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Page 110
Suggested Citation:"Chapter 4 - Design Recommendations." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
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Suggested Citation:"Chapter 4 - Design Recommendations." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
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Suggested Citation:"Chapter 4 - Design Recommendations." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
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Suggested Citation:"Chapter 4 - Design Recommendations." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
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Suggested Citation:"Chapter 4 - Design Recommendations." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
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Suggested Citation:"Chapter 4 - Design Recommendations." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
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Suggested Citation:"Chapter 4 - Design Recommendations." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
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Suggested Citation:"Chapter 4 - Design Recommendations." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
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Suggested Citation:"Chapter 4 - Design Recommendations." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
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110 4.1 Introduction Article 5.4.2.1 of the AASHTO LRFD Bridge Design Specifi- cations (2004) limits the applicability of the specifications for concrete compressive strengths of 10 ksi or less unless physi- cal tests are made to establish the relationships between concrete strength and other properties. A comprehensive article-by-article review of Section 5 of the AASHTO LRFD Bridge Design Specifications (2004), pertaining to transfer and development of prestressing strand and splice length and anchorage of free ends by means of standard hooks for mild reinforcement was performed under NCHRP Project 12-60 to identify all provisions that would have to be revised directly or indirectly to extend their use to high-strength, normal-weight concrete up to 15 ksi. These articles are the following: • Article 5.4.4 Prestressing Steel • Article 5.5.4.2. Resistance Factors • Article 5.11 Development and Splices of Reinforcement • Article 5.11.2 Development of Reinforcement • Article 5.11.2.1 Deformed Bars and Deformed Wire • Article 5.11.2.1.1 Tension Development Length • Article 5.11.2.1.2 Modification Factors that Increase ld • Article 5.11.2.1.3 Modification Factors which Decrease ld • Article 5.11.2.2 Deformed Bars in Compression • Article 5.11.2.2.1 Compressive Development Length • Article 5.11.2.2.2 Modification Factors • Article 5.11.2.3 Bundled Bars • Article 5.11.2.4 Standard Hooks in Tension • Article 5.11.2.4.1 Basic Hook Development Length • Article 5.11.2.4.2 Modification Factors • Article 5.11.2.5 Welded Wire Fabric • Article 5.11.2.5.1 Deformed Wire Fabric • Article 5.11.2.5.2 Plain Wire Fabric • Article 5.11.2.6 Shear Reinforcement • Article 5.11.4 Development of Prestressing Strand • Article 5.11.4.1 General • Article 5.11.4.2 Bonded Strand • Article 5.11.4.3 Partially Debonded Strands • Article 5.11.5 Splices of Bar Reinforcement • Article 5.11.5.3.1 Lap Splices in Tension 4.2 Design Recommendations For prestressing strands, there are five essential recom- mendations stemming from the research: 1. Adoption of the Standard Test Method for the Bond of Prestressing Strands. Heretofore, this test has been known as the NASP Bond Test. 2. Adoption of a transfer length expression incorporating a factor to account for improved bond with increasing con- crete strength. The recommended expression reflects the decrease in transfer lengths as concrete release strengths increase. For release strengths of 4 ksi, the recommended expression would provide a transfer length of 60 strand di- ameters, which is the same value found in prior editions of the AASHTO LRFD Bridge Design Specifications. For release strengths of 6 ksi, the design transfer length would be approximately 50 db. Transfer lengths would be limited to a minimum of 40 strand diameters. 3. Adoption of a development length expression that incor- porates factors to account for improved strand bond as concrete strengths increase. The recommended code expression is founded on the same principles as prior edi- tions of the AASHTO LRFD Bridge Design Specifications, i.e., the development length is the sum of a transfer length expression plus a flexural bond expression. At “normal” concrete strengths, the development length expression requires 60 strand diameters for transfer length and ap- proximately 90 strand diameters for the flexural bond length, for a total of 150 strand diameters. For a concrete release strength of 6 ksi and design concrete strength of 10 ksi, the development length expression provides a development length of 120 strand diameters. For higher C H A P T E R 4 Design Recommendations

