جستجوی مقالات مرتبط با کلیدواژه « دما » در نشریات گروه « عمران »

The concrete surface is the most significant part as it is in direct contact with the outside world. Therefore, increasing the surface strength of concrete could reduce the absorption of harmful substances into the concrete. Permeability and surface strength can be regarded as factors that affect the durability and service life of a concrete structure. As a result, a concrete structure with the desired surface strength and low permeability is of paramount importance. Hence, this study attempted to measure the surface strength and permeability of concrete through “twist-off” and “Cylindrical Chamber” tests under temperature cycling. Relations between the surface strength and permeability of concrete samples were obtained. In addition, a linear function was used to estimate the relation between penetration depth and volume with high precision. According to the twist-off test results, extending the curing age of plain concrete would increase the surface strength; the surface strength at the age of 120 days exceeds that of 7 days by approximately 42%. However, when temperature cycling was performed on the samples, the surface strength of the concrete caused by the twist-off Test decreased. Further, results from the permeability tests using a cylindrical chamber indicate that temperature changes have a negative influence on the permeability and penetration depth in concrete samples. The permeability and penetration depth of concrete under temperature cycling caused an 8-fold and 3-fold increase after 120 days, respectively. According to the results of the study, moderate temperature cycles significantly affect permeability and penetration depth.

Every year, huge costs are spent on the maintenance of road pavements due to premature reflective deterioration of the road surface, a large part of which is related to road surface paving. Accordingly, in order to prevent the propagation of reflective cracks in newly coated pavements, various methods have been used and researched. The results have shown that all the adopted reflective crack control methods have only delayed the propagation of this type of crack. However, some methods have caused a greater impact on the appearance of reflective cracks and reducing the intensity of these cracks, and one of the best methods is the use of geosynthetic products and interlayers. In this research, the performance of 2 different types of geocomposite (with tensile strengths of 50 and 100 kN/m) in asphalt pavements in delaying the initiation of reflective cracks compared to control samples at different temperatures and loading frequencies was investigated. Based on the studies, it was observed that the use of type I geocomposite with high tensile strength is the most effective in increasing the fatigue life and among the mentioned factors, the temperature parameter will have the most negative effect on this performance. After the temperature, in order of the type of geocomposite used in terms of tensile strength and then the loading intensity, they will have the greatest effect on the fatigue life changes (about 53,000 cycles of loading difference, between the temperature of 40 and 0 degrees Celsius) on the reinforced asphalt coatings. Based on the results of variance analysis of the fatigue life model obtained in this research, the value of R² and Adjusted R² are equal to 0.987 and 0.981, respectively, which indicates the high accuracy of the presented model.

Asphalt mortar and asphalt mixture are viscoelastic materials due to the presence of bitumen in their structures. The master curves of dynamic modulus and phase angle developed through the horizontal shifting of the results using the appropriate shift factors are commonly used to describe the viscoelastic properties of asphalt mortar and asphalt mixture. In this study, constructing master curves of dynamic modulus and phase angle of asphalt mortar and asphalt mixture using the least number of test temperatures and frequencies was investigated. The dynamic modulus test was performed on asphalt concrete and asphalt mortar samples at six temperatures and frequencies. Then, the Laboratoire Central des Ponts et Chaussees method was used to determine the shift factors for constructing master curves. Also, the modified Christensen-Anderson-Marasteanu model was fitted on the master curves to predict their viscoelastic properties at an arbitrary temperature and frequency. In addition, the master curves and the fitted models which are created by using test results at two frequencies and six temperatures, six frequencies and three temperatures, or three frequencies and three temperatures, could provide the same predicted values and patterns as the original fitted model with more than 92% accuracy. So, as a result, it is possible to develop the master curves of the viscoelastic properties of asphalt mixture and asphalt mortar with a lower number of test results that have similar accuracy to the original master curves and reduce the time and cost of experiments up to 50%.

