نشریه مهندسی عمران مدرس
سال دوازدهم شماره 3 (پاییز 1391)

The durability of concrete structures against corrosion due to high concentration of chloride ion is considered as a main concern in these kinds of structures in Persian Gulf area. However، increasing the initial temperature its effect on concrete microstructure can intensify this issue & severely increase the concrete corrosion rate. This article deals with the high initial temperature effect on chlorine ion penetration in concrete، particularly concretes consisted of silica fume. Evaluation of high initial temperature has been done by making samples in different temperature till initial setting & keeping in the same conditions in the environment temperature to simulate the environmental conditions of Persian Gulf. For this purpose، the changes of concrete micro structure due to initial temperature increase & its effect on concrete resistance properties against chloride ion penetration have been studied. In this paper، the influence of high initial temperature on chloride penetration and microstructure of concrete containing silica fume was investigated. Two different mixes at three different initial casting temperature (20-40°c) were studied: a control mix in which no cement replacement materials were added and a mix where cement was partially replaced 6% silica fume (by weight)، at a constant water-to-binder ratio of 0. 45 and a cement content of 400 kg/m3. High initial temperature casting was employed to simulate concrete temperature in Persian Gulf hot climate. The results show that chloride penetration at 40°c is directly related to increase in the porosity of the binder phase and the absorption of concrete. The Higher chloride penetration resistance was observed when cement is partially replaced with silica fume. From SEM result recognized silica fume particles make core sites to cement hydration، therefore make more consistency micro structure and decrease pore size distribution. It is concluded that increasing the initial temperature of the concrete in the time of casting & formation، leads to accelerating the hydration & therefore the heterogeneous distribution of products in the mixture. This phenomenon causes the bigger pores in the concrete structure by increasing the temperature and more space for chloride ion penetration. So، high casting and initial temperature leads to increasing chloride ion penetration & reduction of concrete resistance against corrosion. By adding silica fume to the mixture، size & the distribution of pores improve and the temperature increase effect on parameters of chloride penetration & corrosion decrease. Hence، the corrosion behavior (Chlorine ion penetration coefficient & electrical potential) of 40°c silica fume included specimen are improved than reference concrete.

Hydroquinone is one of the most important phenolic compounds، which has a wide application in industries such as rubber، photo developing، cosmetic، and anti-oxidants. Because of its toxic effects، an efficient system should be provided for treating these kinds of wastewater. Therefore in this study، the performance of conventional and packed-cage RBC biological systems in treatment of wastewater containing hydroquinone، and parameters affecting the process were studied. In this study، the removal efficiency of hydroquinone was investigated in two different laboratory scale RBC systems. The first system was a two-stage RBC with rotating discs and the second one was a one-stage packed-cage RBC with bee-cell 2000 biofilm carriers. Each stage of the rotating discs RBC was consisted of 27 parallel plexiglas rotating disks with 15 cm in diameters. The packed-cage RBC had a net drum full of biofilm carriers with specific surface area about 650 m2/m3. Both systems had a total area of 2 m2 and were fed by a peristaltic pump. During the startup، the reactors with sludge seed، daily dosage of 200 mg/L COD as glucose and synthetic wastewater was fed to each reactor. After increasing biofilm mass، the acclimation was started with stepwise substitution of glucose with hydroquinone (CODhdroquinone/CODtotal was increased about 10% in each step). After acclimation stage، the amount of COD was being increased stepwise up to 5000 mg/L. Both RBC systems as advanced biological processes had proper COD removal efficiencies for treating hydoquinone synthetic wastewater. Up to the maximum 90، 93 and 88 percent removal efficiencies were obtained in RBCI، RBCII and packed-cage، respectively for COD concentration of 1000 mg/L، hydraulic loading rate of 1. 5 L/m3. d and optimum rotation speed of 10 rpm. Also، During the experiments up to 4000 mg/L influent COD، RBC with rotating discs had higher removal efficiencies (about 5%-15%) but at higher loading rates، packed-cage RBC had better results. The effect of hydraulic loading، and the rotational speed of disks in the performance of the both systems were studied. The results proved that the amount of hydroquinone removal increased with raising the rotational speed of the disks up to 10 rpm، and increasing hydraulic loading had a negative effect on the COD removal efficiencies in both conventional and packed-cage RBCs. Also COD concentration increased up to 10000 mg/L in one step whille the hydraulic loading rate and rotational speed were 1. 5 L/m2. d and 5 rpm، respectively. The results showed that conventional RBC had a better performance for the organic shock. For ensuring of hydroquinone biodegradation in the RBC systems carbon-13 NMR spectroscopy was also studied.

