نشریه مصالح و سازه های بتنی
سال ششم شماره 2 (پیاپی 12، پاییز و زمستان 1400)

Irregular bridges are bridges that have a complicated dynamic behavior due to differences in the height of the columns, the angularity of the deck with an abutment or the horizontally curved bridges under earthquake, which caused the magnitude and type of damage by the earthquake in them is different. Two parameters of the arc angle and the height of the bases are important factors in the seismic analysis of horizontally curved bridges. In this paper, the effect of the arc angle on the seismic vulnerability of horizontally curved reinforced concrete bridges was studied. For this purpose, 8 bridges with different arc angles (0, 15, 30, 45, 60, 75, 90, 105 degrees) were designed and simulated in CSiBridge software and then were analyzed as nonlinear dynamic with 8 different earthquake records named Northridge, Loma perieta, Landesr, Kocaeli, Imperial valley and Cape mendocino. The effect of the arc angles on performance parameters such as the amount of rotation of plastic hinge, base shear and deck displacement in two directions of longitudinal and transverse directions were investigated. The results showed that by increasing the arc angle from zero to 105 degree, the amount of rotation of plastic hinges of the columns were increased by 16% compared to the straight bridge. And also the results showed that the performance level of straight bridges and arch bridges up to an angle of 75 degrees was categorized in the immediate occupancy level (IO), but the bridges with angles of 90 and 105 degree were categorized at the life safety level (LS). Also, the study of the effect of the arc angle on the base shear indicated that the maximum base shear was obtained at 75 degree with a 33% increase compared to the straight bridge.

In this paper, the seismic behavior of reinforced concrete (RC) high-rise buildings with frame-core tube systems as one of the lateral bearing systems are studied. In order to investigate the effect of the core tube system on the seismic behavior of the high-rise building, the structural model is evaluated in two cases with the inner core tube and without the inner core tube. Three methods of equivalent static analysis, dynamic nonlinear time history analysis and nonlinear static analysis (push-over analysis) are used. Also, a 25-story high-rise building with a frame-core tube system having voids in the last floors is implemented. The results showed that the use of RC core as a central tube significantly reduces the lateral displacement and drift of floors. Also, the results obtained from the nonlinear static analysis indicated that using the central RC core, the ultimate capacity of the building as well as the ductility of the building increase significantly. Finally, the results obtained from the nonlinear time history showed that the nonlinear static analysis have good accuracy in calculating the maximum displacement of the structure.

In this study, 19 different mixtures, including 11 air-entrained mixtures, three flowable mortar mixtures, and five self-consolidating concrete mixtures were fabricated to investigate the effect of different factors, including air content, type and composition of pozzolans, and water to binder ratio(w/b) on electrical resistivity. These mixtures were made by substituting between 30-60% of cement weight by pozzolanic materials with 0.28 to 0.34 w/b ratios. The pozzolanic materials included zeolite, pumice, wollastonite, and slag. Electrical Surface and bulk resistivity tests were performed at different ages on 100 mm x 200 mm cylindrical specimens, and the relationship between the test results was investigated. The results of the study indicated that the electrical resistivity is related to the water to binder ratio, type and composition of pozzolans used, replacement level of pozzolanic materials, and entrained air amount. Among these factors, the w/b ratio and the replacement level of pozzolan used had a greater effect on the electrical resistivity of concrete. The combination of slag and zeolite had the greatest effect, and the combination of slag and wollastonite had the least effect on the development of electrical resistivity results in the mixtures. The result of regression analysis showed that there is a strong correlation between the results of bulk and surface resistivity of concrete at different ages.

Steel reinforced concrete (SRC) columns have become very popular in recent years due to their many advantages. In this study, to investigate the behavior of SRC columns with a cruciform steel section at their core, the ductility index and factors affecting it were investigated. Also, by calculating the necessary equations based on the plastic stress distribution method, a simple and practical method for designing these columns was provided. Accordingly, a finite element model was created using ABAQUS software for numerical study and was then validated with the results of a previous experimental study. A total of 16 columns with cruciform steel cores were analyzed for the present study. The effect of variables such as steel core percentage, concrete compressive strength, stirrup spacing, slenderness ratio, and load eccentricity ratio on the ductility index of these columns was discussed. The results showed that with increasing the eccentricity ratio, the bearing capacity decreases but the ductility index increases due to the change in behavior from compressive to flexural, such that as the eccentricity ratio increased from 0.1 to 0.4, the bearing capacity decreased by 59.1% but the ductility index increased by 28.2%.

