Refined plasticity model for concrete stress-strain relationship part I : Prediction of peak stress and residual stress
- Authors: Piscesa, Bambang , Attard, Mario , Samani, Ali Khajeh
- Date: 2014
- Type: Text , Conference paper
- Relation: 23rd Australasian Conference on the Mechanics of Structures and Materials (ACMSM23) p. 149-154
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- Description: A refined plasticity model for concrete stress-strain relationships is proposed. The proposed failure surface has the ability to evolve its form based on empirical formulation in which being extracted from the experimental results via the frictional driver parameter (). Two main features are highlighted in this paper such as the peak stress prediction and residual stress prediction of the proposed model. In this paper the comparison of proposed models with experimental results weighted on uniaxial-triaxial compression in axial direction. In the next part of the research a non-associative flow rule in which has an inclusion of size effect to be applied in the constitutive driver is proposed and experimental comparison in both axial and lateral direction is discussed.
- Description: A refined plasticity model for concrete stress-strain relationships is proposed. The proposed failure surface has the ability to evolve its form based on empirical formulation in which being extracted from the experimental results via the frictional driver parameter (
Investigation on the fiber based approach to estimate the axial load carrying capacity of the circular concrete filled steel tube (CFST)
- Authors: Piscesa, Bambang , Attard, Mario , Suprobo, Priyo , Samani, Ali Khajeh
- Date: 2017
- Type: Text , Conference paper
- Relation: International Conference of Applied Science and Technology for Infrastructure Engineering 2017, ICASIE 2017; East Java, Indonesia; 5 August 2017; published in IOP Conference series: Materials Science and Engineering Vol. 267, p. 1-9
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- Description: External confining devices are often used to enhance the strength and ductility of reinforced concrete columns. Among the available external confining devices, steel tube is one of the most widely used in construction. However, steel tube has some drawbacks such as local buckling which needs to be considered when estimating the axial load carrying capacity of the concrete-filled-steel-tube (CFST) column. To tackle this problem in design, Eurocode 4 provided guidelines to estimate the effective yield strength of the steel tube material. To study the behavior of CFST column, in this paper, a non-linear analysis using a fiber-based approach was conducted. The use of the fiber-based approach allows the engineers to predict not only the axial load carrying capacity but also the complete load-deformation curve of the CFST columns for a known confining pressure. In the proposed fiber-based approach, an inverse analysis is used to estimate the constant confining pressure similar to design-oriented models. This paper also presents comparisons between the fiber-based approach model with the experimental results and the 3D non-linear finite element analysis.
Lateral strain of confined concrete incorporating size effect
- Authors: Samani, Ali Khajeh , Attard, Mario
- Date: 2012
- Type: Text , Conference paper
- Relation: 22nd Australasian Conference on the Mechanics of Structures and Materials, ACMSM22; Sydney, Australia; 11th-14th December 2012; published in From Materials to Structures: Advancement through Innovation; p. 357-361
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- Description: The lateral and axial strain relationship plays an important part in predicting the confinement of confined columns. Measuring lateral strains in compressive experiments proves to be difficult which mean few reliable results are available. A lateral strain versus axial strain model is proposed based on the supposition that the concrete behaves linear elastically in the early stages of loading, nonlinear hardening up to the peak stress after which the inelastic lateral strain vary linearly with the inelastic axial strain. The lateral to axial inelastic strain ratio is shown to be a function of the lateral confinement level and related to the failure mechanism. Moreover, size effect is also discussed from the lateral strain versus axial strain perspective.
Ductility in concentrically loaded reinforced concrete columns
- Authors: Samani, Ali Khajeh , Attard, Mario , Foster, Stephen
- Date: 2015
- Type: Text , Journal article
- Relation: Australian Journal of Structural Engineering Vol. 16, no. 3 (2015), p. 237-250
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- Description: In recent years, the use of high-strength concrete materials has been regulated into Australian design standards. The use of high-strength concrete is desirable in many cases. For instance, in reinforced concrete columns of high rise buildings, the columns can carry more load with a smaller cross section compared to reinforced concrete columns built of normal strength material. However, there are some disadvantages, one being the reduction of ductility. The Australian Concrete Standard AS3600 deals with this issue by changing the tie arrangement in reinforced columns for different concrete strength grades. This study reviews the ductility index used to measure the ductility of reinforced concrete columns and uses an analytical model to predict the ductility index of several practical example columns. These columns are designed and detailed using the requirements of the Australian Concrete Standard. The outcome of a parametric study shows that the columns designed and detailed using the rules in the Australian Concrete Standard may not necessarily have the ductility index which the code assumes. Another well-known deficiency observed in the behaviour of reinforced high-strength concrete columns is premature spalling of the cover concrete. The Australian Concrete Standard addresses premature cover spalling by modifying a reduction factor which is applied to the strength of the concrete when the squash load of a reinforced concrete column is calculated. This reduction factor accounts for many issues not only premature cover spalling. Using an analytical model, it is shown that the code formula for estimating the squash load is too conservative and needs adjustment for very large columns with small cover to core ratio. © 2015 Engineers Australia.
