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31 match the "perfect fit" line very closely, with an R2 value of 0.92. 3.2.3 Recommendation for the Standard Test Method for the Bond of Prestressing Strands The NASP Bond Test performed in this research program was conducted on ten 0.5 in. diameter and two 0.6 in. diam- eter strands. In the NCHRP testing, round robin tests were performed at Purdue and OSU. As shown in Figure 3.7, the results from the two testing sites closely match. This research builds upon earlier work done by NASP to develop a standard test for bond. The NCHRP research fur- ther refined the testing protocols to the point where the test results are now demonstrably reproducible between testing sites. The refined test is recommended as a standard test method to evaluate the ability of a prestressing strand to bond with concrete. Figure 3.6. NASP Test specimen 3.3 The NASP Bond Test inside loading frame. in Concrete The NASP Bond Test protocol was modified to test the diameter samples are labeled Strand A6 and Strand B6. For strand in concrete in place of mortar. This is important to the example, Strand E had the lowest reported results at both test- overall research because the NASP Bond Test modified for ing sites, 5240 lb at OSU and 6070 lb at Purdue. For Strand C, concrete demonstrates the relationship between bond strength OSU reported an average of 13,715 lb, whereas Purdue and concrete strength. The overarching conclusion from this reported an average of 14,710 lb. Note that the table reports segment of the testing was that bond strength improves in results of testing with both 0.5 in. and 0.6 in. strands. proportion to the square root of the concrete strength. This The results in Table 3.3 are illustrated in Figure 3.7. Figure conclusion stems from an examination of data from three of 3.7 plots the average NASP values from OSU against the av- the 0.5 in. diameter strands and one of the 0.6 in. diameter erage NASP values from Purdue. A linear regression line and strands. The concrete used for the modified NASP Bond Test a "perfect fit" line are plotted in the figure. The test results had 1-day strengths varying from 4 ksi and to 10 ksi. Table 3.2. Round-robin testing at OSU and Purdue. NASP NASP STRAND NCHRP ROUND III ROUND IV DIAMETER (IN) OSU ID OSU Purdue ID ID 0.5 A C x 0.5 B x 0.5 FF C D x x 0.5 II D E x x 0.5 E x 0.5 F x 0.5 AA G A x x 0.5 H x 0.5 I x 0.5 J B x x 0.6 K6 B6 x x 0.6 L6 A6 x x

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32 Table 3.3. Results from round-robin testing--Standard Test for the Bond of Prestressing Strands in Concrete. NASP Test Results NASP Test Results Diameter (in) STRAND ID at OSU at Purdue NCHRP Strand Mortar Strength Pull-Out Force Mortar Strength Pull-Out Force f ci at 0.1" slip f ci at 0.1" slip (psi) (lb) (psi) (lb) C 0.50 4,723 14,130 4,498 14,270 C 0.50 4,927 13,300 4,810 15,150 Avg. 13,715 14,710 D 0.50 4,765 6,870 4,665 7,280 D 0.50 4,484 6,910 4,365 9,770 1 D 0.50 4,767 9,970 Avg. 6,890 8,625 E 0.50 4,303 5,240 4,000 6,070 A 0.50 4,730 20,710 4,847 2,0880 A 0.50 4,815 21,190 4,318 16,470 1 A 0.50 4,638 18,880 Avg. 20,950 19,880 B 0.50 4,723 19,330 4,893 22,700 B 0.50 4,927 21,090 4,798 22,280 Avg. 20,210 22,490 B6 0.60 4,843 22,420 4,356 19,130 B6 0.60 4,933 19,010 B6 0.60 4,153 19,510 1 Avg. 20,715 19,130 A6 0.60 4,933 17,960 4,628 15,450 A6 0.60 4,843 18,610 Avg. 18,285 15,450 1 Value omitted from average because the mortar strength was out of range. The modified NASP Bond Test was conducted in con- NASP tests in concrete were conducted on three 0.5 in. di- crete to understand the effects of varying concrete strengths ameter strands with NCHRP strand designations A, B, and on the bond of prestressing strands. The test procedure was D and on one 0.6 in. diameter strand with an NCHRP identical to the NASP Bond Test protocols discussed in strand designation of A6. The number of NASP tests con- Section 3.2 except that concrete with varying strengths was ducted on concrete for varying concrete strengths is re- used instead of the standard cement-sand mortar. The ported in Table 3.4. Each test listed in Table 3.4 contains six 25000 B Average NASP Pull-out Values (lb) 20000 A B6 Purdue University 15000 C Linear Regression A6 2 R = 0.92 10000 D "Perfect" Test 5000 E 0 0 5,000 10,000 15,000 20,000 25,000 Average NASP Pull-out Values (lb) Oklahoma State University Figure 3.7. Comparison of NASP Bond Tests at OSU and Purdue.

