Improving the Development Characteristics of Steel Reinforcing Bars

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

Bond of Epoxy-Coated Reinforcement: Coefficient of Friction and Rib Face Angle

The coefficients of friction between epoxy-coated and uncoated reinforcing steel and mortar and the effects of rib face angle on the relative bond strength of epoxy-coated bars are determined. Results for 130 test specimens indicate that the average coefficient of friction is about 0.49 between epoxy-coated reinforcing steel and mortar and about 0.56 between uncoated reinforcing steel and mortar. Based on 58 beam-end specimen tests using both machined and rolled 1-in. (25-mm) nominal diameter reinforcing bars with face angles of 30, 40, 45, 60, and 90 deg, epoxy coating has the least effect on the bond strength of steel reinforcing bars to concrete when the rib face angle is greater than or equal to 45 deg

Development Length Criteria for Conventional and High Relative Rib Area Reinforcing Bars

Statistical analyses of 133 splice and development specimens in which the bars are not confined by transverse reinforcement and 166 specimens in which the bars are confined by transverse reinforcement are used to develop an expression for the bond force at failure as a function of concrete strength, cover, bar spacing, development/splice length, transverse reinforcement, and the geometric properties of the developed/spliced bars. Results are used to formulate design criteria that incorporate a reliability-based strength reduction (f) factor that allows the calculation of a single value for both development and splice length for given material properties and member geometry. As with earlier studies, the analyses demonstrate that the relationship between bond force and development or splice length ld is linear but not proportional. Thus, to increase the bond force (or bar stress) by a given percentage requires more than the percentage increase in ld . f ¢ c 1/2 does not provide an accurate representation of the effect of concrete strength on bond strength over the full range of concrete strengths in use today; development/splice strengths are underestimated for low-strength concretes and overestimated for high-strength concretes. f¢ c 1/4 provides an accurate representation of the effect of concrete strength on bond strength for concretes with compressive strengths between 2500 and 16,000 psi (17 and 110 MPa). The most accurate representation of the effect of transverse reinforcement on bond strength obtained in the current analysis includes parameters that account for the number of transverse reinforcing bars that cross the developed/spliced bar, the area of the transverse reinforcement, the number of bars developed or spliced at one location, the relative rib area of the developed/spliced bar, and the size of the developed/spliced bar. The yield strength of transverse reinforcement does not play a role in the effectiveness of the transverse reinforcement in improving development/splice strength. Depending on the design expression selected, for conventional and high relative rib area bars that are not confined by transverse reinforcement, development lengths average 2 to 14 percent higher and splice lengths 12 to 22 percent lower than those obtained using ACI 318-95. For conventional reinforcing bars confined by transverse reinforcement, development lengths average 5 percent lower to 16 percent higher than those obtained using ACI 318-95, while splice lengths average 11 to 27 percent lower than those obtained using ACI 318-95. For high relative rib area reinforcing bars confined by transverse reinforcement, development lengths average 3 to 17 percent lower than those obtained using ACI 318 95, while splice lengths average 25 to 36 percent lower than those obtained using ACI 318-95. When confined by transverse reinforcement, high relative rib area bars require development and splice lengths that are 13 to 16 percent lower than required by conventional bars

Reliability-Based Strength Reduction Factor For Bond

The formulation and calculation of a reliability-based strength-reduction ( <P) factor for developed and spliced bars is described. Conventional and high relative rib area bars, both with and without confining reinforcement, are considered. The cp-factor is determined using statistically-based expressions for development/splice strength and Monte Carlo simulations of a range of beams. A strength-reduction factor of 0.9 is obtained for the design expressions for development/splice length, based on a probability of failure in bond equal to about one-fifth of the probability of failure in bending or combined bending and compression. <P = 0.9 is incorporated into two expressions for development/splice length in a manner that is transparent to the user. A major advantage of each of the final expressions is that they provide identical values for development and splice length, removing the need to multiply development length by 1.3 or 1.7 to obtain the length of most splices