111 concrete strengths, the development length is limited to a minimum of 100 strand diameters. 4. Adoption of a modified bilinear build-up of strand stress consistent with the recommended transfer and develop- ment length expressions. 5. Adoption of additional restrictions regarding the use of debonded, or shielded, strands. The following recommendations, discussed in detail in Section 4.3.3, stemming from the work conducted under NCHRP Project 12-60 address the anchorage of Grade 60 mild steel in tension: 1. Development length of black and epoxy-coated reinforc- ing bars anchored by means of straight embedment length and splices. 2. Anchorage of black and epoxy-coated bars terminated with standard hook. 4.3 Details of the Design Recommendations 4.3.1 Prestressing Strand—Adoption of the Standard Test Method for Bond of Prestressing Strands Table 5.4.4.1-1 reiterates the requirements for mechanical properties for the prestressing steels that are found in the two AASHTO material specifications. Mechanical properties in- clude the grade or type, the size, the tensile strength (ksi) and the yield strength (ksi). Research findings from NCHRP Proj- ect 12-60 support the conclusion that the bond of prestressing strand should be recognized as a material property of the strand and included in Section 5.4.4, “Prestressing Steel,” of the AASHTO LRFD Bridge Design Specifications. The research described in Chapter 3 of this report provides supporting evidence that the Standard Test Method for the Bond of Pre- stressing Strands, found in Appendix H, should be adopted into the LRFD specifications for the purpose of qualifying strand for use in prestressed concrete structures. (Please note that the Standard Test Method for the Bond of Prestressing Strands is also known as the Standard Test for Strand Bond.) There are two issues that require resolution in order to adopt the Standard Test Method for the Bond of Prestressing Strands. First, the repeatability of the test procedure must be clearly shown. Second, minimum threshold values for bond perfor- mance need to be established. Both items are addressed below. 4.3.1.1 Standard Test Method for the Bond of Prestressing Strands Section 3.2 in this report addresses the repeatability and reproducibility of the Standard Test Method for the Bond of Prestressing Strands. The reproducibility of the test is summa- rized in Figure 3.7 where the Standard Test Method for the Bond of Prestressing Strands was performed on identical strand samples at Purdue University and at OSU. Altogether, the data illustrated in Figure 3.7 come from tests performed on five dif- ferent 0.5-in.-diameter strand samples and two different 0.6- in.-diameter strand samples. Figure 3.7 shows that there is a high degree of statistical correlation between the results from the two sites. The coefficient of determination, R2, is 0.92. Perhaps even more importantly, Figure 3.7 helps to show that the test results fall very near to the “perfect” line, where nearly identical results are obtained at the two sites independently. The tests at Purdue University were completely blind, as strand des- ignations for the strands tested at Purdue University did not match the strand designations on the same samples at OSU. The protocols for the NASP Bond Testing used in NCHRP Project 12-60 were based on the NASP Bond Test from May 2004 and are found in Appendix I. However, some refine- ments were made during the early part of the NCHRP testing at OSU, and the round-robin testing between Purdue Univer- sity and OSU more closely matched the protocols that were further refined during the NCHRP testing program. These protocols are now titled, “Standard Test Method for the Bond of Prestressing Strands,” and are recommended as the basis for the material requirements to ensure “bond-ability” of pre- stressing strands with concrete. The final recommended stan- dard is included in Appendix H. Some of the modifications included in the April 2006 edition of the Standard Test Method for the Bond of Prestressing Strands include provi- sions for minimum acceptance and frequency of testing. Therefore, it is recommended that the Standard Test Method for the Bond of Prestressing Strands be adopted and made part of the LRFD specifications. Notably, the North American Strand Producers Committee of the American Wire Producers Association has formally, by unanimous vote, adopted the test procedure as their standard for bond. The recommendation from this report is that the Standard Test Method for the Bond of Prestressing Strands be incor- porated into the AASHTO LRFD Bridge Design Specifications in Section 5.4.4. The LRFD specifications text should state that the material supplier is required to provide certification that the bonding ability of the prestressing strand is accept- able for use in pretensioned and prestressed concrete appli- cations and that the Standard Test Method for the Bond of Prestressing Strands “shall be permitted” to provide accept- ability of the prestressing strand product. 4.3.1.2 Minimum Acceptance Value for Strand Bond Minimum acceptance values for strand bond are incorpo- rated into the Standard Test Method for the Bond of

