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18 terminated with a standard hook are similar to those in the 2005 version of the ACI Code. 2.3 Identification of Issues C1 C2 and Needs The work described in the previous section was used to as- Cc Cs = min{C1, C2/2} semble a comprehensive list of issues pertaining to transfer (a) (b) (c) length, development length, and splice length of strand/ reinforcement to normal-weight concrete with compressive Figure 2.12. Anchorage failure modes: strengths in excess of 10 ksi and up to 15 ksi. In this section, a (a) vertical splitting, (b) splitting in the discussion of the main issues related to bond performance of horizontal plane of the bars, and (c) pull-out without splitting (ACI 408 2003). reinforcement is presented, and gaps found in the existing data- base are addressed. The experimental program described in Chapter 3 of this report was directed at addressing the identi- perimeter of the bar, resulting in a pull-out type failure. Tests fied needs in order to extend the AASHTO LFRD Bridge Design have shown that these two types of failures can take place at Specifications to allow greater use of high-strength concrete. stresses close to the tensile strength of the reinforcement. Pull- In reinforced and prestressed concrete structures, suffi- out failures occur in cases of high confinement and low cient transfer of forces between concrete and reinforcement bonded lengths. However, splitting failures are more common is required for a satisfactory design. The transfer of forces in structural applications. For this reason, it is recommended occurs through a combination of chemical surface adhesion, that experimental data considered for development of design friction, and bearing of bar deformations against the sur- equations should have a minimum embedment length. rounding concrete. Initially, the transfer of forces occurs Another important observation is that transverse reinforce- mainly by chemical adhesion; after initial slip, most of the ment has been observed to rarely yield during splitting failures force is transferred by bearing and friction. In the case of plain (Maeda, Otani, and Aoyama 1991; Sakaruda, Morohashi, and bars or wires, slip-induced friction--resulting from shape Tanaka 1993; and Azizinamini et al. 1999a). Therefore, it is and surface roughness--plays an important role in the force important to limit in design provisions the level of confine- transfer. In the case of deformed reinforcement, as slip in- ment provided by transverse reinforcement. The many factors creases, bearing of the ribs against the surrounding concrete affecting bond performance are presented in two main cate- becomes the principal mechanism of force transfer between gories: member properties and material properties. Some of concrete and steel. The forces on the bar surface are balanced the factors are common to both strand and mild reinforce- by compressive and shearing stresses in the concrete (see Fig- ment while others are unique to one or the other. ure 2.11). The concrete stresses result in tensile stresses that, Initially, in the testing of prestressing strand for bond to con- if high enough, can lead to cracking in planes both parallel crete, the simple pull-out tests were criticized because they did and perpendicular to the reinforcement, as shown in Figure not include the wedging action, or Hoyer's effect, associated 2.12. These transverse cracks can lead to splitting failure. with pretensioned strands in real beams. However, subsequent If the concrete cover, bar spacing, or amount of transverse testing with both the Moustafa Test and the NASP Bond Test reinforcement is sufficient to prevent or delay the splitting fail- have demonstrated that a direct correlation exists between re- ure, then failure can occur along a surface surrounding the sults from these simple pull-out tests and strand performance in pretensioned beams. Therefore, in this testing program the NASP Bond Test was employed as an assessment tool to quan- bearing and friction forces on bar tify the "bond-ability" of prestressing strands that will be employed. Testing sponsored by the NASP has demonstrated that the NASP Bond Test has superior repeatability and repro- ducibility when compared with the Moustafa Test. 2.3.1 Member Properties adhesion and friction forces along the surface of the bar Transfer Length of Prestressing Strand Figure 2.11. Mechanisms of force transfer In the specific case where prestressing strands are bonded between concrete and reinforcement- to concrete, bond stresses are derived through a combination deformed bars. of adhesion, friction, and mechanical interlocking (Hanson

