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Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete (2008)

Chapter: Appendix C - Specifications for Standard Surface Test Methods

« Previous: Appendix B - Evaluation of Mechanical and Chemical Test Methods
Page 123
Suggested Citation:"Appendix C - Specifications for Standard Surface Test Methods." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
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Page 123
Page 124
Suggested Citation:"Appendix C - Specifications for Standard Surface Test Methods." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
×
Page 124
Page 125
Suggested Citation:"Appendix C - Specifications for Standard Surface Test Methods." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
×
Page 125
Page 126
Suggested Citation:"Appendix C - Specifications for Standard Surface Test Methods." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
×
Page 126
Page 127
Suggested Citation:"Appendix C - Specifications for Standard Surface Test Methods." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
×
Page 127
Page 128
Suggested Citation:"Appendix C - Specifications for Standard Surface Test Methods." National Academies of Sciences, Engineering, and Medicine. 2008. Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete. Washington, DC: The National Academies Press. doi: 10.17226/14206.
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Page 128

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123 Introduction Based on the experimental results conducted during this study, four surface and chemical test methods have been rec- ommended for inclusion in a QC program to assess strand bond. The recommended QC methods have been written in AASHTO/ASTM standard method format and presented in this appendix. They are titled: 1. Test Method for the Determination of the Surface Tension of Steel Strand by Contact Angle Measurement, 2. Test Method for Weight Loss on Ignition (LOI) of Steel Strand, 3. Test Method for Change in Corrosion Potential of Steel Strand, and 4. Test Method for Identification and Quantification of Residue on Steel Strand by Extraction, Gravimetric, and Spectroscopical Analyses. Proposed Standard Test Method for Determination of the Surface Tension of Steel Strand by Contact Angle Measurement Scope 1.1 The contact angle of a water droplet on a strand surface is a measure of surface tension (wet-ability). The presence of drawing lubricants will affect this property. A relatively high amount of water-insoluble drawing lubricants will cause a relatively high contact angle value. The contact angle is meas- ured on the projected shadow of a droplet of distilled water applied to the strand surface using a specialized instrument designed for this purpose. 1.2 The contact angle is measured after immersing the strand sample in a saturated calcium hydroxide [Ca(OH)2] solution to simulate the environment surrounding the strand when it is in concrete. Due to the chemistry of Portland cement, concrete mix-water quickly becomes saturated with calcium hydroxide. This calcium hydroxide solution reacts with drawing-compound residues on the strand. This is the form of the residue on the strand when it is in concrete. Thus, this is the form of the residue that affects the bond, and this is the form to be assessed with this test. 1.3 This standard does not purport to address the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Apparatus 2.1 Contact angle meter—The contact angle of a water droplet on a strand sample surface is measured using a contact angle meter.1 The apparatus has a built-in syringe to dispense distilled water droplets of a small, precise size. Also, it has a fiber optic light source and a focusing lens to project the water droplet image onto a graduated screen. A photograph of the apparatus is shown in Figure C-1. Specimens 3.1 The strand segments shall be cut to a length of 12 in., a length suitable to fit on the V-shaped sample pedestal of the apparatus. Test Procedures 4.1 The procedure for preparation of each strand segment is described as follows: 4.1.1 Immerse the entire strand segment, or the portion to be tested, in saturated calcium hydroxide solution for 10 min. A P P E N D I X C Specifications for Standard Surface Test Methods 1 This device is commercially available as Cam Plus Micro contact angle meter manufactured by ChemInstruments, Inc., Fairfield, OH.