Nowadays, concrete is known as one of the most used building materials in the world. Being economical, the availability of constituents, good resistance under the fire and atmospheric factors, the ability to fit in shapes and molds, and also the high compressive strength are factors that have made public acceptance in the use of concrete as a building material. The use of fiber in concrete has increased dramatically over the last few decades. The use of fiber in concrete causes the concrete to become flexible to a considerable extent and the resulting concrete is highly homogeneous. Since the concrete specimens are used worldwide to determine the mechanical properties of concrete, in this study, 160 samples of fiber concrete containing 64 cube samples with dimensions of 5×5×5, 10×10×10, 15×15×15, 20×20×20 cm for testing of compressive strength, 64 cylindrical cylinders with dimensions (diameter × height) of 15 × 30 and 10 × 20 cm for compressive strength and tensile strength tests and 32 samples of beam with dimensions of 10×45×10 and 15×60 ×15 were used for flexural strength testing. Furthermore, in this study, 4 concrete ranges of 20MPa, 25MPa, 30MPa and 35MPa were tested. Of each concrete grade and each dimension, four samples were made; one of which was non-fibrous (as the base sample) and three samples with fibers. The steel fibers used were two-end hooks of 3.5 cm in length and 0.8 mm in thickness, with 0.5% of the volume of concrete used. The results showed that the flexural strength, tensile and compression strength of the concrete increased with the presence of fibers and some new conversion factors were proposed for the fiber-containing specimens.

Reinforcement corrosion is one of the most important factors in reducing the strength and destruction of concrete structures during the lifetime of the structure. Various service life prediction models based on laboratory experiments or numerical analysis are used to predict the behavior of concrete structures in aggressive environments. Models that predict the time to cover cracking induced by corrosion are among the most important service life model in concrete structures. These models have some errors due to the influence of environmental factors and also the existence of uncertainties in the concrete properties. In this study, the effect of environmental conditions that are usually applied in regulations by increasing the thickness of the concrete cover was investigated on existing models. In addition to the concrete cover, the effect of other environmental parameters such as corrosion intensity was also investigated. The effect of uncertainty is applied by applying coefficients of variation to the parameters. The results show the effect of uncertainty of the amount of corrosion current density on the average value of crack initiation time so that by increasing the coefficient of variation of this variable, the predicted time for the occurrence of crack increases. this overestimated time is not conservative. The results also revealed that if the maximum ratio of concrete cover to rebar diameter (c/D) is selected in any environmental conditions, the crack initiation time can be delayed approximately 4.3 times.

Using self-compacting concrete is being developed due to its advantages. Also, the high-performance concrete has attracted a lot of researcher attention because of its special characteristics. In this research, a comprehensive experimental investigation is performed on the mechanical and flowability of high-performance self-compacting concrete. In this article, nine mix compositions were cast and tested. In mechanical properties part, the compressive strength test, splitting tensile strength and flexural strength test were done on the specimens. In the durability and flowability part, J-Ring and ultra-sonic pulse velocity (UPV) tests were conducted on the specimens. According to the results, the effects of fiber on tensile and flexural strength was significantly more than the compressive strength. On the other hand, the addition of fibers had negative effects on the workability and UPV results. Finally, to calculate the tensile and flexural strength of specimens versus compressive strength, the relationships were proposed.

Nowadays, it is a common idea to use decorative concrete panels as an alternative to natural stone. Graphic concrete is one of the different types of decorative concrete using the exposed-aggregate method. Considering that there has been no research for graphic concrete production in Iran so far, the aim of this research is to localize and simplify the method for the first time. For this purpose, concrete samples with 435 kg/m3 and 500 kg /m3 Portland cement and water to cement ratio of 0.40 and 0.45 have been produced using natural sand as aggregate. Also, superplasticizer with dosage of 0.35 % and air-entraining admixture with dosage of 0.02% have been used. Concrete specimens were tested in terms of visual criteria, compressive strengths and water absorption in 28 days in accordance with the requirements of Iran National Standard 12038. The results showed that samples with the water to cement ratio of 0.40, which were produced without using of superplasticizer and air-entraining admixture, due to low workability of the mixture, are not in good condition in terms of compressibility. Also low workability, in addition to disrupting the designs printed on the membrane, causes many holes on the concrete surface after leaving the mold, which is visually rejected. Despite the positive effects of air entraining admixture on workability, plastic viscosity and visual criteria, this type of admixture reduces the compressive strength by 30 to 35% and increases water absorption, which is not recommended. Finally, mixture design with 500 kg/m3 of cement, water to cement ratio of 0.40 and 0.45, which were produced using superplasticizer with 28-day compressive strength of 54.6 and 52.5 MPa and Water absorption of 3.25% and 3.50% while meeting all requirements are suitable for production of graphic concrete decorative panels.

