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

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

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113 Please note the protocols described in the Standard Test found in Figure 3.9 corresponds with a NASP Bond Test Method for the Bond of Prestressing Strands require that a value (in concrete) of about 6,600 lb. Strand D, tested in single bond test consist of six individual tests and that the re- mortar using the Standard Test Method for the Bond of ported "Bond Test Value" is the average value from those six Prestressing Strands, had an NASP value of 6,890 lb. In con- tests. Therefore, the 10,500-lb recommended minimum crete with a strength of 10 ksi, Strand D had a NASP Bond threshold is an average value in conformance with the proto- Test value of nearly 12,000 lb. The data from the specimens cols found in the Standard Test Method for the Bond of in concrete support the conclusion that increasing concrete Prestressing Strands. In addition to the requirement for a strength improves the bond between strand and concrete. minimum average Bond Test Value, the test includes a re- Figures 3.12 and 3.13 illustrate the results from all of the quirement for the minimum value for the lowest value of the NASP Bond Tests in concrete versus concrete strength on four set of six individual tests. This second criteria is established to different strand samples. One of the remarkable features of avoid excessive variations in strand bond quality within the this dataset is that it includes data from both 0.5-in. and same sample of strand. The requirement for the minimum 0.6-in. strands. There are more than 20 data points repre- single test value is 9,000 lb for 0.5-in. strands. This second sented in the figures, all cast within a wide sample of concrete requirement effectively limits the standard deviation, or the strengths. The unifying factor is that the Standard Test statistical variance, for strand produced that may have mod- Method for the Bond of Prestressing Strands (in concrete) was erate bonding properties. performed on each of the samples, and each data point repre- In the NCHRP Project 12-60 research, transfer and devel- sents the average from at least six individual tests. Further- opment length testing was also performed on 0.6-in. strands. more, the coefficient of determination, R2, was a remarkable The NASP Bond Test was also conducted on 0.6-in.-diameter 0.80 for this dataset, which includes two different strand sizes, strands. The results indicated that the NASP Bond Test or the four different strand samples, and a variety of concrete mix- Standard Test Method for the Bond of Prestressing Strands tures. The figures show clearly how the bond of concrete and was suitable in predicting the bond behavior of 0.6-in strands. sand is directly improved by increases in concrete strength. Therefore, the NASP Bond Test is recommended for use for This result is important in determining recommendations for 0.6-in. strands. For 0.6-in. strands, the minimum threshold transfer length and development length equations. values are an average value of 12,600 lb and a single test min- Figures 3.12 and 3.13 show two very important things: (1) imum of 10,800 lb. 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 4.3.2 Transfer and Development Length in various concrete mixtures. As illustrated in Figure 3.12, the Expressions for Prestressing Strand power regression between bond strength and concrete The current ACI and AASHTO design equations for pre- strength suggests the relationship: tensioned transfer length and development length do not in- ( NASPconcrete ) clude concrete strength as a parameter in the design equation. = 0.49139 f ci0.51702 (4.1) However, test results obtained during NCHRP Project 12-60 NASP strongly suggest that the anchorage ability of the strands is The results from these tests further attest to the suitability improved as concrete strength increases. The results from of the Standard Test Method for the Bond of Prestressing both transfer and development length testing support the Strands as a vehicle to measure the bond between prestress- conclusion that concrete strength is an important factor. ing strand and concrete. Further, these results support the ar- These results are supported independently by the results from gument that the variation in transfer and development NASP Bond Tests in varying concrete strengths. lengths should vary with the square root of concrete strength. The Standard Test Method for the Bond of Prestressing The results also support the recommendation for the adop- Strands was used to assess the impact of varying concrete tion of the Standard Test Method for the Bond of Prestress- strengths on the bond between strand and concrete. In these ing Strands by demonstrating that the Standard Test Method tests, the NASP Bond Test was modified simply by casting the for the Bond of Prestressing Strands is useful for various sizes strand in concrete (with varying concrete strengths) instead of strands and can predict differences in bond based on vari- of the sand-cement mortar required in the Standard Test ations in concrete strength. Method for the Bond of Prestressing Strands protocols. Fig- Testing showed that it is possible to describe the relation- ure 3.9 illustrates the data collected from the NASP Bond Test ship between concrete strength and bond strength using the performed on Strand D, in concrete, and the effects of vary- correlations found directly from the NASP Bond Tests in ing concrete strengths. The figure shows that at a concrete concrete. In those tests, it was found that bond strength var- strength of about 4.5 ksi, the prediction curve for Strand D ied nearly in proportion with the square root of concrete