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19 and Kaar 1959). It has been widely believed that a wedging length. In the case of anchored reinforcement by means of effect, called Hoyer's Effect, unique to pretensioned strands, straight embedment, longitudinal splitting will initiate at either creates significant bond stresses in the transfer zone where the a free surface or at a flexural crack location. In the case of spliced effective prestressing force is transferred from the preten- bars, splitting will start at the ends of the splice and move toward sioned strand to the concrete. In those same regions, slip the center. The mode of failure explains the fact that the non- occurs between strand and concrete due to the difference in loaded end of a developed/spliced bar is less effective than the strain condition. Research has indicated that the strand end loaded end in transferring forces between concrete and rein- slip can be used as a quality control measure for the bond of forcement. It can be concluded that there is a non-proportional prestressing strands (Rose and Russell 1997). Furthermore, relationship between development/splice length and bond the relative slip between strand and concrete virtually ensures strength. Thus, even though bond strengths have been meas- that adhesion plays little or no role in the transfer of pre- ured for very short embedment lengths, it is not appropriate to stressing forces to concrete (Russell and Burns 1996). linearly extrapolate such findings to code development lengths. This observation suggests the need for testing at appropriate scale for development of design provisions. Development and Splice Length In beams tested for strand development, it is equally appar- Bond forces are not uniformly distributed over the length of ent that cracking causes the mobilization of the strand relative anchorage (see Figure 2.13). Thus, bond failures are incremen- to concrete. Commonly, bond stresses that develop strand tal, initiating in the region of highest bond force per unit of tension from the transfer zone to the point where flexural capacity is required are called "flexural bond stresses." Flex- ural bond results primarily from a combination of mechani- cal interlocking and friction. The mechanical interlocking bond stresses are derived by the helical windings of the 7-wire prestressing strand, which act similarly to the mechanical de- formations found on rolled, mild reinforcement. Development length testing of pretensioned beams indi- cates that splitting occurs less frequently than in convention- ally reinforced beams (although splitting cracks have been observed in pretensioned bond failures). Issues for strand development are more related to the cracking patterns that occur as the pretensioned beams approach their ultimate strength. In testing on beams with debonded strands, it is clear that cracks that propagate through or near the transfer zone of pretensioned strands cause strands to slip. In many of those tests, cracking in the transfer zones of pretensioned strands caused bond failure of pretensioned strands (Russell, Burns, and ZumBrunnen 1994; Russell and Burns 1994). Additionally, in pretensioned strands with fully bonded beams, it is important to note that sections with narrow webs, specifically I-shaped beams, have failed in bond in concert with web shear cracking that occurs near or through the transfer zones of pretensioned strands (Jacob 1998; Kaufman and Ramirez 1988; Russell and Burns 1993). In contrast, tests on rectangular prestressed beams will not produce web shear cracks, so the behavior of rectangular cross sections can be Figure 2.13. Variation of steel significantly different than cross sections with narrow webs. and bond forces in a reinforced For that reason, the testing program includes testing of both concrete member subjected to pure bending: (a) cracked con- rectangular and I-shaped sections. crete region, (b) bond stresses acting on a reinforcing bar, (c) Transverse Reinforcement variation of tensile force in steel, and (d) variation of bond force Orangun, Jirsa, and Breen (1977) indicated that transverse along the bar (Nilson 1997). reinforcement confines the concrete around anchored bars