Wheat is one of the most important grain crop in the world . This kind of plants getting infected by various natural and human factors. Occurrence of disease in this crop may cause disturbance in the circle of economic and management. Therefore prediction and prevention of this disease is one of the main concerns for managers. The method that can help to predict the conditions suitable for disease outbreak can be highly useful. So to achieve this goal, it is tried to study the environmental parameters that makes this disease occur. Then, based on the obtained parameters and using satellite images, the areas that have the potential of the disease outbreak was detected. The study area in this article is Argentina. MODIS satellite images with a spatial resolution of one kilometer is used. Based on previous researches, the parameters such as air temperature, humidity and greenness are the most important parameters to for occurrence of wheat rust disease. The output of this work is an image with two classes of potential zone for rust outbreak and safe zone. The ranges for air temperature, air humidity and greenness for the occurrence of the disease was introduced. Finally an algorithm for identification of the potential zone was introduced. According to the results obtained in this study, the temperature between 2 and 37 degrees Celsius, humidity more than 19% and the NDVI highre than 0. 3 was suitable for the disease outbreak. The overall accuracy in identification of these zones was 90. 70 percent.

In this study, the effect of penetration and temperature changes on the performance specifications of bitumen was investigated experimentally. In this regard, it was ensured that the bitumens were obtained from a constant raw material and oxidation process at different times from the reactor. Experiments were carried out on bitumen with different penetration grades of 35, 40, 50, 55 and 60 (1⁄10 mm ). The effect of changes in the penetration grades and temperature were investigated on various quality parameters of bitumen, including bitumen specifications at high and medium temperatures, after initial and long-term aging, using rheological behavior tests by dynamic shear rheometer, thin film oven and pressurized aging vessel and the corresponding comparison diagrams were drawn. The results show that the rutting resistance or permanent deformation of bitumen in both binder and residual bitumen of thin film oven test was decreased with increasing temperature in a bitumen with a constant penetration grade and also increasing the penetration grade at a constant temperature. The Fatigue cracking index in the residual bitumen of the thin film oven and pressurized aging vessel, was decreased with increasing temperature in a bitumen with a constant penetration grade and also increasing the penetration grade at a constant temperature.

Cement production is one of the major pollution contributors owing to its large rates of energy consumption and gas emission. Moreover, high temperatures could detrimentally impact the concrete infrastructure and thus, it would be essential to study performance of such structures under exposure to the elevated temperatures. In this paper, post-heat performance of the concrete whose cement has been replaced by zeolite and bentonite at ratios of 6 and 10% (by cement weight) under exposure to temperatures of 28, 150, 300 and 700°C, was studied. For this purpose, cubic specimens with sizes of 100×100×100, 150×150×150 and 200×200×200mm and cylindrical specimens with sizes of 200×100 and 300×150mm, were produced. Based on the results, replacing cement by zeolite and bentonite at the age of 90 days under ambient temperature, increases the compressive strength compared to the control specimen. Moreover, it was observed that heating the cubic and cylindrical specimens containing 10% bentonite at 150°C, increase the compressive strength by 40%.Conversely, the results indicate that when exposed to temperatures of 300 and 700°C, a decreasing trend is seen in the tensile strength of both cubic and cylindrical specimens containing the pozzolans. Besides, to study micro-structure of the specimens, the X-ray diffraction tests were carried out on the specimens at the age of 28 days. The difference between conversion ratio of the cubic and cylindrical specimens in this study, to the values provided by the codes, is less than 10%.

In most studies, the increase in hydraulic conductivity with temperature has been considered due to the decrease in the viscosity of fluid (Habibagahi, 1977; Cho et al., 1999; Delage et al., 2011). However, the changes in hydraulic conductivity with temperature are not only influenced by the changes in water properties, but also by the thermal effect on soil-water interaction at the microstructure level (Towhata et al., 1993; Romero et al., 2001; Villar and Lloret, 2004). In the present study, results of temperature effects on the hydraulic conductivity of compacted clay from the Nazlou region of Urmia City (Iran), and geosynthetic clay liner (GCL) are presented. In this research, experiments were conducted by flexible-wall triaxial permeability apparatus. In order to increase the temperature of the permeability cell to a desired level, a heater and a temperature sensor were used. Results showed that by increasing the temperature, the viscosity of fluid decreases, the soil pore size increases, cross-section of effective flow increases and hence, the soil hydraulic conductivity increases. Increasing the effective stress causes the rate of increase in soil permeability due to temperature to decrease. Results showed that temperature increase does not have a significant effect on the hydraulic conductivity increase of geosynthetic clay liners.

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.

