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205 results on '"Mojtahedi, Alireza"'

20. New concrete composites incorporated with calcium acetate as admixture: The Impact of Curing Age on Strength and the Effect of Temperature on Water Absorption

Author

Abed Ammar Ali, Mojtahedi Alireza, and Lotfollahi-Yaghin Mohamad Ali

Subjects
Environmental sciences, GE1-350
Abstract

In previous work, concrete mixes incorporated with different types of acetates (inorganic and organic) were formulated, cured for 28 days in water, and then tested for compressive strength. The effect of the water-to-cement ratio and the acetate content was optimized using a central composited design based on the response surface methodology approach. The results confirmed that the optimum compressive strength was achieved for the mixes incorporated with calcium acetate (CaA) using of 0.48 water-to-cement ratio. In the current work, the effect of curing age on compressive strength and density was studied for the optimum mixes. Also, the water absorption of the mixes carried out by immersion test was assessed at different temperatures (25-55 °C). Microscopic observations were also noted. The results obtained confirmed that the compressive strength and density of the CaA-concrete mix increase with increasing the curing age. The thermodynamic study of water absorption assured the dependence of water absorption on temperature. The activation energies depicted from the Arrhenius relation and the decrease in the water absorption rate for the concrete incorporated with CaA are considered as an indicator of decreasing the porosity of the concrete mixes compared to the control mixes without CaA. The findings assured that the modified mixes with CaA are promising construction materials owing to their outstanding strength and lower water absorption rates compared to conventional concrete.

Published
2023
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39. Investigation of a combined Monte Carlo simulation and Galerkin approach for probabilistic characteristics modelling the dynamic behaviour of the fluid-conveying pipe.

Author

Bayrami Atashgah, Mahdi, Iranmanesh, Mehdi, and Mojtahedi, Alireza

Subjects
DISTRIBUTION (Probability theory), FINITE element method, CRITICAL velocity, GALERKIN methods, MONTE Carlo method, DYNAMIC models
Abstract

A probabilistic approach is used to compute the probabilistic responses of considered fluid-conveying pipes by combining the Galerkin method and Monte Carlo simulation. Analysing the fluid-conveying pipes is often performed based on the deterministic approaches, but the effect of uncertainty needs to be investigated. The natural frequencies can be represented using probabilistic methods by considering the inherent uncertainty in estimating values. The uncertainty was considered as a random variable. The effects of coefficient of variation (COV) associated with the fluid velocity on the probabilistic characteristics are evaluated. The probability distribution functions of natural frequencies are presented. The measured values were validated by comparing with results of the stochastic finite element method. The results imply that the peak points for the diagrams of the standard deviations of the velocity values gained by the Monte Carlo simulation versus the mean velocities of the related values are equal to the critical fluid velocity. [ABSTRACT FROM AUTHOR]

Published
2023
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45. Damage Parameter Variations of Breakwater along with a Floating Wave Barrier and a Submerged Obstacle

Author

Naseri, Alireza, Vafaeipour Sorkhabi, Ramin, Alami, Mohammad Taghi, Mojtahedi, Alireza, Naseri, Alireza, Vafaeipour Sorkhabi, Ramin, Alami, Mohammad Taghi, and Mojtahedi, Alireza

Abstract

Damage to rubble mound breakwaters (RMBs), both general and partial, causes instability and inconstancy of the structure against waves. This study aimed to investigate the effect of a submerged obstacle on the stability and damage reduction of RMBs as an innovative method and determine the optimal distance of the obstacle from the breakwater and the floating wave barrier based on the damage parameter. The waves affecting the breakwater were assumed to be random using a JONSWAP spectrum. The aggregates' movement and the RMB's exact deformation were recorded using close-range photogrammetric imaging, and the eroded area and the damage parameters were obtained at equal distances in eight cross-sections. According to the results of the tests, by analyzing the effect of the number of waves hitting the breakwater, 3000 waves were considered to bring the structure to a stable state. The results showed that increasing the relative wave height from 0.36 to 0.48 and from 0.48 to 0.6 increased the damage parameters to 39.12% and 44.44%, respectively, and increasing the relative wave period from 0.6 to 0.8 and 0.8 to 1 increased the damage parameters to 22.94% and 28.26%, respectively. Moreover, using a seaward obstacle at 0, 5, 10, 15, and 20 cm distances decreased the damage parameter. The greatest effect, a reduction of 39.15% in the damage parameter, was observed at a distance of 5 cm from the RMB. This number was reduced to 0.735 when a floating wave barrier was used without a submerged obstacle (i.e., 34.14%). Using an obstacle at 5 cm in conjunction with a wave barrier reduced the damage parameter by 54.03% and demonstrated the best function among different models. Hence, this model is proposed based on the experiments carried out in this study.

Published
2022

49. A numerical approach to evaluating the asymmetric ground settlement response to twin-tunnel asynchronous excavation

Author

Mojtahedi, Alireza Seghateh and Nabizadeh, Ali

Subjects
Twin-tunnel, Ground surface settlement, Numerical modeling, Centrifuge test
Abstract

Due to the importance of surface and subsurface settlements to prevent damages to building foundations and sensitive structures in the urban cities, in this study, the ABAQUS finite element software has used to conduct a series of numerical modeling analysis on ground surface settlement caused from the asynchronous excavation of twin-tunnel. The effects of tunnel diameter, center-to-center tunnel spacing, and tunnel depth are discussed in detail and the shape of the surface settlement curves is also plotted. The numerical modeling has been verified by the results of three sequential twin-tunneling centrifuge tests conducted by the City University of London with 94.22%, 98.71% and 99.56% accuracy, respectively. Based on the results of this study, reducing the tunnel diameter decreases the amount of the maximum ground surface settlements and reducing the depth of tunnels and the distance between twin-tunnel to less than 2D (D is the diameter of the tunnels) increase the maximum surface settlements. Installation of 30 cm of tunnel lining can decrease the maximum ground surface settlement up to almost 79%.

Published
2022

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