Bond and Development of Straight Reinforcing Bars in Tension
|Publication Date:||1 November 2003|
The bond between reinforcing bars and concrete has been acknowledged as a key to the proper performance of reinforced concrete structures for well over 100 years (Hyatt 1877). Much research has been performed during the intervening years, providing an ever-improving understanding of this aspect of reinforced concrete behavior. AC1 Committee 408 issued its first report on the subject in 1966. The report emphasized key aspects of bond. that are now well understood by the design community but that, at the time, represented conceptually new ways of looking at bond strength. The report emphasized the importance of splitting cracks in governing bond strength and the fact that bond forces did not vary monotonically and could even change direction in regions subjected to constant or smoothly varying moment. Committee 408 followed up in 1979 with suggested provisions for development, splice, and hook design (AC1 408.1R-79), in 1992 with a state-of-the-art report on bond under cyclic loads (AC1 408.2R-92), and in 2001 with design provisions for splice and development design for high relative rib area bars (bars with improved bond characteristics) (AC1 408.3-01). This report represents the next in that line, emphasizing bond behavior and design of straight reinforcing bars that are placed in tension.
For many years, bond strength was represented in terms of the shear stress at the interface between the reinforcing bar and the concrete, effectively treating bond as a material property. It is now clear that bond, anchorage, development, and splice strength are structural properties, dependent not only on the materials but also on the geometry of the reinforcing bar and the structural member itself. The knowledge base on bond remains primarily empirical, as do the descriptive equations and design provisions. An understanding of the empirical behavior, however, is critical to the eventual development of rational analysis and design techniques.
Test results for bond specimens invariably exhibit large scatter. This scatter increases as the test results from different laboratories are compared. Research since 1990 indicates that much of the scatter is the result of differences in concrete material properties, such as fracture energy and reinforcing bar geometry, factors not normally considered in design. This report provides a summary of the current state of knowledge of the factors affecting the tensile bond strength of straight reinforcing bars, as well as realistic descriptions of development and splice strength as a function of these factors. The report covers bond under the loading conditions that are addressed in Chapter 12 of AC1 318; dynamic, blast, and seismic loading are not covered.
Chapter 1 provides an overview of bond behavior, including bond forces, test specimens, and details of bond response. Chapter 2 covers the factors that affect bond, discussing the impact of structural characteristics as well as bar and concrete properties. The chapter provides insight not only into aspects that are normally considered in structural design, but into a broad range of factors that control anchorage, development, and splice strength in reinforced concrete members. Chapter 3 presents a number of widely cited descriptive equations for development and splice strength, including expressions recently developed by AC1 Committee 408. The expressions are compared for accuracy using the test results in the AC1 Committee 408 database. Chapter 4 summarizes the design provisions in AC1 318, AC1 408.3, the 1990 CEB-FIP Model Code, as well as design procedures recently developed by Committee 408. The design procedures are compared for accuracy, reliability, safety, and economy using the AC1 Committee 408 database. The observations presented in Chapters 3 and 4 demonstrate thatfc'"4 provides a realistic representation of the contribution of concrete strength to bond for values up to at least 16,000 psi (110 MPa), while fc¾ does the same for the effect of concrete strength on the increase in bond strength provided by transverse reinforcement. This is in contrast to fc, which is used in most design provisions. The comparisons in Chapter 4 also demonstrate the need to modify the design provisions in AC1 318 by removing the bar size y factor of 0.8 for small bars and addressing the negative impact on bond reliability of changing the load factors while maintaining the strength reduction factor for tension in the transition from AC1 318-99 to AC1 318-02. Design procedures recommended by AC1 Committee 408 that provide both additional safety and economy are presented. Chapter 5 describes the AC1 Committee 408 database, while Chapter 6 presents a recommended protocol for bond tests. The expressions within the body of the report are presented in inch-pound units. Expressions in SI units are presented in Appendix A.
A few words are appropriate with respect to terminology. The term bondforce represents the force that tends to move a reinforcing bar parallel to its length with respect to the surrounding concrete. Bond strength represents the maximum bond force that may be sustained by a bar. The terms development strength and splice strength are, respectively, the bond strengths of bars that are not spliced with other bars and of bars that are spliced. The terms anchored length, bonded length, and embedded length are used interchangeably to represent the length of a bar over which bond force acts; in most cases, this is the distance between the point of maximum force in the bar and the end of the bar. Bonded length may refer to the length of a lap splice. Developed length and development length are used interchangeably to represent the bonded length of a bar that is not spliced with another bar, while spliced length and splice length are used to represent the bonded length of bars that are lapped spliced. When used in design, development length and splice length are understood to mean the "length of embedded reinforcement required to develop the design strength of reinforcement at a critical section," as defined in AC1 3 18.