3D Finite element modeling of circular reinforced concrete columns confined with FRP using a plasticity based formulation
- Authors: Piscesa, Bambang , Attard, Mario , Samani, Ali Khajeh
- Date: 2018
- Type: Text , Journal article
- Relation: Composite Structures Vol. 194, no. (2018), p. 478-493
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- Description: Strengthening reinforced concrete (RC) columns with external confining devices such as FRP wraps or steel tube is widely used in construction. By using external confining devices, both the strength and ductility of RC columns are significantly improved. However, numerical modeling to predict the capacity of strengthened RC columns is limited and often oversimplified. One of the biggest challenges in numerical modeling is to deal with unequal dilation between the concrete inner core (enclosed by both transverse steel and FRP wraps) and the concrete outer core (between the transverse steel and FRP wraps). Inaccurate modeling on the concrete dilatant behavior can lead to incorrect strength prediction. Sophisticated constitutive models which are able to model concrete dilation and robust modeling techniques are required. In this paper, three-dimensional non-linear finite element analysis (3D-NLFEA) of circular RC columns confined with conventional steel stirrups and FRP wraps is presented. In the FEA, the initial stiffness method with Process Modification (acceleration technique) is used to solve the equilibrium forces in the global solution. The constitutive model is based on the plasticity formulation proposed by the authors, which can capture the effective lateral modulus (EL) of the confining devices. This lateral modulus is obtained by observing the principal incremental stresses and strains at each element gauss point. It was found that, the lateral modulus is greatly affected by the boundary condition, dilatant behavior of the constitutive model and the Poisson's ratio of the external confining device. To validate the performance of the proposed model, several comparisons of the proposed model, using 3D-NLFEA, with experimental results is presented. The comparisons show that the predicted response using 3D-NLFEA and the experimental results of RC columns confined with FRP are in a good agreement.
A lateral strain plasticity model for FRP confined concrete
- Authors: Piscesa, Bambang , Attard, Mario , Samani, Ali Khajeh
- Date: 2016
- Type: Text , Journal article
- Relation: Composite Structures Vol. 158, no. (2016), p. 160-174
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- Description: This paper presents a plasticity constitutive formulation for actively and passively confined concrete. The loading surface is based on Menetrey and Willam's model with an additional frictional driver parameter. The frictional driver parameter controls the prediction of the peak stress and the residual stress level. The proposed flow rule has a plastic dilation rate control parameter which is a function of the restraining device or the local lateral modulus. A non-constant plastic dilation rate formulation is proposed to improve the prediction of the lateral strain behaviour of concrete. The proposed plastic dilation rate formulation is able to model plastic volumetric compaction caused by the use of very stiff confining devices, as well as the initial plastic compaction after the onset of localized cracking. Furthermore, the formulation is able to distinguish between active and passive confinement by monitoring the local lateral modulus. The accuracy of the proposed plastic dilation rate formulation is verified by comparison with experimental results for specimens subjected to either active or passive confinement from a variety of concrete strengths. The comparison between the proposed plasticity model and the experimental results for concrete under passive confinement (specimens with FRP confining material) was excellent. © 2016
Plastic dilation rate characteristic of concrete confined with steel tube
- Authors: Piscesa, Bambang , Attard, Mario , Samani, Ali Khajeh
- Date: 2017
- Type: Text , Conference paper
- Relation: 14th International Conference on Computational Plasticity - Fundamentals and Applications, COMPLAS 2017; Barcelona, Spain; 5th-7th September 2017 p. 436-446
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- Description: The use of external confining devices to confine concrete has become widely used. One of the purposes is to gain additional concrete strength and ductility. Although there are many types of external confining devices, in this paper, the attention is limited to the use of the steel tube as an external confining device. One of the main objectives of this research is to study the plastic dilation rate behavior of concrete-filled-steel-tube (CFST) columns. The experimental data for the plastic dilation rate is extracted, and compared with the authors concrete plasticity model. In the authors’ previous research, the calibration of the plastic dilation rate model was based on confined concrete tested under both active and passive confinement using FRP wraps. Since the behavior of the steel tube and the FRP materials are different, the author’s plastic dilation rate model needs to be re-evaluated for CFST columns. Comparisons of the extracted experimental plastic dilation rates with the model prediction for CFST specimens with normal strength concrete show good agreement and requires no adjustment in the formulation. However, for a specimen with 80 MPa concrete, the proposed formulation shows slightly lower plastic dilation rates. More experimental data for CFST using high strength concretes is required for further investigation. For the sake of completeness, the overall response of two CFST specimens is also evaluated using an in-house three-dimensional non-linear finite element analysis (3D-NLFEA) using the author’s proposed plasticity formulation for confined concrete.
