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50 Table 26. Splice development test results. Design Bar Stress to Experimentally Splice Design be Developed; fy in Observed Bar Stress Spliced Length Specimen f' c Equation 13 Developed Bars (ksi) Stress Stress (in.) Strain Strain (ksi) (ksi) D5-1 10 2 #5 36db 22.5 100 0.0041 161 0.0261 D5-2 10 2 #5 45db 28.2 125 0.0063 160 0.0232 D5-3 15 2 #5 29db 18.4 100 0.0039 152 0.0135 D5-4 15 2 #5 37db 23.0 125 0.0060 163 0.0254 D8-1 10 2 #8 36db 36.0 100 0.0042 140 0.0126 D8-2 10 2 #8 45db 45.0 125 0.0074 152 0.0306 D8-3 15 2 #8 29db 29.0 100 0.0043 133 0.0096 D8-4 15 2 #8 37db 37.0 125 0.0070 139 0.0122 Strain gages were bonded on the bars immediately beyond Equation 12, are more conservative than those given by Equa- their splice length from which the stress developed by each tion 13 (Figure 30). The AASHTO requirements would have splice was determined using experimentally obtained stress- resulted in development lengths 11% and 36% longer for 10 strain relationships (Appendix A). Confinement reinforce- and 15 ksi concrete, respectively. In all cases, the 0.4dbfy limit to ment in the splice region (and along the entire span) consisted Equation 12 controls the development length. Thus, the pres- of #3 stirrups having a nominal yield capacity of 60 ksi spaced ent AASHTO requirements also have been demonstrated to be at 6 in. Based on the confinement provided, a value of (cb + Ktr)/ conservative through this test program. db greater than 2.5 is calculated; thus, (cb+Ktr)/db = 2.5 for all specimens. 2.8.2 Hook Anchorage AASHTO LRFD (2007) provides geometric Splice Development Results requirements for standard 90 or 180 hooked anchorages of All eight specimens developed their design bar stresses of deformed reinforcing bars in tension. The basic development 100 or 125 ksi exhibiting significant reserve capacity (Table 26). length (lhb) of such standard hooked anchorages for bars hav- Nonetheless, all specimens except for specimen D5-4 even- ing fy 60 ksi is as follows: tually exhibited a failure of the splice rather than rupture of the spliced bars. The bars in specimen D5-4 ruptured. Fig- 38 db ure 32 shows the measured load-deflection behavior of all hb = 8db 6.0 in. ( ksi units ) (Eq. 14) fc specimens, and the predicted beam behaviors determined based on a Response 2000 (Bentz 2000) section analysis (see Where: Appendix H) that assumes no splice is present. Since the db = bar diameter in inches and splices did eventually slip, the full ductility of the sections was fc = concrete strength in ksi. not achieved. Also shown on Figure 32 are the displacements at which the primary reinforcing steel achieved the design For bars having a yield strength greater than 60 ksi, Equation stresses of 100 and 125 ksi. Reasonable reserve capacity beyond 14 is modified by the factor fy/60, effectively scaling the devel- these design values is achieved in all cases, particularly for the opment length in an inverse manner with the bar capacity to smaller #5 bars. The improved capacity of the smaller bars is be developed. Factors that increase the basic development accounted for in the ACI 318 version of Equation 13 by the length are prescribed for cases where lightweight aggregate s = 0.8. This factor has been removed from the NCHRP 12-60 (adjustment factor = 1.3) or epoxy-coated reinforcing steel version of Equation 13 and was not applied in the present (1.2) are used. These factors were not relevant in the present work. Spliced beams exhibited good deflection capacity, achiev- work and were taken as unity. For #11 bars and smaller, pro- ing midspan deflections on the order of L/55 at splice failure. vided with sufficient concrete cover, the basic development Splice details focus on developing reinforcing bar strength; length may be reduced as follows: member ductility is achieved in practice through detailing such as staggering splice locations. 1. For side cover normal to the plane of the hook exceeding The splice test series is intended as proof tests of the NCHRP 2.5 in. and back cover to the 90 hook extension exceeding 12-60 straight bar tension development length recommenda- 2 in., the basic development length may be factored by 0.7. tion given by Equation 13. These tests have clearly shown that 2. For hooked bars enclosed by vertical or horizontal ties or this recommendation is adequate to develop up to 125 ksi in stirrups having a spacing not exceeding 3db, the basic 15 ksi concrete. The present AASHTO requirements, given in development length may be factored by 0.8.