4.1.2 Immerse in de-ionized water for 5 min. 4.1.3 Dry by allowing vertical strand to drip while expos- ing to a warm air stream from a minimum 100 W heat gun held at a distance of 12 in. for a period of 2 min. 4.1.4 Set strand on a clean surface and cool to room temperature. 4.2 The procedure for contact angle measurement with the aforesaid apparatus is as follows: 4.2.1 Set the strand sample horizontally on the pedestal of the test apparatus with the surface to be tested facing upward. 4.2.2 Adjust the projection lens so that silhouettes of both the wire surface and the syringe needle are in focus. 4.2.3 Adjust the position of the helical wire surface to achieve a level silhouette on the projection screen under the syringe needle. 4.2.4 Adjust the syringe needle so that it is directly over the high point of the helical wire surface. 4.2.5 Form a distilled water droplet at the tip of the syringe. Rotate the syringe barrel to form a droplet with a diameter of 7 units on the scale imprinted on the projection screen 4.2.6 Raise the pedestal holding the strand sample until the wire surface touches the water droplet, and then lower the pedestal until the water droplet is released from the syringe needle. 4.2.7 Adjust the sample pedestal and the projection screen to align the left-hand side of water droplet silhouette with the y-axis and origin of the scale on the screen. 4.2.8 Adjust the dial of the protractor on the projection screen so that the indicator line intersects the apex of the water droplet silhouette. 4.2.9 Measure the angle from the apex of the drop using the protractor scale. Please note that the contact angle, which is twice the angle measured from the apex of the drop, is already doubled on the projection screen scale. Record this contact angle. Calculation 5.1 Measure and record six individual contact angle readings for each sample. The individual readings are used to calculate a mean average contact angle. Report 6.1 The mean average contact angle and the source of the strand are reported. Interpretation 7.1 A lower contact angle corresponds to better bond per- formance. In our past studies, measured contact angles below 85° after the calcium hydroxide exposure were indica- tive of, but did not guarantee, good bond potential. As was observed when comparisons were made with transfer length, the contact angle measured on the strand after exposure to calcium hydroxide correlated well with concrete pull-out bond stresses (that is, bond stress at first or 0.1-in. slip during con- crete pull-out testing is inversely proportional to contact angle). The contact angle was lower with greater bond stress. Precision and Bias 8.1 Single-operator precision—The single-operator stan- dard deviation was found to be 4°F. Therefore, results of two 124 (a) (b) Figure C-1. Contact angle meter (a) and close-up of drop projection (b).

properly conducted tests by the same operator on the same source are not expected to differ from each other by more than 10°F. (These numbers represent, respectively, the (1s) and (d2s) limits as described in ASTM C670 [ASTM 2003].) 8.2 Bias—Since there is no accepted reference material suitable for determining the bias in this test method, no state- ment on bias is made. Keywords 9.1 strand; contact angle. Proposed Standard Test Method for Weight Loss on Ignition of Steel Strand Scope 1.1 The weight loss on ignition (LOI) represents the weight of compounds that can be volatilized or burned off the strand surface at high temperature. This property is measured with the expectation that the weight loss represents the quantity of volatile residues from organic components of drawing lubri- cants and organic contaminants such as lubricating oils. 1.2 This standard does not purport to address the safety con- cerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limita- tions prior to use. Apparatus 2.1 Balance—An analytical-grade weighing instrument shall have a maximum capacity of at least 300 g and a precision of 0.1 mg. The sample compartment shall be at least 24.5-cm high to accommodate a vertically set sample. 2.2 Graduated cylinder—A 25-mL glass or plastic graduated cylinder is needed to hold the strand sample in a vertical po- sition on the balance pan while weighing. The tare weight of the cylinder is subtracted from the weight reading to obtain the net weight of the sample. 2.3 Oven and furnace—The drying oven and ignition fur- nace instruments shall both have sufficient chamber length and volume to accommodate at least three strand segments. The drying oven shall be capable of heating the samples to 110°C ± 5°C, and the ignition furnace shall be capable of heating the specimens to 415°C ± 5°C. 2.4 Desiccator—A glass desiccating chamber shall use gran- ular calcium chloride desiccant and a ceramic disk insert on which to rest strand segments and to separate samples from desiccant. The chamber shall have sufficient volume to accom- modate at least three strand segments. Specimens 3.1 Three segments per strand source shall be cut and loosely wrapped in uncoated aluminum foil for transport to minimize contamination or abrasion of the strand surfaces. Each strand segment shall be cut to a length of 23.0 cm ± 0.5 cm. The operator shall handle the strand segments with pow- der-free latex gloves. Test Procedure 4.1 The LOI of each strand segment is measured using the following procedure: 4.1.1 Measure the length of the strand segment to the nearest 0.1 cm. 4.1.2 Determine the nominal diameter of the strand. 4.1.3 Dry the strand segment sample for 4 h at 110°C. 4.1.4 Cool sample in a desiccator for at least 12 h. 4.1.5 Weigh the sample three times to 0.1 mg precision. Record these initial weights and calculate the average weight. 4.1.6 Ignite samples for 30 min at 415°C. 4.1.7 Cool samples to room temperature in a desiccator for at least 12 h. 4.1.8 Weigh each sample again three times on the same analytical balance. Record these final weights and calculate the average final weight. Calculation 5.1 The LOI reported for the strand source is the change in average weight divided by the surface area of the strand. The surface area can be calculated by 4π × nominal diameter × tested length. Report 6.1 Report the loss on ignition in milligrams per square centimeter (mg/cm2) of strand and the source of the strand. Interpretation 7.1 In past studies, the mass LOI decreased as the pull-out bond stress increased. Precision and Bias 8.1 Single-operator precision—The single-operator standard deviation was found to be 0.014 mg/cm2. Therefore, results of two properly conducted tests by the same operator on the same source are not expected to differ from each other by more than 0.041 mg/cm2. (These numbers represent, respectively, the (1s) and (d2s) limits as described in ASTM C670 [ASTM 2003].) 125

