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سال پانزدهم شماره 1 (بهار 1401)

Expansion and damage due to the alkaline reaction of aggregates in various concrete structures as one of the major concerns in the durability of concrete structures have attracted the attention of many researchers to identify and study this importance. Aggregates containing active ingredients can cause destructive expansions even when they are small in weight. This reaction occurs between some materials in aggregates with alkaline hydroxide in the concrete pores resulting in a water-absorbing gel. This phenomenon usually causes the concrete to expand and crack through internal changes by creating changes in the boundaries between the aggregate and the cement paste. On the other hand, due to limited credit resources and the importance of environmental issues in most countries, using recycled and unused materials in the concrete industry has flourished. Therefore, in the present study, with the approach of using dredging materials and potentials in the Caspian Sea coast, in the first step, by performing lithographic experiments (microscopic observations) according to ASTM C295 standard, minerals with alkali-siliceous and carbonate reaction potentials have been identified. Then, the reactivity of the materials was studied by making concrete and mortar prisms containing these aggregates in both short-term and long-term states, and finally, by performing chemical experiments, the potential of alkaline-carbonate activity of carbonate aggregates was evaluated. Based on the results obtained from these experiments, the quality of dredged materials from the sea was suitable for making concrete.

In this paper, the results of Experimental Investigation Determination the Moment Redistribution and Plastic Hinge in continuous concrete Beams by replacing normal Concrete with HPFRCC and reducing the spacing of the stirrups are presented. Eight specimens of beam with reinforced concrete and fiber reinforced cement composites with bending distance were considered as three groups of A, B and C. Group A consisted of two conventional bending beams with standard bending intervals (d/2) (reference sample) and compact (d/4). Group B consists of four beams made of reinforced cement composites reinforced as a layer with non-compressive bending intervals (d/2) and Group C comprises two beams made of reinforced cement composites. Strong fibers were fully compressed (d/2) and compressed (d/4). Experimental results showed that using reinforced cement composite reinforced fibers instead of normal concrete in different parts of beams and the use of compacted stirrups increased the ductility of beams. In the specimens made of HPFRCC with compressive stirrups, bearing capacity, displacement ductility and energy ductility increased by 35, 77 and 83%, respectively, compared to the reference sample. The highest amount of moment redistribution, occurred in the specimen made with HPFRCC in the lower part with 1.51 times the reference beam.

In the present study, steel and alumina industrial wastes (ground granulated blast-furnace, electric arc furnace dust, and red mud) were used as a partial replacement for cement to form cement-based repair mortars. By substituting 5%, 10%, and 15% of cement with these materials a total of 14 mixtures were formed and an experimental program comprising three distinct parts was carried out. In the first part of the experimental program, the microstructure and chemical composition of all the constituent materials were examined. In the second part, resistance to acid sulfuric, water absorption, and electrical resistivity tests were conducted to evaluate the durability performance of the mortars, and compressive strength and tensile strength tests were performed to assess the mechanical properties of the mortars. In the last part of the experimental program, one reinforced concrete beam was strengthened in shear using repair mortar and carbon fiber reinforced polymer (CFRP) sheets and it was compared with a reinforced concrete beam without strengthening and a beam strengthened in shear using epoxy and CFRP sheets. The beams were tested by implementing a concentrated load and their shear capacity was compared to evaluate the performance of repair mortar. The results indicate that strengthening using repair mortar resulted in 23% increase in the shear capacity compared to the control specimen, while the shear capacity of the specimen strengthened with epoxy increased by 14% compared to the control specimen.

Calcium aluminate cement (CAC) is considered as a specific cement due to its special properties and maybe have improved performances compared to the Portland cement under some severe conditions. Although alumina cement has many advantages, it is not widely used in the construction industry because it obtains reduced strength in the long term and has not been adequately investigated. In this study, an attempt is made to improve the durability and mechanical properties of mortars with this type of cement using supplementary cementitious materials (SCMs) such as pumice, zeolite and limestone powder in high cement replacement levels. In this study, porosity, rapid chloride migration and acid resistance tests were employed to investigate the durability properties. In addition, the microstructure of the mixtures was investigated using scanning electron microscopy (SEM). The SCMs were used as cement substitution in proportions of 25, 40, and 60 % and a total of 10 mortar mixes were investigated. The results indicated that the pumice and zeolite significantly improved the mechanical strength and durability properties of the mixes. The microstructural studies revealed one of the most important reasons for the reduction of the compressive strength of the plain mixture. The results of this study show that the use of pumice and zeolite mitigates the conversion process, resulting in better performance of the mixes compared to the plain mixture.