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33 Table 3.4. Number of NASP Bond Tests modified was performed (Tessema 2006) to arrive at the concrete mix for concrete with varying concrete target proportions and the target fresh and hardened properties. strengths. The results and discussion on the concrete mix proportions are beyond the scope of this report. The mixture propor- NASP Strand Target 1d Concrete Strengths NCHRP tions reported in Table 3.5 were also employed to make the Round IV Diameter (ksi) ID (inches) ID 4 6 8 10 transfer length and development length beams. G A 0.5 1 1 1 1 The NASP Bond Test, modified to be tested in concrete, J B 0.5 1 1 1 1 conforms to the same protocols for NASP Bond Testing that C D 0.5 2 4 2 1 are found in Appendix I. The only variation is that concrete L A6 0.6 1 1 1 1 is used in place of the sand-cement mortar. Also, concrete slumps of 2 to 3 in. were achieved instead of the mortar flow rates of 100 to 125. The handling and preparation of the or more NASP specimens. The target concrete strengths for strands, the steel casing, and the bond breakers were identi- each of the tests were 4, 6, 8, and 10 ksi. The concrete cal to the NASP Bond Tests conducted in sand-cement mor- mixtures used for making the NASP specimens in concrete tar. The mixing procedures used for the NASP Bond Test included Type III cement from Lafarge North America, conformed to ASTM C 192. The fresh concrete is placed in coarse and fine aggregate from Dolese Bros. Co., cement two layers; each layer is consolidated using a handheld elec- slag from Lafarge North America, and admixtures from De- tric vibrator. The slump, unit weight, and air content are gussa Admixtures, Inc. Admixtures used included HRWRs, measured per ASTM C 143, ASTM C 138, and ASTM C 231, normal range water reducers (NRWRs), and air entraining respectively. The NASP specimens and the test cylinders were admixtures (AEAs). Table 3.5 gives the mix proportions and cured in conformance with ASTM C 192. The compressive the target fresh and hardened properties for the concrete strength testing was conducted during the time of the NASP cast in the modified NASP specimens. The mix proportions Bond Test in concrete, in conformance with ASTM C 39. The were named based on the target 1-day strength. The mix NASP specimens are then kept in a laboratory curing room C-0 targets a concrete strength of 4 ksi at release. Similarly, for 22 to 24 hr from the time of hydration. Curing conditions C-I, C-II, and C-III target strengths of 6, 8, and 10 ksi at near 73.4 F and 100-percent relative humidity were main- release, respectively. The concrete mix C-IA has a target tained. The modified NASP Bond Test is performed at 24 2 release strength of 6 ksi with AEA. Detailed trial batching hr after the hydration of the cement. The NASP specimen in Table 3.5. Concrete mixture proportions for transfer and devel- opment length testing and for the NASP Bond Test in concrete. Concrete Mixture Designations C-0 C-I C-IA C-II C-III Cement (PCY) 650 800 800 800 900 Cement Slag (PCY) 100 Coarse Aggregates (PCY) 1,800 1,703 1,800 1,805 1,747 Fine Aggregates (PCY) 1,243 1,203 922 1,219 1,183 Water (PCY) 298 303 272 277 251 Glenium 3200 (fl oz/cm. wt) 10 14 7 Glenium 3400 (fl oz/cm. wt) 8 5 5.5 Polyheed 997 (fl oz/cm. wt) 3 MB-AE 90 (fl oz/cm. wt) 1.88 Target Properties for Fresh and Hardened Concrete 1-Day Strength (ksi) 4 6 6 8 10 28-Day Strength (ksi) 6 8 8 10 14 56-Day Strength (ksi) n/a 10 10 14 15 Slump (in) 8 8 8 8 9 Unit Weight (pcf) 145 148 148 150 157 Air Content (%) 2 2 6 2 2

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34 concrete is mounted on a rigid steel frame in the same man- (S) for each set of tests are reported for the modified NASP ner described for the NASP Bond Test (in mortar). Bond Test in concrete. 3.3.1 Results from the NASP Bond Tests 3.3.2 Discussion of the Results from the in Concrete NASP Bond Tests in Concrete The NASP Bond Test standardized in mortar was con- Figure 3.8 shows the pull-out values from the Modified ducted in concrete to understand the effect of concrete NASP Bond Test for Strands A and B plotted versus the strengths on the NASP Bond Test. The results from this ex- concrete strength. There are a total of 8 data points, also re- perimental testing are reported in Table 3.6. Table 3.6 reports ported in Table 3.6, found in Figure 3.8. Both linear regres- the NCHRP Strand ID, the NASP Strand ID (for comparison sion and the power regression curves are plotted on the purposes), the 1-day concrete