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سال چهاردهم شماره 3 (پاییز 1400)

The use of lightweight aggregates in concrete has been widely increased to reduce the weight of concrete members. On the other hand, the fracture behavior of lightweight concrete is different with that in normal concrete. Due to the limitations of conventional designing methods for concrete members, researchers and design codes have led to consider the analysis of fracture mechanics in predicting the behavior of concrete structures. In this experimental and analytical study, the fracture behavior of lightweight aggregate concrete (LWAC) is investigated on notched beams using the work of fracture method (WFM) and size effect method (SEM). Also, the mechanical properties and shrinkage of LWAC are studied. A total of 30 bending prismatic specimens and 42 compressive samples were constructed and tested to determine the fracture parameters, mechanical properties and shrinkage of LWAC. The results of experiments and analyses show that the Bazant’s size effect theory can evaluate the fracture behavior of lightweight concrete members, appropriately. The total fracture energy G_F of LWAC is by 68.1% less than that of normal concrete due to its lower load-bearing capacity. However, the initial fracture energy G_f obtained from the SEM in LWAC is approximately higher than that in normal concrete. According to the diagrams obtained from the SEM, the LWAC has more ductility and less brittleness than normal concrete. Additionally, the SEM is more accurate than the WFM to study the fracture behavior of LWAC. The use of lightweight aggregates instead of normal aggregates reduced the concrete shrinkage.

Since a portion of concrete is made of the cement paste, its impact on the concrete permeability can be considerable. For this reason, in this paper, permeability of the concretes containing different types of cements (type 1-425, type 2, type 5, pozzolanic and white cements) and mineral admixtures (microsilica, fly ash, zeolite and limestone powder) was studied, using the results obtained from “Cylindrical chamber” method. Cement replacement levels with mineral admixtures were 5%, 10%, 15 and 20%. Considering the durability problems of the concrete structures located in Iran’s seashores, in addition to the tap water, seawater was used for the permeability assessment of the concrete specimens containing different types of cements. The results tend to show that, except the concrete specimen containing limestone powder with a replacement level of 20%, other concrete specimens have lower permeability than the concrete specimen without any mineral admixtures. Microstructural properties of the concretes containing the mentioned mineral admixtures with replacement levels of 0% and 5%, obtained from analyzing scanning electron microscope images were also consistent with the results obtained. Furthermore, it was observed that water type has great impact on concrete permeability. In this regard, concrete specimens containing type 1-425 and pozzolanic cements had the lowest permeability when exposed to potable water and seawater, respectively. On the contrary, concrete specimens containing pozzolanic and white cements showed the highest permeability against potable water and sea water, respectively. Weak correlation was also seen to exist between permeability and compressive strength of the studied concrete specimens.

Evaluation of the life model of concrete structures has an effective role in determining maintenance programs as well as estimating the probability of failure of these structures. One of the most important factors that affect the durability and strength of concrete structures is the corrosion of rebars, which is generally caused by the penetration of chloride ions in concrete. Due to the corrosion of the rebar, the effective surface of the rebars is reduced and over time, by creating cracks in the concrete section, the effective concrete cross-section is also reduced. In this study, considering the inherent and statistical uncertainties of the parameters affecting corrosion, for different states of chloride ion penetration, the concrete beam life model has been evaluated. For this purpose, considering a concrete beam, different scenarios of chloride ion penetration from different directions of the beam cross-section is considered, and based on each scenario, the corrosion initiation time of rebars, cracks, and scaling of concrete are calculated and the effect of each event on The flexural strength of the beam is evaluated. The results show that considering the effect of concrete scaling causes a difference of up to 20% in the predicted values for the flexural strength of concrete beams and a more realistic assessment of the remaining life of the structure will be obtained. Also, the states that cause the compressive cross-section of the concrete to peel are more critical in the concrete beam life model due to the reduction of the effective depth.

