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99 test maximum stress to calculated stress) using 318 Code (ACI The ratios of measured maximum stress on the spliced bars 2005) without a limit on the square root of the concrete com- to stress calculated using 318 Code (ACI 2005) ranged pressive strength, without a bar size factor, and with a single from 0.98 to 1.96. Ratios calculated using 318 Code (ACI epoxy-coated bar factor of 1.5 with concrete strength (defined 2005) with these two proposed modifications--no bar size as the square root of the concrete compressive strength). In factor and a single epoxy-coated bar factor of 1.5--ranged Figure 3.69, the specimens of Hamad, Jirsa, and D'Abreu de from 1.07 to 2.29. Thus, the procedure in Chapter 12 of the Paulo (1993), Treece and Jirsa (1989), Choi et al. (1991), De- 318 Code (ACI 2005) for splice and development length of Vries, Moehle, and Hester (1991), and the tests on epoxy-coated epoxy-coated bars in tension can be extended up to 17 ksi bar splice specimens carried out under NCHRP Project 12-60 with the modifications suggested in this section. (Purdue [S]) were separated into two groups: test results from The ratios of measured maximum stress on the spliced bars specimens with stirrups and test results from specimens with- to the stress calculated using the AASHTO specification out stirrups over the splice region. As can be seen from Figure ranged from 1.73 to 2.67. 3.69, the bond efficiency values ranged from 0.86 to 3.11 for the The use of transverse reinforcement over the splice region specimens without stirrups and from 1.07 to 2.20 for the spec- resulted in increases both in the test maximum stress and imens with stirrups. The ranges for each of the studies, except deflection at failure. for NCHRP Project 12-60, are listed in Table 3.41. The use of The current contribution of the cover in the 318 Code transverse reinforcement over the splice region increased the (ACI 2005) can be overestimated in the case of higher ACI-calculated stress, causing a decrease in the ratio of test max- strength concrete specimens with large covers. imum stress to ACI-calculated stress, and this tendency was consistent with the tendency of the tests conducted under NCHRP Project 12-60. It is interesting to note as well that for 3.10 Anchorage of Bars Terminated the studies in the literature, the range of stress ratios in the spec- with Standard Hooks imens with epoxy-coated bars and companion specimens with in Tension uncoated bars was similar, as shown in Table 3.41. Thus, on the This section deals with the tensile strength of black and basis of the maximum concrete compressive strength included epoxy-coated reinforcing bars terminated in 90-deg hooks in the experimental evaluation and the evaluation of the data in with and without transverse reinforcement under monotonic the literature, the procedure in Chapter 12 of the 318 Code (ACI loading in normal-weight concrete with uniaxial compressive 2005) for splice and development length of epoxy-coated bars strength up to 16 ksi. As part of this examination, in addition in tension could potentially be extended up to 17 ksi without a to 43 previous tests, the test results of 21 beam-column joint limit of 100 psi to the square root of the concrete compressive type specimens are reported. Variables in the tests conducted strength and with these two modifications--removal of the bar under NCHRP Project 12-60 included bar size (#6 and #11), size factor and use of a single epoxy-coated bar factor of 1.5. concrete strength (10, 14, and 16 ksi), and amount of trans- verse reinforcement in the anchorage region. Codes and spec- 3.9.7 Summary and Conclusions ifications have limits to their applicability to higher strength From the test results of 12 beam splice specimens rein- concretes (ACI 2005, AASHTO 2004). These limits are justi- forced with epoxy-coated bars, the following conclusions can fied on the basis of the empirical nature of code and specifi- be drawn: cation requirements. The requirements for bars in tension Table 3.41. Comparison of test results to calculated results. Research Study Without Stirrups With Stirrups Hamad and Jirsa (1993) (Black) 1.37 to 3.11 1.14 to 1.83 Hamad and Jirsa (1993) (Epoxy) 1.31 to 2.73 1.20 to 1.77 Treece and Jirsa(1989) (Black) 1.07 to 1.82 Treece and Jirsa (1989) (Epoxy) 0.86 to 1.71 Choi et al. (1991) (Black) 1.05 to 1.61 Choi et al. (1991) (Epoxy) 1.19 to 2.01 DeVries, Moehle, and Hester (1991) 1.07 to 2.21 (Black) DeVries, Moehle, and Hester (1991) 1.22 to 2.20 (Epoxy) no data available.