Ceramic wastes are considered as a reason for environmental pollution that are hardly decompose against the natural and chemical factors. Substituting Portland cement with the ceramic waste powder which has a desirable chemical composition and fineness, not only can reduce energy consumption and pollution resulting from cement production, but also mitigate the environmental effects of ceramic waste in the landfills. This study investigates the effect of using ceramic waste as a substitute for cement on the mechanical properties and durability of the structural concrete. To this end, experimental tests were initially performed on small cubic samples of mortar and standard cylinders of concrete. 12 mixed designs were made by replacing different percentages of tile soil with cement possessing water to cement ratios of 0.45 and 0.5. The results confirmed that for water to cement ratio of 0.5, no reduction was observed in the compressive and flexural strength of concrete up to 40% of tile soil replacement. In addition, adding tile waste led to porosity loss, reduction of chlorine ion penetration, shrinkage reduction and improving the electrical resistance ratio. For water to cement ratio of 0.45, a slight decrease in the mechanical strength was observed compared after substituting cement with the ceramic waste compared to the control sample, while the durability parameters were improved.

Concrete due to its excellent properties, low cost, and wide availability is the most used construction product in the world. As the world population grows, cement production has increased as one of the main materials used in concrete. Cement production emits carbon dioxide which increases environmental pollution. One of the ways to prevent pollution is to use Supplementary cementing materials instead of cement. One of the most important alternative materials used in recent years is LC3 cement. This type of cement reduces the amount of clinker in cement, decreases our need for fossil fuels, and therefore reduces the emission of carbon dioxide. Moreover, one of the most important mechanical properties of concrete is its compressive strength, which its estimation is complex since the number of existing parameters is high. Hence, costly laboratory methods are used which consist of high error. In this paper, machine learning models that work based on decision tree model are used which their performance have been improved by genetic algorithm. LightGBM and XGBoost models got a prediction score of 0.958 in the prediction of the compressive strength of concrete and perform better than decision tree and random forest models with 0.91 and 0.932 prediction scores. Also, the feature importance of each model has been presented and a new data set has been used to evaluate the validation of models.

Concrete is an unprecedented porous and highly heterogeneous composite material. The durability of reinforced concrete with steel fibers in chloride and sulfate environments is of interest for engineers, infrastructure owners, maintainers, and researchers. Steel fibers and concrete matrices are bonded together through a weak bond, and the properties of this composite are largely due to the surface bonding of the fibers and the concrete matrix. Less permeability maintains the durability of the structure in contact with harmful substances such as chloride ions, sulfate ions and acids because it cannot easily penetrate In this paper, three types of steel fiber reinforced concrete (1, 1.5 and 2% steel fiber) were prepared and used in 3 environments, i.e. magnesium sulfate, sodium sulfate, and sodium chloride, for 6 and 12 months. Compressive strength, flexural strength, electrical resistance, water penetration, and rapid penetration of chloride were also examined. According to the results, the sample containing 2% steel fiber when exposed to acids showed more mechanical performance deterioration compared to the two other samples, i.e. 1% and 1.5% steel fiber.

Basically, self-compacting concrete mixtures (SCC) are more susceptible to temperature, time, and ingredients compared to the conventional ones. This matter is because of the combination of varied needs in the fresh mode, extremely complexed mixture proportions, and its naturally low yield stress. The rheology provides valuable information about the properties of fresh self-compacting concrete (SCC), but investigation of the procedure for achieving optimal SCC mix using the so-called rheographs can further provide practically useful information. In the present research, having their properties functions of temperature and mixing time, SCC samples were prepared with different amounts of cement, limestone, viscosity-modifying admixture and superplasticizer. In order to measure the rheological properties of the prepared samples, the SCC mix temperature was adjusted according to the dominant environmental conditions in different seasons. Accordingly, the samples were prepared during different seasons of the year to check for their workability and rheology. Rheographs were used to investigate the rheological properties of the SCC mixes during a period of 60 min. According to the results, the use of polycarboxylate superplasticizer and increasing the amount of cement at a given water-to-cement ratio were found to improve the rheological properties of the resultant concrete mix. increasing Portland cement to 440 kg/m3 in SCC mixtures containing viscosity modifying admixtures (VMA) improved rheology. On the other hand, the powder type SCC mixtures had 50% more slump loss compared to VMA type SCC mixtures. Therefore, the powder concrete is not recommended for high-temperature operating conditions.