Splice Strength of High Relative Rib Area Reinforcing Bars

This paper describes the testing and analysis of83 beam-splice specimens containing No. 5, No. 8, and No. 11 ( 16, 25, and 36 mm) bars with relative rib areas (ratio ofprojected rib area nonnal to bar axis to the product of the nominal bar perimeter and the center-to-center rib spacinJ?) ranJ?inJ? from 0.065 to 0.140. Concretes containing two different coarse aggregates were used to evaluate the effect of'ag1veJ?ate properties on bond strength. Sixty specimens contained uncoated bars with confining transverse reinforcement. Thirteen specimens contained uncoated bars without confining reinforcement, and 10 specimens contained epoxv-coated bars, nine without confining reinforcement and one with confining reinfiJrcement. The tests are analyzed to detennine the effect of relati1·e rib area and bar diameter on the increase in bond strength provided by confining reinfiJrcement. The tests also provide a preliminary indication of the effect of high relati1·e rib area on the splice strength of epoxv-coated bars. The splice streni;th of uncoated reinforcement confined by transverse reinf( Jrcement increases with an increase in the relative rib area and the bar diameter of the spliced bars. The increase in splice strength provided by transverse reinforcement increases as the strength of the coarse aggregate increases. The use of reinforcinJ? bars with an averai;e relative rib area of 0.1275, an increase ff-om the average value for conventional bars of'0.0727, can provide up to a 26 percent decrease in splice lenJ?th compared to conventional reinf'orcement when confininJ? reinforcement is used. The savings obtainable with high relative rib area bars is highestf(Jr low covers and bar spacings. Epoxy coating appears to have a less detrimental effect on splice strength for high relative rib area bars than for conventional bars. The results indicate that the maximum development length modification factor used for epoxy-coated reinforcement may be reduced by 20 percent

Development Length Criteria: Bars Without Transverse Reinforcement

An expression that accurately represents development and splice strength as a function of concrete cover and bar spacing is developed and used to establish and evaluate modifications to the bond and development provisions of the ACI Building Code (ACI 318-89) for bars without transverse reinforcement The expression for development and splice strength is similar in form to expressions developed by Orangun, Jirsa, and Breen (1975, 1977), but is obtained using techniques that limit the effects of unintentional bias in the test data. The resulting expression provides a more accurate representation of development and splice strength than do the earlier expressions, and provides better guidance when there is a significant difference between the concrete cover and one-half of the clear spacing between bars. The expression for development and splice strength is used to establish new criteria that follow the format of ACI 318-89 and to evaluate design criteria that are currently under review by ACI Subcommittee 318-B. The new criteria that follow the format of ACI 318-89 are generally conservative and economical. The provisions under study by Subcommittee 318-B are unconservative for No. 6 bars and smaller with minimum covers and close spacings, and are overconservative for most bars with higher covers and wider spacings. Modifications are recommended that increase both the safety and the economy provided by the provisions under study by ACI Subcommittee 318-B

Development Length Criteria: Bars Not Confined by Transverse Reinforcement

An expression that accurately represents development and splice strength as a function of concrete cover and bar spacing is developed. The expression is then used to establish and evaluate modifications to the bond and development provisions of the ACI Building Code (AVI 318-89) for bars in concrete members that are not confined by transverse reinforcement. The expression for development and splice strength is similar in form to expression for development and splice strength is similar in form to expressions developed by Orangun, Jirsa, Breen (1975, 1977) but is obtained using techniques that limit the effects of unintentional bias in the test data. The resulting expression provides a more accurate representation of development and splice strength than do the earlier expressions, and it provides better guidance when there is a significant difference between the concrete cover and one-half of the clear spacing between bars. Proposals for new design cirteria, including one under study by ACI Committee 318, are compared. Each of the new proposals contains design criteria that are superior to the current development length provisions (ACI 318089); however, the criteria differ in terms of relative safety, economy, and ease of application. Side-by-side comparisons in design offices are recommended. In All cases, an additional development length modification factor of 1.1 is recommended for reinforcing steels with specified yield strengths in excess of 60,000 psi (414 MPa)