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51 (a) Splice Specimens Having Nominal Concrete Strength, fc' = 10 ksi (b) Splice Specimens Having Nominal Concrete Strength, fc ' = 15 ksi Figure 32. Load-deflection behavior of splice test beams. Deflection at which the stress in the primary reinforcing achieves the intended design value is noted in each case. Both factors may be applied simultaneously, resulting in a also called upon to serve as interface reinforcement for a cast- reduction factor of 0.56 for well-confined hook regions with in-place deck; or (2) the anchorage of primary reinforcing in sufficient concrete cover. cantilever slabs. The #5 and #8 bars were provided with stan- dard 90 hooks and are intended to represent anchorage of these bars where insufficient length is provided to develop a Hook Anchorage Tests straight bar. This condition may occur in starter bars for piers Eighteen ASTM A1035 hook anchorage specimens were or abutments, wall piers, or in short flexural members such as tested. The specimen details shown in Figure 33b and Table 27 pier caps. Specimen labels begin with "H" (for hook) and indi- include two concrete strengths, nominally 5 and 10 ksi, and cate the bar size followed by the specimen number; a trailing three bar sizes: #4, #5, and #8. The #4 bars were provided with "N" indicates that no confining reinforcement was provided. standard 180 hooks and are intended to represent (1) the All hook development lengths were designed using Equa- anchorage of stirrups in girder sections where the stirrups are tion 14 with all appropriate modifications. In all specimens,

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52 the calculated value of lhb was modified by the selected nom- foam pipe insulation) resulting in the development length inal values of fy, 100 or 125 ksi (see Table 27), using the factor beginning 3 in. below the concrete surface. The debonded fy/60. All specimens were provided with sufficient cover to region was provided to (1) mitigate the pullout of a cone of permit the 0.7 reduction factor to be applied. For all speci- concrete at the concrete surface, which affects the develop- mens having confining reinforcement (all but specimens in ment behavior and slip results; and (2) to provide additional Table 27 ending in N), the confinement was adequate to per- concrete depth (h in Table 27) to mitigate the shear failure mit the 0.8 reduction factor to be applied. The objective of of a "cone" of concrete anchored by the hook itself (this was this limited test series was to serve as a series of proof tests: nonetheless observed in Specimens H4-2 and H8-2, as dis- applying the existing AASHTO hooked bar development cussed below). length requirements to the higher strength A1035 reinforcing Each bar was anchored using a bolted, in-line mechanical steel. The measured material properties of the hooks and con- splice anchor with both sides of the splice anchor engaged. All fining steel are given in Table 25. The measured 28-day con- bolts were fully torqued except for the lower two that were crete strength for the specimens having nominal strengths of provided with only 1/3 and 2/3 of their recommended torque fc = 5 and 10 ksi were 6.02 and 9.71 ksi, respectively. Strain values, respectively. The reduced torque levels were intended gages were applied over the length of hooked embedment (following the test of H5-1, see below) to mitigate failure (see inset in Figure 35) to determine bar stresses. associated with the stress raisers that this anchorage pro- The test setup, shown in Figure 33a, was designed to duces. Although the anchorage performed flawlessly in this replicate as closely as possible (without a full-scale element arrangement, it is not the subject of this study, nor can any test) the stresses in the vicinity of a hook anchorage in ten- conclusions with respect to its performance be drawn. sion. The hydraulic ram places the bar in tension and the lever arm reaction to the right of the bar provides the equil- Hook Anchorage Results ibrating compression. In the setup used, the compression reaction is 1.2 times the bar tension, more than sufficient to The results in terms of bar stress achieved and the failure provide the appropriate reaction force necessary to develop mode observed for all 18 specimens are shown in Table 27. All the hook. A short region of the hook was left unbonded as it test specimens exceeded their design stresses of 100 or 125 ksi entered the concrete block (achieved by wrapping the bar in ( fy in Table 27). Indeed, most specimens achieved their ultimate Table 27. Hook specimen details and test results. ID A* fc ' fy l hd h* D* s1* B* C* Ultimate Failure** bar size hook angle ksi ksi in. in. in. in. bars in. ksi H4-1N #4 180o 5 100 10 16 none n.a. 4 #4 #3 @ 6 179 R H4-4N #4 180o 5 125 12 18 none n.a. 4 #4 #3 @ 6 177 R H4-1 #4 180o 5 100 8 14 5 #3 @ 1.5 1 4 #4 #3 @ 6 177 R H4-4 #4 180o 