ACI 408.2R-12 Report on Bond of Steel Reinforcing Bars Under Cyclic Loads.
3.2—Components of bond resistance Although the concept of average bond stress is convenient, the total bond force is a combination of resistance due to chem- ical adhesion (V a ), mechanical anchorage due to bearing of the ribs (V b ), and frictional resistance (V f ), as indicated in Fig. 3.2. Chemical adhesion between the steel and the concrete is lost at low levels of stress in the reinforcing bar. Frictional resis- tance is associated with micro-irregularities along the surface of the steel, wedging of granular material between the bar and the concrete, and bearing forces oriented perpendicular to the rib face that arise as the bar is loaded. Transverse components of the bearing forces can lead to a bond failure characterized by splitting of the concrete cover.
3.3—Failure modes Under monotonic loading, two types of bond failures are typical. The frst is a pullout failure, as depicted in Fig 3.3a, due to the shearing of the concrete between the ribs. This occurs when suffcient concrete cover prevents splitting or where suffcient transverse reinforcement keeps splitting cracks small. Pullout resistance depends primarily on concrete strength, pattern and geometry of bar ribs, depth of concrete cover, and the size and amount of transverse reinforcement. The second type of bond failure is splitting of the concrete cover. This occurs when cover or confnement is insuff- cient to attain full pullout strength. As depicted in Fig. 3.3b, splitting failure is due primarily to tensile radial stresses caused by deformation bearing forces. In this case, split- ting propagates to the edges of the member, resulting in loss of concrete cover and bond (Brown 1966; Ferguson 1979; Fujii and Morita 1981; Guiriani 1981; Hawkins et al. 1982; Lutz 1970; Lutz and Gergely 1967; Morita and Kaku 1979; Robins and Standish 1982; Robinson 1965; Untrauer and Henry 1965; Watstein and Bresler 1974). Failure modes under low-cycle loading are similar to those under monotonic loading. Under high-cycle loading, similar failure modes can occur, but fatigue failures of both the bar and concrete need to be considered.
3.5.7 Amount and distribution of transverse steel— Splitting tensile failures are strongly dependent on amount and distribution of transverse reinforcement (Eligehausen 1979; Orangun et al. 1977; Tepfers 1973). Properly detailed transverse reinforcement confnes the concrete after cracking and signifcantly increases the resistance to splitting failure, especially under cyclic loads. The upper bound to this improvement is the pullout failure of the bar.ACI 408.2R pdf download.