Concrete structures are always affected by environmental conditions, and hot weather condition during mixing and casting impressively affect the performance of concrete. Concrete producers have been using different methods of reducing the temperature of concrete such as liquid nitrogen through direct injection of it into the drums of concrete trucks. Liquid nitrogen with having temperature of -196°C provide accurate temperature control and is not limited by the water-cement ratio as in the case of chilled water or ice. The purpose of this research was to investigate the effects of liquid nitrogen on concrete properties such as slump, setting time, compressive strength, tensile strength and flexural strength with two different curing methods. In this research, three mixtures were prepared: control, hot and LN-cooled. The temperature in control mixture was considered to be 27°C. Hot mixture which was made from materials stored for 72 hours at the desired temperature (50°C) to simulate hot weather condition, and the temperature in this mixture was obtained 40°C. The process in preparation of the LN-cooled mixture was the same with the hot mixture with the difference that, the temperature of the LN-cooled mixture was lowered to the control mixture (27°C). The results indicate that the liquid nitrogen improved the properties of concrete in hot weather and additionally the results show that the properties of concrete are relatively unaffected when cooled with liquid nitrogen to room temperature. Finally based on findings in this research, liquid nitrogen is an appropriate option in hot weather concreting.

One of the most effective factors regarding the design of airfield concrete pavements is its performance relative to the temperature and static load caused by airplane traffic. This research aims at evaluation of tensions and deformations occurring in concrete slabs at the airfield concrete pavements, arising from temperature variations and the layout of the aircraft wheels design (Airbus 320), in the 3D model and in the SAFE software environment. By analyzing models and outcomes, it is observed that tensions and displacements and deformations created in the slabs under load caused by the temperature, are much higher than the wheel load. Furthermore, by increasing the thickness of the slabs and their dimensions, the stresses caused by the temperature are increased while the stresses caused by the wheels are decreased.

The self-compacting concrete is more sensitive to changes in the temperature with respect to the ordinary concrete. Increase in the temperature affects the cement hydration rate and minor mistakes in the amount of used cement could bring about significant problems concerning the properties of self-compacting concrete, mostly affecting workability. Therefore in order to understand the cement mechanism in the self-compacting concrete, care should be exercised to regulate workability through right consumption of cement. In this research the temperature of the concrete mixtures was selected with respect to the ambient conditions in different seasons. Thus, after reaching the concrete temperature to the ambient temperature, the rheological properties of the self-compacting concrete with different amounts of cement as a function of mixing time, and temperature with water to cement ratio of 0.42 were investigated. According to the obtained results from the rheometer in terms of the yield stress and plastic viscosity during 60 minutes, it was found that addition of cement up to 440 kg/m3 improves the rheology. Furthermore referring to the results of this research it should be stated that for improving the rheology, one should prevent rise of temperature in the concrete and reduce the concrete placing time, otherwise it could result in concrete hardening and consequently reduced rheological properties.

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.

The durability-based design of reinforced concrete structures especially in hot marine environment of Bandar-Abbas became extensively important during the last few years. It has been known that the durability of concrete mainly depends on how easily seawater containing chloride ions can ingress into an unsaturated concrete (Sabir et al., 1998; Hubert et al., 2003; Prabakar et al., 2010; Yoo et al., 2011). Chloride-induced corrosion of embedded reinforcement is more severe in tidal condition because of more ingress of chloride due to moisture convection phenomena. (Broomfield, 1997; Olajumok et al., 2009; Akindahunsi et al., 2010). In is necessary to approximate the moisture distribution in concrete due to wetting and drying cycles for determination of the moisture convection zone. The moisture transfer coefficient (MTC) is one of the most important parameters for prediction of moisture distribution. The moisture distribution is often influenced by environmental conditions such as temperature. The changes of the depth of convection zone can be assessed if MTC is available in various temperatures. In this research, six concrete mixtures were prepared in which four plain concrete mixtures were proportioned with water to cementitious materials (w/cm) ratios of 0.40, 0.45, 0.50, and 0.55. In addition to the plain concretes, one silica fume and one natural zeolite blended concrete mixtures with cement replacement levels of 7.5, and 10% and a w/cm ratio of 0.45 were considered. The moisture loss or water absorption of specimens exposed to wetting and drying were measured using the gravimetric method at temperatures of 23, 43, and 63 ºC. A finite element analysis was performed afterward to fit the experimental data to the governing equations of moisture transfer to determine the MTC. The moisture distribution of concrete exposed to wetting and drying and then the depth of convection zone (DCZ) are determined using approximated MTC and finite element-based model. The amount of water entered (AWE) to concrete surface subjected to tidal condition is also calculated.