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114 strength. It can be observed from the data shown in Figures to include the effects of concrete strength in the design ex- 3.12 and 3.13 that the bond strength of concrete is almost di- pression for development length. rectly proportional to the square root of concrete strength. Current expressions for development length (both ACI and The linear regression shown in Figure 3.13 gives the follow- AASHTO) were developed from the addition of the transfer ing relationship: length and a "flexural bond length." This approach is supported by past and current research. Therefore, it is recommended to ( NASPconcrete ) = 0.51 f ci (4.2) continue with the approach of splitting the development length NASP into a transfer length component and a flexural bond length Testing also demonstrated that both transfer length mea- component. Both components, however, are affected by the surements and development length requirements were short- concrete strength. Following is an outline of the rationale for ened as concrete strength increased. Figures 3.24 through developing the recommendation for development length: 3.32 show that for all sizes of strand and for all the varying qualities of strand bond, the transfer lengths shorten as con- 1. The current AASHTO development length equation can crete release strengths increase. Therefore, the recommended be used to adequately predict required development code equation includes the square root of concrete strength lengths for "normal strength concrete" with release at release in order to be consistent with testing results that strengths in the range of 4 ksi to 6 ksi, provided that the show shorter transfer lengths with higher concrete strengths. strand itself is qualified by the Standard Test Method for The transfer length recommendation provides a transfer the Bond of Prestressing Strands. The results presented in length equal to the current design expression of 60 strand di- Chapter 3 of this report demonstrate flexural failures at ameters when the concrete release strength is 4 ksi. Using the embedment lengths of 73 in. for all 0.5-in. strands tested recommended expression, the transfer length shortens as in this research program. The embedment length of 73 in. the concrete release strength increases. Additionally, the ex- corresponds to 100 percent of the current code provision pression provides a minimum limit of 40 strand diameters. for development length for these specimens. The results Thus, the recommended expression for transfer length ex- included tests on beams made with concrete strength at pression is the following: release of approximately 4 ksi and approximately 6 ksi at the time of the beam test. 120 2. The data demonstrate that development length require- lt = db 40db (4.3) fci ments are shortened as concrete strength increases. 3. The required development length calculated from the cur- where rent code provisions is approximately 150 db, although lt = transfer length (in.), under the 2004, 3rd edition of the AASHTO LRFD Bridge f ci = release concrete strength (ksi), and; Design Specifications, some variations will exist due to db= diameter or prestressing strand (in.). variations in strand stressing, beam geometry, and subse- If the concrete release strength is 4 ksi, then the equation quent variations in computed prestress losses. Only a few results in a transfer length of 60 db. As release strength in- specimens contained prestressed strands at the tops of the creases, the transfer length decreases. The limit of 40db cross sections. For these, beam tests were conducted for ensures that all designs consider transfer lengths of some rea- development length. Therefore, no comment can be made sonable value. The recommendation effectively limits an regarding the "top bar effect" for prestressing strands. additional decrease in transfer length from concrete release Discussions regarding the factor are not included, al- strengths greater than 9 ksi. This minimum limit on transfer though the testing demonstrates that the factor should be length is consistent with the testing conducted as part of this discarded for all sizes of strand. Instead all strands can be research in which the highest release strength achieved on qualified for bond through the Standard Test Method for rectangular beams was 9.7 ksi. the Bond of Prestressing Strands. The current ACI and AASHTO design equations for de- 4. Using a transfer length of approximately 60 db, and a de- velopment length do not include concrete strength as a velopment length of approximately 150 db, the flexural parameter. As with the results on transfer length, results from bond length must be approximately 90 db. development length testing strongly suggest that the bond- ability of the strands is improved as concrete strength Thus, the development length expression can then be increases. The experimental results clearly demonstrate that written as the required development length is shortened as concrete 225db strength increases; higher concrete strength results in shorter ld = lt + (4.4) development length requirements. Therefore, it is important fc

