نشریه مهندسی عمران مدرس
سال بیست و دوم شماره 4 (مهر و آبان 1401)

In recent decades, due to the high cost of running a water supply network, designers have been trying to design networks with the least cost and maximum reliability. Networks that are able to provide good services in the face of demand changes or failure of the pipeline. Several indexes for reliability measurements were introduced and used as one of the objective functions along with cost in water distribution systems design problem. One of the issues that has been highlighted in recent years is which of these indicators are more successful in measuring the reliability of a water supply network. In this study, six famous reliability indicator entitled Minimum surplus head (MSH), total surplus head (TSH), resilience index (RI), network resilience index (NRI) and modified resilience index (MRI), entropy reliability indicator (ERI) and a new presented reliability indicator entitled Ratio of surplus head (MSH) described and used as one of the objective functions of a water distribution system design optimization problem. For this purpose, a multi-objective differential evolution algorithm has been developed in Matlab software and linked to the Epanet as the hydraulic solver. The generated algorithm applied on two different sample networks with different nature (gravitational feeding and feeding with pumping stations). To analysis of real hydraulic and mechanical reliability of obtained networks in optimization processes, a large number of abnormal operating conditions such as water demand uncertainty or pipes burst scenarios have been generated and applied on obtained Pareto Fronts of each optimization process. Then, the percentage of scenarios that each network could not satisfy the design’s constraints or failed in response to them has been calculated. The over demand’s scenarios were sampled using the general normal distribution method. The percentage of scenarios that each answer (water network in Pareto Front) cannot satisfy the design constraint has been measured and called Hydraulic Failure Percentage (HFP). Also, for modeling the abnormal mechanical conditions, lot of scenarios were produced with broken pipes. In each of these scenarios, there is a possibility of one to ten different pipes break. The locations of burst pipes are selected randomly. The percentage of scenarios that each case cannot satisfy the design constraint has been measured and called Mechanical Failure Percentage (MFP). These scenarios would remain constant for all of members of Pareto Fronts. The lower value of HFP and MFP demonstrate the greater ability of the network to deal with changes in nodal demand and the pipe bursts respectively. For deeper analysis, the conditions of failing (Not satisfying the constraints) divides into three sub-state as flowing: State A: Pressure of all nodes is more than the minimum acceptable pressure in all time. State B: Pressure of all nodes is more than 95% of the minimum acceptable pressure in all time. State C: Pressure of 95% of nodes is more than the minimum acceptable pressure in all time. The results of calculations summarized and have been shown in the diagrams. Results show that MSH and RI are the best indicators for optimal design of water supply networks without pumping station and include it.

Currently tons of dye produced per year, about the one sixth tons are converted into wastewater in various industries such as textiles and dyeing, which are among the toxic, carcinogenic and mutagenic wastes due to the presence of aromatic rings in their structure. This issue has attracted a lot of attention to the purification of such compounds. Ozone is one of the strong oxidizers which produces non-toxic compounds due to its decomposition. Ozone can convert many organic materials into simpler compounds through both direct and indirect oxidation mechanisms, including degradation of wastewater that contains double bonds components such as aromatic compounds and dyes. The purpose of this study was to investigate modeling, optimization and the interactions between parameters affecting the ozonation process in removal of Acid Orange 7 in order to achieve the highest removal efficiency for the highest possible initial dye concentration under the lowest ozone injection rate, no change in initial wastewater pH and the shortest reaction time by the use of response surface method. The RSM was performed using 4 parameters pH, initial dye concentration, ozone injection rate and time with 5 levels which ends up in 30 experimental tests. The results showed that correlation coefficients and adjusted correlation coefficients were 96.85 and 94.92, respectively, and p-value for model (less than 0.0001) and lack of fit (0.0507) were obtained as significant and non-significant, respectively. These results indicate the consistency and high reliability of the modeling results. Normal error, error independence and variance stability control were also checked which showed that the closeness between the actual and predicted values and the uniform distribution of the results obtained on the normal line indicates the uniform distribution of the error. The results and predictions of the software, the random distribution and distribution of the results indicate the suitability of the assumption considered by the software regarding the stability of the variance. Based on variance stability control, the effect of the experiments on the responses provided by the software. If one of the experiments is outside the range, this experiment will have a negative impact on the overall results of the software. In the case of experiments performed, this control was also well performed. Based on model equation the most important parameters are the injection rate of ozone (Q(O3)), pH, reaction time (T) and initial dye concentration [Dye], respectively, in which all parameters except the initial dye concentration have a positive effect on dye removal efficiency. After the related tests the optimum condition were the initial dye concentration of 480 mg/L, pH of 7.7, ozonation rate of 0.6 L/min and ozonation duration of 60 min which resulted in 90% dye removal efficiency. It was also found that the most effective factors were injectable ozone rate, time, pH, and dye concentration, respectively. The results showed that determining the appropriate domains can be of great importance in achieving the desired results from the response surface method. Also, the ozonation process is able to purify the dye from high initial concentrations to high removal efficiency, indicating the high strength of this applied process in the decomposition of complex organic compounds. Ozonation kinetic rate is based on pseudo first order which was increased from 0.3 to 0.6 by enhancing injected ozone rate from 0.2 to 0.6 L/min respectively and further increase of ozone injection rate didn’t had any effect on its kinetic rate.