A new model for confined concrete
- Authors: Samani, Ali Khajeh , Attard, Mario
- Date: 2010
- Type: Text , Conference paper
- Relation: 21st Australasian Conference on the Mechanics of Structures and Materials p. 227-232
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- Description: Analytical models for the stress-strain relationship of confined and unconfined concrete in compression are required for modeling the structural behavior of the reinforced concrete structural elements. The presented paper reviews the widely used models by Attard & Setunge (1996) and Binici (2005) and compares them with test results on uniaxial and triaxial tests with different specimen heights and widths and concrete strength. The variation of peak stress and corresponding strain, as well as the residual stress level, under confinement are compared with experimental results and discussed. A new analytical model is introduced which tries to address the limitations in previous models. The proposed model is capable of predicting the behavior of normal strength as well as high strength concretes.
Refined plasticity model for concrete stress-strain relationship part II : Inclusion of size effect
- Authors: Piscesa, Bambang , Attard, Mario , Samani, Ali Khajeh
- Date: 2014
- Type: Text , Conference paper
- Relation: 23rd Australasian Conference on the Mechanics of Structures and Materials (ACMSM23) p. 155-160
- Full Text: false
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- Description: This article will discuss about the implementation of size effect into the proposed plasticity model by adjusting the plastic potential function or flow rule. A new parameter (
Lateral behavior of concrete
- Authors: Samani, Ali Khajeh , Attard, Mario
- Date: 2011
- Type: Text , Journal article
- Relation: International Journal of Civil and Environmental Engineering Vol. 5, no. 11 (2011), p. 533-538
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- Description: Lateral expansion is a factor defining the level of confinement in reinforced concrete columns. Therefore, predicting the lateral strain relationship with axial strain becomes an important issue. Measuring lateral strains in experiments is difficult and only few report experimental lateral strains. Among the existing analytical formulations, two recent models are compared with available test results in this paper with shortcomings highlighted. A new analytical model is proposed here for lateral strain axial strain relationship and is based on the supposition that the concrete behaves linear elastic in the early stages of loading and then nonlinear hardening up to the peak stress and then volumetric expansion. The proposal for the lateral strain axial strain relationship after the peak stress is mainly based on the hypothesis that the plastic lateral strain varies linearly with the plastic axial strain and it is shown that this is related to the lateral confinement level.
A stress-strain model for uniaxial and confined concrete under compression
- Authors: Samani, Ali Khajeh , Attard, Mario
- Date: 2012
- Type: Text , Journal article
- Relation: Engineering Structures Vol. 41, no. (2012), p. 335-349
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- Description: Analytical models for the full stress-strain relationship of confined and unconfined concrete in compression are required for the numerical simulation of the structural behavior of reinforced concrete structural elements. There are many analytical models presented in the literature, which are generally empirical and are based on tests either on plain concrete specimens or reinforced concrete columns. This paper reviews some widely used analytical models calibrated using triaxial test results on plain concrete and compares their predictions with available test data on uniaxial and triaxial compression on specimens with different specimen height, width or diameter and concrete strength. The model prediction's for the peak stress and corresponding strain due to confinement are also compared. The residual stress level and the post-peak fracture energy under confinement are discussed. Estimates of the post-peak fracture energy per unit area are obtained from available experimental data showing that the post-peak fracture energy varies with confinement. The size effect on the softening behavior of uniaxial and triaxially loaded plain concrete specimens with different aspect ratios, heights and level of confinement, are also discussed. A new analytical model for unconfined and confined concrete is introduced which tries to address the limitations in previous models. The proposed model is capable of predicting the behavior of normal strength concrete, as well as high strength concrete and incorporates allowances for size effects dependent on specimen height and aspect ratio. Comparisons are made between the proposed new model, the models of others in the literature, and available compression triaxial and uniaxial test results. © 2012.
Size effect in confined concrete
- Authors: Attard, Mario , Samani, Ali Khajeh
- Date: 2010
- Type: Text , Conference paper
- Relation: 21st Australasian Conference on the Mechanics of Structures and Materials p. 221-226
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- Description: Compressive size effect tests have been mainly conducted on uniaxial compression tests. However, softening in concrete happens not only in uniaxial compression but also under triaxial compression. The present study shows that the post-peak compressive fracture energy per unit area, is influenced by the level of confinement in triaxial loading. The results estimated from the literature show an increasing fracture energy with increasing confinement until a limit is reached, at a confinement ratio of about 10 to 20% after which it decreases until it becomes zero. The fracture energy, the specimen height, aspect ratio and the confinement level is shown to influence the softening behavior of both uniaxially and triaxially loaded concrete. A new stress strain model is proposed for unconfined and confined concrete. The proposed model takes account of size effects dependent on specimen height and aspect ratio. The model is compared to experimental data and shows excellent agreement.