Alkali-activated concretes with slag base as new materials could be used to achieve a healthy environment without pollutants such as greenhouse gases and to solve the problems of water shortage and groundwater resources and use by-products produced during the processing of materials such as iron, steel and copper alloys for construction projects. On the other hand, porous concretes have economic and environmental potentials such as preventing flooding, increasing groundwater reserves, decreasing the flow of surface water. Slag-based alkaliactivated concretes have been synthesized by natural pozzolans and industrial wastes such as sodium silicate with alkaline silicate and alkali hydroxide solutions. In the current study, the performance of alkali-activated porous concrete with the different amounts of sodium hydroxide molarity and the ratio of sodium hydroxide to sodium silicate has been investigated in terms of mechanical properties such as compressive strength. The flexibility and permeability of the specimens were also investigated. The tests have been performed on 9 series of samples with three values of 8, 12 and 16 Molar of sodium hydroxide and three ratios of sodium silicate to sodium hydroxide 1, 2 and 3 under curing times of 7, 14 and 28 days. The test results show that the alkali-activated porous concrete indicated a higher initial strength compared to conventional porous concretes. The compressive strength at curing time of 14 days and flexural strength at curing time of 7 days were about 75% of its strength at curing time of 28 days which is significant. The compressive and flexural strengths have been increased with the increasing of molarity and the ratio of sodium silicate to sodium hydroxide about 20 to 25% and 9 to 13%, respectively. However, the permeability decreases with the increasing of molarity of sodium hydroxide solution or the ratio of sodium silicate to sodium hydroxide.

Considering the important and vital role of dams in the industry and economy of countries and the catastrophic nature of their possible failure, the safety of these huge structures is of special importance. Arched, Karun 4 double arched concrete dam has been selected as a case study. ABAQUS 6.12 finite element software was used for numerical analysis of Karun 4 dam. In this research, three numerical models were modeled and analyzed so that in the linear model, the dam body was completely integrated and homogeneous, and in two nonlinear models, the contraction joints in the dam body with two different and widely used types were considered. The results show that the lack of injection of the joint has caused a significant increase in the maximum stresses upstream of the dam so that most of the upstream of the dam, which was initially working under pressure, has been stretched. Tensile stresses in the upstream support and downstream canopy of the dam have also increased. On the other hand, the amount of vertical compressive stresses between the contact surfaces in the absence of seam injection has increased significantly that these stresses in the vicinity of the injection stop level has almost doubled to the original level. Blocks zero and one, the maximum of which in full injection is 13.5 MPa, has increased to 11.3 MPa at this point without injection. The results show that by increasing the joint thickness and if the joint is not injected, the values of maximum stresses, minimum stresses, barrier displacements and vertical compressive force between the contact surfaces are significantly increased.

A review of the damages caused by previous earthquakes shows that a significant number of the structures constructed in the country are neither earthquake-resistant nor have enough strength. Consequently, reliable, fast, and easy retrofitting methods are needed to improve these structures to withstand lateral forces. In the present study, 9 concrete frames were tested, and yielding steel dampers have been used for the seismic rehabilitation of the concrete moment-resisting frames. TADAS (Triangular-plate Added Damping and Stiffness) dampers are attached to the concrete frame using steel braces. The dampers and concrete beams are connected with concentric steel braces. Moreover, three concentric braces have been installed to investigate the influence of the braces on the strength and behavior of the structure. Compared to the moment-resisting frame, the structure that used TADAS dampers exhibited greater strength and recorded fewer damages. Cyclic displacement loading was applied to these frames, and their characteristics, including strength and crack, were investigated. In comparison with the control frame, the steel braces and yield dampers increased the structural strength of the concrete frames by 48% and 28%, respectively.

This study aims to analytically investigate the impact response of FRP-retrofitted RC slabs. In this regard, based on the orthotropic plate theories, an applicable formulation is derived to assess the displacement of simply supported RC slabs due to the applied impact loading induced by dropping an impactor. The results have then been compared with the experimental data available in the literature. The proposed analytical formulation is proved to be reliable and efficacious to an extent that the displacement can be attained up to seven percent error. The effect of number of FRP layers and compressive reinforcement ratio have also been examined on the impact response of FRP-retrofitted RC slabs. The results indicate that adding 7 layers of GFRP strips to the bottom face of the RC slabs can attenuate the displacement values up to 33 percent. Moreover, the effect of compressive reinforcements on improving the impact response of RC slabs was estimated to be 30 and 23 percent, respectively, for RC slabs being examined.

Common methods of controlling setting, hardness, process of strength concrete are tests for determining the setting time of mortar (ISIRI 392) and concrete (ISIRI 6046) and determining the compressive strength of concrete (ISIRI 3206). Since these physical and mechanical methods do not fully reflect the behavior of concrete and also it is time and money consuming, application of electrical resistance of concrete has been investigated. Therefore, in this study, an extensive laboratory operation was designed. In the first stage, 11 samples of cement were prepared, the phases of which varied, but due to laboratory cementation, their fineness and grading were almost the same. Physical and chemical analysis of cement samples were performed. In the next step, 22 concrete mixtures with 11 cement samples were prepared, so that 11 mixtures with the title of control (without additives) and 11 mixtures with constant dosage of commercial super plasticizer (based on poly carboxylate). Compressive strength tests has been performed at ages of 7, 28 and 90 days. Also, an electrical resistance test was performed, which was performed regularly from the time of cement contact with water until 31 days later. The results showed that at least three peaks of 8, 16 and 23 days in the electrical resistance curve are seen along the test time. In almost all samples, the electrical resistance is reduced to about 3 hours, which indicates the setting time of the concrete. Based on the results of this study, formula was presented