Commonwealth of Pennsylvania
Department of Transportation
PA Test Method No. 630
October 2013
8 Pages
LABORATORY TESTING SECTION
Method of Test for
ASSESSMENT OF CONCRETE TO STRAND BOND FOR PRESTRESSED
APPLICATIONS
1.
SCOPE
1.1
This test method examines bond quality of seven-wire strand used in
prestressed concrete applications by means of a destructive pullout test of a strand from a
concrete prism. The method is developed for examination of both strand quality and
concrete bond properties.
1.2
This test method is applicable primarily under field conditions; however,
controlled laboratory tests are also appropriate.
1.3
The values stated in this test method are in pounds and inches unless
otherwise noted.
2.
REFERENCED DOCUMENTS
2.1
ASTM International. “C 39/C 39 – Compressive Strength of Cylindrical
Concrete Specimens,” West Conshohocken, PA: 2004.
.
2.2
ACI 318-02, Building Code Requirements for Structural Concrete and
Commentary. American Concrete Institute, 2002.
2.3
PCI, “PCI Design Handbook,” Precast and Prestressed Concrete, Fifth
Edition, Chicago, IL, 1999
3.
SIGNIFICANCE AND USE
3.1
This test method is intended to be used to assess the adequacy of bond
between concrete and seven wire prestressing strand using production concrete mixes and
as-delivered strand under plant fabrication conditions. The test method allows for
identification of potential bond problems due to changes in strand surface conditions,
concrete mix proportions, or concrete constituents. The test method is intended for use by
a precast/prestress plant to allow for initial mix design verification or initial strand
supplier qualification prior to use in full production. The test would be performed during
the mix prequalification phase. The test is not intended to be used during production.
PTM No. 630
October 2013
Page 2
4.
APPARATUS
4.1.
Molds for Test Specimens- The molds for the test specimen shall measure
6.5-in. wide by 12-in. deep (Figure 1). The specimen length is equal to the bonded length,
Lb, plus an additional 2-in. un-bonded length. The bonded lengths are tabulated relative
to different strand diameters in Table 1. The molds shall be made of steel or wood and
allow for easy removal after fabrication. The molds shall be made watertight after
assembly around the prestressed strand. Special care shall be taken to seal areas where
the strand extends from the mold. The molds shall be designed to hold the strand at the
center of the cross-section.
Table 1: Specimen Bond Length
Strand Designation
Bond Length, Lb [in.] PCI*
Specimen Length [in.]
3/16
22
24
7/16
26
28
1/2
30
32
1/2
Special
32
34
9/16
34
36
0.60
36
38
*Calculated per PCI Bridge Design Manual (8.3.1.5) using 60 strand diameters
End Elevation
Side Elevation
Figure 1: Test Specimen
4.2
Measuring Apparatus- The movement of the strand with respect to the
concrete shall be measured on the rear (dead) end. A dial gage or displacement transducer
with a graduation of 0.001-in. and a minimum stroke of 0.5-in. shall be used. The load
shall be measured with a calibrated load cell in-line with the strand being tested. Use of a
jack pressure gage is not recommended unless the gage has been calibrated with a load
cell and is capable of providing a resolution of +/- 20 lbs. A suitable displacement
measuring apparatus is shown in Figure 2.
PTM No. 630
October 2013
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Figure 2: Measuring apparatus
4.3.
Testing Apparatus - The testing apparatus shall consist of a through-hole
hydraulic jack or universal testing machine, a reusable strand chuck, and a bearing plate.
The bearing plate shall have an adequate bearing area to prevent crushing of the concrete
surface. The hydraulic jack shall have a controller that allows for a smooth load increase
of 20 kips/min. A schematic of a usable testing configuration is shown in Figure 3. The
jack shall have a minimum stroke of 4-in. to allow for full pullout.
Figure 3: Test apparatus
5.
TEST SPECIMEN
5.1
The test specimens shall be cast in a horizontal orientation on the precast
plant prestressing bed around the stressed strand. An initial jacking stress of 75% of the
strand ultimate strength is recommended. For example, for a 270 ksi ½-in. special strand
a jacking force of 33.8 kips is recommended. No additional reinforcement shall be used.
If possible, the specimens shall be situated at the end of the strand run, inline with the
precast element that is being fabricated. Adequate free strand length shall be provided on
the un-bonded end to allow for jacking (a length of 40 in. is recommended). A minimum
of three specimens shall be fabricated for each combination of strand type and concrete
mix design. All specimens shall be cast from the same batch of concrete using the same
strand lot, within one (1) hour of each other and cured together.
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October 2013
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5.2
Concrete shall be placed and vibrated using form or immersion-type
vibrators. For self consolidating concrete mixes (slump > 10-in.) placement can be
conducted in one lift without consolidation. Once the mold is filled, the top surface shall
be finished with a steel trowel and covered to prevent loss of moisture. Curing methods
shall conform to standard plant practice.
5.3
A minimum of six companion concrete cylinders, 4-in. by 8-in. or 6-in. by
12-in., shall be fabricated and tested according to ASTM C 39 to determine the
compressive strength of the concrete at transfer and at the time of testing. The cylinders
will be match cast and match cured with the specimens to ensure consistency between the
specimen and cylinder strengths.