112 Prestressing Strands in its current form. The minimum values were determined by analysis of data measured from both transfer length and development length beams. Development length beams were tested with three different 0.5-in. strands for their ability to develop the tension force necessary to sup- port flexural failures in the prescribed development length. Strand A had a NASP Bond Test value of 20,950 lb. Strand B had a NASP Bond Test value of 20,210 lb. The bond values of Strands A and B are contrasted with Strand D, which had a NASP Bond Test value of 6,890 lb. From the testing that was done and discussed in Chapter 3 of this report, it can be de- termined that beams made with Strands A and B were able to develop their nominal flexural capacity in embedment lengths much shorter than the current requirements for strand devel- opment length. Conversely, tests on beams made with Strand D indicate that the bond-ability of this strand is marginal for the rectangular beams and insufficient for the I-shaped beams with respect to the strand’s ability to satisfy the current and recommended design provisions for development length. NCHRP Project 12-60 testing indicates that the minimum threshold number resides somewhere between 6,890 lb and 20,210 lb, and that the strand with a bond value of 6,890 lb was acceptable in some of the beams but not in others. Table 3.28 reports the results of development length tests on rectangular beams made with Strand D. From the table, the following can be seen: 1. For concrete strengths with release strengths as low as 4 ksi, Strand D was developed in the AASHTO-prescribed development length of 73 in.; 2. For the same beams, bond failures regularly occurred at 58 in.; and 3. Bond performance as measured by strand development improved dramatically with increases in concrete strength. Similarly Table 3.29 reports the results from development length tests on rectangular beams made with Strands A and B. From the table, the following can be seen: 1. Flexural failures were reported on all concrete strengths and at all development lengths, and 2. Strand A was able to develop adequate tensile strength in only 46 in. of embedment for concrete strengths as low as 6,180 lb at release and 8,500 lb at design. When considering whether the performance of Strand D is adequate for development length, it is important to also con- sider the development length test results from the I-shaped beams. The results from the I-shaped beams are reported in Table 3.26. In this table, Beams ID-6-5-1-N, ID-6-5-1-S, and ID-10-5-1-S all failed in bond. All three of these tests had Strand D at embedment lengths of either 72 in. or 88 in. Bond failures occurred when web shear cracking in I-shaped beams propagated through the transfer zones of Strand D. These results from the tests on I-shaped beams indicate that bond performance of Strand D is inadequate. Comparison of data in the two tables also shows that Strands A and B had superior bond performance when com- pared with Strand D. The data in the tables show that the cor- rect threshold value for the NASP Bond Test lies somewhere between the NASP Bond Test value for Strand D and the value for Strands A and B. The data from NASP Round III research, also discussed in Chapter 3, provide additional data points for strands with varying bonding properties (Russell and Brown 2004). In these tests, four different 0.5-in.-diameter strands were used in a testing program that used the NASP Bond Test (August 2001) to assess the bonding properties of the strands. The strands were also cast into beams where transfer lengths and development lengths were measured. Tables 3.31 through 3.33 summarize the results from the Round III testing. Strand FF from the NASP Round III report is the same as Strand D in NCHRP Project 12-60. The data from NASP Round III testing match the data from NCHRP Project 12-60, where beams made with Strand D failed in flexure at 73 in. of em- bedment but sometimes failed in bond at 58 in. The data from NASP Round III also include testing performed on Strand HH, which had a NASP Bond Test value of 10,700 lb. Beams made with Strand HH failed in flexure at both 73 in. and 58 in., except for one bond failure at an embedment length of 58 in. In this bond failure, the beam achieved nearly 90 percent of its fully developed nominal flexural capacity, exhibited significant ductility, and achieved a flexural strength in excess of that calculated for the beam considering the bilinear stress curves now found in the AASHTO LRFD Bridge Design Specifications and the 318 Code. The perfor- mance of beams made with Strand FF (NCHRP Project 12-60 Strand D) and Strand HH provides important data points in determining the minimum acceptance values for the Standard Test Method for the Bond of Prestressing Strands. Therefore, from these test results that incorporate the find- ings of the NASP Round III tests with those of NCHRP Proj- ect 12-60 testing, it is recommended that the minimum threshold shall be 10,500 lb for 0.5-in. strands. In other words, the performance of Strand HH would be minimally acceptable, and the value of the NASP Bond Test of 10,700 lb can be rounded to 10,500 lb for simplification. Further, the data sup- port the overall conclusion that the development length per- formance improves for strand with improving NASP Bond Test values and therefore support the recommendation that the Standard Test Method for the Bond of Prestressing Strands be adopted into the AASHTO LRFD Bridge Design Specifications.

Please note the protocols described in the Standard Test Method for the Bond of Prestressing Strands require that a single bond test consist of six individual tests and that the re- ported “Bond Test Value” is the average value from those six tests. Therefore, the 10,500-lb recommended minimum threshold is an average value in conformance with the proto- cols found in the Standard Test Method for the Bond of Prestressing Strands. In addition to the requirement for a minimum average Bond Test Value, the test includes a re- quirement for the minimum value for the lowest value of the set of six individual tests. This second criteria is established to avoid excessive variations in strand bond quality within the same sample of strand. The requirement for the minimum single test value is 9,000 lb for 0.5-in. strands. This second requirement effectively limits the standard deviation, or the statistical variance, for strand produced that may have mod- erate bonding properties. In the NCHRP Project 12-60 research, transfer and devel- opment length testing was also performed on 0.6-in. strands. The NASP Bond Test was also conducted on 0.6-in.-diameter strands. The results indicated that the NASP Bond Test or the Standard Test Method for the Bond of Prestressing Strands was suitable in predicting the bond behavior of 0.6-in strands. Therefore, the NASP Bond Test is recommended for use for 0.6-in. strands. For 0.6-in. strands, the minimum threshold values are an average value of 12,600 lb and a single test min- imum of 10,800 lb. 4.3.2 Transfer and Development Length Expressions for Prestressing Strand The current ACI and AASHTO design equations for pre- tensioned transfer length and development length do not in- clude concrete strength as a parameter in the design equation. However, test results obtained during NCHRP Project 12-60 strongly suggest that the anchorage ability of the strands is improved as concrete strength increases. The results from both transfer and development length testing support the conclusion that concrete strength is an important factor. These results are supported independently by the results from NASP Bond Tests in varying concrete strengths. The Standard Test Method for the Bond of Prestressing Strands was used to assess the impact of varying concrete strengths on the bond between strand and concrete. In these tests, the NASP Bond Test was modified simply by casting the strand in concrete (with varying concrete strengths) instead of the sand-cement mortar required in the Standard Test Method for the Bond of Prestressing Strands protocols. Fig- ure 3.9 illustrates the data collected from the NASP Bond Test performed on Strand D, in concrete, and the effects of vary- ing concrete strengths. The figure shows that at a concrete strength of about 4.5 ksi, the prediction curve for Strand D 113 found in Figure 3.9 corresponds with a NASP Bond Test value (in concrete) of about 6,600 lb. Strand D, tested in mortar using the Standard Test Method for the Bond of Prestressing Strands, had an NASP value of 6,890 lb. In con- crete with a strength of 10 ksi, Strand D had a NASP Bond Test value of nearly 12,000 lb. The data from the specimens in concrete support the conclusion that increasing concrete strength improves the bond between strand and concrete. Figures 3.12 and 3.13 illustrate the results from all of the NASP Bond Tests in concrete versus concrete strength on four different strand samples. One of the remarkable features of this dataset is that it includes data from both 0.5-in. and 0.6-in. strands. There are more than 20 data points repre- sented in the figures, all cast within a wide sample of concrete strengths. The unifying factor is that the Standard Test Method for the Bond of Prestressing Strands (in concrete) was performed on each of the samples, and each data point repre- sents the average from at least six individual tests. Further- more, the coefficient of determination, R2, was a remarkable 0.80 for this dataset, which includes two different strand sizes, four different strand samples, and a variety of concrete mix- tures. The figures show clearly how the bond of concrete and sand is directly improved by increases in concrete strength. This result is important in determining recommendations for transfer length and development length equations. Figures 3.12 and 3.13 show two very important things: (1) that the Standard Test Method for the Bond of Prestressing Strands is very robust and (2) that it can be modified with concrete to assess bond performance of strand of all sizes and in various concrete mixtures. As illustrated in Figure 3.12, the power regression between bond strength and concrete strength suggests the relationship: The results from these tests further attest to the suitability of the Standard Test Method for the Bond of Prestressing Strands as a vehicle to measure the bond between prestress- ing strand and concrete. Further, these results support the ar- gument that the variation in transfer and development lengths should vary with the square root of concrete strength. The results also support the recommendation for the adop- tion of the Standard Test Method for the Bond of Prestress- ing Strands by demonstrating that the Standard Test Method for the Bond of Prestressing Strands is useful for various sizes of strands and can predict differences in bond based on vari- ations in concrete strength. Testing showed that it is possible to describe the relation- ship between concrete strength and bond strength using the correlations found directly from the NASP Bond Tests in concrete. In those tests, it was found that bond strength var- ied nearly in proportion with the square root of concrete NASP NASP fci concrete( ) = ′0 49139 4 10 51702. ( . ).

