Reinforcement Stability and Fracture of Cover Concrete in Reinforced Concrete Members

Abstract

ABSTRACT:The main aim of this study is to propose simple and reliable method to predict the buckling length of longitudinal reinforcing bars and also to predict the spalling of cover concrete in reinforced concrete members. Stability analysis is conducted giving due consideration to both geometrical and mechanical properties of the longitudinal reinforcing bars and lateral ties. The tie stiffness required to hold longitudinal reinforcing bars in different buckling modes is derived from energy principles, and it is compared with actual tie stiffness to determine the stable buckling mode. The buckling length is computed as the product of the stable buckling mode and the tie spacing. The proposed buckling length determination method is experimentally verified for various cases. A design method for lateral ties to avoid buckling induced strength degradation is also recommended. The effect of lateral deformation of longitudinal bars is quantitatively evaluated and incorporated in the simulation of cover concrete spalling. Analytical prediction considering spalling and buckling according to the proposed methods showed better agreement with the experimental result.Key words: buckling length, cover concrete, lateral deformation, lateral ties, reinforcing bar, spalling, stable buckling mode, stiffness INTRODUCTIONDuring earthquakes, reinforced concrete members may experience significant lateral deformation of the longitudinal reinforcing bars accompanied by spalling of cover concrete due to large compressive strain. Analytical models neglecting these inelastic material mechanisms cannot capture the post-peak softening behavior accurately, and will consequently overestimate the response ductility (Suda et al. 1996). Hence, average stress-strain relationships of concrete and reinforcing bar including spalling and buckling mechanisms are needed.It is commonly assumed that the behaviors of reinforcing bar in tension and in compression are similar. In reality, average behavior in compression is different from that in tension. This difference is mainly attributed to the geometrical nonlinearity associated with large lateral deformation of buckled reinforcing bars. Various average compressive stress-strain relationships including buckling (Monti and Nuti 1992; Gomes and Appleton 1997; Rodriguez et al. 1999) have been proposed based on experimental and/or analytical studies of bare bar under axial compression. All of these relationships implicitly or explicitly suggest that the average compressive response of bare bar is a function of length to diameter ratio. For practical application in reinforced concrete members with a system of lateral ties, the bar length used in such bare bar constitutive relations should be replaced with the buckling length of longitudinal reinforcing bar. Hence, the potential buckling length should be pre-determined considering geometrical and mechanical properties of reinforcing bars and cover concrete spalling that may also affect the reinforcement stability.Longitudinal reinforcing bars inside RC mem...

Modeling of Reinforcement Buckling in RC Columns Confined with FRP

“…r by , which is defined as the ratio of the length of region l by , where the shear reinforcements yielded, to the buckling length l b , ranged from 0.45 to 0.76 for the test specimens. This experimental observation supports the theoretical validity of the energy approach to the buckling phenomenon (Scribner 1986;Dhakal and Maekawa 2002).…”

“…(4) and (5). The experimental work indicated, however, that the lateral mild steel reinforcement yielded at the beginning of buckling although the reinforcement with high strength over 1,300 MPa did not yield (Sato and Ko 2007;Dhakal and Maekawa 2002). Hence, the moduli E w1 of the lateral reinforcement made of mild steel will be corrected in Section 3.…”

“…This criterion is rational and could be applicable to a column confined with an FRP sheet if some modification were provided. From the viewpoint of analytical practice, however, the cover-spalling criterion may not always be the most suitable for the following two reasons: (i) The cover-separation criterion used in previous research (Dhakal and Maekawa 2002) is considered only in the monotonic case. In reality, the cover concrete is subjected to both large tensile straining and high compressive stressing during cyclic loadings.…”

Elongation of Plastic Hinges in Ductile RC Members: Model Development

“…The cyclic constitutive model for steel springs is based on path-dependent averaged stress-strain relationship developed for reinforcing steel (Dhakal and Maekawa 2002a). Although the original steel cyclic model accounted for buckling of reinforcing bars inside RC members Maekawa 2002b and2002c), buckling related features were switched off for the steel springs in the plastic hinge element. This is justified because incorporating buckling renders the model more complex without adding significant value to it because as shown in Fig.…”

Numerical modeling of rebar buckling in reinforced concrete members using truss analogy

“…However, buckling length can span within a few times stirrup spacing as a result of global buckling. Dhakal and Maekawa [20] proposed the determination of buckling length of longitudinal rebars for RC rectangular sections based on energy method. Determination of buckling mode and buckling length of reinforcing bars are summarized in Figure 6.…”