Panos D. Kiousis

Behavior of cemented sands—I. Testing

This paper is accompanied by a study on constitutive modelling issues of cemented sands. The concentration here is on experimental issues related to the triaxial testing of cemented sands. A preliminary investigation is performed aiming to identify potential effects of specimen size and slenderness on the stress–strain–strength characteristics of cemented sands. A comprehensive experimental study follows where clean sand specimens, as well as specimens with 2, 4 and 6 per cent cement content, are tested. The aim of the study is to examine the effects of cement content and confinement on the shear strength, stiffness, softening and dilation characteristics of cemented sand.

CONFINEMENT EFFECTS ON HIGH-STRENGTH CONCRETE

This paper describes an experimental investigation conducted on the stress-strain characteristics of steel sleeve confined high-strength concrete. The axial load and strains of concrete, and the axial and hoop strains of the confining steel sleeves were measured. From these measurements, accurate stress-strain relations of the concrete core were produced, along with confinement calculations based on von-Mises elastoplastic response of the steel sleeves. Confinements ranging from 5-19 MPa were calculated. This confinement had a profound effect on the concrete strength, as much as tripling its unconfined strength of 70 MPa. The increase in ductility was found to develop slower for low amounts of confining steel due to a lagging development of confining pressure. These findings are discussed.

Behavior of cemented sands—II. Modelling

This is the companion paper to a study the triaxial testing of cemented sands. The focus here is turned to the constitutive modelling of cemented sands. A novel micromechanical approach that considers the multiphase nature of cemented sands, is presented in which the clean sand, the cementing bond and the pore water pressure are modelled independently. The model is verified using a series of triaxial compression experiments on 2, 4 and 6 per cent cemented specimens, that were the subject of the companion paper.

Stress rate and the lagrangian formulation of the finite-strain plasticity for a von mises kinematic hardening model

Abstract The use of the Jaumann stress rate, in kinematic hardening finite-strain plasticity, in the case of simple shear generates oscillations in the stress field. Alternate theories have been introduced to define other stress rates by making use of the polar decomposition and consequently producing increasing shear stress. In this work, a Lagrangian formulation is introduced which, for the case of simple shear produces monotonically increasing stress-strain relationships. The yield criterion is originally expressed in terms of the Cauchy stress and subsequently transformed to the Lagrangian reference frame. The associated flow rule used here preserves the normality rule in the second Piola-Kirchhoff stress space and is equivalent to that of the Cauchy stress space. This approach preserves the accuracy of the interpretation of the material behavior in the Eulcrian reference frame and it also bypasses the use of the Jaumann stress rate in the formulation of kinematic hardening finite-strain plasticity.

Effects of Recycled Concrete Aggregates on the Compressive and Shear Strength of High-Strength Self-Consolidating Concrete

The effectiveness of internal curing of high-strength self-consolidating concrete (SCC) using saturated recycled-concrete aggregates is examined in this study. Tests were performed on SCC using natural rock aggregate (NR-SCC) and recycled-concrete aggregate (RC-SCC). Fresh concrete tests included slump-flow, slump-flow with a J-ring, and L-box tests. All mixes tested were found to be highly flowable and stable. Strength tests included compression loading of cylinders and pushoff specimens. The RC-SCC was found to have superior compressive and frictional characteristics, indicating the beneficial effects of internal curing. However, the unconfined shear strength of NR-SCC was found to be superior to that of the RC-SCC. This was attributed to the fact that high-strength concretes fail in shear in planes that go through the aggregate rather than around it. The higher shear strength of the natural rock aggregate resulted in superior unconfined shear strength of the NR-SCC. However, the superior frictional characteristics of RC-SCC resulted in a reversal of this observation for the clamped (confined) specimens.

Analysis of Residual Stresses in Cylindrically Anisotropic Materials

Metal-forming operations leave residual stresses in formed parts due to nonuniform deformation occurring during the process. An exact method of determining the longitudinal, radial and circumferential (tangential) residual stresses in axisymmetric specimens was proposed by Mesnager1 and further developed by Sachs2. The boring-out technique can be complemented by a similar procedure in which strains are measured on the inner surface of the tube when material is removed from the outer surface.The work proposed in this paper extends previous analyses of residual stresses to the case where the material exhibits cylindrical elastic anisotropy, i.e., the principal axes of anisotropy correspond to the longitudinal, radial and circum-ferential directions of the tube. In addition, the present analysis considers the case in which a residual-shear stress, developed by twisting the tube about its axis, exists in the tube. When such shearing stresses are present, the principal axes of the residual-stress distribution are not parallel to the principal axes of the tube.

Truss Modeling of Concrete Columns in Compression

A truss-based simulation of reinforced concrete columns under compression is presented in this study. The concrete truss elements are modeled based on advanced constitutive equations that take into consideration confinement dependent hardening followed by softening. The computational advantages, as well as the challenges of this approach, are discussed. The effects of confining steel content and spacing on the failure mechanisms of concrete columns under compression are examined using a parametric analysis. This analysis verifies the well-established trends that strength and ductility increase with increasing transverse steel content and decreasing tie spacing. It is also found that improvements on strength and ductility have limits. Finally, it is found that the amount and spacing of transverse reinforcement influences the type of failure, which can be one of shear or concrete crushing.

Design of low residual stress joints between 3D CC composites and Glidcop copper for divertor applications

Abstract An innovative approach to modeling the interfacial residual stress between CC Composites and Glidcop copper is proposed. The model predictions were consistent with experimental observations in ceramic-metal joints where it was easier to visualize cracking, and later extended to carbon-Glidcop copper joints. A variety of interface designs were investigated. The numerical predictions were consistent with experimental results in the Glidcop-ISO63 graphite system but less consistent in the CC composite-Glidcop copper system. This inconsistency is presently attributed to the inhomogeneous nature of CC composite in contrast to ISO63 graphite or Si 3 N 4 ceramic. A joint design based on the numerical results, comprising of grooves in the Glidcop copper and metallic interlayers appeared to alleviate the residual stresses sufficiently enough to allow bonding between CC composite and Glidcop copper.

A note on the reference frame indifference

SummaryIn the recent years a controversy has appeared in the literature in reference to the frame indifference of certain tensorial expressions. In this paper a reexamination of the definition of frame indifference, as it is expressed mathematically, is presented. It is emphasized that frame indifference cannot be examined before a choice of reference frame is made. Finally, a number of frame indifferent constitutive equations are given to illustrate the concepts presented here.

Finite-Element Analyses of Mechanically Stabilized Earth Walls Subjected to Midlevel Seismic Loads

The objective is to examine the performance of specific detailing components of mechanically stabilized Earth (MSE) walls when subjected to midlevel seismic excitations, such as those expected in the State of Colorado. The motivation for this study is the elevated peak ground accelerations mandated by the 2007 4th Edition of AASHTO LRFD bridge design specifications. According to this revision, highway-related projects must be designed for an elevated 1,000-year return period earthquake, as opposed to the earlier editions’ 500-year return period earthquake. Finite-element analyses are performed using LS-DYNA to examine the displacement-based, dynamic behavior of individual MSE wall components, such as geogrid reinforcement and wall facings. Walls at two heights, 4.57 m (15.0 ft) and 9.14 m (30.0 ft), with two types of facings (modular block and segmental panel walls), reinforced using geogrids, are modeled based on the Colorado Department of Transportation drawings. These walls are subjected to three synthetic earthquake motions generated by the USGS 2002 deaggregation tool for three sites spread across the geographical extent of the State of Colorado. The results show that typical MSE walls perform well with respect to connection details when subjected to midlevel seismic loads.