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سال چهاردهم شماره 1 (بهار 1400)

Understanding the physical microstructure, including pores size distribution and water absorption of concrete, because of its effect on long-term durability is important. The aim of the present study is to investigate the effect of physical microstructure of pozzolanic self-compacting concretes on compressive strength and water permeability. Self-compacting concrete mixes were designed with total cementitious material content of 450 kg/m3 with water to cementitious material ratio of 0.35, 0.45 and 0.55. The binary and ternary blends of cement, silica fume and metakaolin were used for the purpose of present study. In order to investigate the difference between the effect of metakaolin and silica fume on the physical microstructure, mixtures containing binary and ternary blends of cement, silica fume and metakaolin with water to cement ratio of 0.45 were considered. Pore physical microstructure characteristics including median pore size, volume of large and small capillary pore in self-compacting concretes were investigated. Pore size distribution of self-compacting concrete samples were measured by mercury porosity test. The compressive strength, water absorption and capillary water sorptivity were performed on samples. The results showed that an important effective factor on compressive strength and water permeability is the medium size of the pores and the volume of large capillary pores. The concrete containing 20% metakaolin and 8% silica fume did not have the lowest porosity despite having the highest compressive strength. The lowest porosity with 8.6% belongs to self-compacting concrete containing silica fume with water to cement ratio of 0.35.

Heat, both in transient and steady state, changes the physical and chemical properties of concrete. In addition, the use of slag with Portland cement is very effective on development of concrete microstructure. The main products of hydration process are cement paste and slag, which plays important role in increasing concrete strength, C-S-H and C-A-Fe-H microstructures. Based on this, this paper has focused on the concrete containing varying degrees of steel slag in order to get a deeper perception of the changing behavior of C-S-H and C-A-Fe-H due to being exposed to high temperature. In this regard, 150 cubic samples of concrete with zero, 5%, 10%, 15% and 20% replacement of steel slag to Portland cement were treated for 28 days in a moisture bath. Then, all samples were exposed for 1 hour to 25 to 800 ºC. The percentage of weight change, compressive strength and cracking behavior of samples were investigated. Furthermore, in order to evaluate the microstructural behavior of test samples in different temperatures, SEM photos and EDS were used. Based on the results, the behavior of concrete samples depends on the variations of C-S-H and C-A-Fe-H microstructures under high temperature. The compressive strength of samples containing 5 to 15% steel slag increased as compared to conventional concrete sample. Based on SEM photos, the reason for this increase is higher density of C-S-H and C-A-Fe-H microstructures and FeO(OH).

AbstractIn this study, the toughness and durability of concrete with recycled aggregates and magnetic water against melting and freezing cycles and mechanical properties such as compressive and tensile strengths are investigated. Variables include the number of different cycles of water in the magnetic field, different amounts of super-plasticizer, steel fibers and micro-silica. A total of 242 samples were made in 11 different mix designs. It should be noted that half of the samples mentioned contains steel fibers. The results of most experiments have shown that magnetic water, although having a very high effect on the mechanical properties of concrete, should not be used alone as a compensating agent for defects caused by recycled aggregates. Using microsilica as a mineral admixture with magnetic water improves the concrete properties, significantly. The toughness indices were determined by ASTM C1018, JSCE and PCS method and concrete durability test by BS EN 140: 2003. Among the recycled aggregate concrete mixes, the best toughness results were for the samples containing microsilica and steel fibers. Also, ten round magnetic water resulted in an increase in the durability of recycled aggregate concrete by approximately 25% compared to one round of magnetization.

Considering the positive attitude of special concrete, the main focus should be on statistical relationships such as acceptance control and regulations. The purpose of this paper was to statistical analysis of the special concretes, the information from previous researches, and the statistical information was collected. The collected information and results were analyzed with the software SPSS and Minitab and the histogram curves were drawn and their standard deviation were calculated. And using the formulas of the coefficients and intervals of reliability and the average compressive strength obtained from the available research and 95 and 90% confidence intervals, these results have been analyzed and compared with the proposed formulas of the design codes. The results of the analyzes showed that the use of fibers can not have a positive effect on the compressive strength of concrete, but the standard deviation and the coefficient of variation were increased and the concrete's reliability were reduced. In the case of geopolymer concrete, it can be seen that the standard deviation and the coefficient of variation were improved. Statistical analysis of the results of compressive strength of samples manufactured in ready-made concrete companies that received quality control labs showed that the deviation standard and coefficient of variation the concretes made in August and September were less than the corresponding interval from May to June, the reason for this was the increased use of concrete precision manufacturing workshops and more customer focus in the summer as well as the quality of testing.

Porous concrete refers to a combination of cement and water, without fine grains or with a small amount of fine grains, which is important in terms of water transferability and permeability. This material can act as a drainage and carry rainwater and enhance groundwater. In the present article, the effect of replacement of pumice mineral additive with aggregate of porous concrete with volumetric percentages of 25, 50, 75 and 100% was studied on compressive strength, permeability coefficient and porosity percentage. Laboratory analysis was performed using SAS 9.4 software at 95% confidence level for all samples. The results show that with the replacement of the pumice additive in the structure of the porous concrete and the corresponding removal of the aggregate, the compressive strength of the samples is reduced due to the porosity of the pumice structure and the permeability coefficient and porosity increase. The reduction of compressive strength with increasing pumice percentage for all samples was 60.49, 83.86, 85.96 and 86.98%, respectively. Increasing the permeability coefficient with increasing pumice percentage for all samples was 10.13, 14.96, 18.7 and 24.94%, respectively.

