Jung-Yoon Lee

RATIONAL SHEAR MODULUS FOR SMEARED-CRACK ANALYSIS OF REINFORCED CONCRETE

This paper presents a new rational shear modulus for the fixed-angle softened truss model (FA-STM) based on the smeared-crack concept. This rational, yet very simple, modulus not only greatly simplifies the solution of the FA-STM but also improves its accuracy. Moreover, this new shear modulus can be used widely to replace the many complicated empirical constitutive laws for shear of cracked concrete, as employed in the various nonlinear analyses of reinforced concrete structures. A notable example is the finite-element analysis.

Compressive Response of Concrete Confined with Steel Spirals and FRP Composites

This article presents the results of an experimental and analytical study on the behavior of concrete cylinders externally wrapped with fiber-reinforced polymer (FRP) composites and internally reinforced with steel spirals. The experimental work was carried out by testing twenty-four 150 × 300 mm 2 concrete cylinders subjected to pure compression with various confinement ratios and types of confining material. The test results show that the compressive response of concrete confined with both FRP and steel spirals cannot be predicted by summing the individual confinement effects obtained from FRP and steel spirals. This is largely attributable to differences in the inherent material properties of FRP and steel. A new empirical model to predict the axial stress-strain behavior of concrete confined with FRP and steel spirals is proposed. Comparisons between experimental results and theoretic predictions show agreement.

Predicting the longitudinal axial strain in the plastic hinge regions of reinforced concrete beams subjected to reversed cyclic loading

The longitudinal axial strain in the plastic hinge regions of reinforced concrete (RC) structures has a significant influence on the behavior of RC structures subjected to reversed cyclic loading. This strain affects energy dissipation in the hysteretic response by causing the sliding along interconnecting wide flexural cracks. In addition, the strain also reduces the effective compressive strength of cracked concrete of the RC members, such as coupling beams, dominated by shear action. The research reported in this paper proposes a model to predict the axial strain in the plastic hinge regions of RC beams. The proposed model includes four path types: Path 1, pre-flexural yielding or unloading region; Path 2, post-flexural yielding region; Path 3, slip region; and Path 4, repeated loading region. In addition, this paper provides an equation to predict the longitudinal axial strain in the plastic hinge regions subjected to reversed cyclic loading of various patterns. To verify the longitudinal strain in the plastic hinge region by the proposed method, twelve RC beams were tested under reversed cyclic loading. The observed longitudinal strains in the plastic hinge regions of the tested beams agreed reasonably with the calculated longitudinal strains.

Effect of Longitudinal Tensile Reinforcement Ratio and Shear Span-Depth Ratio on Minimum Shear Reinforcement in Beams

The equations in design codes for minimum shear reinforcement ratio currently give little consideration to the effects of the longitudinal reinforcement ratio and shear span-depth ratio (a /d) in avoiding abrupt shear failure. This paper presents the effects of longitudinal tensile reinforcement ratio and a/d on the minimum shear reinforcement in reinforced concrete beams. A total of 26 reinforced concrete (RC) beams having the minimum shear reinforcements required by ACI 318-05 were cast, instrumented, and tested. The test results indicate that the reserve shear (shear strength of the beam with minimum amount of shear reinforcement/shear strength of the beam without shear reinforcement) of the tested beams increased as the longitudinal tensile reinforcement ratio increased, but decreased as the a/d increased. These findings suggest that design codes should be amended to include longitudinal reinforcement ratio and a/d for determining minimum shear reinforcement.

SHEAR DEFORMATION OF REINFORCED CONCRETE BEAMS SUBJECTED TO REVERSED CYCLIC LOADING

A method is proposed to predict the ductile capacity of reinforced concrete (RC) beams that fail in shear after flexural yielding. The method considers shear strength deterioration in the plastic hinge region of RC beams. The shear contribution of the concrete in the plastic hinge region decreases after flexural yielding of the beam due to a decrease in effective compressive strength of the concrete. Shear deterioration of RC beams after flexural yielding is predicted by using a compatibility-aided truss model that takes into account the axial strain at the center of the beams cross-section. To verify the shear strength and corresponding ductility of the proposed method, 12 RC beams were tested under reversed cyclic loading. Comparisons between observed and calculated shear strengths and their corresponding ductilities of the tested beams showed reasonable agreement.

