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21 is under small cover conditions and there is no transverse rein- In unconfined bar bond tests, the use of basalt aggregates has forcement. Darwin and Graham observed that under large been shown to increase bond strength by almost 13 percent cover or with transverse reinforcement, bond strength in- over the bond strength of concretes with weaker aggregate such creased with an increase in relative rib area. They also found that as limestone (Zuo and Darwin 1998, 2000). Tests on bars con- deformations parallel to the splitting cracks were more effective. fined by transverse reinforcement (Darwin et al. 1996; Zuo and The bond strength of epoxy-coated bars has been found to Darwin 1998) also indicate an increase in bond strength in the increase under all conditions of confinement as the relative presence of stronger aggregates showing a significant effect on rib area is increased. Zuo and Darwin (1998) recommended the contribution from the transverse reinforcement. that for epoxy-coated bars with relative rib areas greater than Lower strength aggregates, on the other hand, have a or equal to 0.1 and concrete with compressive strength below detrimental effect on the bond strength. Reports by ACI 10 ksi, development and splice length should be increased by Committee 408 (1966, 1970) have emphasized the paucity of 20 percent instead of the 50-percent increase for cover less experimental data on the bond strength of reinforced con- than 3db or clear spacing less than 6db. For concrete strengths crete elements made with lightweight aggregate concrete. The greater than 10 ksi, a 50-percent increase appeared warranted AASHTO LRFD Bridge Design Specifications includes a fac- regardless of the value of Rr. tor of 1.3 for development length to reflect the lower ten- The surface condition is important from the standpoint of sile strength of lightweight aggregate concrete and allows that bond strength because it affects adhesion, friction, and bear- factor to be taken as 0.22 fc / fct 1.0 if the average splitting ing in the transfer of forces between steel and surrounding strength, fct, of the lightweight aggregate concrete is specified. concrete. Items such as cleanliness, rust, and coatings affect For lightweight sand, where fct is not specified, a factor of 1.2 the surface condition of the reinforcement. Specifications is specified. Although design provisions, in general, require require that the reinforcement be free of mud and other longer development lengths for lightweight aggregate con- substances capable of reducing bond strength. It is well crete, test results from previous research are contradictory, in established that the presence of epoxy coatings reduces the part, because of the different characteristics associated with bond strength of reinforcement (Mathey and Clifton 1976; the particular type of aggregate and mix design. The use of Johnston and Zia 1982; Treece and Jirsa 1989; Choi et al. lightweight aggregate concrete is outside the scope of NCHRP 1990, 1991; Cleary and Ramirez 1993). Project 12-60. It has been widely observed that as the concrete compres- sive strength increases, the bond strength of the same con- Concrete Properties crete also increases--albeit at a slower rate--leading to po- Compressive strength and lightweight aggregate are ac- tentially more brittle failures (Azizinamini et al. 1993, 1999a). knowledged in codes and specifications as influencing bond On the other hand, the tensile strength of the concrete is not strength. In addition, tensile strength and fracture energy, the only factor controlling bond strength, as it has been noted mineral admixtures, and consolidation and vibration are also by Zuo and Darwin (1998, 2000) for deformed bars. The Zuo factors affecting bond strength of reinforcement. and Darwin studies recommended the use of f c1/4 instead of Azizinamini et al. (1993, 1999a) noted that for higher the traditional f c1/2 to represent the effect of concrete com- strength concretes, the higher bearing capacity prevents pressive strength on bond strength for unconfined bars. They crushing of the concrete in front of the ribs, thus reducing the also noted that the presence of confinement influenced the local slip. These researchers further noted that the reduced power of the compressive strength and recommended the use slip also limited the number of ribs participating in the load of f c3/4 as a good representation of the influence of compres- transfer between concrete and reinforcement. The reduced sive strength on bond strength. participation of the ribs increases the local tension stresses Most of the work related to bond has focused on the effect and further leads to a non-uniform distribution of bond of silica fume. The studies have shown increases of less than force. Although traditionally fc has been used to reflect the 10 percent on bond strength in the presence of the mineral concrete compressive strength in bond calculations, Zuo and admixture (DeVries, Moehle, and Hester 1991; Hamad and Darwin (1998, 2000) have postulated that f c1/4 for members Itani 1998). without stirrups and f c3/4 for members with stirrups better re- flect the effect of concrete strength on bond. These findings 2.4 Issues Related to Testing indicate that if bond strengths are normalized with respect to Protocols f c1/2, the effect of concrete strength on the bond strength is se- verely overestimated. High-strength concrete has been shown A review of testing protocols for determining bond charac- to improve anchorage of prestressing strand, thus reducing teristics was presented. From our review of available research, the required transfer length and development length. we recommended that the NASP Bond Test be employed