5 125 10 16 6 #3 @ 1.5 1 4 #4 #3 @ 6 177 R H4-2 #4 180o 10 100 6 12 3 #3 @ 1.5 1 4 #4 #3 @ 6 173 C/R H4-5 #4 180o 10 125 8 14 5 #3 @ 1.5 1 4 #4 #3 @ 6 176 R H5-1N #5 90o 5 100 13 19 none n.a. 4 #5 #3 @ 6 168 R H5-4N #5 90o 5 125 16 22 none n.a. 4 #5 #3 @ 6 168 R H5-1 #5 90o 5 100 10 16 5 #3 @ 1.88 1.25 4 #5 #3 @ 6 160 RA H5-4 #5 90o 5 125 13 19 6 #3 @ 1.88 1.25 4 #5 #3 @ 6 168 R H5-2 #5 90o 10 100 8 14 4 #3 @ 1.88 1.25 4 #5 #3 @ 6 167 R H5-5 #5 90o 10 125 9 15 4 #3 @ 1.88 1.25 4 #5 #3 @ 6 168 R H8-1N #8 90o 5 100 20 26 none n.a. 4 #8 #3 @ 6 140 TS H8-4N #8 90o 5 125 25 31 none n.a. 4 #8 #3 @ 6 140 TS H8-1 #8 90o 5 100 16 22 5 #3 @ 3 2 4 #8 #3 @ 6 153 TS H8-4 #8 90o 5 125 20 26 6#3 @ 3 2 4 #8 #3 @ 6 138 TS H8-2 #8 90o 10 100 12 18 3 #3 @ 3 2 4 #8 #3 @ 6 162 C (no R) H8-5 #8 90o 10 125 15 21 4#3 @ 3 2 4 #8 #3 @ 6 166 R Notes: *See Figure 33b. Design yield stress to be developed in Equation 14. See Equation 14. **Failure mechanisms: R = bar rupture; RA = bar rupture affected by bar anchor; C = concrete shear failure; n.a. = not applicable; TS = test stopped prior to failure for safety considerations--in this case, the maximum obtained bar stress is reported. Minimum development length.

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53 Figure 33. Hook test setup and specimen details. capacity ( fu in Table 25) as evident by a bar rupture failure in strains at this final location are all very small indicating that the exposed region of the bar--denoted by "R" in Table 27. All the hooked region is not being engaged in tension. The strain ruptured bars exhibited significant necking and elongation and gages used were very small (0.25 in. overall length); their were unaffected (except H5-1) by the bar anchorage or loading installation does not affect the bond stress development in mechanism. Observed rupture stresses agree well with the pre- any significant manner. The data in Figure 35 are given at viously tested material properties (Table 25). stresses of 60 ksi (yield of mild steel), 100 ksi and 125 ksi Specimen H5-1 (the first tested) exhibited a bar rupture (design values for this test), and 140 ksi (maximum value at affected by the installation of the splice anchor used to react which data are available for all specimens). the applied load (not actually part of the test). Nonetheless, The "slip" of the hook also was measured using displacement this specimen still achieved a stress of 160 ksi. A change was transducers that measured the relative movement of the bar as made to the splice installation and this failure mode was mit- it is "pulled out" of the concrete. Since the slip measurement is igated for all subsequent tests. Only two of the #8 specimens obtained over a distance of exposed bar (about 5 in. in most were tested to bar rupture; the remaining tests were stopped tests), the reported slip is greater than the actual slip due to the prior to failure at a stress of 140 ksi, which was still greater elastic, and eventually inelastic, strain present over this un- than the required proof load. The stoppage was done in the bonded length. Figure 36a shows the slip recorded at stress lev- interest of laboratory safety (a rupturing #8 A1035 is a signif- els of 60, 100, 125, and 140 ksi. The "ultimate" stress is the slip icant projectile). In two specimens having very short devel- reported at the maximum stress obtained as given in Table 27. opment lengths, the ultimate failure was a shear "cone" in the concrete (denoted by "C" in Table 27). This failure mode (1) took place at loads that significantly exceeded the required proof loads; (2) occurred at loads very close to those expected to cause bar rupture; and (3) is an artifact of the test specimen and would not be expected in real-world applications. Figure 34 shows an example of such a "C" failure. Strain profiles demonstrate that the hooks are well devel- oped and transfer stress to the concrete through bond. Fig- ure 35 plots the bar strains with length along the #8 hooks (reported in units of bar diameters (db) for the sake of normalization). The uppermost data point on each curve (db = 0) is obtained from the clip gage mounted a few inches above the concrete specimen. The next data point (db = 3) is obtained from the strain gage located 3db into the concrete (see the right-hand inset in Figure 35). As would be expected, these first two strains are similar since little development has yet been engaged. The next data point down is obtained from the strain gage located 5db from the hook bend and the final Figure 34. Typical concrete shear failure data point is 5db around the bend on the hook itself. The (Specimen H8-2).

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54 clip gage "0db" concrete surface 3" debonded region 3db ldh 5db strain gage 5db locations (a) #8 90o hook specimens (b) #5 90o hook specimens (c) #4 180o hook specimens Figure 35. Strains along hook embedment at selected bar stresses.