Experimental and analytical investigations on bond of reinforcement and nonlinear response of reinforced concrete columns

Bond of reinforcement has a major influence on the behavior of reinforced concrete (RC) structures. Even though bond has been extensively studied over the past decades, there are still a number of knowledge gaps requiring investigation. This dissertation presents three studies related to the basic bond behavior of reinforcement and the influence of bond-slip on the seismic response of RC columns. The first investigation characterizes the bond behavior of iron-based shape-memory alloy (Fe-SMA) bars. Fe-SMA reinforcement provides new opportunities for design and strengthening of concrete structures thanks to their shape memory effect. However, there is currently little data on its bond performance. Experiments were conducted to study the bond-slip behavior of ribbed Fe-SMA bars embedded in concrete specimens with different levels of passive confinement. Fe-SMA bars presented a similar bond performance as conventional reinforcement, but their bond strength was reduced with heat activation in low-confined specimens. An analytical model is also proposed to calculate the transfer length of Fe-SMA bars in prestressing applications. The second investigation proposes nonlinear finite element models to accurately simulate the cyclic response of RC columns, including the effect of bond-slip. Concrete is modeled using a triaxial constitutive model recently proposed in the literature. A commonly used uniaxial steel model is modified to account for low-cycle fatigue rupture using a phenomenological criterion. The bond-slip behavior of bars is modeled using a zero-thickness interface element and a bond stress-slip law that predicts bond deterioration caused by generalized slip demands, tensile yielding of steel, and concrete damage. The proposed models accurately predict the cyclic response and failure of previously tested column components. The third investigation focuses on the effects of strain penetration and bond modification on the lateral response of RC columns. Two large-scale column specimens were tested under cyclic loading. One specimen had conventional reinforcement details, while the other had headed bars that were partially debonded along the footing to alleviate strain concentrations. Both specimens formed plastic hinges and failed due to rupture of longitudinal bars. The specimen with partial debonding presented less damage and a larger deformation capacity. Experimental data indicated that the debonding strategy reduced peak tensile strains, and increased bar slip and fixed-end rotations. However, it also reduced the lateral load resistance of the column. Companion finite element analyses have indicated that the reduction in peak lateral strength is caused by lower steel stress demands when concrete cover fails.Civil, Architectural, and Environmental Engineerin

Three-dimensional finite element analysis of the cyclic response of RC columns

This paper presents three-dimensional finite element models for the accurate response simulation of RC columns subjected to severe cyclic loading. The models integrate advanced constitutive models for concrete, steel reinforcement, and bond-slip behaviour of bars. Concrete is modelled with a triaxial constitutive capable of simulating material degradation due to cracking and crushing, crack opening and closing behaviour, and the increase of strength and ductility under multiaxial compression conditions. The cyclic response of steel reinforcement is modelled with a Menegotto-Pinto law modified to account for the low-cycle fatigue of bars using an energy criterion. The bond-slip behaviour of longitudinal bars is modelled using a concrete-steel interface element previously proposed by one of the authors of this paper. The interface element has a bond stress-slip constitutive law that predicts bond deterioration caused by bar slip, cyclic loading and tensile yielding of steel. The finite element models predict well the damage mechanisms and force-displacement responses of cantilever columns tested under fully-reversed lateral cyclic loading.Postprint (published version

Analytical model for bar slip and pullout capacity of straight bars

Publisher Copyright: Copyright © 2019, American Concrete Institute. All rights reserved,This paper presents an analytical model to determine the bar slip and pullout capacity of straight bars embedded in well-confined concrete. The model provides a closed-form solution of the slip experienced at the loaded end of a bar, which can be used to compute the end rotation of flexural members. The slip equations are obtained from the strain penetration resulting from a set of predefined bond stress distributions along the embedment length of a bar. For bars with relatively short lengths, the slip at the unloaded end is also considered. Additionally, the model can inform the pullout capacity of a bar. Its accuracy in predicting the loaded-end slip, strains distributions, and pullout strength has been verified with experimental data. Model predictions have been also compared with results obtained with two similar models available in the literature. Finally, the model has been used to determine the base rotations and lateral displacements of hinging columns.Peer reviewe

