Headed Reinforcing Bars: CCT Node Tests, Design Provisions, and Evaluation of a Granular Micromechanics Model for Use in Finite Element Analysis of Bond

The anchorage behavior of headed bars at the end of beams within compression- compression-tension (CCT) nodes subjected to monotonic loading is assessed. The test parameters for the 10 specimens tested include the embedment length, number and spacing of the anchored bars, and the presence or absence of a head at the anchored end of the bar. The nominal compressive strength of the concrete was 5000 psi. The test results, along with that of members other than beam- column joints available in the literature, are compared with strengths based on a descriptive equation developed for headed bars anchored in beam-column joints. The test results for specimens with bars without heads are compared with anchorage strengths predicted for straight bars. More broadly, test results for headed bars anchored in beam-column joints are compared with design provisions for the development length of headed bars in ACI 318-14 and ACI 318-19 and those 1 proposed by Shao et al. (2016) and Darwin and Dolan (2021) , along with the anchorage provisions in Chapter 17 of ACI 318-19. These comparisons are based on tests of 178 beam-column joint specimens containing headed bars with bearing areas between 3.8 to 9.4 times the area of the bar. In comparisons with the anchorage provisions in Chapter 17 of ACI 318-19, three modes of failure were checked–breakout, side-face blowout, and strength of the anchor reinforcement. Forty of the specimens (18 without confining reinforcement and 22 with confining reinforcement), had a ratio of effective depth of the beam to embedment length of 1.5 or more. In addition to the design provisions for development and anchorage, test results for these specimens are compared with strengths based on the strut-and-tie method in ACI 318-19. Finally, a granular micromechanics model and associated model for reinforcing steel-concrete interaction are evaluated for their general applicability for use in finite element modeling of anchorage of headed and straight reinforcing bars to concrete. A key point in the evaluation is to determine the importance of representing the local interaction between deformed bars and the surrounding concrete, which is not represented in this case, to obtain a fully objective model. Finite element results are compared with those from tests of headed bars embedded in slabs and straight bars embedded in concrete blocks. The strength of the CCT node specimens was limited by anchorage failure, either side-face blowout for headed bars or pullout for straight bars. Anchorage type (headed bars and straight bars) had a minimal effect on initial load-deflection behavior, but did affect strength. The descriptive equation developed for the anchorage strength of headed bars in beam-column joints is very conservative (test-to-calculate ratios ranging from 1.37 to 2.68 with an average of 2.05 for the current study test-to-calculated ratios ranging from 1.67 to 2.21 with an average of 1.89 for a study by Thompson 2006a) for headed bars in CCT nodes that have a compressive force placed perpendicular to the bar, as is the descriptive equation developed by ACI Committee 408 (ACI 408R-03) for straight bars (test-to-calculated ratios ranging from 1.72 to 2.76 with an average of 2.25). The provisions in ACI 318-14 for the development length of headed bars do not accurately estimate the anchorage strength of headed bars with high steel strength or concrete compressive strength. The equation, however, is generally conservative. The development length design provisions proposed by Shao et al. (2016) can be safely used for the design of the development length of headed bars for steel strengths at least up to 120 ksi and concrete compressive strengths at least up to 16,000 psi, while those in ACI 318-19 for headed bars do not fully capture the effects of confining reinforcement, bar spacing, and concrete compressive strength for compressive strengths above 6000 psi. The provisions proposed by Darwin and Dolan (2021) accurately reflect the anchorage strength of headed bars and provide a similar level of accuracy as that provided by those proposed by Shao et al. (2016). The strut-and-tie method in ACI 318-19 should be used to design joints with ratios of effective depth to embedment length of 1.5 or greater. The anchorage provisions in ACI 318-19 are very conservative when compared to any of the other methods evaluated in this study and, if used, would lead to nearly unbuildable designs. The granular micromechanics model and associated model from reinforcing steel-concrete interaction provide a good representation of the anchorage strength in cases where behavior of these specimens is dominated by the compressive and tensile properties of concrete, which are well represented by the granular micromechanics model. The combined model, however, does not provide a good representation of the behavior from members with strength that depends on splitting of concrete caused by to slip of the bar. Lack of representation of the local interaction between deformed bars and the surrounding concrete prevents the model from being generally applicable for use in representing reinforced concrete members, especially in cases where strength is governed by bond between straight reinforcing steel and concrete.Electric Power Research InstituteConcrete Reinforcing Steel Institute Education and Research FoundationBarSplice Products, IncorporatedHeaded Reinforcement CorporationLENTON® products from Pentair