Metal foams are a new class of materials with interesting structural properties; however no comprehensive understanding of their inelastic behavior has been established yet. Since the experimental studies of these materials have their own limitations، there is a growing research interest towards the mesostructural modeling of these materials. Accordingly many researchers have been trying to generate realistic and representative numerical models of the foams and prepare computational labs in which different aspects of foams mechanical behavior can be thoroughly investigated. The following three kinds of mesostructures have been commonly employed: (1) models based on a unit cell or a building block، (2) random Voronoi diagrams، and (3) CAD structures provided by the X-ray micro-computed tomography. In the current study، the physically representative circle set Voronoi diagrams are employed to define the geometry of 2D metallic foams. It is assumed that the minimum and maximum radii of the circular generators are 0. 5 and 1. 5 mm، respectively. The first sample is generated using linear distribution of cell size while، compared to the first sample، the second and third specimens have less and more small cells. An extra specimen (the forth sample) is also created with the same structure of the first one unless its edges are straight. In the next step، the FE models of the specimens are created using second order Timoshenko beam elements. Finally، the effects of microstructural features (e. g. strut curvature and cell size distribution) on the initial yield surface، elastic properties، and failure modes of the foams are numerically investigated under various biaxial loading conditions. Displacement-controlled loading is used. A newly energy-based approach developed for the identification of initial yield points has been incorporated. The results show that: (a) the size of the initial yield surface is significantly influenced by the curvature of the cell struts، (b) in the principal stresses space، the initial yield surface is bigger in the tension-tension region، (c) for a constant relative density، the presence of more big cells in a sample increases the size of the yield envelope، and (d) the macroscopic yield properties of the specimens can be interpreted according the microscopic failure mechanisms of the plastic yielding، elasto-plastic buckling، and plastic hinging of the struts. Furthermore، it is found that the previously proposed energy-based method for the identification of yield initiation under multiaxial loading conditions has serious shortcomings and needs revision.

Steel is considered to be one of the desirable materials used for reinforcing concrete structural members. However، the corrosion of steel bars has been always a threat for the service life of reinforced concrete members in corrosive environments. Fiber Reinforced Polymer (FRP) bars can be used as reinforcing materials due to their corrosion resistance. FRP reinforcing bars are available in different grades of tensile strength and modulus of elasticity. These bars have high tensile strength and durability and display linear elastic behavior up to their failure. The behavior of concrete beams reinforced with FRP bars is different from that of steel reinforced concrete beams. Concrete beams reinforced with glass fiber reinforced polymer (GFRP) bars exhibit large deflections and crack widths as compared with steel reinforced concrete beams due to the low modulus of elasticity of GFRP. In addition، the bond between concrete and FRP bars is different from steel bars because of the difference in their surface geometries and mechanical characteristics. This paper proposes an equation for the bond strength of lap-spliced concrete beams reinforced with FRP bars. First، equations for displacement modulus and local bond strength of FRP bars are formulated by pullout test results، tested by other researchers. Then، using the local bond strength equation and based on the experimental results of lap-spliced FRP reinforced concrete beams، an equation for bond strength of splices is derived. In the formulation of this equation، the non-uniform distribution of the bond stress along the splice length is considered. The effects of concrete cover and transverse reinforcement are also taken into account in the proposed equation. Transverse reinforcement has an important role in the bond strength of beams with spliced bars. Transverse reinforcement confines developed and spliced bars by limiting the progression of splitting cracks and increases the uniformity of bond stress distribution along the splice length and thus، increasing the bond strength. The bond strengths calculated by the proposed equation are compared with the experimental values. The comparison shows that the proposed equation predicts the splice strength accurately. Also، calculated bond strengths are compared with the values predicted by different code provisions and other models. The average and standard deviation of the experimental over calculated bond strength ratios obtained by the proposed equation are 1. 00 and 0. 14، respectively. These ratios are 0. 65 and 0. 19 for the ACI440. 1R-06 code، 0. 55 and 0. 15 for the CAN/CSA-S6-00، 0. 67 and 0. 16 for the CAN/CSA S806-02 code and 0. 99 and 0. 36 for the Aly equation. Compared to Aly equation and design guidelines، the proposed equation for calculating the bond strength shows better agreement with experimental values. In addition، code equations overestimate the bond strength of GFRP bars in splices of beams.