8.2 Bias—Since there is no accepted reference material suitable for determining the bias in this test method, no state- ment on bias is made. Keywords 9.1 strand; ignition; mass loss. Proposed Standard Test Method for Change in Corrosion Potential of Steel Strand Scope 1.1. This test method covers the estimation of change in the electrochemical corrosion potential of prestressed strands in de-ionized water, for the purpose of indirectly estimating the bond performance of prestressing strand in concrete. 1.2. This standard does not purport to address the safety con- cerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limita- tions prior to use. Referenced Documents 2.1 ASTM Standards: • G3 Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing • G15 Standard Terminology Relating to Corrosion and Corrosion Testing Summary of Test Method 3.1 Specimens of strands are immersed in de-ionized water at room temperature. Corrosion resistance of strands is char- acterized by measuring corrosion potential change from initial immersion after 6 h of immersion. Continuous potential monitoring with a potentiostat is optional. At least triplicate specimens shall be used and specimens shall be tested either as received or after ignition. Significance and Use 4.1. This test method is intended for use as a rapid quality control method to indirectly assess the bonding characteristics of prestressing strand. Laboratory testing has demonstrated that strands with greater changes in potential have reduced bond strength. 4.2. Specimens are tested in as-received condition. It is also possible to test specimens after ignition. 4.3 Contamination of testing solution shall be avoided. 4.4 It is recommended that the test is to be performed in a temperature-controlled environment (e.g., an air-conditioned room), so that variation of solution temperature throughout the test is less than 3°C. Interferences 5.1. Extended exposure of reference electrodes with ceramic tips of inferior quality may introduce aggressive ions such as chloride into the testing solution, and such contamination might alter testing results. Immersion time of reference elec- trodes shall be minimized. Copper/copper sulfate reference electrodes with porous wood plugs shall not be used. Apparatus 6.1 Reference electrode—either a standard calomel electrode or Ag/AgCl electrode with controlled rate of leakage (about 3 μL/h) can be used. Such electrodes are durable, reliable, and commercially available. 6.2 Voltmeter—The division on the scale shall be such that a potential difference of 0.02 V or less can be read without interpolation. Test Solution 7.1 De-ionized water shall be used. Test Specimens 8.1 Strand specimens shall be 9-in. long. 8.2 The cut-end to be immersed in solution shall be sealed with a two-component epoxy. No more than 1/2 in. of the side of the strand shall be coated. 8.3 Specimens shall be tested in as-received condition, and surface contamination shall be avoided. Procedure 9.1 Place the strand specimen(s) in a 2-L beaker and sup- port firmly. A plexiglass plate with drilled holes shall be used to hold the specimens in place. When multiple specimens are used, the specimens shall be widely separated to avoid any direct contact with each other. 9.2 Introduce de-ionized water to the 2-L mark. Insert a reference electrode into the solution without disturbing any strand. 9.3 Measure the potential of each strand specimen at the start of testing and then after 1 h and after 6 h of the immersion. 9.4 If the potential of strand is being monitored with a po- tentiostat, the reference electrode shall be introduced before monitoring starts. 126