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100 anchored by means of a standard hook are an example of such other cases, these two factors are taken equal to 1.0. Other pa- specifications. rameters are db, which is the bar diameter of the hooked bar; In 1975, Marques and Jirsa reported a series of tests to de- f c, which is the concrete compressive strength in psi; and the termine capacities of uncoated hooked bars. Twenty-two square root of the concrete compressive strength, which shall specimens simulating exterior beam-column joints were not exceed 100 psi as per Section 12.1.2 of the 318 Code (ACI tested to evaluate the capacity of uncoated anchorage beam 2005). The modification factors of Section 12.5.3 of the 318 reinforcements subjected to varying degrees of confinement Code (ACI 2005) are all less than 1.0 and thus reduce the cal- at the joint. The types of confinement included vertical col- culated length on the basis of cover, presence of ties where the umn reinforcement, lateral reinforcement through the joint, first tie encloses the bent portion of the hook within 2db of side concrete cover, and column axial load. To simulate beam the outside of the bend, and where anchorage or development moment acting on the column, tension was applied to an- for specified minimum yield strength, fy, is not specifically chored bars and a reaction assembly transferred compression required. These modification factors are the following. load to the specimen. Failure in most tests was sudden and resulted in the entire side cover of the column spalling away For #11 bar and smaller hooks with side to the level of the hooked anchorage. The maximum concrete cover (normal to the plane of the hook) compressive strength in these tests, which served as the basis not less than 2.5 in. and for 90-deg hooks for the current anchorage requirements, was 5.1 ksi. with cover on the bar extension beyond Anchorage of epoxy-coated hooked bars was evaluated by the hooks that are not less than 2 in.: 0.7 Hamad, Jirsa, and D'Abreu de Paulo (1993) in a series of tests. Twenty-four hooked-bar specimens simulating exterior For 90-deg hooks of #11 and smaller bars beam-column joints were tested. It was reported that #11 that are enclosed within ties or stirrups hooked bars (coated or uncoated) were consistently less stiff perpendicular to the bar being developed, than #7 hooked bars. Epoxy-coated hooked bars consistently spaced not greater than 3db along ldh; or developed lower anchorage capacities and load-slip stiffness enclosed within ties or stirrups parallel to than companion uncoated hooked bars. The companion the bar being developed, spaced not hooked-bar specimens that had ties in the beam-column greater than 3db along the length of the joint region improved both the anchorage capacity and load- tail extension of the hook plus bend: 0.8 slip behavior of both coated and uncoated bars. For 180-deg hooks of #11 and smaller bars To date, there has been little work on the anchorage that are enclosed within ties or stirrups performance of hooked bars, black and epoxy-coated, in perpendicular to the bar being developed, high-strength concrete. In the 2005 ACI Building Code, the spaced not greater than 3db along ldh: 0.8 equation for the basic development length (lhb) of a hooked bar is limited to concrete strength of 10 ksi. Therefore, fur- Where anchorage or development for fy is ther investigation on anchorage strength of hooked bars in not specifically required, reinforcement in (As required / high-strength concrete is needed. excess of that required by analysis: As provided) 3.10.1 U.S. Design Specifications The factor As required/As provided, also referred to as the factor for excess reinforcement, applies only where