Thesis is Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Civil Engineering - Structure - دانشکده فنی و مهندسی
Thesis is Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Civil Engineering - Structure
نوع: Type: Thesis
مقطع: Segment: masters
عنوان: Title: Thesis is Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Civil Engineering - Structure
ارائه دهنده: Provider: alireza mousivand
اساتید راهنما: Supervisors: Dr. Amir Rezaei Sameti
اساتید مشاور: Advisory Professors:
اساتید ممتحن یا داور: Examining professors or referees: Dr. Fereydoun Rezaei - Dr. Mojtaba Nili
زمان و تاریخ ارائه: Time and date of presentation: 2025
مکان ارائه: Place of presentation: دپارتمان عمران
چکیده: Abstract: Reinforced concrete is one of the most important composite materials in civil engineering, offering significant improvements in the performance and durability of structures when combined with steel fibers. These fibers, uniformly distributed within the concrete matrix, control microcracks and enhance properties such as tensile, flexural, and impact strength. Advances in science and technology have enabled the production of ultra-thin steel fibers at micro- and nanoscale dimensions. These nanoscale fibers, when uniformly dispersed in the concrete matrix, improve the composite structure by modifying its microstructure, reducing overall porosity, accelerating the formation of C-S-H gel, and increasing the Young's modulus, thereby enhancing the density and quality of the cementitious composite. Considering the applications of thin steel fibers in concrete, this study examines the mechanical behavior of these elements using molecular dynamics simulations. Generally, due to the high reactivity of iron with oxygen, a thin oxide layer forms on the surface of steel fibers when exposed to air. In the alkaline environment of concrete, the presence of hydroxide ions leads to the formation of a new iron hydroxide layer on the fiber surface, that can significantly affect the mechanical behavior of these elements. Therefore, in the modeling process, after constructing the initial structure, simulations of oxidation and hydration processes on the surface of thin steel rods under various temperature conditions are performed. The samples are subjected to environmental stabilization and energy minimization before undergoing various mechanical tests. The results indicate an inverse relationship between fiber diameter and strength, which may be attributed to the reduced surface-to-volume ratio with increasing diameter. Additionally, the formation of an oxide layer on the steel fiber surface leads to a marked decrease in yield stress, and this decline becomes more pronounced with an increase in the oxide layer thickness. However, for fibers with larger diameters, the negative impact of the oxide layer on strength is less significant due to the lower surface-to-volume ratio. Structural defects exhibit varying intensities of influence on the mechanical behavior of samples under compressive and tensile tests. In compressive tests, severe stress drops are observed due to greater stress concentration in defective areas. Moreover, temperature increases accelerate the oxidation process, resulting in reduced strength and Young's modulus of the fibers. The findings reveal that the formation of a hydroxide layer on the steel fiber surface significantly decreases stress levels. However, in samples with thicker oxide layers subjected to hydration, the negative impact of hydroxide is less pronounced. Furthermore, with increasing rod diameter and decreasing surface-to-volume ratio, the effect of hydroxide on stress reduction diminishes considerably.