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20 and limits the progression of splitting cracks. An additional strength of vertical bars seems to be reduced only by 25 per- beneficial effect of transverse reinforcement is that increases cent with respect to the bond strength of horizontal bars. A in transverse reinforcement lead eventually to pull-out failures single factor of 1.3 is recommended for all vertical bars where rather than splitting-type failures. However, the Orangun, the center of the splice or the development length has more Jirsa, and Breen study also noted that transverse reinforce- than 24 in. of concrete cast below. ment in excess of the amount required to cause the change in mode of failure is not as effective and eventually leads to no further increase in bond strength. These observations and the Concrete Cover and Spacing observations by Maeda, Otani, and Aoyama 1991; Sakaruda, of Reinforcement Morohashi, and Tanaka 1993; and Azizinamini et al. 1995 that As shown in Figure 2.12, splitting failure is expected to con- the transverse reinforcement confining the anchored bar sel- trol in the majority of structural applications. In this type of fail- dom yields in splitting failure indicates the need for an upper ure, the actual location of the splitting cracks in the case of bot- limit on the improvement in bond strength provided by the tom cast reinforcement depends on the relative values of the presence of transverse reinforcement. concrete bottom cover, concrete side cover, and one-half of the clear spacing between bars. If the bottom cover is less than the Casting Position side cover and one-half the spacing between bars, splitting occurs through the cover to the bottom free surface. If either the It has been observed by various researchers that top cast side cover or one-half the bar spacing is smaller than the bot- bars have lower bond strengths than bottom cast bars. Clark tom cover, then splitting of the concrete occurs either through (1946), using pull-out type specimens cast in a horizontal the side cover or between the reinforcement. This observation position, noted that in the top position, bars were two-thirds supports the need to modify the current AASHTO LRFD Bridge as effective in bond as in the bottom position. The depth of Design Specifications for bond and development length of mild the concrete under the bar in the top position was 15 in., and reinforcement to incorporate the effects of cover, bar spacing, the depth of the concrete under the bar in the bottom posi- and transverse reinforcement. tion was 2 in. The concrete slump was 4.25 in., and the com- pressive strength averaged 5.6 ksi. Ferguson and Thompson (1965) noted that with 12 in. of concrete below the bar, the 2.3.2 Material Properties strength dropped from 3 to 13 percent as the slump was in- Reinforcement Properties creased. They noted that for the beam depths tested, from 13 to 22 in., the 1.4 factor used in the specifications was conser- For a given bonded length required to achieve a given steel vative. This observation is currently recognized in the 318 stress level, reinforcement of different areas will achieve dif- Code where a 1.3 factor is used to increase the development ferent levels of force at the onset of splitting failure, with the length or splice of bars cast horizontally with more than 12 in. larger area reinforcement achieving higher forces. Therefore of fresh concrete cast in the member below the bar (ACI larger area reinforcement will require longer development/ 2005). A 1.4 factor is currently prescribed in the AASHTO splice length than smaller area reinforcement for the same LRFD Bridge Design Specifications to cover this case, and it is degree of confinement. The size of the reinforcement being thus conservative if the effects of cover and transverse rein- developed also plays a role in the contribution of the confin- forcement are included in the specifications. ing reinforcement for the case of deformed bars. As large bars Additional research (Jirsa and Breen 1981) indicates that slip, higher strains are mobilized in the transverse reinforce- the concrete slump plays an important role in determining ment, thus the beneficial effect of transverse reinforcement the effects of casting position, and this is most significant on the bond strength of deformed bars increases as the area when very large depths of concrete are cast below the bars or of the bar increases. splices. The 1981 study by Jirsa and Breen further indicated It is now customary to relate bond performance to bar that the so-called top bar factor should vary with the depth of geometry by means of the relative rib area factor, Rr, defined as: concrete cast below the reinforcement and recommended a projected rib area normal to bar axis maximum factor of 1.3 for slumps of less than 4 in. For Rr = erimeter) (center - to - center rib spacing) (nom. bar pe slumps between 4 and 6 in., a maximum factor of 1.35 is rec- (2.6) ommended for depths below 24 in., and a maximum factor of up to 1.6 is recommended for depths greater than 48 in. Typical bars currently used in the United States have relative For slumps greater than 6 in., a factor of 1.8 for depths below rib area factors ranging between 0.057 and 0.087 (Choi et al. 24 in. is recommended, and a factor of 2.2 for depths below 1990). Darwin and Graham (1993a, 1993b) concluded that the 48 in. is recommended. It is further stated that the basic bond bond strength is independent of deformation pattern if the bar