anchor- 3.10.1.1 318 Code (ACI 2005) age for full fy is not specifically required because the area of Development length for deformed bars in tension termi- steel required to resist the factored flexural moment at the nating in a standard hook, ldh, is determined using Section section, As required, is less than the area of steel provided, As 12.5.2 and applicable modification factors of 12.5.3, as shown provided, at the same section. in Equation 3.22. However, ldh shall not be less than the larger of 8db and 6 in. as indicated in Section 12.5.1 of the 318 Code 3.10.1.2 2004 AASHTO Specifications (Section (ACI 2005). 5.11.2.4 Standard Hooks in Tension) 0.02 e f y The 1995 318 Code provisions for anchorage of bars termi- ldh = db (3.22) fc nated in a standard hook in tension are the current procedure in the AASHTO LRFD Bridge Design Specifications (ACI 1995). In Equation 3.22, e is the coating factor, taken as 1.2 for The 1983 provisions for development of standard hooks in epoxy-coated reinforcement; is the factor reflecting the tension in the 318 Code were a major departure from the 1977 lower tensile strength of lightweight concrete, which is 1.3. In 318 Code in that they uncoupled hooked bar anchorages from

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101 straight bar development provisions and measured the Reinforcement has a yield strength hooked bar embedment length from the critical section to the exceeding 60 ksi: fy/60 outside end or edge of the hook. The development length of the hooked bar is represented by the product of a basic devel- Side cover for #11 bar and smaller, opment length and appropriate modification factors. In the normal to the plane of the hook, is not 1995 edition of the 318 Code, a factor of 1.2 was introduced in less than 2.5 in., and cover on bar the calculation of development lengths of epoxy-coated bars extension beyond 90-deg hooks is not terminated in a standard hook (ACI 1995). less than 2 in.: 0.7 The development length, ldh (in.), for deformed bars in ten- Hooks for #11 bar and smaller that are sion terminating in a standard hook specified in Article enclosed vertically within ties or stirrup 5.10.2.1 shall not be less than the following: ties spaced along the full development length, ldh, at a spacing not exceeding 3db: 0.8 The product of the basic development length and the applicable modification factor or factors, as specified in Anchorage or development of full Article 5.11.2.4.2; yield strength is not required, or 8.0 bar diameters; or reinforcement is provided in excess (As required / 6.0 in. of that required by analysis: As provided) Lightweight concrete is used: 1.3 Basic development length, lhb, for a hooked-bar with yield strength, fy, not exceeding 60.0 ksi shall be taken as: Epoxy-coated reinforcement is used: 1.2 lhb = 38.0db (3.23) f c 3.10.2 Experimental Program where 3.10.2.1 Test Specimens db = diameter of the hooked bar (in.) and The experimental program reported in this research con- f c = specified compressive strength of concrete at 28 days, sisted of the monotonic loading in tension only (see Figures unless another age is specified (ksi). 3.70 and 3.71) of 20 specimens with two bars terminated in Below, cases in which basic hook development length, lhb, 90-deg standard hooks (see Figure 3.72). should be multiplied by a factor are given, as well as the ap- Key test parameters are given in Table 3.42. The test plicable factor. specimens were cast using normal-weight concrete (see Concrete Column (15"x15") Strong Column (W14x99) Loading Plate Load Cell 17.5" Hydraulic Ram 15" Stiffener Bar Lock Anchorage Plate Compression Plate 17.5" Strong Girder 11" (2-MC10x33.6) Figure 3.70. Test setup for beam-column-type specimens (1 in. 25.4 mm).