One of the main objectives of infrastructure managers is the timely and rapid operation of airports and freeways. A goal that is challenging when utilizing concrete pavements, due to their different behavior during the initial stages of implementation. This research aims to improve the mechanical characteristics of concrete pavements and increase their durability against the combined effects of the freeze-thaw cycle and surface desalination, especially during early ages. This paper examines the use of cementitious material in combination with hydrated lime, metakaolin, and zeolite to remove the hurdles to the early operability of concrete pavements. To this end, micro-structural studies have been performed using XRD and SEM analysis and comparisons in two states of water processing and exposure to freeze-thaw cycle. During which replacing cement with zeolite and metakaolin in calcareous concrete resulted in reduced porosity and homogeneous density with the formation of CSH in the concrete structure.  Accordingly, improvements in the mechanical properties and durability of concrete pavements against the combined effects of freeze-thaw cycle and surface desalination were studied and analyzed in four mixtures of Control Concrete (CC), 15% lime (CL), 15% lime, and 15% Metakaolin (CLM) and 15% lime and 15% Zeolite (CLZ). It was noted that at age of 7 days the CLM, CL, and CLZ samples showed an increase of 20%, 32%, and 48% respectively compared to the CC sample. This increase continued throughout the study. During the freeze-thaw test and after 55 cycles the CLM and CLZ samples always exhibited lower degradation and showed a weight loss of 48.7% and 75.2% less than the CC sample. In addition, as per the results of the capillary absorption test the CLM and CLZ mixtures had at lower ages had less permeability than the CC mixture and this behavior continued with better performance at older ages.    Also, the results of flexural strength indicate the positive effect of additives in all samples over time, and at 28 days, the CL, CLM, and CLZ samples increased flexural strength by 39%, 42%, and 57% respectively in comparison to the CC sample. The positive effect of hydrated lime due to its high paste property in increasing the flexural strength of mixtures containing metakaolin and zeolite is quite evident and has increased the mechanical properties at all ages of the samples, but has weakened the durability performance compared to the control sample. This issue has been addressed in composite mixtures containing lime with metakaolin or zeolite, and the results of durability tests indicate a significant improvement in both pozzolans, especially in the zeolite. It can therefore be concluded that with improving mechanical characteristics and durability of CLM and CLZ mixtures, utilizing metakaolin and zeolite in concrete containing hydrated lime is a suitable solution to eliminate the challenges of early usage in concrete pavements.