Experimental study on bond of ribbed Fe-SMA bars in concrete

This paper presents an experimental study on the bond strength and bond-slip behavior of ribbed Fe-SMA bars embedded in concrete. A total of 19 pullout tests were conducted on 16-mm Fe-SMA bars embedded in concrete specimens fabricated with different levels of passive confinement. The bars had been pre-strained at 4% or 8% and were activated by resistive heating 28 days after concrete casting. Different activation temperature conditions were studied: 160℃, 300℃, and no heating. A limited number of pullout tests were also conducted on conventional steel bars for comparison. Pullout specimens with low confinement failed by splitting of concrete, while well-confined specimens presented pullout failures and higher bond strengths. Fe-SMA bars activated at 160℃ in well-confined specimens had a bond strength 15% lower than that of the control steel bars and a very similar bond stress-slip response. The bond strength of Fe-SMA bars embedded in low-confined specimens was significantly reduced with heat activation. It is concluded that with sufficient passive confinement, the overall bond performance of the Fe-SMA bars is adequate and comparable to that of conventional steel bars.Postprint (published version

Three-dimensional finite element modeling of RC columns subjected to cyclic lateral loading

This paper presents a detailed finite element modeling scheme to simulate the cyclic response of RC columns considering low-cycle fatigue and bond-slip of reinforcement. The modeling scheme includes a triaxial constitutive model recently proposed in the literature to simulate concrete failure under cyclic loading. For steel, a commonly used uniaxial model is enhanced to account for bar rupture using a new low-cycle fatigue criterion, which has been validated with data from fatigue tests on reinforcing bars. The bond-slip behavior of vertical reinforcing bars is modeled using a zero-thickness concrete-steel interface element. The interface element has a bond stress-slip constitutive law that predicts bond deterioration caused by generalized slip demands, tensile yielding of steel, and compression damage in concrete. The finite element models are validated with experimental data from cyclic tests on large-scale column and pile specimens which exhibited flexure-dominated responses. The models accurately simulate the lateral force–displacement response and failure of the specimens, and provide peak tensile strain demands along longitudinal bars which are similar to those measured experimentally. Sensitivity analyses are also conducted to study the effect of modeling assumptions related to low-cycle fatigue and bond-slip behavior.Peer ReviewedPostprint (author's final draft

Experimental evaluation of bar slip and strain penetration in reinforced concrete columns

An experimental study on the effects strain penetration on the lateral performance of reinforced concrete columns is presented. Two cantilever columns with a diameter of 610 mm and an aspect ratio of 5.5 were tested under quasi-static cyclic loading. The specimens were nominally identical except for the anchorage of the column longitudinal steel in the adjacent footing. The first column had straight bar anchorages, while the second column had headed bars which were partially debonded along the footing to increase the effects of strain penetration and alleviate tensile strains in vertical steel. Optical deformation techniques were employed to monitor bar slip and base rotations during testing. Both columns presented a ductile response and failed by rupture of the vertical bars. The base rotation in the column with fully-bonded straight bars accounted for 25% to 35% of the total drift. In the column with partially-debonded headed bars, bar slip at the base of the column was more than doubled and base rotations accounted for 40% to 60% of the total drift. Partial debonding also resulted in more limited concrete damage and a larger deformation capacity. Experimental observations are complemented with nonlinear finite element analysis of the tests.Postprint (published version

Synthesis of Precast Column Designs for Texas Bridges [Project Summary]

0-6978Using prefabricated bridge elements and systems minimizes on-site operations and closure times during bridge construction and contributes to improving durability and reducing the environmental impact of construction. Prefabrication of bridge columns has been very limited as compared to bridge superstructures and bent caps. Nevertheless, some states have started to develop and implement design concepts for precast concrete columns

Predictors of drug‐resistant epilepsy in childhood epilepsy syndromes: A subgroup analysis from a prospective cohort study

International audienceObjective: Previous studies assessing factors associated with drug-resistant epilepsy (DRE) were constrained by their amalgamation of all epilepsy syndromes in their analyses and the absence of uniform criteria for defining DRE. Our objective was to identify predictors of DRE among the four primary childhood epilepsy syndrome groups within a cohort of children with new onset seizures, using the International League Against Epilepsy (ILAE) definition of DRE and the recent classification of epilepsies.Methods: This is a prospective study of 676 children with new onset seizures initiated on antiseizure medication. Patients were monitored for the occurrence of DRE according to the ILAE criteria and were categorized into one of four epilepsy groups: self-limited focal epilepsies (SeLFEs), genetic generalized epilepsies (GGEs), developmental epileptic encephalopathies (DEEs), and focal epilepsies.Cox regression analysis was performed to identify predictors of DRE within each epilepsy group.</div