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The data obtained during the transfer testing are not re- solution, and (3) after an ignition process. The calcium hy-
ported as a length, but instead as an average bond stress over droxide exposure (also called a lime dip) will convert sodium
the transfer length. This was done to enable comparisons of soaps (e.g., sodium stearates) to insoluble calcium salts. For
stress transfer behavior between the tested strand sources, example, water-soluble sodium stearate (a soap or wetting
since this approach eliminates complications from strands of agent) is converted to a film of insoluble calcium stearate
varying sizes and varying initial stress conditions. The aver- (a wax-like, water repellent that increases the surface energy
age bond stress over the transfer length, Ut, is calculated as of the strand). This conversion reaction was chosen to simu-
late the reaction of concrete with surface residues of soaps and
f se Aps is intended to produce a condition where the effect of similar
Ut = (Eq. 1)
C p Lt calcium stearate compounds on the contact angle are com-
pared, even if the original residue did not result from a calcium
where fse is the effective prestress after transfer, Aps is the cross- stearate-based lubricant. The ignition process was performed
sectional area of the strand, Cp is the circumferential perimeter on samples to volatilize organic compounds expected to be
of the strand (4/3 db) and Lt is the transfer length. Average present in the drawing lubricants.
bond stress is thus dependent both on effective prestress as
well as transfer length for a given strand geometry. For the
Examination under Ultraviolet Light
purpose of this calculation, fse was taken as the difference be-
tween the stresses in the strand before release and the elastic Certain lubricant additives (e.g., hydrocarbon oils, fluo-
losses only. The elastic loss was determined based on the rescein additives, and some inorganic deposits) will fluoresce
strain measured immediately after release in the central region under ultraviolet (UV) radiation. An examination under UV
of the test prism over which the strain is approximately con- light was conducted using a range of light sources, the most
stant, assuming no relaxation losses in the strand. promising of which was a 366-nm wavelength.
Proposed Quality Control Test Methods Testing pH
The test methods that were proposed and conducted as part Testing of the pH of the surface was attempted with each
of the screening and correlation test program are summarized of the strand sources to see if alkalinity of a solution generated
in Tables 2 and 3. These tables also list the QC levels for these by placing drops of water on the residue could be linked to
tests, if applicable. These test methods consist of (1) surface and bond. Testing of the pH of the surface was conducted using
chemical testing, (2) pull-out testing, and (3) transfer length indicator papers, indicator solutions, and a pH meter.
testing. Since insufficient lengths of strand were available for
pull-out and transfer length testing during the Screening Round
Loss on Ignition
from the historic sources, these tests were conducted only on
the recently manufactured strand. The surface and chemical The weight loss on ignition (LOI) represents the weight of
testing program has been conducted on both the historic compounds that can be volatilized or burned off the strand
strand samples and on the recently manufactured samples. surface at high temperature. This property was measured with
the expectation that the weight lost would consist mainly of the
Surface and Chemical Testing organic component of residues, such as drawing lubricants.
The surface and chemical test methods that were attempted
Loss in Hot Alkali Bath
are described briefly. More complete descriptions are given in
Appendix B. The weight loss after hot alkali bath (LAB) represents the
weight of compounds that can be washed off the strand sur-
face in a hot sodium hydroxide solution. As with the LOI test,
Contact Angle Measurement
this property was measured with the expectation that the
The contact angle is a measure of surface tension (wet- weight lost would consist mainly of drawing lubricants.
ability). It was anticipated that the presence of drawing lubri-
cants would affect this property. The contact angle is measured
Change in Corrosion Potential
on the projected shadow of a small droplet of distilled water
applied to the strand surface. Measurements were taken with Past studies of the corrosion resistance of prestressing strand
the strand: (1) in an as-received condition, (2) after immersing in concrete have suggested that strand with a coating of residue
the strand sample in a saturated calcium hydroxide [Ca(OH)2] does not corrode as readily as a clean strand. To assess the
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Table 2. Test methods conducted during screening and correlation testing programs.