Report 10.1 Record the test procedure used, specimen size, temper- ature, and potentials. 10.2 Calculate changes in potential from the initial meas- urement to the potential measurement after 1 h and 6 h of exposure. 10.3 Report following information: 10.3.1 Strand identification, 10.3.2 Type of reference electrode, and 10.3.3 Change in potential at 1 h and at 6 h. 10.3.4 When a potentiostat is used to monitor potential change, a plot of potential change versus time shall be reported. Precision and Bias 11.1 Single-operator precision—The single-operator stan- dard deviation was found to be 0.047 V. Therefore, results of two properly conducted tests by the same operator on the same source are not expected to differ from each other by more than 0.133 V. (These numbers represent, respectively, the (1s) and (d2s) limits as described in ASTM C670 [ASTM 2003].) 11.2 Bias—Since there is no accepted reference material suitable for determining the bias in this test method, no state- ment on bias is made. Keywords 12.1 Corrosion potential, strand, reference electrode. Proposed Standard Test Method for Identification and Quantification of Residue on Steel Strand by Extraction, Gravimetric, and Spectroscopical Analyses Scope 1.1 The test methods for identification and quantification of organic drawing-compound residues on strand surfaces are based on solvent extraction procedures, together with gravi- metric and Fourier transform infrared spectroscopy (FTIR) analyses. Essentially, the amount of material extracted from a defined length of strand is determined by weighing the extraction residue on an analytical balance. The material(s) in the extraction residue are then identified by FTIR spectro- scopical analysis. 1.2. This standard does not purport to address the safety con- cerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limita- tions prior to use. Apparatus 2.1 Balance—An analytical-grade weighing instrument shall have a maximum capacity of at least 300 g and a precision of 0.1 mg. The weighing instrument shall have a maximum capacity of at least 50 g and a precision of 0.1 mg. 2.2 Glassware—The test procedure requires the use of the following glassware: a glass cylinder (Pyrex No. 2962), a 1000-mL glass separatory funnel (Pyrex No. 6402), a 400-mL glass beaker, and a 50-mL glass beaker. 2.3 FTIR—Use an FTIR spectroscope capable of analyzing the chemical make-up of the organic residue. Spectroscopic grade potassium bromide (KBr) plates will be required for FTIR analysis. 2.4 Reagents—Reagent grade hydrochloric acid (HCl), de- ionized water, and HPLC grade chloroform are required for this procedure. Specimens 3.1 Each strand sample shall be cut to a minimum length of 35.5 cm (14 in.). A preferable strand segment length is in the range of 40.6 cm (16 in.) to 45.7 cm (18 in.) long. The upper boundary of the test portion area shall be marked by cutting a small notch at a length of 30.5 cm (12 in.) from the bottom of the strand segment. Test Procedure 4.1 The organic drawing-compound residues on the strand segment sample are extracted, quantified, and identified using the following procedure: 4.1.1 Perform all steps involving hydrochloric acid and chloroform under a fume hood. Place a straight strand seg- ment with a minimum length of 35.5 cm (14 in.) in a clean Pyrex No. 2962 glass cylinder, which is about 31.8 cm (12.5 in.) deep by 3.6 cm (1.4 in.) inside diameter. While wearing powder-free latex gloves, handle the strand segment only above the test portion length of 30.5 cm (12 in.). 4.1.2 Fill glass cylinder containing strand sample to a 12-in.-deep mark with 10% hydrochloric acid solution (about 280 mL) pre-heated to 120°F. Two-liter quantities of acid solution can be prepared in advance by adding 472 mL of ACS-grade concentrated hydrochloric acid (37.2% hydrochlo- ric acid assay, 1.19 specific gravity) to 1528 mL of de-ionized water to achieve a 10% hydrochloric acid solution. 4.1.3 Allow 12-in.test portion of strand segment to set in 10% hydrochloric acid solution for 10 min, with frequent agitation of solution by stirring with strand segment. Carefully transfer acidic solution to a clean 1000-mL glass separatory funnel (Pyrex No. 6402). Rinse strand and cylinder with 20 mL of 10% hydrochloric acid solution, and transfer acidic solution to separatory funnel. 127