Hilbert-Huang transform (HHT) consists of two main parts: (1) empirical mode decomposition (EMD) to extract intrinsic mode functions (IMFs) and (2) Hilbert spectral analysis to obtain time-frequency characteristics of the IMFs through the Hilbert transform. Recently، a new enhanced HHT is proposed by the authors in which، two mathematical limitations that restrict the application of the Hilbert transform are circumvented and also an additional smoothing parameter is applied to decrease noise effects on the results. In this paper based on the HHT approach، a simple method for output-only identification of natural frequencies of linear structures is proposed in which HHT or enhanced HHT can be employed. In the proposed method، ambient response data measured at all degrees of freedom of the structure are used to obtain an averaged marginal spectrum. The averaged marginal spectrum is used for identifying the natural frequencies of the structure. In order to validate the effectiveness of the proposed identification method، ambient response data of an arch bridge and a 15-story building are examined. In the first case، the first six natural frequencies of the bridge in vertical direction are extracted. And in the second case، the first three natural frequencies of the building in East-West، North-South and torsional directions are identified. From the results، first، it is found that the enhanced HHT by employing the smoothing parameter is more efficient than the HHT in increasing the readability of the time-frequency-amplitude spectrum and also is capable to provide more accurate amplitude-frequency distribution; second، by comparing results of the proposed method with those obtained from other valid methods، it is concluded that the proposed identification method by using the enhanced HHT is accurately able to estimate the natural frequencies of structures. Regarding to simplicity of the proposed method، it can be applied as an efficient tool for identification of structures or employed to extract changes in frequencies due to occurrences of damages during strong ground motions.

Confined masonry consists of load-bearing walls with some slender cast-in-place tie columns connected with tie beams. These confining elements are usually made of reinforced concrete and especially located around the critical points such as openings and corners. Although the response of confined masonry walls has been extensively studied in experimental tests worldwide، the analytical models capable of capturing deformation and strength characteristics of these walls and also technical guidelines which can help engineers to numerically evaluate the seismic resistance of confined masonry structures are rate. In this study، the micro-modeling strategy is adopted for the numerical simulation of unreinforced masonry walls confined by reinforced concrete tie columns and beams. A modified version of general path dependent contact density model is used to simulate the complicated response of brick-mortar interaction in the mixed mode of shear and axial deformations. A nonlinear finite element analysis program called “WCOMD_SJ” is used for this purpose. This program has been developed at University of Tokyo and is an analytical tool for two-dimensional static and dynamic nonlinear analysis of reinforced concrete structures based on fixed smeared crack approach. This program has been modified by the second author for nonlinear macro and micro analysis of masonry structures. In order to validate the analytical approach، experimental test results and gathered data from literature are used. The comparison between experimental and analytical results shows good agreement between analytical and experimental findings. Then through a parametric study، the effect of opening and also the interactional effect of adjacent walls on the lateral response and strength of confined masonry walls are numerically investigated. Finally a simple but rational method for modeling the nonlinear behavior of confined masonry walls is proposed. The comparison between this model and numerical results confirms the reliability of the proposed model. Since available experimental results are rare، an analytical study is also performed for investigating the accuracy of the proposed relations.

