Top PDF A Plasticity Model to Predict the Effects of Confinement on Concrete

A Plasticity Model to Predict the Effects of Confinement on Concrete

A Plasticity Model to Predict the Effects of Confinement on Concrete

An existing finite element (FE) program, written by John F. Hall, is adapted for use with this model. The original program was a linearly elastic, static analysis FE program for solid and structural mechanics. It was designed to solve the equation [K]{x} = {f} for {x} given a linear elastic stiffness matrix and constant force values. It contained only linear springs and four node isotropic plane stress or strain elements. Modifications of the program are necessary to model reinforced concrete columns. The first step is to add or modify elements to be used for representing reinforced concrete columns confined by steel rebar or fiber reinforced polymer (FRP) sheets. Capabilities for six node tetrahedral and eight node rectangular brick elements are added to the existing program; both using an eight point integration scheme. Each node of the solid element has three translational degrees of freedom. The solid ele- ments are to be used for modeling the concrete. The material model defined in this thesis is implemented into the newly added solid elements. Three material parameters need to be provided as input to these solid elements: the modulus of elasticity, E; Poisson’s ratio, ν; and the unconfined compressive strength, f c 0 . Further, the linear spring element is modified to be bilinear elastic. The loading behavior of the spring element is bilinear, and the unloading behavior is along the current stiffness value, not the initial stiffness value. Four input quantities are required for the spring el- ements: the initial modulus of elasticity, E; area, A; yield stress, f y ; and the yield
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A simple and efficient plasticity-fracture constitutive model for confined concrete

A simple and efficient plasticity-fracture constitutive model for confined concrete

A plasticity-fracture constitutive model is presented for prediction of the behavior of confined plain concrete. A three-parameter yield surface is used to define the elastic limit. Volumetric plastic strain is defined as hardening parameter, which together with a nonlinear plastic potential forms a non-associated flow rule. The use of non-associated flow rule improves the prediction of the dilation behavior of concrete under compressive loading. To model the softening behavior, a fracture energy-based function is used to describe strength degradation in post-peak regime. The Euler-forward algorithm is used to integrate the constitutive equations. The proposed model is validated against the results of triaxial compressive experiments. Finally, the behavior of plain concrete confined by layers of carbon fiber reinforced polymer is studied to show capability of the model for passive confinement.
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Partial Confinement Utilization for Rectangular Concrete Columns Subjected to Biaxial Bending and Axial Compression

Partial Confinement Utilization for Rectangular Concrete Columns Subjected to Biaxial Bending and Axial Compression

In this study, a partial confinement model is developed for rectangular reinforced concrete column sections under general eccentric loading. The model realizes an inverse correlation between the compression zone to the entire section ratio and the eccentricity of the axial compression force due to biaxial moment resultant. Accordingly, the partially confined strength of eccentric loading is morphed between the fully confined case under pure axial com- pression and the unconfined case under pure bending. Therefore, incrementing the resultant moment and the axial compression takes place proportionally through radial loading to sustain constant eccentricity throughout the loading until failure. The uniaxial moment–axial compres- sion versus uniaxial curvature–axial strain relationship is extended, within the framework of the moment of area concept, from a 2 9 2 to a 3 9 3 stiffness matrix in the case of biaxial bending. The non-linear numerical proce- dure introduced successfully-predicted the confined capac- ity of rectangular reinforced concrete columns. The generalized moment of area concept is benchmarked against experimental data, to verify its reliability in pro- viding accurate predictions. The partial confinement effects
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Mathematical Model to Predict the Permeability of Water Transport in Concrete Structure

Mathematical Model to Predict the Permeability of Water Transport in Concrete Structure