Today, in order to reduce the harmful effects of the environment and increase the mechanical properties and durability of concrete, particles with high pozzolanic properties are used as a suitable alternative to ordinary cement in concrete. And filler, as an alternative to cement, has attracted the attention of researchers. In this laboratory study to investigate the effects of slag and nanosilica slag consumption on the microstructure of geopolymer concrete and compare it with the characteristics of control concrete containing Portland cement, 1 mixing design of control concrete and 3 mixing designs of geopolymer concrete containing 92, 96 and 100% composite kiln slag was fabricated with 0, 4 and 8% nanosilica, respectively. X-ray fluorescence (XRF) was performed. In order to investigate the effect of microstructural changes on the macro structure of concrete, compressive strength and tensile strength tests were performed on concrete samples at 90 days of age. Examination of the images obtained from the SEM test shows the superiority of the microstructure of the geopolymer cement matrix in all designs, compared to the microstructure of the control concrete containing Portland cement. Celsius), the effects of improvement and cohesion in the microstructure of geopolymer concrete are evident due to the presence of silica nanoparticles, in this regard, the presence of 8% nanosilica in mixture 4 (geopolymer concrete), accelerates the reactivity process and increases the volume of hydrated gels Geopolymerization was compared to other geopolymer concrete mixtures (containing 0 and 4% nanosilica). Images of concrete samples heated to 500 ° C show signs of weakening of the concrete microstructure compared to images taken of concrete at room temperature. The results of XRF test indicate the presence of the highest amount of oxidilica and aluminum oxide (the main factors in improving the density in the microstructure of concrete), in the combination of designs 4 and 2 by 36 and 8%, respectively. The high peaks created in the XRD spectrum diagram often occur in areas with angles (θ2) of 28 °, and their height varies according to the presence of aluminosilicate particles in the concrete mix. The application of high heat to the concrete specimens caused a decrease in the results of the XRD test. Evaluations performed on the results of the test to determine the compressive strength and tensile strength in concrete, showed coordination and overlap with the results of microstructural tests in this study.

Alkali-Aggregate Reaction (AAR) is a type of destructive and time-dependent chemical process in concrete that occurs between alkali ions in the cement and reactive minerals in certain types of aggregates. First recognized in 1930s, AAR is divided into two major categories of Alkali-Silica Reaction (ASR) and Alkali-Carbonate Reaction (ACR), both of which produce an expansive gel in the concrete which expands as a result of water absorption. The expansion of the AAR gel exerts significant internal pressure in the concrete, which may lead to internal and external cracks. With the occurrence of such cracks, many parameters affecting the stiffness and strength of the structure, such as compressive strength, tensile strength, and modulus of elasticity are diminished. As a result, the safety and serviceability of the structure may be seriously impacted. While advances in concrete materials science have led to means to prevent AAR in new construction, numerous existing structures worldwide, such as dams, power plants, and bridges, are affected by these reactions, the replacement of which may be impractical, or in some cases, impossible. As a result, it is crucial to simulate the behavior of such structures for reliable estimation of their safety and providing rehabilitation measures as necessary. One of the major indicators of AAR is the anisotropic expansion it generates inside the concrete member, which changes drastically based on the boundary conditions and internal and external restraint imposed on the expansions. As a result, the prediction of the anisotropic expansions is of utmost importance in successful simulation of AAR-affected reinforced concrete structures. This paper presents a practical simulation methodology for estimating the directional distribution of AAR expansions. The methodology makes use of the user subroutine capability in the finite element software Abaqus. A mathematical model is used to simulation AAR-related expansion based on the stress tensor, whereas concrete damage is simulated using the concrete damage plasticity model. The model is used to simulate a variety of AAR-affected reinforced and plain concrete cube and beam specimens for which the directional expansion data have been reported in the literature. Comparison between numerical and experimental results shows that the proposed methodology is capable of reliably simulating the AAR-induced expansions and the interaction between AAR expansions and the ensuing damage for a variety of reinforcement configurations. The model showed that the yield strength of reinforcing bars plays a major role in the directional distribution of expansion. However, changes in the mechanical properties of concrete were found to be inconsequential in the distribution of the expansions. Moreover, changes in distance between reinforcing bars and the reinforcement ratio in each direction were observed to affect the accuracy of the model. However, the model was found to be successful in reasonably capturing the trends in all case studies investigated. The results of this study are of great value to the simulation of AAR-affected reinforced concrete structures.

