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
- Full Text: false
- Reviewed:
- 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
- Full Text: false
- Reviewed:
- 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.
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
- Full Text: false
- Reviewed:
- 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.
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
- Full Text: false
- Reviewed:
- 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
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
- Full Text: false
- Reviewed:
- 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.
Bio-reinforced self-healing concrete using magnetic iron oxide nanoparticles
- Authors: Seifan, Mostafa , Sarmah, Ajit , Ebrahiminezhad, Alireza , Ghasemi, Younes , Samani, Ali Khajeh , Berenjian, Aydin
- Date: 2018
- Type: Text , Journal article
- Relation: Applied Microbiology and Biotechnology Vol. 102, no. 5 (2018), p. 2167-2178
- Full Text: false
- Reviewed:
- Description: Immobilization has been reported as an efficient technique to address the bacterial vulnerability for application in bio self-healing concrete. In this study, for the first time, magnetic iron oxide nanoparticles (IONs) are being practically employed as the protective vehicle for bacteria to evaluate the self-healing performance in concrete environment. Magnetic IONs were successfully synthesized and characterized using different techniques. The scanning electron microscope (SEM) images show the efficient adsorption of nanoparticles to the Bacillus cells. Microscopic observation illustrates that the incorporation of the immobilized bacteria in the concrete matrix resulted in a significant crack healing behavior, while the control specimen had no healing characteristics. Analysis of bio-precipitates revealed that the induced minerals in the cracks were calcium carbonate. The effect of magnetic immobilized cells on the concrete water absorption showed that the concrete specimens supplemented with decorated bacteria with IONs had a higher resistance to water penetration. The initial and secondary water absorption rates in bio-concrete specimens were 26% and 22% lower than the control specimens. Due to the compatible behavior of IONs with the concrete compositions, the results of this study proved the potential application of IONs for developing a new generation of bio self-healing concrete.
Mechanical properties of bio self-healing concrete containing immobilized bacteria with iron oxide nanoparticles
- Authors: Seifan, Mostafa , Sarmah, Ajit , Samani, Ali Khajeh , Ebrahiminezhad, Alireza , Ghasemi, Younes , Berenjian, Aydin
- Date: 2018
- Type: Text , Journal article
- Relation: Applied Microbiology and Biotechnology Vol. 102, no. 10 (2018), p. 4489-4498
- Full Text: false
- Reviewed:
- Description: Concrete is arguably one of the most important and widely used materials in the world, responsible for the majority of the industrial revolution due to its unique properties. However, it is susceptible to cracking under internal and external stresses. The generated cracks result in a significant reduction in the concrete lifespan and an increase in maintenance and repair costs. In recent years, the implementation of bacterial-based healing agent in the concrete matrix has emerged as one of the most promising approaches to address the concrete cracking issue. However, the bacterial cells need to be protected from the high pH content of concrete as well as the exerted shear forces during preparation and hardening stages. To address these issues, we propose the magnetic immobilization of bacteria with iron oxide nanoparticles (IONs). In the present study, the effect of the designed bio-agent on mechanical properties of concrete (compressive strength and drying shrinkage) is investigated. The results indicate that the addition of immobilized Bacillus species with IONs in concrete matrix contributes to increasing the compressive strength. Moreover, the precipitates in the bio-concrete specimen were characterized using scanning electron microscope (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDS). The characterization studies confirm that the precipitated crystals in bio-concrete specimen were CaCO3, while no precipitation was observed in the control sample.
The role of magnetic iron oxide nanoparticles in the bacterially induced calcium carbonate precipitation
- Authors: Seifan, Mostafa , Ebrahiminezhad, Alireza , Ghasemi, Younes , Samani, Ali Khajeh , Berenjian, Aydin
- Date: 2018
- Type: Text , Journal article
- Relation: Applied Microbiology and Biotechnology Vol. 102, no. 8 (2018), p. 3595-3606
- Full Text: false
- Reviewed:
- Description: Recently, magnetic iron oxide nanoparticles (IONs) have been used to control and modify the characteristics of concrete and mortar. Concrete is one of the most used materials in the world; however, it is susceptible to cracking. Over recent years, a sustainable biotechnological approach has emerged as an alternative approach to conventional techniques to heal the concrete cracks by the incorporation of bacterial cells and nutrients into the concrete matrix. Once cracking occurs, CaCO3 is induced and the crack is healed. Considering the positive effects of IONs on the concrete properties, the effect of these nanoparticles on bacterial growth and CaCO3 biosynthesis needs to be evaluated for their possible application in bio self-healing concrete. In the present work, IONs were successfully synthesized and characterized using various techniques. The presence of IONs showed a significant effect on both bacterial growth and CaCO3 precipitation. The highest bacterial growth was observed in the presence of 150 μg/mL IONs. The highest concentration of induced CaCO3 (34.54 g/L) was achieved when the bacterial cells were immobilized with 300 μg/mL of IONs. This study provides new data and supports the possibility of using IONs as a new tool in designing the next generation of bio self-healing concrete.