mahdi mahdikhani

Fracture toughness is one of the most important properties of concrete that controls the conditions for crack propagation and ultimately concrete failure. This research uses the Brazilian disk test to prediction of crack propagation and fracture toughness in ordinary concrete samples without micro-silica and lime powder and ordinary concrete samples containing micro-silica and lime powder has been investigated. Micro-silica replaces 10% by weight of cement and limestone powder replaces 5% by weight of cement. The crack propagation process was investigated from pre-existing cracks in the specimens as well as fracture toughness in modes I, II and hybrid mode I-II. Fracture toughness tests have been performed on Brazilian disk specimens at angles of 0, 15, 28.83, 45, 60, 75 and 90 degrees relative to the pre-existing crack direction. After laboratory studies, it was found that the onset of fin cracks at angles less than 60 degrees (0 <α <60) occurs from the pre-existing crack tip and approaches the loading direction by continuing to load and propagate the crack path. However, for angles of 60 degrees or greater, the crack starts at a distance d from the tip of the crack. This distance is more in ordinary concrete samples without micro-silica and limestone powder than in ordinary concrete samples containing micro-silica and limestone powder. Samples containing micro-silica and limestone powder have higher fracture toughness of modes I, II and mixed state (I-II) than samples without micro-silica and limestone powder.

Tehran, the capital city of Iran, is a dense city with a more than 12 million dynamic population. The city is located next to the seismically active zone of the Alborz Mountains, with many active faults surrounding it. Because of Tehran's significant economic and political function at the national level, earthquakes in this city or its vicinity could largely affect the whole country. Together, these factors can explain how important the identification and assessment of seismic risk in this city can be. In this study, an OpenQuake-engine has been employed to quantify the earthquake risk of one of the districts of Tehran. Initially, a probabilistic hazard assessment was carried out for Tehran; then, after disaggregating the result of a 475-year PSHA, the main contributing earthquake scenario was determined as the hazard input. In addition, an exposure model was developed for the residential buildings of the study area, indicating building typology and locations. Further, a set of fragility and vulnerability functions that are consistent with the exposure model was selected from past studies. Finally, the seismic loss for residential buildings in district 2 of Tehran municipality was assessed in the event of the 475-year hazard scenario. The results show the intensity and spatial distribution of damages and losses in various subdivisions of the study area. Such analysis can provide essential information for disaster management decision-makers to prepare for possible future events.

Execution of construction projects despite safety regulations, there are always irreparable risks, including physical and financial accidents for human resources and the project, these events adversely affect the balance of cost, time and quality of projects. Therefore, the existence of safety management system can play an effective role in improving the project management process. This research includes identifying and evaluating the most important levels and safety criteria in construction projects, evaluating and developing a safety management maturity model in workshops based on a case study, statistical analysis of the data obtained from the questionnaire and applying the combined multi-criteria decision making method. It is based on combining Analytical Hierarchy Process techniques, Technique for Order of Preference by Similarity to Ideal Solution and Decision Making Trial and Evaluation. Based on the results of the study, twelve important safety criteria were identified and ranked according to the weighting performed and then among the three projects studied, one of them was selected as the project closest to the ideal solution and at the end, the most effective criteria were prioritized in four pillars. The main project, which includes the leadership and management of the project organization, project partners and stakeholders, key personnel and project staff, policies and strategies, was identified.

Concrete is the most widely-used material in civil engineering and often contains the cheapest and most common matter. It can therefore cause irreparable damages due to cracks and fractures. The creation of fibre-reinforced concrete in recent years has largely rectified the aforementioned shortcomings. This study uses the direct crack Brazilian disk test to evaluate fracture toughness and crack propagation in fibre-free and glass fibre concrete samples in 0.2, 0.35 and 0.5 volume percentages. Moreover, fracture toughness and crack propagation from pre-existing cracks were calculated for samples in mode I, mode II, and mixed mode (I-II). The samples were subjected to the Brazilian disk test at 0, 15, 28.83, 45, 60, 75 and 90-degree angles relative to the pre-existing crack’s trajectory. Laboratory investigations showed that the wing crack at sub 75-degree (0<α <75) angles was initiated from the pre-existing fracture’s tip and approached loading trajectory as the load on the crack growth and propagation trajectory continued. At the same time, crack initiation at 75-degree angles and above begins at a distance of d from the tip, which is greater in fibre-free samples. The results also showed that using 0.2% glass fibre in mode I, II and mixed mode (I-II) resulted in a higher fracture toughness than fibre-free samples.