Response modification factors are used to reduce the lateral loads in "force-based design" method. Naturally the calculated lateral displacement (drift) of the structures in the linear static analyses is smaller than actual values. Hence, deflection amplification factor (Cd) is needed to consider a realistic estimation of nonlinear displacements. Most seismic design codes such as ASCE7 and standard No. 2800- 4th version propose this factor for different lateral bearing systems. This paper evaluates the proposed deflection amplification factor for dual system of special reinforced concrete moment-resisting frame with/without shear wall. For this purpose, a set of 2D reinforced concrete frames with 3, 7 and 11 story are designed based on standard No. 2800 (4th version) and implemented in Opensees software in each case without considering the soil- foundation- structure interaction. In this regard, beams and columns are modeled using concentrated plasticity method with “Elastic Beam Column Element” in the middle and “Zerolength Element” at the end of elements. Moreover, “SFI-MVLEM” element is used for modeling of shear walls. Nonlinear behavior in two ends of the beams and columns is assigned by “Modified Ibarra- Medina- Krawinkler Deterioration Model with Peak-Oriented Hysteretic Response” model which has been developed by Ibarra et al. (2005). This model is defined using the proposed equations by Haselto et al. (2007). Uniaxial behavior of steel reinforcements and concrete sections are simulated by Steel02 and ConcreteCM, respectively. Studied frames are verified using Hatzigeorgiou and Liolios (2010) and Liu et al. (2020) study for special moment-resisting frame with/without shear wall, respectively. In addition to linear static analysis (LSA), linear and nonlinear dynamic analyses (LDA and NDA) are applied to 3, 7 and 11 story frames with two lateral bearing systems. In this regard, 22 far-field ground motion records which have been introduced in FEMA P695 are used as seismic scenarios. These records are scaled based on Standard No. 2800 to have identical spectral acceleration with the design spectrum for the fundamental period (T) of each studied frames. For this purpose, each record is normalized to its peak ground acceleration and records are scaled so that the average acceleration spectrum of all records was above the design spectrum in 0.2T to 1.5T range. In order to evaluate the deflection amplification factor and Cd/R, maximum drift of roof and other stories is used for each frames due to concentration of structural damage in certain floors of a multi-story structures and, consequently, creating larger lateral displacements in those floors. The calculated Cd coefficients are compared to the proposed values in ASCE7 and standard No. 2800 (4th version) for all special reinforced concrete moment-resisting frames with/without shear wall. This comparison shows that the Cd coefficients which have been proposed in above-mentioned seismic design codes are not appropriate and more realistic estimate of the structural performance in earthquake has demanded larger Cd values. Moreover, Cd and Cd/R values are changed with the height of special reinforce concrete frames with/without shear wall. Finally, adequate values of deflection amplification factors are proposed for these frames with/without shear wall in this paper.

Using modal analysis is a lot easier and more widespread among structures, but the important question is about the number of modes should be considered in the modal analysis method to reach an answer with an inevitable error but in logical tolerance. In this regard, the ratio of the dominant period of the earthquake to the main period of the structure is used as a criterion for selecting the number of modes in the modal analysis method. On the other hand, although the maximum displacement of the structure occurs above it, but when the period of the pulse is less than the main period of the structure, due to wave motion along the structure, the maximum shear strain can occur not only at the base but also in other places along the structure. In this paper, some limitations of modal analysis versus Dchr('39')Alembert solution have been studied in analysis of shear beam under impulsive loads. For this purpose, the structure is modeled with a shear beam with linear material and zero damping, and it is analyzed by discrete (modal analysis) and continuous (Dchr('39')Alembert solution) methods. The time response of modal analysis has been done by the fourth-order Runge-Kutta method. The shear beam is subjected to short, medium, and long period half-sine pulses, relative to the main period of the structure, as well as two near-field earthquakes with distinct pulse. The envelope of maximum induced displacement and shear strain (drift) along the beam have been selected to compare the two methods. The necessary number of modes in modal analysis are determined in such a way that its difference with the exact method (Dchr('39')Alembert solution) would be in acceptable range. For shear beam with linear material and zero damping, as it is expected, the results indicate that for convergence of shear strain (drift) response to the exact solution more number of modes are needed than convergence of displacement response in the modal analysis. Under short period pulse , when the ratio of the period of the pulse or the predominant period of earthquake to the main period of the beam is less than , if the minimum number of modes in modal analysis would be 20 and 50 modes for displacement and shear strain, respectively, then the percentage of error of envelope of maximum induced displacement and shear strain (drift) in beam, calculated by modal analysis, would be less than 10 percent, respect to Dchr('39')Alembert solution. Under medium period pulse , when the ratio of the period of the pulse or the predominant period of earthquake to the main period of the beam is greater than  and less than , for having ten percent difference between two methods of analyses, the necessary number of modes in modal analysis of beam would be  and  modes for displacement and shear strain, respectively. For the beam under long period pulse , when the ratio of the period of the pulse or the predominant period of earthquake to the main period of the beam is greater than , the necessary number of modes in modal analysis would be 1 and 5 modes for displacement and shear strain, respectively.