The correct estimation of compressive strength of concrete plays a key role in evaluating the performance of reinforced concrete (RC) structures under gravity and seismic loads. According to the provisions of Chapter 9 of the Iranian National Building Regulations, sampling and performing compressive strength tests are required to determine the average compressive strength of concrete. In this study, by statistically studying the results derived from analyzing the archive of the compressive strength of concrete samples extracted from reinforced concrete (RC) buildings constructed over the past ten years in Tabriz, the values of statistical parameters affecting the average compressive strength of concrete such as the coefficient of variation (COV) and the standard deviation were calculated. The probability density function (PDF) and the cumulative distribution function (CDF) of the above-mentioned statistical parameters were determined for the purpose of comparison with the values provided in the Iranian National Building Regulations and other international standards as well as for use in future studies. The results show that in 30% of the buildings studied, the standard deviation of compressive strength of concrete exceeds the amounts presented in Chapter 9 of the Iranian National Building Regulations. A similar result is obtained in 18% of the high-rise buildings studied. The coefficient of variation of compressive strength of concrete in approximately 22% of the buildings examined is higher than the allowable values of FEMA356 (2000) and this problem was observed in a higher percentage of the low-rise buildings.

Progressive failure is the spread of a chain of damage from one member to another in a structure that, following local damage caused by the removal of one or more load-bearing members, begins and is disproportionately transferred to the other members of the structure. The whole or destruction becomes a large part of the structure. In the discussion of progressive failure, determining the element that has the greatest potential for progressive failure is important. Because by strengthening this element, the performance of the structure can be improved. In this paper, the progressive failure of two reinforced concrete structures with a different number of floors (10 and 16-storey buildings) using nonlinear dynamic analysis is investigated. The analysis and design of this structure has been done in SAP 2000 software and modeling has been done by creating local failure in the middle column and corner column under the influence of three different acceleration angles. The relative displacement of the floors, the viscosity index based on the maximum base cut and the formation of plastic joints, and the possibility of progressive failure based on existing standards and design based on the GSA guideline alternative method have been investigated. The results of this study show that in reinforced concrete buildings, the bending frame of the corner columns has the highest potential for progressive failure. In addition, the results show that structures with higher heights perform better against progressive failure.

The bond between concrete and reinforcing steel is essential for the composite action in reinforced concrete structures. In the present study, the effects of super-plasticizer additive on the steel-concrete bond strength is experimentally investigated. A total of 18 pullout samples are prepared in two concrete mix design cases, including ordinary and high-strength concrete. The used steel bars are of diameters 8, 10 and 12 mm, which are embedded inside the concrete in a length five times the rebar diameter. The steel-concrete bond strength is evaluated using direct pullout test method with the universal testing machine. Besides preparing the pullout specimens, 5 standard cube samples of concrete are casted to examine the concrete compressive strength and dimensions of the hardened concrete cubes. The specimens are tested at the 42-day ages and the slip-force curves are obtained using the pullout test. The maximum force and the corresponding slip, bond stress and other important parameters are determined and discussed for the different studied conditions. The results indicate that the rebar-concrete bond strength decreases by increasing the rebar diameter and the super-plasticizer additive has an important effect on the bond strength. For instance, the average bond strength for the concrete mix containing superplasticizer is 2.3 time of that for the normal concrete.

Value engineering is a creative approach to optimizing life cycle costs, saving time, increasing profits, improving quality, and optimizing the use of resources. This management approach in Iran also has numerous project backgrounds, and many of the country's executive agencies use it in projects. Implementing a value engineering approach in hydraulic structures enables systematic behavior to balance project costs and performance, relying on teamwork and creativity to eliminate unnecessary costs and increase efficiency. And take more effective steps to make projects more efficient. Implementation of this method has yielded tangible results worldwide and even in Iran, from which the Maroun Regulatory Dam, the Ramshir Dam Control Channel, the Kuhrang Dam diversion tunnel, and more can be mentioned. Reservoir dams are major national and multipurpose projects that aim to conserve water for drinking, agriculture, industry, power generation, flood control, tourism, cultural and recreational activities. The major problem most large projects face is the lack of value engineering. The purpose of this research is to apply value engineering in the implementation of Kowsar Gachsaran reservoir dam located in Kohgiluyeh and Boyer Ahmad province. In this research, value engineering methodology was used by "Analytical Hierarchy Process" method. The results showed that the cost of implementation of dam body with value engineering application decreased by 8.47%, equivalent to 5.8 billion Tomans; about 20.25% decrease in execution time and 9.25% increase in performance quality.

The effect of hot climate on the performance of fresh self-compacting concrete causes a number of problems including the reduced workability. This issue is due to the various demands of the fresh self-compacting concrete, much more complicated mix design and low yield stress of this type of concrete. Therefore, in design of the self-compacting concrete, in addition to the constituent elements the ambient conditions also are taken into consideration. In this research, the self-compacting concrete specimens with different amounts of limestone and viscosity-modifying admixture were made. Also the temperature of the concrete mixtures was selected with respect to the ambient conditions of different seasons. After the concrete specimen temperature reached the ambient temperature, the rheological properties of the fresh self-compacting concrete specimens containing limestone or viscosity-modifying admixture being a function of mixing time, and temperature with water to cement ratio of 0.42 were investigated. According to the obtained results the viscosity-modifying admixture has a better effect on the plastic viscosity with respect to limestone and reduces the slump loss by 50%. Thus in the hot climate where our goal is to maintain workability, the concretes with viscosity-modifying agents are the best options.