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102 Column Section (9"x15") Column Section (15"x15") Ldh=6.5" Ldh=12.5" 3" 3" 3" 3" 6.5" 9" 6.5" 9" 3" 3" 3" 3" 9" 15" Anchorage Plate Anchorage Plate Figure 3.71. Specimen details for anchorage tests of bars terminated with standard hooks (1 in. 25.4 mm). Table 3.43). Specimens I-1 to I-6 contained black hooked epoxy-coated bars. In NCHRP Project 12-60, a similar test bars. Specimens II-7 to II-12 had epoxy-coated hooked setup was used in the evaluation of these provisions in bars. In Specimens III-13 to III-20, transverse reinforce- higher strength concretes. ment was provided in the joint area to confine the concrete along the anchorage length of the hooked bars. The speci- 3.10.2.2 Test Setup and Procedure mens were provided with an anchorage length, ldh, as per 318 Code (ACI 2005) (see Table 3.42). The test setup used in this investigation is shown in Figures Test specimens contained two #6 hooked bars or two #11 3.70 and 3.73. A force couple consisting of a tensile force in hooked bars. The concrete column size of the specimens the test bars (applied by two center-hole hydraulic rams) and with the #6 hooked bars was 9 by 15 in. The column cross a compressive force concentrated at a distance of 15 in. below section of the specimens reinforced with the #11 bars (such the centerline of the bars was applied. The compression force as I-2) was 15 by 15 in. The width of the column was kept at the face of test specimen was applied through two plates the same in all specimens, but the depth was changed to ac- (3 in. and 3/4 in. thick) attached to the reaction column sim- commodate the different development lengths. In both ulating a 6 in. deep compression zone of the assumed beam. types of specimens, concrete cover was 2.5 in. Each concrete The reaction column consisted of a W14x99 column column was reinforced with five or seven #8 main vertical welded to a base plate 1 in. thick and bolted to the strong bars and 4 stirrups spaced at 6 in.--two at the top and two girder on the floor. Pull-out load was applied in 3.5-kip in- at the bottom of the column as shown in Figure 3.72. The crements to the #6 bar specimens and in 10-kip increments 318 Code (ACI 2005) anchorage requirements for uncoated to the #11 bar specimens until failure occurred. Two strain bars anchored by a combination of standard hook and gages were affixed to each bar, and the slip of the anchored straight embedment length were based on the test results of reinforcing bar relative to the concrete surface was measured Marques and Jirsa (1975). These provisions were later using LVDTs. extended by Hamad, Jirsa, and D'Abreu de Paulo (1993) to 3.10.2.3 Materials The two concrete mixes ordered from a concrete ready- mix company were proportioned to yield a concrete com- pressive strength of 10 ksi (Mix I) and at least 14 ksi (Mix II). Table 3.43 shows a typical concrete mix. The water-to- cement ratio was 0.32 for Mix I and 0.20 for Mix II. A stress versus age relationship is shown in Figure 3.74. The modulus of rupture was 566 psi and 834 psi at 28 days for Mix I and Mix II, respectively. Each size of reinforcing bar was from the same heat of steel, and all bars had the same deformation pattern. The relative rib area of #6 and #11 bars was 0.091 and 0.135, respectively. Grade 60 steel was used for both black and epoxy-coated bars. The yield strength obtained from tensile tests was 81.9 ksi and 63.1 ksi for the #6 and #11 black bars, respectively. For the Figure 3.72. Detail of Specimen I-2. epoxy-coated bars, the yield strength calculated by 0.2-

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103 Table 3.42. Description of key test parameters. Name Bar Concrete Bar Type ldh Concrete Stirrup Size Strength (psi) (in.) Cover (in.) Spacing I-1 #6 8,905 Black 6.5 2.5 None I-2 #11 8,905 Black 12.5 2.5 None I-2' #11 9,535 Black 15.5 2.5 None I-3 #6 12,455 Black 6.5 2.5 None I-4 #11 12,455 Black 12.5 2.5 None I-5 #6 12,845 Black 6.5 2.5 None I-6 #11 12,845 Black 12.5 2.5 None II-7 #6 9,535 Epoxy-coated 6.5 2.5 None II-8 #11 9,535 Epoxy-coated 12.5 2.5 None II-9 #6 13,670 Epoxy-coated 6.5 2.5 None II-10 #11 13,670 Epoxy-coated 12.5 2.5 None II-11 #6 14,800 Epoxy-coated 6.5 2.5 None II-12 #11 14,800 Epoxy-coated 12.5 2.5 None III-13 #6 13,980 Black 6.5 db 3db III-14 #11 13,980 Black 12.5 db 3db III-15 #6 16,350 Black 6.5 db 3db III-16 #11 16,500 Black 12.5 db 3db III-17 #6 13,670 Epoxy-coated 6.5 db 3db III-18 #11 13,670 Epoxy-coated 12.5 db 3db III-19 #6 16,350 Epoxy-coated 6.5 db 3db III-20 #11 16,500 Epoxy-coated 12.5 db 3db 1 in. = 25.4 mm; 1 psi = 6.89 kPa percent offset from tensile tests was 72.5 ksi and 74.7 ksi for in Figure 3.76. Load was measured using a load cell attached the #6 and #11 bars, respectively. The average coating thick- to each bar terminated with a 90- deg standard hook. ness measured with a dry film thickness gage for all epoxy- In almost all the specimens, the gages placed on the bar at coated bars was around 12 mils. a distance of 2 in. from the column surface showed yielding before reaching the maximum pull-out load, with less than 3.10.3 Experimental Results 0.05-in. slip between hooked bar and concrete surface on the loaded side. As can be seen from Figure 3.75(a) and (b), spec- 3.10.3.1 Load versus Slip Behavior and Cracking imens without shear reinforcement in the test region, II-9 and Pattern II-10, had a significant decrease in load at a 0.2-in. relative slip Pull-out load versus slip responses for Specimens II-9, II-10, between hooked bar and concrete surface. However, in the III-17, and III-18 are shown in Figure 3.75. The pull-out load specimens with shear reinforcement in the test region, III-17 versus slip responses for Specimen III-19 and III-20 are given and III-18 (see Figure 3.75[c] and [d]), the load decrease (20 Table 3.43. Typical concrete mix ratio (per 1 cubic yard). Contents 10-ksi Mix 14-ksi Mix Cement (lb) 780 900 Silica fume (lb) 50 200 Water (lb) 265 220 Coarse aggregate (lb) 1,600 1,800 (3/8" pea gravel) (1/2" crushed limestone) Fine aggregate (lb) 1,240 1,000 High-range water reducer (oz) 190 520 Normal-range water reducer (oz) 35 38 3 3 1 lb = 0.454 kg. 1 oz = 28.35 gr. 1 yd = 0.765 m . 1 ksi = 6.89 MPa.

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104 the behavior for the #6 bar specimens, III-17 and III-19, recorded in Figure 3.75(c) and Figure 3.76(a), respectively, it can be observed that the increase in concrete compressive strength from about 13.5 ksi to 16.5 ksi resulted in an increase in pull-out strength. The same increase in concrete strength in the case of the specimens anchoring #11 bars, III-18 and III-20, also resulted in an increase in pull-out strength (see Figure 3.75[d] and Figure 3.76[b]).The same type of finding was ob- served for the specimens anchoring uncoated bars. In almost all of the tests, the cracking sequence was simi- lar. The first flexural (horizontal) crack occurred on the back face of the specimens at a load of 20 kips for the #6 bar spec- imens and 60 kips for the #11 bar specimens. The crack ap- peared near the tail end of the hook. After the initial flexural crack, a shear crack appeared on the side of the specimen as Figure 3.73. Beam-column-type specimen test setup shown in Figure 3.77. With the increase in the pull-out load, and instrumentation. the gage near the hook showed signs of yielding. At 90 per- cent of the peak load, the vertical cracks appeared along the column main bar. Finally, the concrete block near the hooked percent of the peak load) was not as severe as the load bar pushed out in Type I and II specimens that had no stir- decrease observed in the specimens without shear reinforce- rups in the joint (see Figure 3.78). In the Type III specimens ment (almost 50 percent of the peak load). In Specimens containing stirrups over the anchorage length, with the fail- III-17 and III-18, at 0.2-in. slip, the sustained anchorage force ure of the concrete near the hook, some of the side concrete was more than 80 percent of the maximum pull-out force. It cover spalled off (see Figure 3.79). Within the range of these can be concluded that the #6 epoxy-coated bar specimen with tests, there was no significant difference on the pull-out char- shear reinforcement in the test region and smaller cover was acteristics of the hooked bars with different concrete com- able to reach a higher peak load than its companion specimen pressive strength up to 16 ksi. It must be noted that