Long-lasting structure is very important if it is to survive the harsh environment that it is often exposed to [8]. In Nigeria, where there is no maintenance culture, it is all the more imperative. Nigerian hot marine coastal waters, constitute an aggressive environment that has been found to be deleterious to concrete [9], leading to premature deterioration that affects the strength and durability characteristics of concrete structures. One of the major forms of chemical attack on concrete is the chloride ingress. This ingress leads to corrosion of reinforcement, reduction in strength, unserviceable structures, and structures that are aesthetically poor. According to Stanish [8], corrosion products put surrounding concrete in tension thereby causing tension cracking and spalling of the cover of concrete. The attendant adverse structural influences are: loss of bond between the reinforcement and concrete, loss of steel area, and loss of stiffness. The total effects of these are serious durability problems because of reductions in the strength, serviceability and aesthetics of concrete structures.
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A confinement dependent load induced thermal strain constitutive model for concrete subjected to temperatures up to 500°C

A confinement dependent load induced thermal strain constitutive model for concrete subjected to temperatures up to 500°C

Commonly, 3D LITS models are based on the assumption of the superimposition principle [10–15]; in other words it is assumed that the LITS state that develops in the case of multiaxial confinement equals the sum of the LITS states corresponding to each compressive stress component applied individually. However, the few multiaxial tests that are available in the literature show that the situation is more complex and LITS depends on the confinement of the stress state [8,16]. For this reason, a novel approach for modelling LITS as a stress-confinement strain component has been presented by the authors in [17]. The model was validated for temperatures up to 250°C, temperatures representative of nuclear Prestressed Concrete Pressure Vessels (PCPVs) subjected to partial fault of their water cooling system. The validation studies were performed using experimental data from Petkovski and Crouch [8]. This work showed that accounting for the effects of stress confinement was important for accurately calculating stress-states in bulk concrete structures such as PCPVs used in nuclear power stations. In the present paper, the 3D extension method proposed by the authors in [17] is adopted for formulating a LITS model able to capture the behaviour of multiaxially confined concrete subjected to transient temperatures up to 500°C, temperatures representative of nuclear PCPVs subjected to complete fault of their water cooling system. The approach is validated and discussed in the light of the experimental studies by Kordina et al. [16].
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A physically motivated element deletion criteria for the concrete damage plasticity model

A physically motivated element deletion criteria for the concrete damage plasticity model

Figures 9 and 10 show the confinement stress dependent plastic strain cutoff value in compression and the strain rate dependent cracking displacement cutoff value in tension, respectively. Both curves are constructed based on a choice of cohesive stress threshold value of 1Pa. This choice of threshold value is small enough for the element under consideration to be able to dissipate more than 99.99% of its energy before being removed. Notice, however, that in case of element deletion in a compression state, the cutoff strain has been given an upper bound of 40%. This upper bound is necessary to avoid deformations that are so large that the explicit time integration solver fails at some point during the impact simulation.
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Splitting of concrete cover in steel fiber reinforced concrete: Semi-empirical modeling and minimum confinement requirements

Splitting of concrete cover in steel fiber reinforced concrete: Semi-empirical modeling and minimum confinement requirements

Splitting probability surfaces left and minimum C/D values to avoid splitting failure, for 45-MPa concrete reinforced with fibers of slenderness 65.... Function to relate min C/D to comp[r]

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Improvement of ductility in shear failures of reinforced concrete deep beams by diagonal confinement.

Improvement of ductility in shear failures of reinforced concrete deep beams by diagonal confinement.

The vast majority of structural members in reinforced concrete have to resist shearing forces. The shear forces seldom act on their own but rather in combination with flexure, axial loads and torsion. The behavior of reinforced concrete beams at failure in shear is distinctly different from their behavior in flexure. Shear capacity of an RC beam is defined as the maximum shear force that can be sustained by a critical section of the beam. Though there are different mechanisms that govern shear failure in beams, it is confirmed that shear transfer in reinforced concrete beams relies heavily on both tensile and compressive strength of concrete. Shear reinforcement is provided in order to carry that portion of the shear force which cannot be sustained by concrete alone. The failure of member in shear is relatively more brittle. Consequently an attempt must be made to suppress such drastic failures.
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Design of 300 Years Durable Concrete for Nuclear Confinement Zone and Casting of Tunnel Form with Self Compacting Concrete

Design of 300 Years Durable Concrete for Nuclear Confinement Zone and Casting of Tunnel Form with Self Compacting Concrete