5.4
The initial prestress shall be transferred in accordance with plant
procedures. Slow transfer of stress is recommended; however, flame cutting of strands
can be used.
5.5
The release strength of the concrete shall conform to the design
application. Initial testing, for the purpose of developing historical data of accepted
and/or approved mix designs used for prestressed concrete beams must be performed
when the concrete compressive strength at release is not less than 6800 psi or greater than
7500 psi. That strength is to be verified before release in accordance with the plant's
normal procedures.
5.6
After release of prestress the strand shall be cut flush with the surface of
the specimen at the dead end and a 40-in. length of strand shall be left on the live end (debonded side). The cut side shall be ground flat with an abrasive wheel to provide a
smooth bearing surface for the displacement measurements.
5.7
Testing of the specimens shall take place within two (2) hours after
release. Pullout tests of the three specimens shall be conducted sequentially over a
duration of less than one (1) hour to ensure consistency in concrete strengths.
6.
PROCEDURE
6.1
The specimens shall be tested in the horizontal position. The displacement
gage shall be attached to the strand at the dead end of the specimen as shown in Figure 3.
At the dead end of the specimen the displacement gage shall be attached to one of the
outer strand wires. Avoid the inner wire as relative slip may occur between the inner and
outer wires of the strand resulting in an inaccurate measurement.
6.2
The jack and bearing plates shall be supported with blocking prior to
loading to ensure that the strand is centered in the test setup. Apply a load at a maximum
rate of 20 kips/min., or if displacement control of the ram is available, at a rate less than
0.02 in./min. Take proper safety precautions against potential flying debris that may be
generated during failures. Safety guidelines are beyond the scope of this document and
shall be discussed with the plant safety committee prior to use.
PTM No. 630
October 2013
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6.3
Record the applied load (in pounds) and displacements regularly such that
a minimum of 10 points are recorded prior to a dead end displacement of 0.01-inch.
Record the load to the nearest pound and the displacement to the nearest 0.001-inch. Be
careful to note the load level when the dead end reaches a displacement of 0.01-inch, Fi,
and 0.1-inch, Fu. Continue recording until: 1) a decrease in load by 25% is measured, 2)
the strand fractures, or 3) a displacement of 0.5-in. is measured on the live end.
7.
CALCULATION
7.1
Bond quality is computed relative to lower bound transfer and
development lengths measured for the chosen concrete properties and strand type. The
lower bound estimate of transfer length is computed according to Equation 1. The lower
bound bond length required to achieve the fracture strength of the strand can be computed
according to Equation 2.
Lti = Lower bound transfer length [in.]
Lult = Lower bound bond length required to fracture strand [in.]
fpi = Initial stress in strand at transfer [ksi]
fpu = Ultimate strength of strand [ksi]
Fi = Measured force corresponding to 0.01-in. slip at dead end of specimen [kips]
Fu = Measured force corresponding to 0.10-in. slip at dead end of specimen [kips]
2
As = Cross-sectional area of strand [in. ]
Lb = Bonded length of strand [in.]
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October 2013
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7.2
To compare the bond qualities of the results with concretes of different strengths,
compute a normalized transfer and ultimate bond length. Normalize the lengths relative
to the tensile concrete strength by dividing the computed lengths by the square root of the
concrete compressive strength as shown in Equations 3 and 4.
f’c = Concrete compressive strength measured at the time of the pullout test [psi]
8.
REPORT
8.1
Include in the report the following information:
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Date and Time of Concrete Placement
Date and Time of Stress Transfer
Date and Time of Pullout Test
Concrete Mix Batch Weights
Mix Design Identification No.
Concrete Constituents
Strand Size and Type
Strand Manufacturer
Strand Heat Number and Lot ID
Strand Yield Strength [ksi] (from mill certification)
Strand Ultimate Strength [ksi] (from mill certification)
Concrete Compressive Strength at Release [psi] (average and
standard deviation)
Concrete Compressive Strength at time of Pullout Test [psi]
(average and standard deviation)
Initial Strand Jacking Stress [ksi]
Strand Stress Prior to Transfer, fpi [ksi]
Measured Pullout Force at Initial Slip, Fi [kips] per test
Measured Maximum Pullout Force, Fu [kips] per test
Failure mode
Lower bound transfer and bond lengths
Normalized transfer and bond lengths
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October 2013
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9.
ACCEPTANCE
9.1
Until historical data is generated using this test method it is recommended
that passing performance be assessed relative to the performance of historically good
mixes. For example, to qualify a new self consolidating concrete (SCC) mix the
following procedure can be adopted. The SCC shall be considered passing if the average
of the three (3) tests is greater than 90% of the average of a high early strength (HES)
concrete that has historically good performance.
9.2
With continued use of this test method it is expected that the results can be
correlated directly with ACI/PCI transfer and development lengths.
10.
ACKNOWLEDGMENTS
10.1 This test method was developed through a project financed by Schuylkill
Products Inc. and a grant from the Commonwealth of Pennsylvania, Department of
Community and Economic Development, through the Pennsylvania Infrastructure
Technology Alliance (PITA) and through the support of, the Pennsylvania Department of
Transportation, the Precast Association of Pennsylvania (PPA), and DeGussa
Construction Chemicals. (PITA Project PIT-457-04, Dr. Clay Naito, Lehigh UniversityPrincipal Investigator).
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October 2013
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