strength. It can be observed from the data shown in Figures 3.12 and 3.13 that the bond strength of concrete is almost di- rectly proportional to the square root of concrete strength. The linear regression shown in Figure 3.13 gives the follow- ing relationship: Testing also demonstrated that both transfer length mea- surements and development length requirements were short- ened as concrete strength increased. Figures 3.24 through 3.32 show that for all sizes of strand and for all the varying qualities of strand bond, the transfer lengths shorten as con- crete release strengths increase. Therefore, the recommended code equation includes the square root of concrete strength at release in order to be consistent with testing results that show shorter transfer lengths with higher concrete strengths. The transfer length recommendation provides a transfer length equal to the current design expression of 60 strand di- ameters when the concrete release strength is 4 ksi. Using the recommended expression, the transfer length shortens as the concrete release strength increases. Additionally, the ex- pression provides a minimum limit of 40 strand diameters. Thus, the recommended expression for transfer length ex- pression is the following: where lt = transfer length (in.), f ′ci = release concrete strength (ksi), and; db= diameter or prestressing strand (in.). If the concrete release strength is 4 ksi, then the equation results in a transfer length of 60 db. As release strength in- creases, the transfer length decreases. The limit of 40db ensures that all designs consider transfer lengths of some rea- sonable value. The recommendation effectively limits an additional decrease in transfer length from concrete release strengths greater than 9 ksi. This minimum limit on transfer length is consistent with the testing conducted as part of this research in which the highest release strength achieved on rectangular beams was 9.7 ksi. The current ACI and AASHTO design equations for de- velopment length do not include concrete strength as a parameter. As with the results on transfer length, results from development length testing strongly suggest that the bond- ability of the strands is improved as concrete strength increases. The experimental results clearly demonstrate that the required development length is shortened as concrete strength increases; higher concrete strength results in shorter development length requirements. Therefore, it is important l f d dt ci b b= ′ ≥ 120 40 4 3( . ) NASP NASP fci concrete( ) = ′0 51 4 2. ( . ) 114 to include the effects of concrete strength in the design ex- pression for development length. Current expressions for development length (both ACI and AASHTO) were developed from the addition of the transfer length and a “flexural bond length.” This approach is supported by past and current research. Therefore, it is recommended to continue with the approach of splitting the development length into a transfer length component and a flexural bond length component. Both components, however, are affected by the concrete strength. Following is an outline of the rationale for developing the recommendation for development length: 1. The current AASHTO development length equation can be used to adequately predict required development lengths for “normal strength concrete” with release strengths in the range of 4 ksi to 6 ksi, provided that the strand itself is qualified by the Standard Test Method for the Bond of Prestressing Strands. The results presented in Chapter 3 of this report demonstrate flexural failures at embedment lengths of 73 in. for all 0.5-in. strands tested in this research program. The embedment length of 73 in. corresponds to 100 percent of the current code provision for development length for these specimens. The results included tests on beams made with concrete strength at release of approximately 4 ksi and approximately 6 ksi at the time of the beam test. 2. The data demonstrate that development length require- ments are shortened as concrete strength increases. 3. The required development length calculated from the cur- rent code provisions is approximately 150 db, although under the 2004, 3rd edition of the AASHTO LRFD Bridge Design Specifications, some variations will exist due to variations in strand stressing, beam geometry, and subse- quent variations in computed prestress losses. Only a few specimens contained prestressed strands at the tops of the cross sections. For these, beam tests were conducted for development length. Therefore, no comment can be made regarding the “top bar effect” for prestressing strands. Discussions regarding the κ factor are not included, al- though the testing demonstrates that the κ factor should be discarded for all sizes of strand. Instead all strands can be qualified for bond through the Standard Test Method for the Bond of Prestressing Strands. 4. Using a transfer length of approximately 60 db, and a de- velopment length of approximately 150 db, the flexural bond length must be approximately 90 db. Thus, the development length expression can then be written as l l d f d t b c = + ′ 225 4 4( . )