Bond Strength of Reinforcing Bars with Deformation Spacings that Exceed Maximum Specified in ASTM A615

The bond strength of two sets of No. 7 reinforcing bars was evaluated in accordance with ASTM A944. One set satisfied the criterion for maximum deformation spacing specified in ASTM A615, while the other had deformations that exceeded the maximum spacing. All bars exceeded the requirements for minimum deformation height. Research related to the effect of deformation properties on bond strength, including the research used to establish the requirements for deformations in ASTM A615, is also reviewed. The test results match earlier research and demonstrate that (1) the bond strength of the bars with deformation spacings that exceed those specified in ASTM A615 is similar to the bond strength of the bars that meet the specification, and (2) the differences in bond strength observed in the tests are not statistically significant. The bars tested in this study with deformation spacings that exceed those specified in ASTM A615 will provide satisfactory bond performance and can be used in all concrete construction

Anchorage of Conventional and High-Strength Headed Reinforcing Bars

Headed bars are often used to anchor reinforcing steel as a means of reducing congestion where member geometry precludes adequate anchorage with a straight bar. Currently, limited data on the behavior of headed bars are available, with no data on high-strength steel or high-strength concrete. Due to a lack of information, current design provisions for development length of headed reinforcing bars in ACI 318-14 limit the yield strength of headed reinforcing steel to 60,000 psi and the concrete compressive strength for calculating development length to 6,000 psi. Current design provisions for developing headed bars in ACI 349-13, which are based on ACI 318-08, apply the same limits on the material strengths (60,000 psi and 6,000 psi, respectively, for headed bars and concrete). These limits restrict the use of headed bars and prevent the full benefits of higher-strength reinforcing steel and concrete from being realized. The purpose of this study was to establish the primary factors that affect the development length of headed bars and to develop new design guidelines for development length that allow higher strength steel and concrete to be utilized. A total of 233 specimens were tested, with four specimen types used to evaluate heads across a variety of applications. Two hundred two beam-column joint specimens, 10 beam specimens with headed bars anchored near the support in regions that are known as compression-compression-tension (CCT nodes, 15 shallow embedment specimens (each containing one to three headed bars for a total of 32 tests), and 6 splice specimens were evaluated. No. 5, No. 6, No. 8, and No. 11 bars were evaluated to cover the range of headed bar sizes commonly used in practice. Concrete compressive strengths ranged from 3,960 to 16,030 psi. A range of headed bar sizes, with net bearing areas between 3.8 and 14.9 times the area of the bar, were also investigated. Some of these heads had obstructions larger than allowed under current Code requirements. In addition, the amount of confining reinforcement, number of heads in a specimen, spacing between heads, and embedment length were evaluated in this study. The results of this study show that provisions in ACI 318-14 and ACI 349-13 do not accurately account for the effect of bar size, compressive strength, or the spacing of headed bars in a joint. The effect of concrete compressive strength on the development length of headed bars is accurately represented by concrete strength raised to the 0.25 power, not the 0.5 power currently used in the ACI provisions. Confining reinforcement increases the anchorage strength of headed bars in proportion to the amount of confining reinforcement per headed bar being developed. Headed bars with obstructions not meeting the Class HA head requirements of ASTM A970 (heads permitted by ACI 318-14 and ACI 349-13) perform similarly to HA heads, provided the unobstructed bearing area of the head is at least 4.5 times the area of the bar. Headed bars exhibit a reduction in capacity for values of center-to-center spacing less than eight bar diameters. These results are used to develop descriptive equations for anchorage strength that cover a broad range of material strengths and member properties. The equations are used to formulate design provisions for development length that safely allow for the use of headed reinforcing bars for steels with yield strengths up to 120,000 psi and concretes with compressive strengths up to 16,000 psi. Adoption of the proposed provisions will significantly improve the constructability and economy of nuclear power plants and other building structures.Electric Power Research Institute, Concrete Steel Reinforcing Institute Education and Research Foundation, BarSplice Products, Incorporated, Headed Reinforcement Corporation, and LENTON® products from Pentair