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10 In the past, the quality of bond has been evaluated using was to eliminate those tests that were not helpful for predict- mechanical methods, such as pull-out or transfer length tests. ing bond performance. Thus, the first step of the analysis in However, such methods are expensive, time consuming, and this round of testing was to estimate the correlation between conducted infrequently; currently, routine pull-out tests are each surface or chemical test and bond performance, and the conducted quarterly by strand manufacturers. A main goal second step was to identify those methods where some degree of this project was to develop fast, accurate, reproducible, of correlation was indicated. For each source of strand, bond simple to conduct, and inexpensive QC test methods for performance was measured in terms of pull-out stresses, detecting and measuring the level of deleterious residues transfer lengths, or both. Accordingly, for the purpose of this on strand that could be performed frequently (i.e., weekly). analysis, the bond performance was treated as the independ- A number of methods were proposed involving testing sur- ent variable. The best experimental design for estimating face and chemical properties of the strand that could be linked a correlation is to place the design points as far apart as pos- to strand bond. All tests were intended as part of a routine sible in terms of the independent variable. Thus, the optimal testing program that could be conducted by strand manufac- statistical design is to run each test on strands that show a turers, precasters, or other interested parties. range of bonding performance. For the screening experi- ments high, medium, and low bonding sources were desired. However, efforts to obtain a very low bonding strand were Quality Control Program Overview not successful. Although reports of low bonding strand inci- The individual tests that were proposed required a varied dents continue to surface in the precast concrete industry, range of time, expertise, and equipment. Therefore, it was "unused" samples of such strand remained elusive. There- envisioned that they would be applied as part of a two-tiered fore, the screening tests on new strands were run on what are QC program, in which the value of the test is proportionate to essentially high and intermediate bond strands. the test complexity, with the following distinct components: Correlation Testing · Level I QC component (QC-I) and · Level II QC component (QC-II). The second round of experiments was performed for confirmation and calibration purposes. This round involved The Level I QC component consists of relatively quick, running additional tests using those methods that showed simple, and inexpensive tests that can be conducted by strand promise in the screening experiments. These selected tests manufacturing personnel. These tests would be performed on were conducted on five new strand sources. This complete a daily basis. Each test would take less than one-half hour to dataset was then used to assess the correlation between the QC perform. tests and bond performance, and to determine if the tests The Level II QC component consists of tests that require were able to accurately identify good and bad strand. It was more in-depth training and advanced equipment, and could also used as a basis for discussing pass/fail criteria for accept- be performed by testing laboratories on behalf of strand man- able bond performance. ufacturers. These tests would be performed at longer intervals, with changes in processes, or as dictated by the Level I QC test Precision Testing results. A third round of testing was conducted to determine the precision (i.e., repeatability) of those methods, showing good Evaluation of Proposed Surface correlation with bond strength. This was used to develop pre- and Chemical Test Methods cision statements included in the published test methods. Several chemical and surface tests were proposed to predict poor bonding characteristics for strand. The purpose of the Basis for Evaluation--Transfer Length experimental program conducted in this research program and Pull-Out Tests was to determine if these proposed tests would be applicable for use in a QC program. To do this, several rounds of exper- Transfer length is the most reliable and realistic measure imentation were needed. of bond performance. During the screening testing, the eval- uation of correlations between the pull-out tests and bond performance were based on performance as measured with Screening Testing transfer length tests conducted on the same sources of strand. The first round of experiments consisted of "screening" ex- Pull-out testing was conducted as part of the screening periments. The objective for the screening experimentation studies using three materials as the test matrix: a concrete,
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11 a Portland cement mortar, and a gypsum plaster mortar. Based pull-out stresses from at least six individual pieces of strand. on comparisons with transfer length tests conducted in this The bond stresses are calculated from the measured loads study and described in Appendix B, the concrete pull-out test based on the actual surface area and the embedment length showed the best correlation with bond quality. The surface and of the strand. chemical test methods were evaluated in the Screening Round These strand sources fall into three groupings: historic, re- based on the results of pull-out tests from concrete, again, on cently manufactured, and OSU (Oklahoma State University) strand samples from the same source. However, the evaluation strand. of correlation of test results to bond in the correlation round of testing was based on results from a mortar pull-out test pro- Historic Strand--This study initially identified samples of gram associated with NCHRP Project 12-60. The principle in- strand for testing from prior unpublished tests conducted at vestigator from that project supplied the strand samples for this Kansas State University (KSU) by Bob Peterman and at portion of the study. No pull-out testing was conducted in the StressCon Corporation, Inc. by Don Logan that cover a wide Correlation Round of the experimental program. range of pull-out behavior. These are referred to as "historic" strand and were manufactured between 1997 and 2004. Strand Samples Figure 2 is a plot of first slip bond stress or bond stress at 0.1-in. end slip versus maximum bond stress from the data To assess the effectiveness of the mechanical and surface