Condition/Type QC
Test Method Property Measured Objective
of Test Level
As received Detect presence of materials that reduce water surface tension
Contact Angle Measurement After Ca(OH)2 dip I Surface energy of strand (Na-based soaps) or increase steel surface energy (Ca-based
salts)
After ignition
Identify lubricant additives such as hydrocarbon oils, some
Examination under UV Light - I Presence of fluorescing materials inorganic deposits, or possibly fluorescing-based tracers that
may fluoresce under UV light
Universal indicator
Indicator solutions Detect presence of pretreatment lubricant residues containing
pH Testing I pH of surface
pH meter alkaline salts or alkalies
High-res. indicator
Weight of material burned off Determine amount of material that can be oxidized on the
Weight Loss on Ignition (LOI) - I
strand strand surface at 415°C, expected to be largely organic
Method 1 Determine amount of material that can be washed off the strand
Weight Loss in Alkali Bath I Weight washed off strand
Method 2 surface after soak in a NaOH solution
As received
After Ca(OH)2 dip Assess the potential for corrosion by comparing the corrosion
Change in Corrosion Potential I Average change of potential
potential to a reference cell monitored versus time
After ignition
Surface Roughness - I Roughness parameters Ra, Rz, Pc Quantify surface profile
As received
Determine the shift in potential of a metal sample from a stable
Corrosion Rate After Ca(OH)2 dip II Corrosion current
corrosion potential due to an external current
After ignition
Warm water/acid-
Determine amount of individual components of strand
chloroform wash Weight of extracted organic
Organic Residue Extraction II manufacturing lubricants from a warm/hot water wash
Hot water/acid- residue
procedure then an acid/solvent-wash procedure
chloroform wash
Sodium
Calcium
Atomic Absorption (AA) Potassium Concentrations of inorganic Quantify inorganic elements (sodium, calcium, potassium, zinc,
II
Spectroscopy Boron components of extraction residue and boron) in residue
Zinc
Phosphate
Maximum bond stress and stress at Mechanically measure stresses required to break bond with
Pull Out from Large Concrete Block - II
0.1 in. displacement (or first slip) concrete
Pull Out from Portland Cement Maximum bond stress and stress at Mechanically measure stresses required to break bond with
- II
Mortar 0.1 in. displacement (or first slip) mortar
Pull Out from Hydrocal-Based Maximum bond stress and stress at Mechanically measure stresses required to break bond with
- II
Mortar 0.1 in. displacement (or first slip) Hydrocal-based mortar
Length over which the prestress is Directly measure bond performance in prestressed concrete
Transfer Length - Analytical
transferred to a concrete beam beam
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Table 3. Coefficient of determination (R2) Organic Residue Extraction
from linear regression with average bond
stress over transfer length. The tests for identification and quantification of organic
drawing-compound residues were based on solvent extrac-
Coefficient of tion procedures, together with gravimetric and Fourier trans-
Determination (R2)
from Regression form infrared spectroscopical (FTIR) analyses. Essentially,
Test Method QC Level
with Average Bond the amount of material extracted from a defined length of
Stress over
Transfer Length
strand was determined by weighing the extraction residue on
Concrete Pull Out II 0.98 an analytical balance. The material in the extraction residue
Mortar Pull Out II 0.85 was then identified by FTIR analysis of the residue. The FTIR
Hyrdocal Mortar Pull Out II 0.36 spectrum obtained is like a fingerprint of the material.