In the previous study, optimization based on genetic algorithms on the design specifications of isolators and reinforced concrete piers of a famous multi-span highway bridge was performed, and an optimized bridge with a semi-isolated deck was proposed. In this study, using incremental dynamic analysis, by selecting two cases of displacement of the semi-isolated deck and displacement of the isolators or the shear strain of the isolators, the incremental dynamic analysis curves are obtained under the records proposed in FEMA P695. Then, while performing fragility analysis based on the results of incremental dynamic analysis, fragility curves are determined for the displacement of the semi-isolated deck and the displacement of isolators or the shear strain of the isolators in longitudinal and transverse directions of the bridge. Based on these curves, the values of the limit states for structural collapse, life safety, moderate damage, slight damage and first yield has been identified and proposed.

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.

In recent years, new methods for improving the mechanical and durability properties of concrete have been studied and researched. One of these methods is the use of a special type of bacteria that can repair the concrete with their performance inside the concrete.For this purpose, Bacillus subtilis PTCC 1254 strain was used. Adding any unusual material to the concrete should not adversely affect the properties of the concrete. To investigate these effects, 6 mixing designs were investigated. The control design, which is expressed in the text as N; It has been proposed without any bacteria or additives. In the mixing desing called BU, bacteria are added to the concrete mix water with a concentration of 108 cell/ml. Also for bacteria anaerobic function and urea decomposition for produce calcium carbonate, Yeast extract, urea and calcium nitrate are added to the mixture. The mixing design, called BL, contains bacteria with a concentration of 108 cell / ml, yeast extract and calcium lactate. In other mixing design, the bacterial effect alone (B), the lactate effect (L) and the urea effect without the presence of the bacterium (U) are investigated. To investigate the effectiveness of these materials, 28- and 90-day compressive strength tests as well as concrete water absorption were taken. The results of this study showed that the bacterium did not have a negative effect on the properties of concrete. Also, the bacteria with calcium lactate improved the mechanical properties and permeability of concrete.

Fiber Reinforced Polymer (FRP) bars are corrosion-resistant and non-magnetic and have high tensile strength. However, using this type of bar in concrete beams reduces their ductility and increases their cracking. In this paper, the effect of using FRP bars as an alternative to steel bars has been investigated, and the percentage and arrangement of GFRP reinforcements, increase the cover of concrete, and use of compression reinforcement on the behavior of deep concrete beams have been evaluated. Deep concrete beams are common members of structures. These beams are used in shear walls, girder bridges, offshore structures, and tall buildings. Due to the different behavior of deep concrete beams compared to bending beams, these beams were evaluated. The results showed that by increasing the concrete cover from 38 mm to 98 mm, the strength of the beam decreased to 31.7%. Additionally, the use of compression bars did not change the bearing capacity of the beam. Also, increasing in GFRP bars, the bearing strength of the beam increased to about 46% compared to the beam with one row of bars.

Looking at the terrorist incidents in recent years, the protection of buildings under explosions is not only a concern of military institutions but also an important concern for design engineers. Due to the importance of the slabs during the explosion in the building, their correct design according to technical and security standards, especially passive defense standards, is essential. In designing slabs, it is necessary to consider openings for various purposes, including the passage of pipes, installation and electrical ducts, and elevators. In general, opening changes the behavior of the slabs. Therefore, in this study, the behavior of the slabs with the opening was dynamically analyzed by ABAQUS and to achieve the optimal state, the effect of the shape and position of the opening and also the effect of increasing the compressive strength of concrete on the performance of the slab containing the opening was investigated. In this study, the dimensions and thickness of the slab, the explosive load and the abutment conditions were considered the same and the maximum displacement and damage were investigated. The results show that circular openings have better performance than square openings and when the opening is in center, it creates less displacement than the opening is in corner. In addition, in the slab with circle opening in center, less amount and surface is damaged. Therefore, it is recommended to be located the opening in the central areas of the slab. The results also show that as the compressive strength increases, the displacement decreases.