available from historic concrete pull-out tests. chemistry-based testing procedures, it was essential that When suggested minimum pull-out loads for acceptable samples representing the range of possible performance be bonding performance (suggested by Logan, based on a limited evaluated. Since neither precasters nor strand suppliers were number of flexural beam tests conducted in the mid-1990s enthusiastic about associating themselves with poor-bonding and his engineering judgment) are converted to bond stresses, strand, obtaining samples of strand from the lower end of the they are 425 and 955 psi for the first slip and maximum performance spectrum was difficult. stresses, respectively. These thresholds have been reproduced The strand sources included in testing for this program are in Figure 2. listed in Table 1. This table also includes a result from con- crete pull-out tests or mortar pull-out tests (the bond stress Recently Manufactured Strand--Figure 2 also shows the at the observed first slip or after 0.1-in. slip at the non-loaded concrete pull-out performance of recently manufactured sam- end of the strand). Each pull-out stress is the average of the ples identified during this project. These recently manufactured Table 1. Strand sources. Strand Geometry Mortar Pull-Out Testing Concrete Pull-Out Testing (LBPT) Strand Measured Lay Source ID Pitch 0.1-in. Slip 0.1-in. Slip Size (in.) Diameter (Handed- Location Date Location Date (in.) Stress (psi) Stress (psi) (in.) ness) Historic Strand KSU-F 1/2 Special 0.524 7 5/8 Left -- -- -- KSU Mar 2004 241 KSU-H 1/2 Special 0.523 7 1/2 Left -- -- -- KSU Mar 2004 209 SC-F 1/2 0.503 8 Left -- -- -- SC May 1997 223 SC-H 1/2 Special 0.530 7 1/4 Left -- -- -- SC Nov 2002 472 SC-IS 1/2 0.501 7 Left -- -- -- SC Mar 2003 682 101 6/10 0.601 8 1/2 Left -- -- -- SC Oct 2004 241 Recently Manufactured Strand 102 1/2 0.501 7 1/2 Left KSU Jun 2005 315 KSU Jun 2005 441 103 1/2 0.503 8 Left KSU Jun 2005 397 KSU Jun 2005 944 151 1/2 Special 0.517 7 1/2 Left KSU Jun 2005 273 KSU Jun 2005 541 153 6/10 0.588 9 Right -- -- -- KSU/SC Jun / Aug 2006 142/406 OSU Strand 349 1/2 0.505 8 3/4 Left OSU Jun 2004 156 -- -- -- 548 1/2 0.500 7 5/8 Left OSU Jan-Feb 2004 623 -- -- -- 697 1/2 0.503 7 1/4 Left OSU May 2004 606 -- -- -- 717 1/2 0.500 8 Left OSU Feb 2004 206 -- -- -- 478 * 1/2 0.499 7 5/8 Left OSU May-June 2004 409 -- -- -- 960 * 1/2 0.500 7 1/2 Left OSU May-June 2004 409 -- -- -- * Samples designated 478 and 960 were from same source. KSU = Kansas State University; OSU = Oklahoma State University; SC = StressCon Corporation, Inc.
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12 U -11/02 103-B-0.5 1200 D IT-11/02 TW-5/96 E A S3 S5 IS-3/03 J B G S4 C 102-A-0.5 S1 S2 103-B-0.5 1000 151-Z-0.5 F 102-A-0.5 H-11/02 NC-05/01 Max Stress (psi) H 101-A-0.6 800 hollowcore F-5/97 600 Historical results at first slip NCHRP tests at 0.1 in slip Max Threshold D-5/96 1st Slip Threshold 400 200 200 300 400 500 600 700 800 900 1000 Bond Stress at 1st Slip or 0.1 in (psi) Figure 2. Correlation between maximum stress and first observed or 0.1-in. slip stress for historic and recently manufactured strand. samples were obtained in large quantities for the purpose of were actually the same strand source, a fact that was not this research and were used in the screening experiments. known by the research team before the testing was com- Notice that the greatest variation in the recently manufac- pleted. This was intended to test the repeatability of the tured strand is not in terms of maximum stress but in terms surface and chemical test methods. Complete mortar pull- of stress at 0.1-in. slip. out and some transfer length test results were provided in The recently manufactured strand sources (102, 103, tabular form by Russell after the chemical and surface testing and 151) were selected because initial testing indicated that had been completed. they represented a range of first-slip pull-out performance. None of these strands had significantly low maximum load Transfer Length Testing pull-out performance. Source 103 is the strand used by StressCon Corporation, Inc. in their ordinary production of The transfer length test is not proposed as a QC test precast/prestressed concrete and has a proven record of good method, but was conducted as a basis for evaluating the pro- bond from pull-out tests, flexure beam tests, transfer length posed QC test methods, since the transfer length quantifies tests, and end slips observed in hollow core precast/prestressed bond performance under the most realistic conditions. concrete members. Transfer length is defined as the distance over which the The bond stress at 0.1-in. slip of Source 102, measured in effective prestressing force is transferred to the concrete ele- concrete pull-out testing performed as part of this project, is ment. In other words, this is the distance from the end of the slightly above Logan's 425 psi threshold. Of the 31 results strand where no stress is applied to the concrete to the point from historic and other sources presented on this plot, 13 are where the maximum amount of stress has been transferred to the left of Source 102. Only one of these (D-5/96) was into the concrete. The test for transfer length involves casting available in enough quantity to enable additional testing, and a prism of concrete around a stressed strand or strands and the condition of this strand was variable. then measuring the strain profile along the length of the prism after the stress is released. The transfer length is defined OSU Strand--The sample sources used for the Correlation as the length over which the measured strain in the prism Round of testing were selected by Bruce Russell of Oklahoma increases from zero at the ends of the prism to the edge of the State University (OSU). These sources of strand had been strain plateau region in the middle of the prism. The end slip tested in work performed by Russell for NCHRP Project (i.e., the distance that the end of the strand moves relative to 12-60: Transfer, Development, and Splice Length for Strand/ its original position) is proportional to transfer length and Reinforcement in High-Strength Concrete, the Oklahoma also was measured. The strain profiles of the transfer length Department of Transportation, and the NASPA (also known prisms were monitored over time, starting with the initial as the Committee of the American Wire Products Association reading immediately after release; additional measurements [AWPA]). Two of the six strand sources provided by Russell were taken at 28 days, 6 months, and 18-22 months.