Road pavements are one of the most important assets of any country, and tremendous amounts of budgets are allocated for their maintenance every year. Unexpected distresses in asphalt pavement cause many financial losses. Winter maintenance of roads and infrastructures and the study of the effects of anti-icers and deicers on the asphalt pavements have always been of interest to researchers, departments, and agencies in the field of roads and transportation. As a contribution to this task, the present study was conducted to evaluate the effect of Zycotherm on the fracture behavior of asphalt mixtures in the presence of moisture and deicers. In order to achieve the research objectives, PG58-22 bitumen and siliceous materials were used to prepare the asphalt mixture and also Zycotherm was used to modify the asphalt binder. Data were collected by testing on laboratory samples. The asphalt mixture samples were conditioned in the presence of distilled water and solutions of brine, calcium magnesium acetate, and potassium acetate in their normal concentration for 96 hours at 60°C. Then, the fracture toughness of the specimens at low temperatures (K1c) and the critical strain energy release rate (Jc) at intermediate temperatures were measured by performing a semi-circular bending test (SCB). The results showed that simulation of low-temperature environmental conditions in the vicinity of distilled water and all deicers reduces the fracture toughness of asphalt mixtures compared to the dry sample. Brine solution has the most negative effect among all the deicers and reduces the K1c parameter by approximately 30%. On the other hand, Zycotherm maintains the fracture toughness of the asphalt mixture at low temperatures in the vicinity of distilled water and deicers at an almost constant level and recovers about 70% of the lost fracture strength of the sample conditioned in the brine solution. The effect of Zycotherm at intermediate temperature is different and causes the softening of bitumen and the reduction of the critical fracture force and the reduction of the critical strain energy release rate. This reduction is 34% and 32% for the dry sample and the specimen in the presence of brine solution, and 23% and 12% for the samples in the presence of calcium magnesium acetate and potassium acetate, respectively, compared to the sample made of neat bitumen. Also, samples in the vicinity of distilled water and potassium acetate solution showed no significant change in their critical strain energy release rate compared to samples in dry conditions. Visual inspection also revealed that calcium magnesium acetate causes additional stress and cracking in the samples. In a general summary and based on the obtained results, Zycotherm has a positive effect on the fracture toughness of the asphalt mixture at low temperatures but reduces the Jc parameter at intermediate temperatures. All specimens have the minimum critical strain energy release rate recommended by ASTM D-8044 at intermediate temperatures. Potassium acetate has no effect on the fracture toughness of asphalt mixtures at low and medium temperatures and can be an appropriate alternative in comparison with other deicers in winter road maintenance.