where ld = development length, (in.), lt = transfer length, (in.), db = nominal diameter of the prestressing strand, (in.), and f ′c = design concrete strength, (ksi). If a concrete design strength, f ′c, of 6 ksi is seen as a reason- able approximation for “normal concrete strength,” then a coefficient can be computed to correspond with a flexural bond length of 90 strand diameters. That coefficient is 225 (225/ ≈ 90). A minimum value is also recommended for the develop- ment length expression. The recommended expression for development length, therefore, is based on a limiting concrete strength of approximately 14 ksi, which is slightly less than the maximum concrete strength attained in beams tested in the research program (14.9 ksi). Thus, the recommended development length equation is as follows: The proposed development length equation is plotted against the development length test results in Figures 3.49 to 3.51. The result of each beam test, whether flexural failure or bond failure is plotted on a chart showing concrete strength versus embedment length. The curves representing the rec- ommended design equations for development length are also shown on each of the charts. Note that the development length expression is now dependent on the concrete strength. For the purpose of computing the values within the equation and charting the results, the release strength is taken as 66.7 percent of the design strength. Figure 3.49 shows the results of development length tests on Strand D. Strand D demonstrated below average bond performance with a relatively low NASP Bond Test value of 6,890 lb. Strand D also had measurably longer transfer lengths than Strands A/B. Figure 3.49 indicates that beams made with Strand D experienced bond failures in rectan- gular beams with embedment lengths of 58 in. for concrete strengths of about 8 ksi and lower. Note that in Figure 3.49, the flexural failures are represented by open symbols (tri- angles for R-beams and diamonds for I-Beams) and the bond failures are represented by solid symbols. Figure 3.49 clearly shows that as concrete strength increased, Strand D was able to move from bond failures to flexural failures a 58-in. embedment length. Furthermore, Figure 3.49 indi- cates that I-beams made with Strand D failed in bond at embedment lengths where the proposed design equation would predict adequate development. Therefore, Figure 3.49 shows that Strand D, with an NASP Bond Test value of only 6,890 lb, does not provide adequate bond-ability with concrete. l f f d dd ci c b b= ′ + ′ ⎡ ⎣⎢ ⎤ ⎦⎥ ≥ 120 225 100 4 5( . ) 6 115 In contrast to the results from Strand D, Figure 3.50 plots results from development length tests on Strands A/B versus the proposed development length equation. The chart clearly shows that all of the beams tested resulted in flexural failures except for one single shear failure that occurred in an I-shaped beam. Thus, Strands A/B, high performers with an NASP Bond Test value in excess of 20,000 lb, can develop adequate tension force even in relatively short distances. In Figure 3.52, the results obtained during NASP Round III with Strand HH are shown. Strand HH had a NASP Bond Test value of 10,700 lb. The chart shows that in the beams with two strands, flexural failures occurred at 73 in. (the AASHTO development length expression) and 58 in. The chart indicates that one bond failure occurred at an embed- ment length of 58 in. on a single strand beam. An embedment length of 58 in. corresponds to about 80 percent of the AASHTO requirement for development length. So in fact, a bond failure is the expected result. These beam test results from Round III provide important support to the recom- mendation that 10,500 lb should be the minimum average bond value for acceptance of prestressing strand. Clearly, Strand HH performed adequately when the ACI and AASHTO development lengths are provided. Finally, shown in Figure 3.51 are the results of development length tests on the 0.6-in. strand, Strand A6. Strand A6 demon- strated good bond performance with a NASP Bond Test value of 18,290 lb. That NASP Bond Test value is comparable with the recommended minimum average value for 0.6-in. strands of 12,600 lb. Figure 3.51 indicates that rectangular beams made with Strand A6 experienced bond failures at an embedment length of 58 in., for concrete strengths of about 8 ksi and lower. Note that in the case of 0.6-in. strand these bond failures oc- curred when the embedment length provided was only 66 per- cent of the development length required. At an embedment length of 73 in. or so, Strand A6 was able to achieve adequate tension for all concrete strengths tested. In summary, the results on beam tests clearly demonstrate support for the proposed development length expression for 0.5-in. and 0.6-in. strand. The inclusion of concrete strength is an important parameter that should be included in the design expressions for transfer and development lengths. 4.3.2.1 Additional Requirements for the Use of Debonded Strands Past research includes some behavioral models that more ac- curately describe the behavior of beams containing debonded strands. Russell and Burns (1994) and Russell et al. (1994) de- scribed the behavior of beams containing debonded strands. Their research indicates that some additional provisions limit- ing the overall length of the debonded strands should be incor- porated into the LRFD Specifications. Early testing performed