The extraction procedure used is a modification of a pro-
cedure found in ASTM C114 for organic materials in cement.
potential for corrosion, the strand samples were placed in
Multiple extractions were used to differentiate between various
a solution of deionized water, and the corrosion potential
forms of drawing-compound residue. The strand was first
measured with a reference cell (saturated calomel reference
washed with warm or hot water to remove water-soluble
electrode), was monitored versus time. This corrosion poten-
materials, such as sodium stearate. Then, the strand was ex-
tial is determined by the amount of ferrous ions in solution
posed to hydrochloric acid and chloroform to extract water-
surrounding the sample, and a greater drop in this potential
insoluble residues such as calcium stearate and stearic acid.
is indicative of a greater tendency to corrode. Measurements
At the conclusion of the Screening Round, it was observed
were taken with the strand in an as-received condition, after
that the water temperature had little effect in most cases, but
immersing the strand sample in a saturated Ca(OH)2 solution,
that the residue concentrations measured with the warm water
and after an ignition process.
method seemed to generally correlate better with bond tests.
Therefore, only a warm-water wash was used in the Correla-
Surface Roughness tion Round. In addition, to minimize the effort spent on per-
forming the time-consuming chloroform organic extraction,
Microscopic examinations of sectioned portions of wire
the wash solutions from the warm water and acid-chloroform
taken from strand have indicated that an observable difference
washes were combined, and a single separation was performed.
in the surface roughness of the good- and poor-bonding strand
Therefore, only one FTIR scan and residue weight determi-
sources exists. Based on images captured using a scanning elec-
nation was made per piece of strand in this round of testing.
tron microscope, the depth of the roughened surface features
However, this is considered essentially equivalent to the com-
is typically 3 µm (0.0001 in.) or less. Trials with a portable
bination of sequential warm water and acid-chloroform washes
profilometer suitable for a QC setting were conducted to de-
performed in the Screening Round. Since quantifying the
termine whether these physical measurements could accurately
water-soluble materials was still of interest, because it might
represent the surface roughness and to investigate the corre-
provide insight into possible cleaning methods, separate warm
lation with bond performance.
water washes were performed on additional pieces of strand.
This system works by measuring the deflection of a diamond
This wash solution was acidified and saved for elemental
probe, with a 2-µm tip radius, as it is dragged 2 mm across the
analysis.
surface of the sample.
Corrosion Rate Atomic Absorption and Colorimetric Analysis
To further explore the interaction between strand bond To identify the chemical composition of residual inorganic
and corrosion, the instantaneous rate of corrosion of samples components of pretreatment chemicals and drawing com-
of strand in a salt solution was measured with a polarization pounds, chemical analyses of the acidified water extract and
resistance technique. The polarization resistance technique acid/solvent extract solutions, which had been obtained dur-
measures the corrosion current, which quantifies the rate at ing the organic residue extraction procedure and had been
which the electrochemical corrosion reaction is occurring. separated from the chloroform, were performed. Either zinc
This is a much faster test than the test for change in corrosion phosphate or borax (sodium borate) is often applied to the wire
potential, but requires specialized equipment (a potentiostat). before the drawing process begins to help drawing lubricants
Measurements were taken with the strand in an as-received stick to the surface of the rod stock. Most common drawing
condition, after immersing the strand sample in a saturated lubricants are expected to include stearate salts, particularly
Ca(OH)2 solution and after an ignition process. sodium and calcium stearates. The elemental concentrations
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of sodium, potassium, calcium, and zinc were determined by end of the strand were monitored throughout testing. To allow
atomic absorption spectroscopy. The solutions were also comparison of data among strand of different sizes, the bond
scanned for detectable quantities of aluminum during the stress has been calculated from the measured loads based on
Screening Round. Colorimetric analyses of the wash solutions the nominal surface area (equal to 4/3 db l, where db is the
-
for boron and phosphate ( PO3 4 as total phosphate) were per- nominal strand diameter and l is the embedment length) of
formed using visible light spectroscopy. the embedded section of the strands. Two characterizations
of performance are determined during strand bond pull-out
tests. The first characterizes the early part of the bond stress-
Pull-Out Testing
slip relationship, while the second is based on the maximum
The original project scope included the development of a stress measured throughout the test. In concrete pull-out tests
performance-based test method for use in evaluating strand performed on historic strand, the early performance was char-
bond. As a result, in the initial phases of this study, efforts acterized in terms of the stress at which movement is first vi-
were made to develop a procedure for quantifying bond using sually observed at the loaded end of the strand, called the stress
a pull-out test conducted on untensioned strand embedded in at "first observed slip" or "first slip." For the tests conducted
some material. Two types of pull-out tests have been commonly as part of this experimental program, the stress selected to
used to evaluate strand bond. The first method involves characterize the early part of the bond stress-slip relationship
pulling untensioned strand out of a block of concrete. The is the bond stress at 0.1-in. slip, measured at the non-loaded
second method involves pulling untensioned strand out of a end of the strand. This 0.1-in. slip criterion was adopted to
steel cylinder filled with mortar. give a more precisely defined location on the stress-slip curve.