Maximum Shear Reinforcement of Reinforced Concrete Beams

The ACI 318-08 Code requires the maximum amount of shear reinforcement in reinforced concrete (RC) beams to prevent possible sudden shear failure due to over reinforcement. The design equations of the maximum amount of shear reinforcement provided by the current four design codes—ACI 318-08, CSA-04, EC2-02, and AIJ-99, differ substantially from one another. ACI 318-08, CSA-04, and EC2-02 provide an expression for the maximum amount of the shear reinforcement ratio as a function of the concrete compressive strength, but AIJ-99 does not take into account the influence of the concrete compressive strength. For high-strength concrete, the maximum amount of shear reinforcement calculated by EC2-02 and CSA-04 is much greater than that calculated by ACI 318-08. This paper presents the effects of shear reinforcement ratio and compressive strength of concrete on the maximum shear reinforcement in RC beams. Eighteen RC beams that have various shear reinforcement ratios were tested. Although designed to have considerably more shear reinforcement than that required in ACI 318-08, the test beams failed in shear after the yielding of shear reinforcement.

Shear Behavior of Reinforced Concrete Beams with High-Strength Stirrups

ACI Structural Journal, V. 108, No. 5, September-October 2011. MS No. S-2010-237 received August 3, 2010, and reviewed under Institute publication policies. Copyright

Torsional Strength of RC Beams Considering Tension Stiffening Effect

Stress-strain relationship for a steel bar embedded in concrete differs somewhat to that of a bare steel bar because the surrounding concrete is bonded to the bar. This behavior is known as tension stiffening. This paper presents the results of an analytical and experimental study on the performance of reinforced concrete beams subjected to pure torsion. In particular, the effect of the tension stiffening was discussed and included in the analytical study. Nine RC beams having different torsional reinforcements were tested. Although the torsional strength of RC beams according to the existing design codes (ACI 318-05, EC2, and JSCE-02) depends on neither the average yield stress of steel bars nor the tension stiffening effect, the test results indicated that the steel stress of the beams at peak load increased as the total percentage of reinforcement decreased due to the tension stiffening effect. A new equation including tension stiffening effect was proposed to predict the torsional moment capacities of RC beams. Comparisons between tested and calculated torsional moments of the seventy-one beams showed reasonable agreement.

Effect of transport properties of fiber types on steel reinforcement corrosion

This study investigated the transport properties of fiber types in concrete to evaluate their effect on the corrosion of steel reinforcement. The fibers used in this research are polypropylene (PP), polyvinyl alcohol (PVA), and hooked-end steel fiber (Steel). The addition of PVA fibers having relatively good resistance to transport properties indicated the best resistance to the initiation time of corrosion. On the other hand, the addition of PP fibers showed a relatively good resistance to corrosion, even though the specimen had a fast rate of absorption. The addition of hooked-end steel fibers that have the best ability to resist mass transport showed the earliest failure time. The localized corrosion effect of steel fibers from the repeated wet/dry cycles seems to have a considerable effect on the acceleration of corrosion nearby steel reinforcement. In addition, fiber types do not significantly resist further acceleration following the initiation of corrosion. It is worth noting that transport properties alone are not necessarily a good indicator of the effect of fibers on resistance to corrosion.

Nonlinear Analysis of Shear-Critical Reinforced Concrete Beams Using Fixed Angle Theory

This paper proposes a solution methodology for the application of the fixed-angle theory to predict the shear response of reinforced concrete (RC) beams subjected to the combined actions of shear and flexure. The proposed solution, based on the fixed-angle theory, takes into account the effect of flexural moment on the shear strength of RC beams and calculates the concrete constitutive relationships by transforming the concrete stresses and strains from the principal direction of concrete stresses to that of the applied stresses. To verify the effectiveness of the proposed solution method, seven shear-critical RC beams were tested and their corresponding experimental shear stress-strain relationships were compared with the predicted ones by using the proposed methodology. Furthermore, the shear strengths of 150 RC test beams, reported in the literature with various shear span-to-depth ratios, steel reinforcing ratios, and support conditions, were compared with the predicted shear strengths obtained by the proposed method and other existing truss models. The results presented in this paper show that the proposed formulation can predict the shear response of RC beams with reasonable accuracy.