---------------------------------------------------------------------***--------------------------------------------------------------------- Abstract - In this modern era, civil engineers and architectures build many innovative monumental structures. As these structures are constructed by using conventional concrete, the durability of the structure could not be more. Even though the structural works are stunning at the time, their durability will be very low when compared to the efforts put on those structures. Hence a technical revolution has to be made to overcome the durability problem. Self-Compacting Concrete is a flowable concrete which can compact under its own weight throughout the formwork and need not any vibrator. SCC is used in the reinforcement congested area where the manual compaction can not be adopted. Adopting these properties of a concrete, we moved further to produce a high durable concrete which can last for nearly 300 years. In SCC, displacement of aggregates and water due to external compaction is avoided. Hence chances of air-entrapment and honey-combing will be reduced. By reducing water cement ratio and using ultra-fine materials, we are producing a concrete which can last for 300 years and the durability can be of that concrete can be checked as a prediction by using Life-365 software.
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Effect of Confinement on Load – Moment Interaction Behavior of Reinforced Concrete Column

Effect of Confinement on Load – Moment Interaction Behavior of Reinforced Concrete Column

The effects of confinement directly influence the safe and magnitude of stress-strain curve of concrete. This in turn will affect the compressive force per unit width of concrete. The increase in compressive force of concrete will automatically improve the nominal capacity of column subjected to axial force and bending moment, or in other words, the interaction diagram of the column is enlarged. In the present study the effect of lateral confinement on reinforced concrete column is investigated in terms of load- moment interaction curve (including the partial safety factors). The lode-moment interaction curves are developed by considering above confined stress-strain models and compared with the lode-moment interaction curve for unconfined concrete as per the IS code.
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Development of Scheffe’s Model to Predict the Compressive Strength of Concrete using SDA as Partial Replacement for Fine Aggregate

Development of Scheffe’s Model to Predict the Compressive Strength of Concrete using SDA as Partial Replacement for Fine Aggregate

emitted for every 600kg of cement produced. They therefore replaced 10% of cement with SDA which did not negatively affect the chloride permeability and thaw resistance of the concrete, but decreased the drying shrinkage, and increased the water absorption. This also established the pozzolanic ability of SDA. Similarly, [6] found in their research that replacing 5 to 15% cement content with saw dust increased the compressive, flexural, and split tensile strengths of the concrete for 28days curing period and beyond. It also decreased the weight and cost. However, fewer researchers such as, [7] have carried out research work on replacement of fine aggregates with SDA. Their research findings revealed that 10% replacement of fine aggregate with SDA will result in an acceptable tensile, flexural, and compressive strengths as well as reduce the amount of wastes in the environment. SDA has different particles that are mostly angular in shape. According to [7] SDA has a specific gravity of 2.5, fineness modulus of 1.78, water absorption of 0.56%, and bulk dry density of 1300kg/m 3 as against sand with specific gravity of 2.65, fineness modulus of 2.21, water absorption of 0.45%, and bulk dry density of 1512 kg/m 3 . When 10% of SDA is added to the sand, these properties became 2.67, 2.2, 0.5%, and 1436kg/m 3 for specific
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Dynamic Modular Networks Model Mediated by Confinement

Dynamic Modular Networks Model Mediated by Confinement

The rewiring phase increases the link density inside the confined space. As the number of links is conserved in the whole network, nodes outside P tend to have their connec- tivity degree reduced, eventually reaching k = 0. Therefore, the outside confinement sub-graph may eventually fragment, decreasing the size of the largest connected compo- nent (LCC). This fragmentation is counterbalanced by the constant influx and outflux of random nodes, so an orphaned node or a separated cluster may be reconnected to other nodes, when confined to P . In order to follow and characterize the level of fragmentation of the network, we measure the time evolution and stable states of the LCC and show the results in Fig. 8.
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A model to predict the reflectance from a concrete surface as a function of the sun-object-image angular relationship

A model to predict the reflectance from a concrete surface as a function of the sun-object-image angular relationship

E Uncorrected Versus Corrected Plots 23 Measurement Positions Within Projected Spot of = aR +6 Size Plot Plot of 26 of Measured Voltages Versus Percent of 84 Standardized Residuals Versu[r]