In its current form, the concrete pull-out test resembles a
method developed by Moustafa (1974). The method was pri- Large Concrete Block Pull-Out Test
marily developed to judge the capacity of strand to be used as
lifting loops to handle product during shipping and erection. The large concrete block pull-out test (LBPT) involves
The test developed by Moustafa was modified by Logan pulling six untensioned strands bonded over 18 in. from a
(1997) to judge the bond quality of strand in pretensioned large (2 ft × 2 ft × 2 ft 8 in.) block of concrete. This concrete
applications. Further developments of the method have oc- was produced from a conventional mix design used by a pre-
curred and are the basis for the testing reported herein. caster, and is produced with a coarse aggregate with Mohs
In its current form, the mortar pull-out test method resem- hardness greater than 6.0. The strength of the concrete at the
bles a method originally developed for the Post Tensioning time of the test is 3500 to 5900 psi. The test is conducted in
Institute in 1994 (Hyett et al. 1994, Post-Tensioning Institute load-rate control with a load rate of 20 kips per minute. The
1996). The method was primarily developed to judge the bond bond stress at 0.1-in. slip and the average maximum bond stress
quality of prestressing strand used in rock anchors. The method for each strand tested are reported and averaged. In addition,
became the basis of ASTM A981-97 (2002) Standard Test for the concrete pull-out tests conducted as part of this study,
Method for Evaluating Bond Strength for 15.2 mm (0.6 in.) Di- the load at which slip at the loaded end was first observed
ameter Prestressing Steel Strand, Grade 270, Uncoated, Used in visually also was recorded. This "observed first slip" was
Prestressed Ground Anchors (ASTM 2002). Later, this method determined because it relates back to historic pullout data
was modified by Russell and Paulsgrove (1999) for NASPA recorded by Logan (1997) and others.
and became known as the NASPA test. The NASPA test has
been modified slightly by this research project to make it less
Mortar Pull-Out Test
sensitive to the test apparatus.
One of the goals of this project was to try to eliminate vari- In the mortar pull-out test, each prestressing strand is em-
ables by using standardized and universally available embed- bedded in 5-in. diameter by 18-in. long steel cylinders filled
ment media referred to in the NCHRP Project 10-62 Request with mortar (Portland cement, sand, and water). The top 2 in.
for Proposal (RFP) as a "surrogate homogeneous material." of the embedded portion of the strand is debonded, leaving
Accordingly, a third type of pull-out test was attempted: 16 in. of strand in contact with the mortar. Six cylinders are
pulling untensioned strand out of a steel pipe filled with a tested for each source of strand. The mortar is produced with
modified gypsum plaster (Hydrocal). a Type III cement-to-sand ratio equal to 2:1 by weight, and
The three types of pull-out tests were performed on the three the required mortar strength at time of the test is 3500 to
sources of strand at KSU in March and May of 2005. Each of 5000 psi. The bond stress at 0.1-in. slip and the average
these test procedures and their results are described briefly maximum bond stress for each strand tested are reported
below. More complete descriptions are given in Appendix B. and averaged.
In each of these methods, the load applied to pull out the The test was conducted in load-rate control with a load rate
strand and the movement (or slip) of the non-loaded (free) of 5 kips per minute, which was reportedly similar to the rates