There is a very few experimental data on the mechanical behavior of unsaturated sand-fine mixture، particularly in constant water content conditions، because of the technical difficulties and time-consuming nature of measuring suction and deformation. This paper presents the results of a series of constant water constant triaxial tests on the specimens of an unsaturatedclayey silty sand in low confining pressure. The effective confining pressures selected in this study are 25، 50 and 100kPa. In comparison with the majority of experimental works، existing in the literature، these levelsof confining pressures are relatively low and this aspect is fewer focused. The effect of matric suction on stress-strain behavior of selected soil in different density and confining pressure is the main aim of the present study. The tests were carried out as Constant water content (CW test). This type of test represents field un-drained condition، where، the rate of loading is much quicker than the rate at which the pore water is able to drain out of the soil. Axis translation technique and double-walled triaxial cell have been used to measure the soil matric suction and variation of pore air volume respectively. Test specimens were prepared at two different compaction conditions prior to testing to achieve different initial density. According to the obtained results، it is found that the mechanical behavior of soil mainly depends on the initial density، the mean net stress and the initial matric suction. Also the volume and pore water pressure changes are totally different in specimens with different initial condition. However، the results of tests indicated that the shearing strength of sand-fine mixture increase non-linearly with matric suction. The results of tests show that the soil shear strength is increased with increasing in both effective confining and matric suction. However the effect of confining pressure is more significant than matric suction. The matric suction and confining pressure play an important role on volume changes of unsaturated soil، however the effect of confining pressure is more considerable. For all of the tests، the specimens were subjected to large axial strain. This allowed us to calculate the deviatoric and mean net stresses at critical state. The comparison of results in q: p-uacoordinate، indicate that، the position of critical state lines for unsaturated soil changes as a function of matric suction. However، these lines can be considered as parallel to each other. Deviatoric stress (q) intercept of critical state line increases with increasing matric suction. Besides، initial density for unsaturated specimens influences the shear resistance corresponding to large strains. However the tests results on saturated specimens، according to the classical critical state soil mechanics، indicated that، soil resistance at critical state is not a function of soil density.

Concrete sleeper as an important component of super-structure has a remarkable role in proper performance of ballasted track. Nowadays، railroads companies of the world are willing to increase strength of concrete sleepers and use new solutions for repairing and rehabilitation of these elements with low cost. Sleeper damage is usually occurred in forms of cracks that have appeared by repetitive loadings and thermal effects. Corrosion of reinforcement tendons in some special regions is another problem that causes serious damage in sleeper. Since these concrete elements are being exposed to various weather conditions، enhancing their resistance against corrosion and improving their durability has a great importance as well. there are many ways to improve concrete characteristics like using FRP by covering concrete in regions that have high stress، but this solution will not increase corrosion resistance. Using epoxy resins that can be sprayed as nano material on concrete is another way to reduce permeability، but it is an expensive way. One of the useful and cost benefit ways to increase the corrosion resistance and reduce the cracks extension is using Poly-Propylene fibers in mixture of concrete. Many researchers have been studied on using Poly-Propylene fibers in mixture of concrete and shotcrete. these reasearches are presented below: reaseraches have been accomplished on using Poly-Propylene fibers in shotcrete. the results have shown that rebounding of shotcrete has been being decreased 30% volume of cement، and cracks have been being decreased about 20%. In case of fiber reinforced concrete also reasearches have been accomplished the result of test on a concrete cube; the compression strength of specimens till 28 days، decreases but it increases till the day 90. As mentioned، many researches have been accomplished on using Poly-Propylene fibers in mixture of concrete and shotcrete. Moreover the Poly-Propylene fibers can control the propagation and extension of the surface cracks. Regarding SEM results، this increase is due to enlargement of Poly-Propylene fibers that can fill micro cracks، and of course the bonding of the concrete structure will be enhanced. Considering the significant effect of Poly-Propylene fibers on concrete behavior، the current research is devoted to investigating the influence of Poly-Propylene fibers on improving the electrical resistance، tensile and flexural strengths of the concrete in sleeper. In this matter an optimal value of 0. 7 kg/m3 of Poly-Propylene fibers was achieved during the laboratory tests on fresh and hardened concrete، including 72 laboratory specimens.