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22 throughout the experimental program to quantify the bond transfer and development of prestressing strands. Through characteristics of individual strand samples. The testing the years, several researchers have included concrete strength program includes "round robin" testing at both Purdue Uni- as a variable in transfer and development length equations. versity and Oklahoma State University (OSU) to validate the However, the lack of consistency in the strand products repeatability of the test procedure. The NASP Bond Test themselves has worked against developing a consensus re- procedure has been refined through this research and is now garding the effect of concrete strength. By quantifying strand recommended for adoption into the AASHTO LRFD Bridge bond characteristics through the NASP Bond Test, this re- Code as the Standard Test Method for the Bond of Prestress- search has been able to assess the effects of concrete strength ing Strands. The testing protocols for splice/development on transfer and development of pretensioned strands. lengths and bars terminated with standard hooks are also pre- Recommendations to include concrete strength in the design sented. The beam-splice test for splice/development length equations have been made. and the Marques and Jirsa (1975) and Hamad, Jirsa, and D'Abreu de Paulo (1993) exterior beam column joint setup Influence of Water-Reducing Admixtures for bars in tension anchored by means of standard hooks are recommended for use in this study. There is a lack of data available to assess what effect, if any, water-reducing admixtures have on the bond of pretensioned strands. This contrasts directly with the fact that more than 2.4.1 Testing Protocols for 99 percent of the prestressing plants in North America use Prestressing Strand HRWRs (the source for this information is an informal, Since 1994, three new test procedures or protocols have unpublished committee report on a survey of precast/ been developed for assessing the bonding characteristics of prestressing plants done by the Prestressing Steel Committee prestressing strand: the Moustafa Bond Test, the Post- of PCI circa 1998). For historical perspective, it is noted that Tensioning Institute (PTI) Bond Test, and the NASP Bond the bulk of development regarding the Moustafa Test em- Test. Testing has demonstrated that the NASP Bond Test de- ployed concrete that did not contain HRWRs. A majority of livers the greatest degree of repeatability and reproducibility the Moustafa testing has been performed at Stresscon of the three tests. Therefore, the testing program for NCHRP Corporation in Colorado Springs, where HRWRs are not Project 12-60 employed the NASP Bond Test as the standard commonly employed. Yet, others that have participated in test to assess the relative "bond-ability" of prestressing Moustafa Testing have employed HRWRs as part of the stan- strands. Previous experience with research on strand bond dard casting procedures used in the local prestressing plants. demonstrates the importance of quantifying the strand bond- Variations that result from the use of HRWR have not been ing properties prior to or concurrent with testing programs measured or quantified. These data were compiled informally for transfer and development length of strands. through the work of the Prestressing Steel Committee of the Precast/Prestressed Concrete Institute and are not available for publication. Prestressing Strand up to 0.6 in. in Diameter Engineers and contractors concerned with the bond of pre- Influence of Air Entrainment stressing strand used for rock anchors developed the PTI Bond Test. In conformance with standard practice for rock anchors, There is a lack of data available to assess what effect, if any, the test protocol indicates explicitly that testing should be con- air entrainment has on the bond of pretensioned strands. ducted on 0.6-in. strand. The test protocol has been modified The experiences of the states are mixed with regard to to accept 0.5-in. strand, but the acceptance value has not been whether air entrainment is required in pretensioned beams. adjusted or evaluated using 0.5-in. strand. Both the Moustafa The NASP Bond Test was employed to examine what effects, Test and the NASP Bond Test were developed using 0.5-in. if any, air entrainment has on bond. Results indicate that strand. In the experimental program, the NASP Bond Test was concrete strength is more important to bond strength than performed using 0.6-in. strand. The testing demonstrated that air entrainment. the Standard Test Method for the Bond of Prestressing Strands is suitable for 0.6-in. strands as well as 0.5-in. strands. Top Bar Effects There is a small database in existence available to examine Influence of Concrete Strength the "top bar effect" on transfer and development of pre- As noted in the literature review, concrete strength has stressing strands. This information is primarily available from long been described as an important variable affecting the testing programs on prestressed concrete piling. The top bar

