The effect of deformation pattern on bond strength is studied using I in.(25 mm) diameter machined bars with deformation heights of0.05, 0.075, and 0.10 in.( 1.27, 1.91, and 2.54 mm) and deformation spacings ranging from 0.26 to 2.2 in.(6.7 to 56 mm).The combinations of rib height and spacing produce relative rib areas (ratio of projected rib area normal to bar axis to product of nominal bar perimeter and center-to-center rib spacing) of 0.20, 0.10, and 0.05 for each deformation height.Conventional reinforcing bars, with a relative rib area of0.07, are also studied.The effect of deformation pattern is evaluated using beam-end specimens with varying degrees of confinement provided to the test bars.Degrees of confinement are: 1) 2-in.(51-mm) cover without transverse stirrups, 2) 2-in.(51-mm) cover with confining transverse stirrups, and 3) 3-in.(76-mm) cover without confining transverse stirrups.Bars with 2-in.(51-mm) cover have an initial unhanded length of 1 /z in.( 13 mm) and a bonded length of 12 in.(305 mm).Bars with 3-in.(76-mm) cover have an initial unhanded length of 4 in.( 102 mm) and a bonded length of 8 1 /z in.(216 mm).The bond force-slip response of reinforcing bars is a function of the relative rib area of the bars, independent of the specific combination of rib height and rib spacing.Under all conditions of bar confinement, the initial stiffness of load-slip curves increases with an increase in the relative rib area.Under conditions of relatively low confinement, in which bond strength is governed by splitting of the concrete, bond strength is independent of deformation pattern.Under conditions in which additional bar confinement is provided by transverse reinforcement or higher cover, bond strength increases compared to the bond strength of bars with less confinement.The magnitude of the increase in bond strength increases with an increase in the relative rib area.
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Darwin et al. (1993) studied this question.