strength (f ci), the NASP Bond figure. The coefficient of determination (R2) value for both Test result (from the Standard Test for Strand Bond in mor- the regressions is 0.82. The linear and the power best-fit tar), and the NASP Bond Test when modified and performed equations are reported in Figure 3.8. Figure 3.8 clearly shows in concrete. The table reports the w/cm (watercementitious that increases in concrete strength result in a higher NASP materials) ratio because there were pozzolanic materials pull-out value for NCHRP Strands A and B. Note that the added for some of the concrete mixtures reported in Table NASP Bond Test pull-out value for the standardized test in 3.5. The concrete strengths reported in Table 3.6 are averages mortar is 20.95 kips for NCHRP Strand A and 20.21 kips for of three or more concrete specimens tested during the NASP Strand B. Also, note that the regression plots cross the 4 ksi test. The number of NASP Bond Test specimens (N) that concrete strength at a corresponding NASP Bond Test were included as part of the test and the standard deviation (modified) value of about 23 kips. Table 3.6. Results of NASP Bond Tests in concrete. Strand ID Concrete NASP Test Results NCHRP ID in Mortar NASP STRAND Diameter NASP IV S Strand NASP (kips) f ci Value (in.) f ci ID w/cm N (ksi) (ksi) (kips) ksi A G 0.5 0.425 4.52 2.13 23.58 6 0.66 A G 0.5 0.38 7.02 2.65 26.35 6 1.44 20.95 A G 0.5 0.36 8.05 2.84 30.68 6 1.77 A G 0.5 0.235 11.79 3.43 35.29 6 2.33 B J 0.5 0.46 3.56 1.89 22.55 6 5.57 B J 0.5 0.4 5.58 2.36 30.8 6 1.04 20.21 B J 0.5 0.32 7.11 2.67 28.78 6 4.55 B J 0.5 0.24 10.06 3.17 34.33 6 4.17 D C 0.5 0.45 4.71 2.17 7.48 6 2.76 D C 0.5 0.46 4.56 2.13 6.66 6 2.52 D C 0.5 0.36 6.99 2.64 8.96 6 2.23 D C 0.5 0.38 7.34 2.71 9.51 6 2.64 6.89 D C 0.5 0.4 6.13 2.48 6.74 6 0.25 D C 0.5 0.3 8.67 2.94 10.26 6 0.26 D C 0.5 0.32 8.34 2.89 9.97 6 1.06 D C 0.5 0.26 9.95 3.15 11.56 6 0.84 A6 L 0.6 0.46 2.23 1.49 11.6 6 0.61 A6 L 0.6 0.38 5.02 2.24 23.13 6 1.24 18.29 A6 L 0.6 0.28 8.79 2.96 24.84 6 0.82 A6 L 0.6 0.235 10.42 3.23 28.74 6 1.39

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35 45.00 NASP Pull-out Value in Concrete (kip) 40.00 35.00 30.00 y = 1.5293x + 18.02 25.00 R2 = 0.8153 20.00 15.00 y = 14.078x0.3734 10.00 R2 = 0.8222 5.00 0.00 0 2 4 6 8 10 12 14 Concrete Strength (ksi) Figure 3.8. Pull-out values from the modified NASP Bond Test for Strands A/B versus concrete strength. Figure 3.9 illustrates the pull-out values from the Modified that the regression plots cross the 4 ksi concrete strength at NASP Bond Test for NCHRP Strand D plotted against con- a corresponding NASP Bond Test (modified) value of about crete strength. NCHRP Strand D had a NASP Bond Test 6 kips. value of 6.89 kips in the standardized test, which was lower Figure 3.10 makes the same comparison as Figures 3.8 and than the standardized NASP Bond Test values of Strands A 3.9, but for 0.6 in. diameter strand, NCHRP A6. The data and B. There are a total of 8 data points for Strand D, and the shown in Figure 3.10 are also reported in Table 3.6. Both data shown in Figure 3.9 correspond to data reported in Table linear regression and the power regression curves are plotted 3.6. Linear regression and the power regression curves are on Figure 3.10 for NCHRP Strand A6. Please note that the ex- plotted on Figure 3.9 for Strand D. The coefficient of deter- ponent in the best-fit power curve is approximately 0.56. As mination (R2) values are 0.89 for the linear regression and in Figures 3.8 and 3.9, Figure 3.10 clearly shows that increases 0.84 for the power regression. The linear and the power best- in concrete strength result in higher NASP pull-out values for fit equations are also reported in the figure. Figure 3.9 clearly NCHRP Strand A6. Please note that the NASP Bond Test shows that increases in concrete strength result in a higher pull-out value for the standardized test in mortar is 18.29 kips NASP pull-out value for NCHRP Strand D. Please note that for the NCHRP Strand A6, and that the regression plots cross the NASP Bond Test pull-out value for the standardized test the 4 ksi concrete strength at a corresponding NASP Bond in mortar is 6.89 kips for the NCHRP Strand D. Also, note Test (modified) value of about 17.5 kips. 14.00 NASP Pull-out Value in Concrete (kip) 12.00 10.00 y = 0.8833x + 2.6335 8.00 R2 = 0.8917 6.00 4.00 y = 2.4161x0.6677 2.00 R2 = 0.8431 0.00 0 2 4 6 8 10 12 14 Concrete Strength (ksi) Figure 3.9. Pull-out values from the modified NASP Bond Test for Strand D versus concrete strength.