with large without shear reinforcement but with a larger cover (see Fig- slips and with the tendency of the bar to straighten under ten- ure 3.75[a] and [c]). This was not the case for specimens with sion, the tail end of the hook tended to kick out, thus splitting #11 epoxy-coated bars anchored by standard hooks (Figure the concrete behind the hook. However, these cracks were 3.75[b] and [d]). However, the specimens with shear rein- very small, implying that a cover of 2.5 in. over the tail end of forcement (Figure 3.75[c] and [d]) were able to sustain the hook used was sufficient for design purposes in the range almost 80 percent of the peak load at a slip of 0.2 in. regard- of concrete strengths considered in this study. less of the bar size. The load versus slip behavior of Specimens III-19 and III-20 are shown in Figure 3.76(a) and (b), respectively. Comparing 3.10.3.2 Maximum Pull-Out Stress and Failure Mode 20 Table 3.44 shows a comparison of maximum pull-out 18 Compressive Strength (ksi) stress and the calculated stress on the basis of Equation 3.22. 16 This equation gives the straight embedment length calculated 14 in accordance with the 318 Code (ACI 2005) and measured 12 from the critical section to the outside portion of the hook. In 10 this equation, fy is the yield strength of hooked bar, e is the 8 6 coating factor (epoxy-coated reinforcement = 1.2, uncoated 4 10 ksi reinforcement = 1.0), is the lightweight aggregate concrete 2 factor (for lightweight concrete = 1.3, for normal concrete 14 ksi 0 =1.0), and f c represents the concrete compressive strength. 0 20 40 60 80 100 Substituting fs in place of fy, stress in the bar for a given Age (days) anchorage length, and solving for s with a given design an- Figure 3.74. Concrete compressive strength chorage length, as in Equation 3.24, it is possible to obtain the versus age relationship in standard hook tests calculated stress shown in Table 3.44. The maximum stress (1 ksi 6.89 MPa). corresponding to the peak pull-out load is obtained by

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105 Load- Slip of II-9 (Right) Load - Slip of II-10 (Right) 30000 100000 25000 80000 Load (lb) 20000 Load (lb) 60000 15000 40000 10000 5000 20000 0 0 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 Slip (in.) Slip (in.) (a) II-9 (#6, fc' = 13.5 ksi, cover = 2.5") (b) II-10 (#11, fc' = 13.5 ksi, cover = 2.5") Load- Slip of III-17 (Right) Load - Slip of III-18 (Right) 40000 80000 35000 70000 30000 60000 Load (lb) 50000 Load (lb) 25000 20000 40000 30000 15000 20000 10000 10000 5000 0 0 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 Slip (in.) Slip (in.) (d) III-18 (#11, fc' = 13.5 ksi, cover = db) (c) III-17 (#6, fc' = 13.5 ksi, cover = db) Figure 3.75. Effect of transverse reinforcement in the anchorage region on the pull-out load versus slip response (1 in. 25.4 mm; 1 kip 4.448 kN). averaging the values from the two load cells attached to each 1.23). However, most of the specimens anchoring # 11 bars at hooked bar divided by the area of the bar. failure reached a stress less than or equal to the calculated stress (ratio of test result to calculated result of 0.83 to 1.02). ( ) ldh f s = db * fc (3.24) In the case of specimens anchoring epoxy-coated bars (Series II), the tendency was the same as in Series I specimens rein- 0.02 e forced with black bars. In Series III, the specimens anchoring Specimens with #6 bars, except Specimens II-9 and III-15, #11 bars reached failure stress levels less than or equal to at failure reached a stress equal to or greater than the calcu- the calculated stress values, yielding a ratio of test to calcu- lated stress (ratio of test result to calculated value of 0.99 to lated stress ranging from 0.83 to 0.98 while the specimens Load- Slip of III-19 (Left) Load - Slip of III-20 (Right) 45000 140000 40000 120000 35000 100000 30000 Load (lb) Load (lb) 25000 80000 20000 60000 15000 40000 10000 5000 20000 0 0 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 Slip (in.) Slip (in.) (a) III-19 (#6, fc' = 16.5 ksi, cover = db) (b) III-20 (#11, fc' = 16.5 ksi, cover = db) Figure 3.76. Effect of high-strength concrete in the anchorage region on the pull-out load versus slip response (1 in. 25.4 mm; 1 kip 4.448 kN).