by Rabbat et al. (1979) and Kaar and Magura (1965) also sup- port a behavioral approach toward limitations on debonded strands. The experimental programs were reviewed, and rec- ommendations are made to amend the current code provisions. The experimental program did not incorporate testing on beams containing debonded strands. Recommended changes are based on experimental work already performed and identi- fied within the existing body of knowledge. The current AASHTO LRFD Bridge Design Specifications provide limitations on debonding strands. There are limita- tions on the total percentage of strands that can be debonded and the total number of debonded strands per row, as well as limitations on which strands within the cross section can be debonded. In addition to these existing limitations, the authors recommend three more: 1. Where pretensioned beams are not simply supported, debonding shall not be permitted except where it can be shown that cracking will not occur through the regions where debonding is placed nor through the transfer zones of debonded strands. 2. Debonding shall be limited in length from the end of a member to a distance equal to 0.15 times the span mea- sured from center of bearing to center of bearing. 3. Debonding shall be limited in number and in length to sections where debonding is required to meet the require- ments of Article 5.9.4.1. At sections where debonding is not required to meet the requirements of Article 5.9.4.1, debonding shall not be permitted. 4.3.3 Mild Reinforcement Development and Splice Lengths and Anchorage with Standard Hooks in Tension In Section 12.1.2 of the 318 Code (ACI 2005) an upper limit of 100 psi on the term in the anchorage and devel- opment length provisions is imposed; in Section 5.4.2.1 of the Interim 2008 AASHTO LRFD Bridge Design Specifications, it is stated that design concrete strengths above 10 ksi shall be used only when allowed by specific articles or when physical tests are made to establish the relationships between concrete strength and other properties. The experimental program on mild reinforcement described in NCHRP Project 12-60 was designed to determine, in conjunction with the data already available in the literature, whether these limitations can be re- moved for concrete compressive strengths up to 15 ksi. The 1971 provisions in the 318 Code (with slight modifica- tions because fy and f ′c are stated in terms of ksi and the intro- duction of epoxy-coated bar factors) are the current provisions for development and splice length of mild reinforcing bars in tension in the AASHTO LRFD Bridge Design Specifications. Thus, there are differences between the current 318 Code ′fc 116 provisions and the AASHTO LRFD Bridge Design Specifications (2004). In the 1995 edition of the 318 Code, the procedures for calculating development lengths for deformed bars and de- formed wire in tension were extensively modified (ACI 1995). The changes resulted in an increase in the development lengths for closely spaced bars and bars with small covers. The basic development length was modified to reflect the influence of cover, spacing, transverse reinforcement, casting position, type of aggregate, and epoxy coating. The basic development lengths remained essentially the same as they were in the 1971 edition of the ACI Code and the AASHTO LRFD Bridge Design Specifications, with a few exceptions. One exception is the equa- tion for the basic development length in tension for No. 18 bars. This equation was revised on the basis of a review of avail- able test results on large bars. This change resulted in an increase of 12 percent over the values given by the current AASHTO LRFD Bridge Design Specifications for bars of the same size. Also, the top bar factor, which is 1.3 in the 318 Code, is 1.4 in the LRFD specifications. Another important change to the 318 Code, introduced in 1989, was to limit the value to a maximum of 100 psi, regardless of the compressive design strength of the concrete (ACI 1989). This limitation meant that development lengths would no longer decrease with concrete strengths greater than 10 ksi. It was noted that research on de- velopment of bars in high-strength concretes was not sufficient to substantiate a reduction beyond the limit imposed. This is also the reason given for the limitation imposed in Section 5.4.2.1 of the current LRFD specifications. In 1977, the 318 Code provisions for tension lap splices of deformed bars and deformed wire encouraged the location of splices away from regions of high tensile stresses and to places where the area of steel provided at the splice location is at least twice that required by analysis. A lap splice of any portion of the total area of steel in regions where (As pro- vided/As required) is less than 2.0 had to be at least 1.3 times the development length of the individual bar in tension (Class B splice) in length. If more than one-half of the rein- forcement was to be spliced in such regions, lap splices had to be at least 1.7 times the development length of the indi- vidual bar (Class C splice) in length. Class A splices in which the length of bar is equal to the development length of the individual bar were only permitted in regions where (As provided/As required) is less than 2.0, and no more than 25 percent of the total area is spliced within one lap length. These same provisions are currently in the AASHTO LRFD Bridge Design Specifications. The Class C splice has been re- moved from the 318 Code. It must be noted that the splice factor is associated with the potential mode of failure when multiple bars are spliced at the same location and does not relate to the actual strength of the spliced bar. The work by Treece and Jirsa (1989) is the basis of the de- velopment length modification factors for epoxy-coated bars ′fc