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Gradient   Plasticity Approach To Size Effects

Gradient Plasticity Approach To Size Effects

Under non-uniform stress distributions the phenomenon of increased yield stress (sometimes called "delayed yielding") compared to the yield in quasi-homogeneous stress distributions (tensile tests) has attracted considerable attention in the past; a review is contained in Ref. [1]. It is observed especially for steels with a pronounced upper and lower yield stress, and it has been related to the formation of flow layers of finite thickness or to a supporting effect of the material neighborhood of stress peaks, their thickness characterized by an intrinsic material length. These interpretations imply a size influence on the initiation of yielding of geometrically similar specimens under macroscopically non-uniform stress distributions such that small specimens should have a greater resistance against yield initiation than larger ones. This effect has been observed in experiments for thick cylinders under internal pressure, beams with circular or rectangular cross-sections under pure bending, torsion of circular rods, flat strips with a central circular bore-hole under tension, and indentation tests; for a review see Ref. [1]. Typical dimensions (diameter or depth) ranged from a few mm (<5 mm) to dimensions not larger than 10-times. Qualitatively, all tests showed a decrease of the yield stress when the size is increased. Size effects were also studied recently in the domain of micro-plasticity. The micro-hardness indentation size effect (increase of hardness with decreasing indenter size) has lately attracted ample attention (e.g. Ref. [6]). Also the increase of the flow stress in plastic deformation with the decrease in diameter was demonstrated for torsion of very thin copper wires (Ref. [7]) and for the micro-bending of thin foil specimens (Ref. [8]).
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Cell motility in confinement: a computational model for the shape of the cell

Cell motility in confinement: a computational model for the shape of the cell

The basis of the model is to consider the lamellipodium as an “active fluid”: the complex material consisting of a mix of intracellular liquid and actin network will be described by a continuous medium. To describe the properties of this medium, we have to note that the scale of the setting allows us to use low Reynolds number approximation. Now following the discussion in [21] we argue as follows: the intracellular liquid is an incompressible Newtonian fluid and the actin network is an elastic system. We could then go and model our lamellipodium as a viscoelastic material, inheriting properties of both its constituents. But we argue that the scale of the outer mechanical forces that would act on the lamellipodium are too large to see the elastic response, driving us to describe the lamellipodium as an incompressible Newtonian fluid with an apparent viscosity µ 1 .
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Study of Stress – Strain Behaviour of Glass Fibre Self–Compacting Concrete With & Without Confinement

Study of Stress – Strain Behaviour of Glass Fibre Self–Compacting Concrete With & Without Confinement

Subramanian and Chattopadhyay (2002) are research and development engineers at the ECC Division of Larsen & Toubro Ltd (L&T), Chennai, India. They have over 10 years of experience on development of self-compacting concrete, underwater concrete with antiwashout admixtures and proportioning of special concrete mixtures. Their research was concentrated on several trials carried out to arrive at an approximate mix proportion of self-compacting concrete, which would give the procedure for the selection of a viscosity modifying agent, a compatible superplasticizer and the determination of their dosages. The Portland cement was partially replaced with fly ash and blast furnace slag, in the same percentages as Ozawa (1989) has done before and the maximum coarse aggregate size did not exceed 25mm.
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Effects of Confinement on Structural Stability and Electronic Structure of Sodium Clusters

Effects of Confinement on Structural Stability and Electronic Structure of Sodium Clusters

where the ground state geometries remain unchanged, HOMO-LUMO gap increases as expected from the simple particle in a box model. Further, it is observed that the strong confinement leads to significant isomeric transitions.[6,7] However, the structural stability and electronic structure have not been discussed. Then question arise whether the clusters would be stable under high confinement. If so, then what would be its limiting value. To gain some insight into the stability of clusters and electronic structure under confinement, we have carried out systematic DFT calculations on a series of sodium clusters.
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STUDY ON THE STRESS-STRAIN BEHAVIOR OF FIBER REINFORCED SELF-COMPACTING CONCRETE WITH & WITHOUT CONFINEMENT