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106 (a) Specimen I-1 (b) Specimen I-2 (c) Specimen III-13 (d) Specimen III-14 (e) Specimen III-19 (f) Specimen III-20 Figure 3.77. Crack patterns.

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107 Figure 3.78. Concrete block push off in specimens Figure 3.79. Failure region in the case of Specimen without stirrups in the anchored hooked bar III-13 with stirrups. specimens. anchoring #6 bars reached failure stresses greater than the cal- the case of #6 bars, and 15 percent more force in the case of culated values, i.e., ratios ranging from 1.01 to 1.14, except #11 bars. Each pair of specimens had the same dimensions Specimen III-15, which failed at a lower level. A similar com- and material properties, but had different details, such as hav- parison was conducted in terms of force developed in the bar. ing ties, having no ties, or having different concrete cover. Comparison of the results of Specimens II-9 and II-10 with Taking into account these similarities and differences in the Specimens III-17 and III-18 indicated that the use of ties over specimens, it can be concluded that the confinement (with the joint region developed 56 percent more force in the bar in ties) of the anchorage region produced stronger bond char- Table 3.44. Comparison of maximum pull-out bar stress compared with calculated stress using the 318 Code (ACI 2005) method with a modification factor of 0.7 (ksi). Specimen db (in.) f c (psi) ldh (in.) Calculated Test Ratio (T/C) Force (kips) I-1-9 0.75 8905 6.5 58.4 68.2 1.17 30.0 I-2-9 1.41 8905 12.5 59.8 56.4 0.94 88.0 I-2'-10 1.41 9535 15.5 76.7 67.3 0.88 105.0 I-3-12 0.75 12455 6.5 69.1 68.2 0.99 30.0 I-4-12 1.41 12455 12.5 70.7 63.5 0.90 99.1 I-5-13 0.75 12845 6.5 70.2 69.3 0.99 30.5 I-6-13 1.41 12845 12.5 71.8 73.1 1.02 114.0 II-7-10 0.75 9535 9.5 73.6 90.9 1.23 40.0 II-8-10 1.41 9535 15.5 63.9 56.4 0.88 88.0 II-9-14 0.75 13670 6.5 60.3 56.1 0.93 24.7 II-10-14 1.41 13670 12.5 61.7 53.5 0.87 83.5 II-11-15 0.75 14800 6.5 62.8 64.8 1.03 28.5 II-12-15 1.41 14800 12.5 64.2 54.5 0.85 85.0 III-13-14 0.75 13980 8.3 92.9 93.8 1.01 41.3 III-14-14 1.41 13980 13.5 80.9 67.3 0.83 105.0 III-15-16 0.75 16350 8.3 100.5 87.5 0.87 38.5 III-16-16 1.41 16500 13.5 87.8 76.9 0.88 120.0 III-17-14 0.75 13670 8.3 76.6 87.5 1.14 38.5 III-18-14 1.41 13670 13.5 66.6 61.5 0.92 95.9 III-19-16 0.75 16350 8.3 83.7 89.8 1.07 39.5 III-20-16 1.41 16500 13.5 73.2 71.8 0.98 112.0 1 in. = 25.4 mm; 1 kip = 4.448 kN; 1 psi = 6.89 kPa; 1 KSI = 6.89 MPa

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108 120 Project 12-60 test results follow in general the same trend as those of previous researchers. Thus, it is plausible to propose to extend the current ACI procedure for hooked bars up to 16 Maximum Experimental Stress (ksi) 100 ksi without a limit on the fc term. 80 In Figure 3.81, the ratio of test to calculated stress for hooked bars is shown versus the concrete compressive strength of the specimen. It can be seen that the ratio de- 60 creases as the concrete compressive strength is increased in both black and epoxy-coated bars terminated with a standard 40 hook and subjected to direct tension. To increase the values of the ratio of test to calculated stress in specimens with 20 Marques & Jirsa (1975) higher concrete strengths, it is proposed that a 0.8 modifica- Kim Hamad, Jirsa & D'Abreu de Paulo (1993) tion factor be used instead of the current factor of 0.7 [for 0 hooks with side cover not less than 2-1/2 in. and for 90-deg 0 20 40 60 80 100 120 hooks with cover on bar extension beyond hook not less than Calculated Stress (ksi) 2 in. in ACI Code 12.5.3(a) in concrete strengths above 10 Figure 3.80. Maximum experimental ksi]. The calculated results using the proposed modification stress versus 318 Code calculated stress factor and current factor are shown in Figure 3.82. (1 ksi 6.89 MPa). 