Improving the Development Characteristics of Steel Reinforcing Bars

Abstract

The bond characteristics of deformed reinforcing bars are investigated ctnd design equations for development and splice lengths are obtained with the goal of improving the bond strength of steel reinforcing bars to concrete. The research includes both experimental and analytical studies. The experimental studies involved evaluating the performance of deformed steel reinforcing bars with different deformation patterns and the effects of epoxy coating on these bars, using friction, beam-end, and splice tests. The friction tests are used to determine the coefficient of friction between reinforcing steel and mortar, for both epoxy-coated and uncoated steel. The results indicate that the coefficient of friction is about 0.49 between epoxy-coated reinforcing steel and mortar and about 0.56 between uncoated reinforcing steel and mortar. The beam-end tests are used to study the effects of deformation pattern on bond strength as affected by epoxy coating. Fifty-eight beam-end specimens, containing No. 8 bars with different deformation patterns (relative rib· area and rib face angle) were tested. Epoxy coating appears to have a less detrimental effect on bond strength for high relative rib area bars than for previously tested conventional bars. Bars with high rib face angles also appear to be affected less by epoxy coating. The splice tests are used to study the effects of deformation pattern on splice strength as affected by epoxy coating and confinement by transverse reinforcement. Fifty-four splice specimens containing No. 8 bars with different deformation patterns were tested. Concretes containing two different coarse aggregates were used to evaluate the effect of aggregate properties on bond strength. Epoxy coating appears to be less detrimental on splice strength for high relative rib area bars than for conventional bars. The splice strength of uncoated reinforcement confined hy transverse reinforcement increases with an increase in the relative rib area. The increase in splice strength provided by transverse reinforcement increases as the strength of the coarse aggregate increases. The results indicate that current development/splice lengths can be reduced by an average of 9 to 16% if high relative rib area bars are used with confinement provided by transverse reinforcement. The analytical study focused on obtaining splice and development length expressions for bars with and without transverse reinforcement. The analyses demonstrate that the relationship between bond force and development or splice length is linear but not proportional. ( 112 does not provide an accurate representation of the effect of concrete strength on bond strength. Development/splice strengths are underestimated for low strength concretes and overestimated for high strength concretes. r; 114 provides an accurate representation of the effect of concrete strength on bond strength. The yield strength of transverse reinforcement does not play a role in the effectiveness of the transverse reinforcement in improving development/splice strength. The effectiveness of the transverse reinforcement depends on the total area of stirrups crossing the potential plane of splitting. LRFD concepts and Monte Carlo techniques are applied to the bond strength expressions to obtain a strength reduction (¢) factor of 0.85 which, together with the bond strength expressions, are used to obtain prototype design equations for splice and development length. For high relative rib area bars confined by transverse reinforcement, development/splice lengths average 9 to 16% lower than obtained for conventional bars. For high relative rib area bars confined by transverse reinforcement, development lengths average 9 to l7'7c lower and splice lengths average 30 to 36% lower than those obtained with ACI 318-95. depending on the value of R, for the high relative rib area bar.