STUDY ON THE STRESS-STRAIN BEHAVIOR OF FIBER REINFORCED SELF-COMPACTING CONCRETE WITH & WITHOUT CONFINEMENT

Concrete is the most common construction material used throughout the world for infrastructure, civil engineering and housing applications, followed by wood, steel and a number of miscellaneous materials. Worldwide, over ten billion tons of concrete are being produced each year (Mehta 2002)(Lafarge.com 2005). Fresh concrete is an inorganic material consisting of a mixture of particles suspended in water. Particles include hydraulic and inert powders, and fine to coarse aggregates . Hydraulic powders comprise Portland cements and semi-hydraulic mineral admixtures like fly ash and silica fume, which react with the water and solidify into a matrix phase bonding the components together to create a stone-like material. Chemical admixtures may be applied to enhance the control of the workability,hardening process, and air void distribution (Herholdt, Justesen, Christensen & Nielsen 1985)(Neville & Brooks 1990).
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EFFECTS OF CONFINEMENT/CROWDING ON UREOGENESIS IN THE GULF TOADFISH OPSANUS BETA

EFFECTS OF CONFINEMENT/CROWDING ON UREOGENESIS IN THE GULF TOADFISH OPSANUS BETA

et al. 1990). Although these treatments might be useful tools for activating ureogenesis, it is not clear how often toadfish experience these conditions in nature and whether these treatments would elicit underlying biochemical changes in a biologically relevant way. Gulf toadfish spend up to several weeks in confined water volumes while breeding and caring for young (Breeder, 1941), and they have been observed to occur in small volumes of water (similar to the confinement conditions in the present study) and, more rarely, to undergo air-exposure during low tides (personal observations of T. E. Hopkins for the West Coast of Florida; personal observations of D. Gleeson, Gulf Specimen Marine Laboratories, Panacea, FL, for the Florida Panhandle), although we know of no useful water ammonia data in this regard. The present study suggests that the cues of increased water ammonia level or air-exposure may be secondary to the cue of confinement in activating ureogenesis in the toadfish in nature in that (1) confinement to small volumes of water would presumably happen in advance of ammonia build-up or air-exposure, and (2) substantial ammonia build-up was not necessary to activate ureogenesis (Figs 1C, 2A). Our results suggest that a classic stress response may be important in activating ureogenesis, but this interpretation should be considered as speculative for three reasons. First, although confinement and crowding conditions normally associated with eliciting the stress response in fish (Barton and Iwama, 1991) clearly activated ureogenesis, we have yet to examine in detail the hormonal (e.g. cortisol) basis of this response. Second, although some of our treatments involved minimal intervention, very different patterns of nitrogen metabolism and excretion are possible in toadfish which choose to enter (and possibly defend) confined spaces rather than those that have the condition imposed externally, as in the present study (and in most studies of stress in fish). Also
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Application of a plane stratified emission model to predict the effects of vegetation in passive microwave radiometry

Application of a plane stratified emission model to predict the effects of vegetation in passive microwave radiometry

This paper reports the application to vegetation canopies of a coherent model for the propagation of electromagnetic radiation through a stratified medium. The resulting multi-layer vegetation model is plausibly realistic in that it recognises the dielectric permittivity of the vegetation matter, the mixing of the dielectric permittivities for vegetation and air within the canopy and, in simplified terms, the overall vertical distribution of dielectric permittivity and temperature through the canopy. Any sharp changes in the dielectric profile of the canopy resulted in interference effects manifested as oscillations in the microwave brightness temperature as a function of canopy height or look angle. However, when Gaussian broadening of the top and bottom of the canopy (reflecting the natural variability between plants) was included within the model, these oscillations were eliminated. The model parameters required to specify the dielectric profile within the canopy, particularly the parameters that quantify the dielectric mixing between vegetation and air in the canopy, are not usually available in typical field experiments. Thus, the feasibility of specifying these parameters using an advanced single-criterion, multiple-parameter optimisation technique was investigated by automatically minimizing the difference between the modelled and measured brightness temperatures. The results imply that the mixing parameters can be so determined but only if other parameters that specify vegetation dry matter and water content are measured independently. The new model was then applied to investigate the sensitivity of microwave emission to specific vegetation parameters.
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