In the present era, Climate change and its impact on available water resources are one of the main challenges. In this regard, temporal and spatial analysis of temperature and precipitation, which are important parameters in determining the status of water resources, can be used to assess the hydro-climatological conditions of the watershed and appropriate management policies. In this research, the trend in precipitation and temperature distribution over 30 years testing period of 1981-2010 was investigated using non-parametric tests such as Man-Kendal, Spearman, Sen’s Slope, and Pettit. Afterward, interpolation techniques, such as IDW, LPI, GPI, and RBF were used to detect spatially trends at the watershed. The results showed that precipitation decreased by 14.3% during the period 1981-2010 and the temperature increased by 3.5%, with changes in precipitation and temperature occurring in 2004 and 1985, respectively. However, the negative trend in precipitation was not significant in contrast to the positive temperature trend during the study period. A comparative analysis of interpolation techniques shows that Ordinary Kriging and Radial Basis Functions with least error are the best methods for spatial analysis of precipitation and temperature, respectively

Investigation of the effect of temperature on soil strength is one of the issues which has been considered by many researchers in recent decades. In this study to investigate the effect of temperature on the undrained shear strength (Cu) of clay soils, a cell with the capability of both changing and keeping fixed the temperature of sample, was designed and constructed. After determining the index properties of samples for Kaolinite clay soil, undrained shear strength (Cu) test was carried out on saturated clay soil at 10, 20, 30, 40, 50, 60 and 70 ° C. Repeatability of the results was confirmed by repeating tests on samples with the same properties a given temperature. The results showed that by increasing the temperature, the Cu values decreased so that, as the temperature raised from 10 ° C to 70 ° C, the values Cu were reduced from 26.6 to 10.94 kPa. Accordingly, in the studied temperature range, an empirical relationship between temperature and Cu (with R2= 0.96) was proposed. The general shape of the stress–strain curves of the samples in different temperature was the same and in strain level of 20% was linear. Increasing the temperature caused to decline in the range of elastic deformation and enhancement in the range of plastic deformation of the samples; in addition, by increasing the temperature, the angle of the failure plane was decreased. The measurement of axial expansion stress (AES) indicated enhancement of the stress by increasing the temperature.

Designing and developing rheological properties of self-compacting concrete is an important issue in concrete technology. This matter is because of the combination of varied needs in the fresh mode, extremely complexed mixture proportions, and its naturally low yield stress. Selection of chemical admixtures, especially super-plasticizers plays an important role in Designing of self-consolidating concrete (SCC). In this paper, the rheological properties of SCC samples made with various chemical admixtures in various mixing time and temperatures were examined. Super-plasticizers are strongly influenced by the ambient temperature, due to the fact that the temperature dependent on the hydration rate affects the absorbance rate of the super-plasticizer as the ability to absorb the polymer content. The rheological properties of the SCC were examined in various mixing time and temperature. After changing the temperature of the SCC to the ambient temperature, the rheology test was conducted using rheometer in 60 min. In this research, the dosage of rheological parameters and its growth rate with time have been compared in various temperatures. According to the obtained results, Naphthalene sulfonate cause to increase yield stress and to decrease plastic viscosity more than poly-carboxylate in SCC, in other words, poly-carboxylate cause better viscosity in concrete. Also, concrete containing naphthalene sulfonate has less yield stress growth rate and viscosity by increasing the time. On the other hand, slump loss of the SCC containing naphthalene sulfonate is less than the one containing poly-carboxylate. That is to say, naphthalene sulfonate protects the fluidity more than poly-carboxylate in SCC.

Present study illustrates the importance of simulated 3D finite element in order to calculate the flexible pavement response with viscoelastic behavior under moving load and different tempratures. 3D finite element was simulated by Abaqus software. In the simulated model, regarding to load speed, moving load was applied using separated load in sequential nodes in the path direction. In order to simulate the silent boundaries, the outer boundaries were meshed through infinite elements. Viscoelastic behavior of asphalt layer was defined by generalized Maxwell model with Prony series. Firstly, in this study, the accuracy of simulated model in Abaqus was verified using the paper of Chen and et al. that studied a layered viscoelastic pavement using analytical approach and then flexible pavement response was studied under different moving vehicle speed and different temperature. The results show that the pavement response increases with increasing the temperature and with decreasing the speed.

