Page 10
Previous Page