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36 45.00 NASP Pull-out Value in Concrete (kips) 40.00 35.00 30.00 25.00 y = 1.8439x + 9.8805 20.00 R2 = 0.8566 15.00 10.00 y = 7.9751x0.5565 5.00 R2 = 0.9203 0.00 0 2 4 6 8 10 12 14 Concrete Strength (ksi) Figure 3.10. Pull-out values from the modified NASP Bond Test for Strand A6 versus concrete strength. Table 3.6 also reports values for the square of the 1-day The data presented in Table 3.6 are normalized and pre- concrete strength. The NASP pull-out values and the square sented all together in Figure 3.12. The NASP Bond Test val- root of the concrete strength are presented for all the strands ues were normalized by dividing by the NASP Bond Values in tested in concrete in Figure 3.11. The linear best fit-lines are concrete by the Standard NASP Bond Test (in mortar) values. plotted in the figure with the corresponding R2 values for the Figure 3.12 includes data from all three 0.5 in. diameter four strands tested in the modified NASP test in concrete. In strands and the one 0.6 in. diameter strand. Concrete Figure 3.11, the best-fit curves tend to have a steeper slope for strength at 24 hr from the modified NASP Bond Test is plot- strands with higher NASP values in the same range of con- ted against normalized NASP values. The data are plotted crete strengths. The NASP value increases with increases in against a best-fit power regression curve, also shown in Fig- concrete strength, and the high-performing strands have a ure 3.12. The R2 value for the test data is 0.80, indicating that steeper best-fit line. Thus, for a given change in the concrete the power regression equation closely agrees with the test strength, the NASP results can have a higher variation for the data. The best-fit equation is given in Equation 3.1. high-performing strands (strands with higher NASP values) when compared with the moderately performing strands ( NASPconcrete ) = 0.49139 f ci0.51702 (3.1) (strands with lower NASP values). NASP 40.00 35.00 Strand B 2 R = 0.7003 Strand A R2 = 0.9351 30.00 NASP Pull-out Value (kip) 25.00 20.00 Strand A6 2 R = 0.9053 15.00 10.00 Strand D R2 = 0.8166 5.00 0.00 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00 fci 0.5 (ksi) for all strands. Figure 3.11. NASP pull-out values versus fci

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37 2.00 1.80 NASP in Concrete Normalized by NASP in Mortar (kips/kips) 1.60 1.40 1.20 1.00 y = 0.49139x0.51702 0.80 R2 = 0.79826 0.60 0.40 0.20 0.00 0 2 4 6 8 10 12 14 Concrete Strength at 24 hours (ksi) NCHRP A NCHRP B NCHRP D NCHRP A6 (0.6") Figure 3.12. Normalized NASP pull-out values versus concrete strength for all strands. where The result of the regression is remarkable for two reasons. NASPconcrete = the value obtained from the NASP Bond One, the data's best fit regression demonstrates a coefficient Test in concrete, and of determination of 0.79, illustrating that the data set is fairly NASP = the value obtained from the Standard NASP well predicted by the regression; two, the data demonstrate Bond Test (NASP Bond Test in mortar). that bond improvements are directly proportional to the con- crete strength at 1 day of age. Furthermore, the normalized The equation is modified to fit the NASP values as a func- value of 1.0 is achieved at an f ci of 4 ksi. These significant re- tion of the square root of concrete strengths. In Figure 3.13, sults are used later in the recommendation for transfer and the normalized NASP pull-out values are plotted against the development length code expressions. Also note that the square root of the concrete strength. The linear regression modified NASP Bond Test in concrete nearly matches the results in the following equation: Standard NASP Bond Test if the concrete strength is only ( NASPconcrete ) 4 ksi, as compared to the requirement for mortar strength of = 0.51 fci (3.2) NASP 4,500 to 5,000 psi. 2.00 1.80 NASP in Concrete Normalized by 1.60 NASP in Mortar (kips/kips) 1.40 1.20 y = 0.5096x R2 = 0.7889 1.00 0.80 y = 0.51 f ci 0.60 0.40 0.20 0.00 0 1 2 3 4 5 Square Root of Concrete Strength at 24 hours ( ksi) NCHRP A NCHRP B NCHRP D NCHRP A6 (0.6") Linear (all) Figure 3.13. Normalized NASP pull-out values versus fci .