In this paper, the eect of multi-walled carbon nanotubes on the properties of concrete was evaluated in the post-heat treated condition. Following this, a number of cylindrical specimens (10 x 20 cm) including multiwalled carbon nanotubes in dierent percentages of 0.5, 1 and 1.5% by weight of cement were cast. Later on, concrete specimens in the electric furnace were exposed to temperatures of 25, 100, 250,500 and 7000C and after cooling down, compressive and tensile strength tests were carried out on them. The results showed that by increasing multi-walled carbon nanotubes in concrete, compressive and tensile strengths of concrete were increased up to 138% and 88%, respectively. In addition, dissipation of energy and modulus of elasticity of concrete specimens were up to 2 times more than that of control concrete specimens. The SEM test results indicated that a strong bond between concrete particles exists at the room temperature and upper than that.

In recent years, the construction of high - security buildings against the conditions of explosion and fire is a very important issue in the world. Therefore, one of the appropriate strategies in this field is to use steel - concrete composite columns to be used in the core of the concrete column to maintain its durability and stability against high temperatures. This paper studies the impact of temperature rise on the performance of concrete-filled double skin tubular steel columns with prismatic geometry. In doing so, a number of columns whose interior core was square, diamond and circularly-shaped and their exterior core was prismatic with a square cross-sectional area increasing with the slope of 2/1 degrees from top to the bottom, were constructed and exposed to the temperatures of 25, 250, 500 and 700 °c. Accordingly, the temperature rates up in compliance with ISO-834. Afterwards, all column specimens were subjected to cyclic loads adopted from the ATC-24 loading protocol which proceeded until the specimens failed. Firstly, the 28 - day compressive strength of concrete, measured, and steel used is tested under tensile test and the stress and strain characteristics were obtained. The results indicate that although the failure mode of the columns with interior core of square or diamond shape resembles each other, the columns whose interior core was circularly-shaped, experienced more intensive damages compared to the latter specimens. Failure mode of the columns base was developed as a diagonally with degree of 450 by temperature of 500 °c but at the temperature of 700 °c, the damages have occurred horizontally at the level of 10 cm from the column base. Moreover, initial stiffness and ductility ratio of the columns with diamond-shaped interior core was approximately 2 times greater than the other columns. in all columns, the maximum damage in the leg area is caused by the maximum anchor in that area caused by 25 to 700 °C, and with increasing temperature, the central and middle regions are also completely destroyed, so that at 700 °C in all specimens, concrete is completely crushed in such a way that it has virtually no role in the load - bearing capacity of the column. The destruction of concrete in columns is almost identical in all columns of the inner circle of the circle, squares and diamond. In general, the load - bearing capacity of the columns decreased by about 50 % after 15 cycles at a temperature of 700 °C after 9 or 12 cycles, and the loading process was halted. In the meantime, the columns of the inner circle of the circle had broken out earlier than the other columns and did not have proper functioning. also in the diamond - shaped columns, the rupture process and the severe reduction of the carrier capacity are initiated at 500 °C, indicating that this column does not indicate that this column is due to heterogeneous geometrical distribution at the height of the column.

The use of externally bonded fiber reinforced polymer (FRP) has been proven to be an effective and efficient method to strengthen deficient concrete components and structures. This paper presents numerical investigations on the de-bonding behavior of concrete beams strengthened with externally bonded CFRP fabrics, at elevated temperatures. The numerical study was carried out, using the commercial package, “Abaqus”, for finite element analysis. The simulation was validated against laboratory four point bending tests of nine plain concrete beams previously performed by the authors. The behavior of concrete beams with three different design concrete strength of 20, 30 and 40 MPa which were externally bonded by CFRP fabrics were investigated at -20, +20, +50 and +80 Degree Celsius. Linear elastic isotropic and orthotropic models were used for the CFRP layer and a cohesive bond model was used for the concrete – CFRP interface. A plastic damage model was also used for the concrete. In the finite element analysis, similar to the laboratory tests, the specimens were first heated up to the elevated temperatures, and then loaded up to failure. The results show good agreement with the experimental data, regarding the failure loads, load–displacement response, and crack patterns. The finite element results of thermal strains in CFRP and stresses in the concrete are also compared with the thermal strains and stresses, determined by the analytically models.