In this research, the effects of temperature and number of heating–cooling cycles on the mode I, mode II and the effective value of the mixed-mode I-II fracture toughness of concrete were investigated. In the first series, the effect of temperature was studied in a heating–cooling cycle at ambient temperature (25°C) and 60, 150, 200, 300, 500, and 700°C. The highest and lowest mode I, mode II and the effective value of the mixed-mode I-II fracture toughness were, respectively, observed at 150 and 700°C. In the second series of tests, the effect of number of heating–cooling cycles was investigated on the mode I, mode II and the effective value of the mixed-mode I-II fracture toughness of concrete specimens at 150°C and a crack inclination angle of 45°. According to the results, the mode I, mode II and the effective value of the mixed-mode I-II fracture toughness increased in the first cycle and decreased with increasing the number of heating–cooling cycles. As the crack inclination increased, the effective value of the mixed-mode I-II fracture toughness of the concrete specimens increased. The mode II fracture toughness increased up to a crack inclination angle of 45° and then decreased. Moreover, with increasing the crack inclination angle, the mode I fracture at the inclination angle of 0° was changed into the mixed-mode (tension–shear) fracture at inclination angles smaller than 28.8°. The mixed-mode tension–shear fracture was changed into the mixed-mode compressive–shear fracture at crack inclination angles larger than 28.8°.

Roller Compacted Concrete (RCC), is a concrete with zero slump, which is used in damming and road pavement. Roller Compacted Concrete Pavement (RCCP) bear heavy traffic loads and severe weather conditions. This type of concrete is due to its economic and environmental capabilities, including reduced construction and maintenance costs, longer durability and longer lifetime, as well as environmental compatibility. This paper presents the results of laboratory studies on the effect of natural pozzolan (zeolite) on the mechanical properties of RCC specimens. In this study, cement materials of were used The rheological performance test (VeBe) and compressive and tensile strengths at the age of 7, 14 and 28 days and the flexural strength tests at 14 and 28 days were performed on the specimens. The results show that the higher amount of pozzolan reduced concrete efficiency (the time of VeBe increased). In addition, the compressive, tensile and flexural strength of the samples are decreased, significantly. However, reducing resistance does not mean compliance with the requirements of the Code.

The use of concrete in construction projects is increasing day by day. Making concrete with different properties, compatible with the conditions of each project, is necessary and unavoidable. So far, researchers have conducted research on the characteristics of this type of concrete. However, the role of water consumption in the mixing and curing of this type of concrete has not been studied much. Also, in some countries, studies are being conducted on the use of magnetic water and its effect on concrete. The purpose of this article is to investigate the changes in the properties of self-compacting mortars in the fresh state, due to the use of magnetic water in the construction phase, and therefore small slump tests and small v-shaped funnels are used as important criteria using normal and processed waters. It has been done for 5, 10, 15 and 20 minutes. The results of the tests show that magnetic water improves the rheological properties of fresh mortar.

Under acid rain conditions, physical and chemical reactions occur in the concrete structure, which leads to a change in pH. As these reactions continue, the concrete begins to lose its mechanical strength, leading to cracking, weight loss, and eventual structural damage. Since the control of acid rain and its effects on the environment is inevitable in some cases, researchers have done many studies on this topic and have proposed solutions to eliminate or control its effects. One of the new solutions in this field is the use of nanoparticles. In recent years, studies have focused on silica nanoparticles, with the aim of using this material to further enhance the properties of concrete. The addition of nanosilica to concrete reduces the water permeability of the concrete as well as higher resistance to chemical attacks. In this paper, the mechanical properties and durability of nanosilica-containing concrete including weight loss, compressive strength, electrical resistance and water absorption under acidic conditions are investigated. By increasing nanosilica to concrete under normal conditions, the mechanical properties of concrete are improved, but according to the observed results, when the samples are placed in acidic conditions, the nanosilica in the samples reacts with acid and the performance of nanosilica improves the properties. The mechanical properties of the concrete deteriorate due to the mechanical replacement of the percentage of cement with nanosilica.

Self-compacting concrete is one of the most important and significant achievements of concrete technology. In some countries, studies are being conducted on the use of magnetic water and its effect on concrete. According to the results of this research, by applying a magnetic field to ordinary water, the structure of its molecules changes so that it can be said that as water molecules pass through a magnetic field, there is a significant change in the arrangement of them in the liquid. In this paper, changes in the properties of hardened self-compacting mortars due to the use of magnetic water in the fabrication and processing stage have been investigated. It has been done for 5, 10, 15 and 20 minutes using ordinary water as well as processed water. The results of the experiments show that magnetic water improves the strength standards of hardened concrete. Therefore, in order to achieve a certain level of efficiency and strength, if the magnetic water is used, the ratio of water to concrete or superplasticizer can be reduced.

In the conditions of acid rain, chemical reactions occur in the concrete structure, which leads to a change in pH. When these reactions continue, concrete begins to lose its mechanical strength, which leads to cracking, weight loss, and finally the destruction of the structure. Since the control of acid rain and its effects on the surrounding environment is unavoidable in some cases, so far researchers have conducted many studies on this category and have provided solutions to eliminate or control its effects. One of the new solutions in this field is the use of nanoparticles. In recent years, studies have been focused on silica nanoparticles, with the aim of increasing the properties of concrete by using this material. Adding nano-silica to concrete in non-acidic (neutral) conditions reduces water permeability in concrete and also increases resistance against chemical attacks. In this article, the mechanical characteristics and durability of concrete containing nano-silica, including weight loss, compressive strength, electrical resistance, and water absorption rate under acidic conditions are investigated. According to the obtained results, with the increase of nano-silica in concrete, the mechanical properties and durability of concrete improve, but with the increase of water acidity, the durability and mechanical properties of concrete decrease.