4.1.4 Fill glass cylinder containing strand sample to a 12-in.-deep mark with HPLC-grade chloroform (approxi- mately 280 mL). Allow 12-in. test portion of strand segment to set in chloroform for 10 min, with frequent agitation of solvent by stirring with strand segment. 4.1.5 Wearing powder-free latex gloves and handling the strand segment only above 12-in. test portion, very slowly lift the strand sample from the solvent while scrubbing the strand at solvent surface using a rubber policeman (a spatula-shaped, latex-rubber tip on the end of a glass rod). The elapsed time for the scrubbing process of the entire 12-in. test portion should be about 3 min. Briefly immerse the 12-in. test portion into solvent and agitate to remove as much solvent-soluble mate- rial as possible. Remove strand sample from solvent. 4.1.6 Carefully transfer solvent to the glass separatory fun- nel that contains the acidic solution. Rinse strand sample over glass cylinder with about 20 mL of chloroform, swirl solvent in glass cylinder, and pour rinse solvent into same separatory funnel. Stopper the separatory funnel, and shake it for 5 min. Vent funnel occasionally to release vapor pressure. After shaking, allow separatory funnel with liquid contents to rest in an upright position so that chloroform settles to bottom of funnel. The acidified aqueous solution becomes the top liquid layer. 4.1.7 Place the beaker below the separatory funnel and open stopcock of separatory funnel slowly and carefully to drain chloroform only into the beaker. Place beaker on a hot plate set at less than 70°C to evaporate chloroform in a controlled manner. The evaporation process can be aided by directing the air flow of a fan over the top of the beaker. 4.1.8 The 400-mL beaker with dried residue is rinsed with about 10 mL of chloroform, and the chloroform with solvent- soluble residue is transferred to a clean, pre-weighed 50-mL beaker. The chloroform rinse procedure is repeated twice (a triple rinse procedure). The chloroform with solvent- soluble residue in the 50-mL beaker is evaporated at less than 70°C. The beaker is reweighed to determine the net weight gain due to solvent-soluble, dried residue. 4.1.9 The dried residue, if any, is transferred with chloro- form to a spectroscopical-grade potassium bromide salt plate for analysis by FTIR spectroscopy. The FTIR spectrum is in- terpreted to identify the residue. Calculation 5.1 The extraction residue weight shall be determined by weighing on an analytical balance once to the nearest 0.l mg. This weight shall be divided by the surface area of the strand sample’s test portion, in square centimeters (cm2). The surface area of the strand shall be calculated in cm2 as follows: 30.5 × 4/3 π × D where D = nominal diameter of the strand. After weighing, the residue is identified by FTIR spectroscopical analysis, and the FTIR spectrum interpretation is reported. Report 6.1 The weight of the residue shall be reported in units of mg/cm2 of strand surface. Our past studies have found that a relationship exists between the extracted organic residue and the average bond stress over the transfer length. As bond stress increased, extraction residue decreased. The relationship is especially evident in the total extraction residue (combined from water and acid/chloroform wash solutions) plotted for both of these procedures. The report shall also include the infrared spectrum of the residue, and the interpretation of the spectrum by the spec- troscopist. Precision and Bias 7.1 Single-operator precision—The single-operator stan- dard deviation was found to be 0.013 mg/cm2. Therefore, re- sults of two properly conducted tests by the same operator on the same source are not expected to differ from each other by more than 0.037 mg/cm2. (These numbers represent, respec- tively, the (1s) and (d2s) limits as described in ASTM C670 [ASTM 2003].) Bias—Since there is no accepted reference material suit- able for determining the bias in this test method, no state- ment on bias is made. 128

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TRB's National Cooperative Highway Research Program (NCHRP) Report 621: Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete explores tests to identify and measure residues on the surface of steel pre-stressing strands and to establish thresholds for residue types found to affect the strength of the strand's bond to concrete.

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