Due to considerable carbon removal, high on-site biomass, and lipid production compared to traditional agriculture, and a wide range of final products, recent research has focused on the facile commercialization of Microalgae by increasing productivity and cost-effectiveness. Nowadays, wastewater is used as an inexpensive and easy-accessible culture medium rather than expensive culture medium synthesis on large scale, therefore, simultaneous wastewater treatment and production of biodiesel from microalgae can be considered sustainable, cost-effective, and environmentally friendly approach. In this regard, the present study is aimed to optimize the growth and biomass quality of Chlorella sorokiniana pa.91 microalgae from Sari wastewater culture medium using Magnesium Aminoclay nanomaterial (MgAC). In this study by application of the surface response method - central design, the effect of temperature, light intensity, and nanomaterial concentration was investigated on the parameters including the dry weight of biomass on the seventh day, specific growth rate, chlorophyll, and carotenoids in wastewater after 12 h exposure to visible light. Under 37 μmol photons, m-2 s-1 radiation intensity, and in the presence of 0.05 g/L of magnesium aminoclay NM at 20 °C, the optimal condition including biomass dry weight, specific growth rate, chlorophyll, and carotenoids increased by 47.13, 30.26, 81.33 and 36.47% respectively compared to the control sample. Also, to make the present study feasible, the test conditions were performed in real wastewater.

In this research, a truss model is introduced to simulate the in-plane behavior of masonry walls in different failure modes. In the proposed model, the structural members are divided into smaller two-dimensional panels and then a truss system is used to model each panel. How to determine the geometry, dimensions and behavioral model of these truss elements are introduced according to the behavioral mechanism and failure modes of the masonry wall. The dimensions of the truss members are determined in such a way that the stiffness of the truss model is the same as the stiffness of the initial panel, under different loads. In this regard, the computational method introduced by Aghababaei is used. The elasto-plastic-fracture model developed at the University of Tokyo is used for compressive behavior of elements. Barimani et al. evaluated and verified the accuracy of this model. Also for tensile and post cracking behavior of masonry, the tension softening model of Maekawa et al. is used. In order to apply the effects of cracking on the compressive behavior of the diagonal elements, a softening model dependent on the crack width is introduced. The idea used is generally based on the smeared crack approach. In this model, after identifying the pair of each diagonal element and as soon as a crack occurs in one diagonal element, the behavior of the other diagonal element is modified by multiplying the force values ​​in a proposed function. Then, the results of the analysis are evaluated based on ASCE41 code in different failure modes. Also, for verification, two I-shaped masonry walls, three masonry walls with rectangular cross-section and an arched walls are numerically analyzed using the proposed model and the results of numerical and experimental tests are compared. The results show that the proposed model, while simple, has a good ability to estimate the nonlinear behavior of masonry wall.

Concentrically braced frames are among the prevalent seismic force-resisting systems used in the construction of steel structures. This type of system provides a suitable level of stiffness for structures under low and intermediate seismic oscillations. However, under strong motions, it has noticeable deficiencies such as stiffness loss under compressive force, the unacceptable difference between the tensile and compressive strength of the brace, low energy-dissipating capacity, and overall poor cyclic behavior. To overcome these deficiencies, the idea of the Buckling Restrained Brace (BRB) was proposed a few decades ago.  Since the invention of BRB, extensive studies have been carried out to optimize the new brace system. These studies have resulted in the emergence of different generations of buckling restrained braces. In the first generation of BRB, a concrete-filled sheath had been used around the inner core of the brace. To upgrade that heavy brace, the researchers developed an all-steel brace system that was considerably lighter in weight, faster to build, and easier to inspect its yielded core after an earthquake. Later on, the idea of reducing the length of the core, as well as the sheath, was proposed which led to an even lighter brace, while keeping all the major advantages of the traditional BRB. In this paper, twelve all-steel BRB samples, based on a reduced fuse length, have been investigated numerically. Each brace sample is composed of three boxes, which include the main box, the outer sheath, and the inner box. The outer sheath and the inner box are used to prevent the local buckling of the core in the fuse zone. The outer sheath and the inner box are connected to the brace core at one end only. In this study, the cross-sectional area of the brace core in the fuse zone was considered to be less than half the total cross-sectional area of the original brace section. The samples were loaded by the quasi-static cyclic loading protocol of AISC. The numerical analysis showed that the proposed brace withstood an axial strain level of around 4%. The numerical modeling of the proposed brace was verified by the data reported for an earlier experiment that had been carried out in the laboratory of the Housing and Urban Development Research Center (BHRC). In the numerical study, the effect of influential parameters of the proposed brace on its cyclic behavior was investigated. These parameters included the ratio of the fuse length to the total brace length, the gap between the core and the inner/outer boxes, the inner/outer box thickness, and the friction coefficient between the core and the contact surfaces of the boxes. Using the hysteretic curves of the brace, obtained from the numerical analyses, the ductility parameters, and the amount of dissipating energy were evaluated. The results showed that the obtained amount of the relative lateral displacement of the proposed brace is acceptable according to the code regulations. Moreover, the cumulative inelastic deformation of the proposed brace surpasses the minimum requirement of the code for the predefined loading protocol. The studied samples were stable and had relatively symmetric cyclic behavior in the compression and tension zones. The study showed that the proposed bracing is suitable for the rehabilitation of buildings.