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115 where In contrast to the results from Strand D, Figure 3.50 plots ld = development length, (in.), results from development length tests on Strands A/B versus lt = transfer length, (in.), the proposed development length equation. The chart clearly db = nominal diameter of the prestressing strand, (in.), and shows that all of the beams tested resulted in flexural failures f c = design concrete strength, (ksi). except for one single shear failure that occurred in an I-shaped beam. Thus, Strands A/B, high performers with an If a concrete design strength, f c, of 6 ksi is seen as a reason- NASP Bond Test value in excess of 20,000 lb, can develop able approximation for "normal concrete strength," then a adequate tension force even in relatively short distances. coefficient can be computed to correspond with a flexural In Figure 3.52, the results obtained during NASP Round III bond length of 90 strand diameters. That coefficient is 225 with Strand HH are shown. Strand HH had a NASP Bond (225/ 6 90). Test value of 10,700 lb. The chart shows that in the beams A minimum value is also recommended for the develop- with two strands, flexural failures occurred at 73 in. (the ment length expression. The recommended expression for AASHTO development length expression) and 58 in. The development length, therefore, is based on a limiting concrete chart indicates that one bond failure occurred at an embed- strength of approximately 14 ksi, which is slightly less than ment length of 58 in. on a single strand beam. An embedment the maximum concrete strength attained in beams tested in length of 58 in. corresponds to about 80 percent of the the research program (14.9 ksi). Thus, the recommended AASHTO requirement for development length. So in fact, a development length equation is as follows: bond failure is the expected result. These beam test results 120 225 from Round III provide important support to the recom- ld = + db 100db (4.5) mendation that 10,500 lb should be the minimum average f ci f c bond value for acceptance of prestressing strand. Clearly, The proposed development length equation is plotted Strand HH performed adequately when the ACI and against the development length test results in Figures 3.49 to AASHTO development lengths are provided. 3.51. The result of each beam test, whether flexural failure or Finally, shown in Figure 3.51 are the results of development bond failure is plotted on a chart showing concrete strength length tests on the 0.6-in. strand, Strand A6. Strand A6 demon- versus embedment length. The curves representing the rec- strated good bond performance with a NASP Bond Test value ommended design equations for development length are also of 18,290 lb. That NASP Bond Test value is comparable with shown on each of the charts. Note that the development the recommended minimum average value for 0.6-in. strands length expression is now dependent on the concrete strength. of 12,600 lb. Figure 3.51 indicates that rectangular beams made For the purpose of computing the values within the equation with Strand A6 experienced bond failures at an embedment and charting the results, the release strength is taken as 66.7 length of 58 in., for concrete strengths of about 8 ksi and lower. percent of the design strength. Note that in the case of 0.6-in. strand these bond failures oc- Figure 3.49 shows the results of development length tests curred when the embedment length provided was only 66 per- on Strand D. Strand D demonstrated below average bond cent of the development length required. At an embedment performance with a relatively low NASP Bond Test value of length of 73 in. or so, Strand A6 was able to achieve adequate 6,890 lb. Strand D also had measurably longer transfer tension for all concrete strengths tested. lengths than Strands A/B. Figure 3.49 indicates that beams In summary, the results on beam tests clearly demonstrate made with Strand D experienced bond failures in rectan- support for the proposed development length expression for gular beams with embedment lengths of 58 in. for concrete 0.5-in. and 0.6-in. strand. The inclusion of concrete strength strengths of about 8 ksi and lower. Note that in Figure 3.49, is an important parameter that should be included in the the flexural failures are represented by open symbols (tri- design expressions for transfer and development lengths. angles for R-beams and diamonds for I-Beams) and the bond failures are represented by solid symbols. Figure 3.49 4.3.2.1 Additional Requirements for the Use clearly shows that as concrete strength increased, Strand D of Debonded Strands was able to move from bond failures to flexural failures a 58-in. embedment length. Furthermore, Figure 3.49 indi- Past research includes some behavioral models that more ac- cates that I-beams made with Strand D failed in bond at curately describe the behavior of beams containing debonded embedment lengths where the proposed design equation strands. Russell and Burns (1994) and Russell et al. (1994) de- would predict adequate development. Therefore, Figure scribed the behavior of beams containing debonded strands. 3.49 shows that Strand D, with an NASP Bond Test value of Their research indicates that some additional provisions limit- only 6,890 lb, does not provide adequate bond-ability with ing the overall length of the debonded strands should be incor- concrete. porated into the LRFD Specifications. Early testing performed