in the 318 Code and the AASHTO LRFD Bridge Design Speci- fications. In both, development length is multiplied by a factor of 1.5 for the epoxy-coated bars with a cover of less than 3db or clear spacing between the bars that is less than 6db. Devel- opment length is multiplied by 1.2 for other cases. In either case, the product of the top-bar factor and epoxy-coating fac- tor should not exceed 1.7. The 1.2 factor was selected based on the work of Johnston and Zia (1982). DeVries, Moehle, and Hester (1991), Hadje-Ghaffari, Darwin, and McCabe (1991), and Hadje-Ghaffari et al. (1994) found the current maximum of 1.7 for the product of the top-bar factor and epoxy-coating factor to be too conservative and recommended a value of 1.5. Cleary and Ramirez (1993), based on experimental observa- tions on slab-type specimens, noted that since the experimen- tal data on splitting type failures included only up to maxi- mum cover to bar diameter ratio of 2.67 due to the increase of rib-bearing forces with epoxy-coated reinforcement, the limit of 3 used in the 318 Code for transition between splitting and pull-out failures should be examined experimentally. The 1983 provisions in the 318 Code for development of bars in tension terminated with standard hooks were a major departure from the 1977 version of the 318 Code as hooked- bar anchorage provisions were uncoupled from provisions for straight-bar development. In the 1983 version of the 318 Code, the hooked-bar embedment length was measured from the critical section to the outside end or edge of the hook. The development length of the hooked bar was calculated as the product of a basic development length and appropriate mod- ification factors. In the 1995 edition of the 318 Code, a factor of 1.2 was introduced in the hooked anchorage requirements (ACI 1995). The requirements in the 3rd edition of the AASHTO LRFD Bridge Design Specifications (2004) for an- chorage of hooked bars in tension are the same as those in the 2005 318 Code (2005). Equation 12-1 of the 318 Code (ACI 2005), used for calcu- lating tension splice and the development length require- ment, is the following: In Equation 12-1, ψt is a reinforcement location factor of 1.3 to reflect the adverse effects of the top reinforcement cast- ing position; ψe is a coating factor—1.5 with cover less than 3db or clear spacing less than 6db and 1.2 for all other cases. The product of ψt and ψe need not be taken greater than 1.7. The parameter ψs is a reinforcement size factor—0.8 for No. 6 bars and smaller and 1.0 for all other cases (the square root of the concrete compressive strength shall not exceed 100 psi as per Section 12.1.2). Other parameters are defined as follows: λ is a factor reflecting the lower tensile strength of l f f c K d d y t e s c b tr b = ′ +⎛⎝⎜ ⎞⎠⎟ ⎡ ⎣ ⎢⎢⎢⎢ ⎤ ⎦ ⎥⎥⎥ 3 40 ψ ψ ψ λ ⎥ db ( . )4 6 117 lightweight concrete, db is the bar diameter, Ktr is 40 Atr/snand represents the contribution of confining reinforcement across potential splitting planes, where Atr is the area of trans- verse reinforcement within the spacing s that crosses the potential splitting plane, s is the spacing of stirrups and n is the number of bars being spliced or developed along the plane of splitting; cb is the spacing or cover dimension using the smaller of either the distance from the center of the bar or wire to the nearest concrete surface or one-half the center-to- center spacing of the bars being developed. The ratio of (cb+ Ktr)/db is not to be taken greater than 2.5. However, ld shall not be less than 12 in. In addition, when calculating anchorage length requirements for tension lap splices, these should be as required for Class A or B splices but not less than 12 in., where a Class A splice is 1.0 ld and a Class B splice is 1.3ld. Development length for deformed bars in tension terminat- ing in a standard hook (as per Section 7.1 of the 318 Code), ldh, is determined using the equation below, found also in Section 12.5.2 of the 318 Code and applicable modification factors of Section 12.5.3, but ldh shall not be less than the smaller 8db and 6 in. as indicated in Section 12.5.1 of the 318 Code (ACI 2005). ψe is a coating factor taken as 1.2 for epoxy-coated reinforce- ment; λ is a factor reflecting the lower tensile strength of lightweight concrete taken as 0.75 (for other cases, these two factors are taken equal to 1.0). Other parameters include the bar diameter of the hooked bar, db, concrete compressive strength, f ′c, in psi (the square root of the concrete compres- sive strength shall not exceed 100 psi as per Section 12.1.2 of the 318 Code). The modification factors of Section 12.5.3 are all less than 1.0 and thus reduce the calculated length on the basis of cover, presence of ties where the first tie encloses the bent portion of the hook within 2db of the outside of the bend, and when anchorage or development for specified minimum yield strength, fy, is not specifically required: 1. For #11 bar and smaller hooks with side cover (normal to the plane of the hook) not less than 2.5 in. and for 90-deg hook with cover on bar extension beyond hook not less than 2 in, the factor is 0.7; 2. For 90-deg hooks of #11 and smaller bars that are either en- closed within ties or stirrups perpendicular to the bar being developed, spaced not greater than 3db along ldh or enclosed within ties or stirrups parallel to the bar being developed, spaced not greater than 3db along the length of the tail extension of the hook plus bend, the factor is 0.8; 3. For 180-deg hooks of #11 and smaller bars that are en- closed within ties or stirrups perpendicular to the bar being developed, spaced not greater than 3db along ldh, the factor is 0.8; and l f f ddh e y c b= ′ ⎛ ⎝⎜ ⎞ ⎠⎟ 0 02 4 7 . ( . ) ψ λ