Chloride penetration resistance of concrete is considered a key parameter affecting durability of structures exposed to marine environments. Chloride diffusivity of concrete can be determined by immersion or ponding tests. However such tests are time consuming and costly. These facts have lead researches to develop accelerated tests such as the rapid chloride penetration test RCPT (ASTM C1202). In the RCPT test، the electrical charge passing during six hours through a 50 mm concrete disc، under potential difference of 60 volts is measured. A main concern expressed for the RCPT test has been the rise in temperature of concrete specimens which results in a reduction in concrete resistivity. The other criticism towards the RCPT test is the role of ions other than chloride ion، particularly the (OH) - ion in conductance of electrical charge. It has therefore been suggested that some complementary cementitous materials can cause a reduction in the electrical charge passed، by reducing the concentration of (OH) - ions in pore solution. Another quick method for measuring chloride resistance of concrete is the Rapid Chloride Migration test RCMT (AASHTO TP64). This test is in general similar to the RCPT method. However for avoiding the heating of specimen، the applied voltage is adjusted in accordance with the initially passed charge. Also in the RCMT test، for avoiding the influence of other charge carrying ions، the actual chloride ion penetration into the specimen is determined. A further method proposed for appraisal of chloride resistance of concrete، is determination of electrical resistance. For avoidance of heating of concrete specimens، electrical conductivity measurements are carried out for short durations. This research was carried out with the aim of comparing the results of various quick methods for determination of chloride resistance of concretes containing various supplementary cementitous materials. In the experimental study the control mix had a water cement ratio of 0. 38 and cement content of 420 Kg/m3. Mixes containing various amounts of supplementary cementitous materials including silica fume، fine fly ash، pumice، fly ash and slag were studied. For the RCPT test، the temperature effect resulted in considerable overestimation in the effect of using of supplementary cementitous materials in improving chloride resistance of concrete. Temperature rise did not occur during the RCMT test، and this test therefore does not suffer In the case of optimum fiber value، the obtained results from laboratory tests have shown a reduction in compressive strength and value of ultrasonic test around 8. 7% and 2. 5% respectively. Moreover it has shown an increase in tensile strength، flexural strength، electrical resistance and value of VB test about 33%، 10%،11% and 51% respectively. Consequently utilizing such fibers in sleeper concrete could be suggested for practical applications.

In recent years، many large ground motions occurred very close to modern cities and caused severe damage to buildings. Damage to modern engineering structures is beyond engineers’ expectation، because the structures are thought to have been designed according to proper ground motion-resistant design standards. In these buildings، although the distribution of strength for all stories of designed structures was considered uniform، but the distribution of overall damage of stories was non-uniform and considerable damage for some stories was observed. In this study، 8، 12 and 15-story dual steel moment-resisting frames with concrete shear wall were designed as a residential building that was located in seismic zone 4 (very high relative seismic risk region). These structures were designed with uniform strength ratio for all elements according to the modal response spectra analysis and static equivalent lateral seismic load pattern that were typically recommended by most building codes. Then six severe ground motions recorded in soil type III with magnitude greater than 6. 2 on the Richter scale، without forward directivity، were selected. Finally، all structures subjected to these six severe ground motions and distribution of damage was examined by using nonlinear dynamic analysis. According to the results، despite uniform distribution of strength for all elements، element types and stories in each structure، the distribution of overall damage for element types (e. g. beams، columns and shear wall) and stories and the distribution of local damage for elements are non-uniform and among different stories، first and last stories have minimum overall damage and among different elements types، the damage of the beams is more than columns and shear wall and increasing the number of stories has not been effected on the distribution of damage of elements، and for 8، 12 and 15-story distribution of damage of elements is similar. Evaluation of distribution of damage on elements illustrate that the beams and columns near the shear wall have been damaged more than the beams and columns that are located far from the shear wall. On some stories، the beams and columns near the shear wall have been damaged considerably، but the beams and columns that are located far from the shear wall have been remained elastic. Also، these results conclude that the distribution of overall damage on stories and element types are not adequate for evaluation of seismic damage and the local damage of each element should be examined separately، because from distortion of the results caused by the temperature effect. As the complexity of equipment and speed of testing for the RCPT and RCMT tests are similar، the RCMT test can be recommended for more realistic appraisal of concrete. There was good correlation between the results of the RCMT and electrical resistance test which shows a good potential for utilization of the electrical resistivity methods for appraisal of chloride resistivity of concrete.