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

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117 in the 318 Code and the AASHTO LRFD Bridge Design Speci- lightweight concrete, db is the bar diameter, Ktr is 40 Atr/sn and fications. In both, development length is multiplied by a factor represents the contribution of confining reinforcement of 1.5 for the epoxy-coated bars with a cover of less than 3db across potential splitting planes, where Atr is the area of trans- or clear spacing between the bars that is less than 6db. Devel- verse reinforcement within the spacing s that crosses the opment length is multiplied by 1.2 for other cases. In either potential splitting plane, s is the spacing of stirrups and n is case, the product of the top-bar factor and epoxy-coating fac- the number of bars being spliced or developed along the tor should not exceed 1.7. The 1.2 factor was selected based on plane of splitting; cb is the spacing or cover dimension using the work of Johnston and Zia (1982). DeVries, Moehle, and the smaller of either the distance from the center of the bar or Hester (1991), Hadje-Ghaffari, Darwin, and McCabe (1991), wire to the nearest concrete surface or one-half the center-to- and Hadje-Ghaffari et al. (1994) found the current maximum center spacing of the bars being developed. The ratio of (cb+ of 1.7 for the product of the top-bar factor and epoxy-coating Ktr)/db is not to be taken greater than 2.5. However, ld shall not factor to be too conservative and recommended a value of 1.5. be less than 12 in. In addition, when calculating anchorage Cleary and Ramirez (1993), based on experimental observa- length requirements for tension lap splices, these should be as tions on slab-type specimens, noted that since the experimen- required for Class A or B splices but not less than 12 in., where tal data on splitting type failures included only up to maxi- a Class A splice is 1.0 ld and a Class B splice is 1.3ld. mum cover to bar diameter ratio of 2.67 due to the increase of Development length for deformed bars in tension terminat- rib-bearing forces with epoxy-coated reinforcement, the limit ing in a standard hook (as per Section 7.1 of the 318 Code), ldh, of 3 used in the 318 Code for transition between splitting and is determined using the equation below, found also in Section pull-out failures should be examined experimentally. 12.5.2 of the 318 Code and applicable modification factors of The 1983 provisions in the 318 Code for development of Section 12.5.3, but ldh shall not be less than the smaller 8db and bars in tension terminated with standard hooks were a major 6 in. as indicated in Section 12.5.1 of the 318 Code (ACI 2005). departure from the 1977 version of the 318 Code as hooked- 0.02 e f y ldh = db (4.7) fc 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 e is a coating factor taken as 1.2 for epoxy-coated reinforce- the critical section to the outside end or edge of the hook. The ment; is a factor reflecting the lower tensile strength of development length of the hooked bar was calculated as the lightweight concrete taken as 0.75 (for other cases, these two product of a basic development length and appropriate mod- factors are taken equal to 1.0). Other parameters include the ification factors. In the 1995 edition of the 318 Code, a factor bar diameter of the hooked bar, db, concrete compressive of 1.2 was introduced in the hooked anchorage requirements strength, f c, in psi (the square root of the concrete compres- (ACI 1995). The requirements in the 3rd edition of the sive strength shall not exceed 100 psi as per Section 12.1.2 of AASHTO LRFD Bridge Design Specifications (2004) for an- the 318 Code). The modification factors of Section 12.5.3 are chorage of hooked bars in tension are the same as those in the all less than 1.0 and thus reduce the calculated length on the 2005 318 Code (2005). basis of cover, presence of ties where the first tie encloses the Equation 12-1 of the 318 Code (ACI 2005), used for calcu- bent portion of the hook within 2db of the outside of the bend, lating tension splice and the development length require- and when anchorage or development for specified minimum ment, is the following: yield strength, fy, is not specifically required: 3 f y t e s 1. For #11 bar and smaller hooks with side cover (normal to ld = db (4.6) the plane of the hook) not less than 2.5 in. and for 90-deg 40 fc cb + K tr db hook with cover on bar extension beyond hook not less than 2 in, the factor is 0.7; In Equation 12-1, t is a reinforcement location factor of 2. For 90-deg hooks of #11 and smaller bars that are either en- 1.3 to reflect the adverse effects of the top reinforcement cast- closed within ties or stirrups perpendicular to the bar being ing position; e is a coating factor--1.5 with cover less than developed, spaced not greater than 3db along ldh or enclosed 3db or clear spacing less than 6db and 1.2 for all other cases. within ties or stirrups parallel to the bar being developed, The product of t and e need not be taken greater than 1.7. spaced not greater than 3db along the length of the tail The parameter s is a reinforcement size factor--0.8 for No. extension of the hook plus bend, the factor is 0.8; 6 bars and smaller and 1.0 for all other cases (the square root 3. For 180-deg hooks of #11 and smaller bars that are en- of the concrete compressive strength shall not exceed 100 psi closed within ties or stirrups perpendicular to the bar being as per Section 12.1.2). Other parameters are defined as developed, spaced not greater than 3db along ldh, the factor follows: is a factor reflecting the lower tensile strength of is 0.8; and

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