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23 effect is expected to be assessed during casting of the scale situations encountered in structures. Transverse compression model and full-sized specimens by including pretensioned has a beneficial effect on bond strength and yields an overly strands in the top half of the cross section. optimistic assessment of the actual performance of struc- tures. For this reason, various testing schemes have been pro- posed to eliminate transverse compression (see Figure 2.14[f] 2.4.2 Testing Protocols for Mild and [g]). In the case of semi-beam specimens, such as those Reinforcement shown in Figure 2.14(f), it is critical to properly account for A review of testing protocols was conducted to determine the increase in the length over which splitting resistance tends the appropriate testing protocol(s) for addressing gaps in the to be mobilized due to the confining pressure at the end of the experimental data. It is well established that testing protocols bar (if the bar end is not shielded). ACI Committee 408 to evaluate development and splice length requirements for (1964) prepared a detailed guide for the determination of deformed bars and wire in tension must be of an appropriate bond strength in beam specimens. The more popular varia- scale, containing more than one bar or wire; testing protocols tion, the so-called beam splice test with the splice located in should also show due regard for a realistic transfer of force the constant moment region (the most critical condition is between concrete and steel reinforcement, as well as cover one where both bars in the splice are subjected to high and bar spacing effects. The more commonly used testing stresses), can be seen in Figure 2.14(i). configurations are shown in Figure 2.14. Although they Splice tests have been realistic simulations of real conditions are economically appealing, pull-out tests used by earlier in structures, but development length tests have been con- researchers to evaluate bond performance of various ducted largely using pull-out tests in which splitting failures reinforcing bars embedded in concrete of different strengths are purposely avoided. As a result, the bond stresses developed (Figures 2.14[a] through [e]) present the problem of intro- along splices are low compared with the bond along a bar in a ducing transverse compression, a compression not typical of pull-out test. This difference in test methods is responsible for (h) (i) (a) (b) (j) (k) (l) (g) (e) (d) (c) (f) (m) P = applied load. Figure 2.14. Testing methods to evaluate bond strength.

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24 large differences in code-required anchorage lengths for splices and development of single bars. Pull-out failures occur in cases of high confinement and short bonded lengths. In most structural applications, however, splitting failures tend to control. On this basis, the data developed for extending the current AASHTO LRFD specification for splice/development length had a minimum bonded length to bar diameter ratio of 15 in order to avoid unrealistically high values of bond strength. A similar concept of minimum embedment length should be included in any proposed specifications. Splice specimens such as those shown in Figure 2.14(i) are deemed to represent larger-scale specimens designed to directly measure development and splice strength in full-scale mem- bers. Because of the relative ease of fabrication and the realistic state of stress achieved during testing, splice specimens were used in the development of experimental data on development/ splice length of mild reinforcement in this research. Review of experimental data on anchorage of bars termi- nated using standard hooks indicates the need for additional testing to extend the current AASHTO LRFD specifications to concrete strengths up to 15 ksi (see Section 2.2.2). Tests have Figure 2.15. Exterior beam-to-column joint setup to shown that the bar force is transferred rapidly into the con- evaluate bond performance of bars developed crete, and the portion following a hook is generally ineffective using standard hooks. and can potentially be limited by the tensile strength of the concrete. Further study of failures of hooked bars indicates that mineral admixtures; splitting of the concrete cover is the primary cause of failure chemical admixtures, including specific gravity and and that splitting originates at the inside of the hook, where the percent solids; and local stress concentrations are higher. Thus, it has been deter- fine and coarse aggregates and their properties (e.g., mined that hook development is a direct function of bar di- specific gravity [SSD] and absorption). ameter, db, which governs the magnitude of compressive the concrete compressive strength, as obtained from a stresses on the inside of the hook. The experimental work sup- standard concrete cylinder (which should be cured side- porting the current requirements for development of standard by-side with, and in the same manner as, the bond/splice hooks in tension was conducted using the test setup shown in specimens), and including: Figure 2.15. In NCHRP Project 12-60, a similar specimen and size of the compressive strength specimens, test setup was used in the evaluation of uncoated and epoxy- type and thickness of the cylinder caps used on the spec- coated bars terminated with standard hooks in tension to imens, and normal-weight concrete with compressive strength up to 15 ksi. age of the specimen at testing. A useful test protocol to help understanding the bond the concrete flexural strength, including: strength of mild reinforcement in concrete members must size of the flexural strength specimens, define a minimum level of information to be provided. The age of specimen at testing, and recommended level of information is described in the flexural test method used. following subsections. Reinforcement Properties Concrete Properties The properties of the reinforcing steel are required for The following information on concrete properties should basic identification and, in most cases, are needed to fully be provided: characterize the steel used in the tests. The following infor- mation should be provided for each heat or production run the source of the concrete. of reinforcing steel: the standard (ASTM) under which the the mix proportions, including identification of the bars were manufactured, the nominal diameter, bar designa- components: tion, yield strength, tensile strength, proof strength (if appli- cement type; cable), elongation at failure, weight (mass) per unit length,