118 4. Where anchorage or development for fy is not specifically required, reinforcement in excess of that required by analysis, the factor is (As required/As provided). 4.3.3.1 Development and Splice Length of Uncoated and Coated Bars Article 5.11 of the 3rd edition of the AASHTO LRFD Bridge Design Specifications (2004), “Development and Splices of Reinforcement,” contained provisions for development length of reinforcement that were essentially the same as those included in editions of the 318 Code up to the 1989 edition. The provisions of the 318 Code were extensively modified for the 1995 edition with a view to formulating a more “user friendly” format while maintaining the same gen- eral agreement with professional judgment and research re- sults. Tests on splices of uncoated bars (Azizinamini et al. 1993, 1999a) have indicated that in the case of high-strength concrete some minimum amount of transverse reinforce- ment is needed to ensure adequate ductility out of the splice at failure. Based on these tests, a proposed modification (Azizinamini et al. 1999b) to the 1999 318 Code calls for the determination of a basic, straight development length for bars in tension, without including the presence of transverse rein- forcement, together with a minimum area of transverse steel in the form of stirrups, Asp, crossing potential splitting planes. In these studies, over 70 specimens were tested with concrete compressive strengths ranging between 5 ksi and 16 ksi. The experimental work conducted in NCHRP Project 12-60 aimed to fill the gaps in the existing data to extend the appli- cability of the LRFD provisions for development and splice length of uncoated and epoxy-coated bars (ASTM A 775) in tension to normal-weight concrete with compressive strengths up to 15 ksi. Based on the observations from tests conducted during NCHRP Project 12-60 on 18 top-cast beam splice specimens and the examination of an extensive database of previous tests compiled by ACI Committee 408 (presented in previous chapters of this report), it is proposed to extend the AASHTO LRFD Bridge Design Specifications to concrete strengths up to 15 ksi using the approach in the 318 Code (ACI 2005), with the following exceptions: • Remove the bar size factor for No. 6 and smaller bars; thus, ψs = 1.0 in all cases. • Use a single factor for epoxy-coated bars of 1.5 regardless of the cover-to-bar diameter ratio. Beam splice specimens with bottom cast bars were not eval- uated in this study. The ACI Committee 408 (2003) indicated that the current approach in the 318 Code overestimated the bar force at failure in many specimens with bottom bars that are available in the ACI Committee 408 database, especially for specimens with concrete compressive strengths greater than 10 ksi. The ACI Committee 408 proposed a modified ex- pression for development and splice length in addition to the removal of the bar size factor to address this issue. In the eval- uation of test data conducted under NCHRP Project 12-60, the researchers found that the use of a bottom cast modifica- tion factor of 1.2 for uncoated bars anchored in concrete with compressive strengths greater than 10 ksi appeared to address the safety concerns raised by ACI Committee 408. This factor would not be needed for bottom cast epoxy-coated bars (be- cause of the single modification factor of 1.5) or for uncoated top bars. This approach could be used as an alternative to the approach suggested by ACI Committee 408. The researchers note that additional testing of bottom cast uncoated splices is justified with higher strength concretes. 4.3.3.2 Anchorage in Tension of Uncoated and Coated Mild Reinforcement Using Standard Hooks Article 5.11.2.4 of the AASHTO LRFD Bridge Design Specifi- cations was verified for high-strength concrete in the proposed experimental Work Plan for NCHRP Project 12-60 with the exception of the lightweight aggregate factor. Based on the analy- sis of tests conducted during NCHRP Project 12-60 (21 full-scale tests of hooked-bar anchorages) and of tests of additional spec- imens in the literature, it is possible to support the extension of the approach in the 318 Code (ACI 2005) provision for anchor- age of bars terminated with standard hooks, black and epoxy- coated, to normal-weight concrete with concrete compressive strengths of up to 15 ksi, with the following modifications: • A minimum amount of transverse reinforcement (at least #3 U bars at 3db spacing) should be provided in the anchor- age length to improve the bond strength of both uncoated and epoxy-coated No. 11 and larger bars terminated in a standard hook. • A modification factor of 0.8 instead of the current factor of 0.7 for No. 11 and smaller hooks with side cover (normal to the plane of the hook) not less than 2.5 in. and for 90- degree hooks with cover on bar extensions beyond the hook of not less than 2 in.

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Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete Get This Book
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TRB's National Cooperative Highway Research Program (NCHRP) Report 603: Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete explores recommended revisions to the American Association of State Highway and Transportation Officials Load and Resistance Factor Design (LRFD) Bridge Design Specifications, which are designed to extend the applicability of the transfer, development, and splice length provisions for prestressed and non-prestressed concrete members to concrete strengths greater than 10 ksi.

Appendices A and B are published as part of NCHRP Report 603. Appendices C through I are available online via the links below:

* Appendix C: Rectangular Beam Summaries-Strand D

* Appendix D: Rectangular Beam Summaries-Strands A&B

* Appendix E: Rectangular Beam Summaries-Strand A (0.6 in.)

* Appendix F: I-Beam Summaries-0.5-in. Strand

* Appendix G: I-Beam Summaries-0.6-in. Strand

* Appendix H: AASHTO Mxxx-Standard Test Method for the Bond of Prestressing Strands

* Appendix I: NASP Test Protocols

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