Given the damages to concrete structures caused by different factors, some materials should be used to repair and strengthen them. In general, cement-based mortars are used to repair concrete structures. The shrinkage and inappropriate compaction of repair mortars can reduce the bond strength between mortar and concrete substrate. the shrinkage is an important problem that has an adverse effect on the adhesion between repair mortar and concrete substrate. The shrinkage can create tensile stress inside mortars, leading to cracking due to their low tensile strength. The mortar-substrate interface is of great importance since the improper compaction can create fine pores and lower the bond strength.The suitable compaction of repair mortar while applying it on the concrete substrate is an effective factor in increasing the adhesion between the mortar and concrete. The mortar-substrate interface is of great importance since the improper compaction can create fine pores and lower the bond strength. Therefore, in this study, different prestresses were imposed on repair mortar to evaluate their effects on the shear and tensile bond strength between repair mortar and concrete substrate using friction-transfer and pull-off tests. The role of curing in reducing the shrinkage of mortars was also evaluated. The semi-destructive friction-transfer and pull-off tests were then used to assess the in-situ compressive strength of cement-based mortars. By calculating the correlation coefficient and plotting the calibration curves, relationships were provided to convert the measurements obtained from the semi-destructive tests to the compressive strength of the mortars. Eventually, the cracks and stresses that appeared in the specimens were presented using finite element ABAQUS software. The obtained results indicated the effect of prestress on increasing the shear and tensile bond strength between the concrete substrate and repair layers. Moreover, a high correlation coefficient was found between the measurements of the in-situ and laboratory tests. A good agreement was also observed between the finite element modeling and the experimental results. In some researches, friction transfer and pull-off tests have been used to compare the compressive strength of fiber-reinforced mortars and polymer-modified mortars with the results of the above tests, which has resulted in a very high correlation coefficient between the results.Imposing a prestress of 0.5 kg/cm2 resulted in an increase of 30.1% and 31.4%, respectively, in the shear and tensile bond strength between the repair mortar and the concrete substrate at the age of 90 days. Imposing a prestress of 0.1 kg/cm2 resulted in an increase by 7.5% and 5.8%, respectively, in the shear and tensile bond strength between the repair mortar and the concrete substrate at the age of 90 days. Keeping the specimen in the free space resulted in 64% more shrinkage compared to the one cured in water. The friction-transfer and pull-off test results had a correlation coefficient of more than 0.98 with the repair mortar's compressive strength. Therefore, these tests can be used to evaluate the in-situ compressive strength of mortars. The compressive strength of repair mortar can be calculated by substituting x for the friction-transfer and pull-off test results, respectively, in the equations y=10x-0.805 and y=17.32x+1.83.