Small cracks in concrete may turn into large cracks over time, reducing the useful life of concrete structures. Therefore, the rate of growth and expansion of cracks should be limited. Repairing cracks in concrete is one of the effective ways to prevent the spread of cracks. Therefore, from the past until now, different methods for repairing cracks created on concrete due to different factors have been studied and compared. This study presents a biological method to improve the self-healing effect of concrete beams by the microbial activity of Bacillus subtilis. In this study, self-healing begins with the injection of Bacillus bacteria through an insulin syringe into the crack. For this purpose, 6 concrete beams with dimensions of 15 × 15 × 120 cm were made and the self-healing property by creating cracks by applying static torsional load was evaluated. In the end, according to the final torsional anchor of the beams, this result was obtained. The beam repaired with this method has increased the torsional capacity of the original control sample by 93.13% and the torsional capacity of the cracked beam has increased 7.6 times.

In recent decades, the use of self-consolidating concrete has become widespread. Hence, recognizing the various properties of self-consolidating concrete are essential. In this study, several mixture designs have been tested and final mixture design of crushed tile aggregates which were replaced by 0%, 25 %, 50%, and 100% volume percentage of natural aggregates were conducted. To evaluate fresh properties of SCC, slump flow and rheometer tests were carried out. Results show that the percentage of fine aggregates has a significant impact on the properties of self-consolidating concrete. In addition, the results of rheometer test show by increasing the percentage of recycled aggregates increases yield stress and plastic viscosity, significantly.

Nowadays, waste production is increased due to the growth of technology and excessive use of natural resources as well as production of new chemicals, among which the hazardous wastes are no exception of that stabilization and solidification method is one of the hazardous waste management methods through which the waste materials are stabilized and confined. High flexibility for various pollutant compounds, ease of use, and being economical for large volumes of waste has led researchers to use this method to treat various wastes. In this research, sludge ash from sewage treatment plantsuses as waste in order to perform the stabilization and solidification process. Nine mix designs with a water to cement ratio of 0.46; and 0, 6, and 10 percent dried sludge residue of the Alborz Industrial City in Qazvin as the replacement of fine aggregate; 0, 2, and 4 percent of silica fume; and cement content of 600 kg/m3are produced. The mini slump flow, mini V-funnel, compressive strength, electrical resistancetests were carried out on the solidified samples to investigate the effect of waste on the performance of the mortar.The results show that the use of this waste as a substitute for fine aggregate could be taken into consideration.

Characteristics of produced concrete become changedthrough changingthe characteristics of materials constituting the concrete. These changes include: The quality of cement, the real ration of water to cement: with neglecting the further water within the aggregates or lack of that, the consideredratio fluctuates and the characteristics of the aggregate which are related to the following dimensions: quality of aggregate, shape of aggregate and aggregate grading. The calculated strength in the mix design is the mean target strength which is marginally further than the specified strength. This further margin lowers the ration of water to cement in calculations and consequently, for a stable flow, the amount of consumption or alloy of cement in concrete increases. On the other hand, the amount of the margin depends on the following factors: The sensitivity level of the concrete project (with K coefficient) and standard deviation of concrete production. The purpose of defining this laboratory project is to obtain an optimal concrete mix design for different consumptions and consequently comparison of theoretical and workshop results in the mix design, lowering the cement consumption and the costs of concrete productions in batching and increasing or stabling the specified strength of concrete through lowering the level of cement consumed in concrete. In this procedure, concrete is produced through common method that is segregating sand from cement to compare that with the results obtained, and to calculate standard deviation fall, strength margin and cement alloy fall in order to produce concrete which is statistically suitable for the required strength.

Recently, Nanotechnology has caused a vast breakthrough worldwide and scientists in different fields have used it. Nano particles have been gaining increasing attention and been applied in many fields to fabricate new material with novel function due to their unique physical and chemical properties. Therefore, using Nano-silica as a Nano-scale additive in concrete is under further inspection by researchers. Recent contribution to the development of building materials compromise adding colloidal Nano-silica to concrete. In this paper, the influence of Nano-silica addition on mechanical and durability properties of concrete as compared with silica fume has been studied. Tests include compressive strength, water permeability, rapid chloride penetration and electrical resistance test. Moreover, microstructure of the cement paste incorporating Nano-silica and Silica-fume were studied through X-ray diffraction and Scanning electron microscope tests. The experimental results show that Nano-silica as an artificial pozzolanic material can improve the compressive strength, water penetration resistance and electrical resistivity of concrete and reduce the chloride diffusion more effectively than SF at early ages. In addition, utilizing NS in cement paste improve its microstructure at early days.