3.10.4 Summary and Conclusions acteristics in hooked bars than no ties with a 2.5 in. cover. Based on the review of over 40 specimens in the literature This tendency is observed with both the epoxy-coated bars and the results from 21 tests of hooked bar anchorages in and black bars. beam-column specimens with normal-weight concrete strengths up to 16 ksi, the following conclusions can be drawn: 3.10.3.3 Comparison with Other Tests The approach in the 318 Code (ACI 2005) provision for and Recommendations anchorage of bars terminated in standard hooks in tension, The comparison of the stress calculated using Equation black and epoxy-coated, can be extended to concrete com- 3.24 for NCHRP Project 12-60 tests (Kim) and test results pressive strengths up to 15 ksi. However, a minimum reported by Hamad, Jirsa, and D'Abreu de Paulo (1993) and transverse reinforcement (3db spacing) should be provided Marques and Jirsa (1975) are plotted against the experimen- in higher strength concretes to improve the bond charac- tal values in Figure 3.80. The comparison shows that NCHRP teristics of both epoxy-coated and black #11 hooked bars. 1.6 1.6 1.4 1.4 1.2 1.2 Test / Calculated Test / Calculated 1.0 1.0 0.8 0.8 0.6 Marques & Jirsa (1975) 0.6 0.4 0.4 Kim 0.2 0.2 Hamad, Jirsa & D'Abreu de Paulo (1993) 0.0 0.0 0 5000 10000 15000 20000 0 5000 10000 15000 20000 Concrete Compressive Strength (psi) Concrete Compressive Strength (psi) (a) Black Bar Specimens (b) Epoxy-Coated Bar Specimens Figure 3.81. Test to calculated stress ratio versus concrete compressive strength (1 psi 6.89 KPa).

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109 120 The epoxy-coated hooked bars developed lower anchorage capacities than uncoated hooked bars. In the #11 hooked Maximum Test Stress (ksi) 100 bar specimens, the ratios of measured stress to calculated stress were 0.85 to 0.88. 80 Transverse reinforcement in the anchorage length of a bar terminated with a standard hook improves the max- 60 imum pull-out strength and load versus slip behavior. In the #11 epoxy-coated hooked bar specimens, the 40 ratios of measured stress to calculated stress increased up to 0.98. 20 Cal.-0.7 While the minimum concrete cover of 2.5 in. at the end of Cal.-0.8 the hook appeared to be adequate to prevent kicking out of 0 the tail end of the hooked bar, it is proposed that a modi- 0 20 40 60 80 100 120 fication factor of 0.8 be used instead of 0.7. The use of a 0.8 Calculated Stress (ksi) modification factor eliminated almost the entire test to cal- Figure 3.82. Maximum test stress versus culated stress ratios less than 1.0. This value of minimum calculated stress using factors of 0.7 and concrete cover can be reduced to db if transverse reinforce- 0.8 for Marques and Jirsa (1975), Hamad ment is used in the anchorage length of a bar terminated et al. (1993), and those tested in NCHRP with a standard hook. Project 12-60 (1 ksi 6.89 MPa).