Tubular members, due to their convenient equipment installation and high-strength performance, are widely applied in the support system of offshore platforms such as jack-ups and jackets. In most steel tubular structures; the circular hollow section (CHS) members are mainly joined using welding. Commonly, one or more braces are welded directly onto the surface of a chord member to form that so named welded connection. So far, some techniques to improve the performance of tubular connections have been proposed. Most of these methods (e.g., internal ring, doubler plate) can only be used for structures during the design, but there are only a few techniques (e.g., outer ring, FRP) which can be applied during both fabrication and service. This paper studies the static strength of CHS X-joints reinforced with external ring subjected to axially tensile load. The SOLID186 in ANSYS version 21 was used to establish the finite element (FE) models of the tubular X-joints. Validation of the FE model with experimental data showed that the present FE model can accurately predict the static behavior of the external-ring stiffened and un-stiffened tubular X-joints under tension. Afterwards, 143 FE models were generated and analyzed to investigate the effect of the joint geometry and the external ring size on the ultimate strength, failure mechanisms, and initial stiffness through a parametric study. In these models, both geometric and material non-linearity were considered. Moreover, the welds joining the chord and brace members were modeled. Results indicated that the ultimate strength of the ring stiffened X-joints under brace tension can be up to 289% that of the ultimate strength of the corresponding un-stiffened joint. Also, the increase of the β (the ratio of the brace diameter to chord diameter) results in the increase of the ultimate strength and initial stiffens (in a fixed chord diameter). Because, the increase of the β leads to the increase of the brace diameter.  The increase of this member results in the increase of the joints stiffness. In addition, the decrease of the γ (the ratio of the chord radius to chord thickness) leads to the remarkable increase of the ultimate strength. Also, the increase of the τ (the ratio of the brace to chord thickness) leads to the increase of the ultimate strength (in a fixed chord thickness). However, it is not remarkable. Moreover, the comparison between failure modes of reinforced and un-reinforced joints showed that the ring can significantly improve the failure mechanisms. Also, the ring can remarkably increase the initial stiffness. Despite this significant difference between the ultimate strength, failure mode, and initial stiffness of unreinforced and ring reinforced X-joints under brace tension, the investigations on this type of the reinforced joints have been limited to only three X-joint tests. Also, no design equation is available to determine the ultimate strength of X-joints reinforced with the external ring. Therefore, the geometrically parametric study was followed by the nonlinear regression analysis to develop an ultimate strength parametric formula for the static design of ring stiffened X-joints subjected to brace tension. The proposed formula was evaluated based on the UK DoE acceptance standard.

These days, concrete is used as an all-purpose material in various industries. Depending on the usage of concrete, different properties can be expected from it. Plentiful usage of concrete in different industries such as bridge construction, landscaping, construction structures, ports, piers and special structures have made this valuable material the center of attention of many researchers. Depending on the type and place of using concrete, this material has limitations and problems. Concrete demolition in long period of time is one of the causes of irreparable damage to the industrial and economic cycle of countries. High durability and reliability concretes reduce irreparable damage to the environment also increase the service life of structures. Factors affecting the service life of concrete structures, environmental conditions and concrete quality. The attack of sulfates on concrete is one of the important factors in reducing the life of the structure and the durability of concrete. Sodium, magnesium and calcium sulfates are salts, which usually found in soils and groundwater that hydrated with various phases of cement paste, such as hydrated aluminum, monosulfate hydrate, and calcium hydroxide and form needle-bearing crystals.The volume of these crystals is more than the volume of the raw materials of cement paste, so they cause cracking in hardened concrete. Magnesium sulfate has a more destructive effect than other sulfates because it destroys calcium silicate hydrate (C-S-H) as well as hydrated C3A and Ca(OH)2. Then, due to the attack of sulfates on concrete, magnesium silicate hydrate is formed, which has no adhesive properties. The effect of sulfate attack on concrete depends on their concentration and permeability of concrete. If the concrete is very permeable, water will easily penetrate into it and Ca(OH)2 will be lost. Evaporation on the concrete surface leaves calcium carbonate deposits, which are formed by the reaction of Ca(OH)2 with carbon dioxide. This deposit is observed due to the attack of sulfates on concrete with a white appearance. The loss of Ca(OH)2 will increase the porosity of the concrete, making it consistently weaker and more susceptible to chemical attacks by sulfates. In the present study, weight change and resistance pressure tests for placement in magnesium sulfate solution of six different concrete designs and replacement of cement with silica fume, glass powder and ground granulated blast furnace slag at the ages of 7, 28, 56 and 90 days have been investigated. Also, in order to evaluate the permeability of each mixture, depth of penetration of water under pressure and water absorption were performed on cubic samples. The overall results indicate the performance in favor of silica fume, glass powder and ground granulated blast furnace slag in reducing the rate of sulfate consumption and water reduction. The rate of weight gain and compressive strength against sulfate depends on the properties of cement consumables and their consumption with cement.