Journal of Advanced Materials in Engineering (Esteghlal)

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The paper presents the results of casting and testing of 264 GFRC specimens. The glass fibers were 25 mm long, with the aspect ratio (L/D) ranging between 1250 and 3570. The parameters studied were the ratio (by weight) of fibers to cement, i.e. F/C=0%, 1.5%, 3%, and 4.5%, and the ratio of coarse to fine aggregates (gravel to sand), i.e. G/S=1.1, 0.7 and 0.2. In total, 12 mix designs were selected for GFRC specimens while the water-cement ratio was constant and equal to W/C=0.4. The balling of glass fibers in the mix was overcome by using adequate and sufficient antistatic agents. The specimens were tested under compressive, tensile and flexular loading at the ages of 7 and 28 days. Furthermore, the modulus of elasticity and the absorption of the concretes were determined. Finally, the mechanical and physical properties of the GFRC specimens were analysed and an empirical expression describing the modulus of elasticity of the GFRC was proposed.

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In most cases, structural engineers assume a concrete floor to be a rigid diaphragm. Although this simplification is in most cases acceptable, it should be noted that such an assumption may be distrusted due to certain problems. Concrete structures with staggered shear walls are among those whose analysis should be conducted with special concern for the behavior of their floor diaphragms. However, in the structures with staggered shear walls, the horizontal shear due to lateral loads is transmitted to the lower stories through the floor diaphragm since the walls are not usually located over each other in consecutive stories. Therefore, the rigidity of the floor diaphragm is of great importance. In the present study, a parametric analysis was performed to investigate the effect of the rigidity of the floor diaphragm on the load-carrying procedure of the structures with staggered shear walls. The investigated parameters were the number of stories, the ratio of length to width of the plan, and the thickness of walls and diaphragms. Furthermore, the study was carried out for both rectangular and I-shaped plans. All analyses were dynamically performed by ANSYS 5.4 using acceleration spectrum recommended by Iranian Building Code Standard No. 2800. Finally, the behavior of these structures and comparison of the frequencies, the maximum lateral displacements and the shear in the walls and columns as the responses of rigid and flexible diaphragms were highlighted and outlined. Keywords: Reinforced concrete, staggered shear wall, load carrying, floor diaphragm, rigidity.

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In the Present study, attempt will be made to propose a new method for prediction of long-term essential creep of concrete utilizing some short-term creep tests under high temperature. To do so, regarding the similarities between essential creep of concrete and creep in viscoelastic materials, the time–temperature equivalence relation in viscoelastic materials is evaluated for concrete. This relation states that experimental curves of creep at different temperatures fit into a single curve when shifted along the axis of logaritmic time. To develop the model, an equation was first developed taking into account the effect of temperature and the maturity of concrete. Then, an appropriate method was proposed for transmission of the creep curve of concrete under a specific temperature to fit in the creep curve of the same concrete under a temperature. The proposed model was verified using existing experimental data which very good agreement was observed.

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Silica fume has been largely used in concrete in recent decades due to its effect on improvement of strength and durability of concrete. On the other hand, attention has been recently paid to the use of limestone powder as a substitute for part of cement in concrete, basically because of its low price and its positive effect on the durability of concrete. The aim of the current study is the investigation of the interactive effect of silica fume and limestone powder on the compressive strength of concrete and the optimization of the mix design. To do so, 27 mix designs including 3 water-to-cementitious materials ratios (W/CM=0.25, 0.3 and 0.4) 3 silica fume-to-cementitious materials ratios (SF/CM=%0, %5 and %10) and 3 limestone powder-to-cement ratios (LP/C=%0, %15 and %30) were used and 28-day compressive strength of the cubic concrete specimens were determined. Then, the interactive effect of silica fume and limestone powder on compressive strength of concrete was investigated using isoresponse curves. Furthermore, the optimization of the mix design for concretes containing silica fume and limestone powder was carried out using “cost effective factor” (CEF) which is defined compressive strength divided by cost of concrete.

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It is well known that the characteristics of concrete components greatly affect the durability of high strength/high performance (HS/HP) concrete against frost action. Undoubtedly, precise recognition of this relationship leads to appropriate selection of the type and proportions of concrete components in any particular project. In the current study, the aim is to investigate the possibility of developing some mathematical-experimental models to explain the frost resistance of high-performance concrete, regarding the role of some of its main components. To do so, the effects of four key elements, i.e. water, silica fume, coarse aggregate, and number of freeze-thawing cycles, were studied on the frost resistance of HS/HP concrete were studied. 24 concrete mix designs including 3 ratios of water to cementitious materials, i. e. 0.4, 0.3, and 0.25 4 ratios of silica fume to cementitious materials, i.e. 0, 5, 10, and 15 percent and 2 types of coarse aggregates, i. e. Limestone and Quartzite were utilized for HS/HP concrete. Overall, about 432 concrete cubes were cast, cured and tested under freeeze-thaw cycles. Finally, some models were proposed for describing the frost resistance of high strength concrete.

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Although studies on RC beams under shear have a history record of more than 100 years, many important issues in this context still remain that have evaded attention. The aim of the current study is to study a number of these less investigated aspects of the behavior of RC beams under shear. For this purpose, and based on the modified compression field theory, a computer program has been written to study the effects of transverse and longitudinal steel reinforcement and shear span, a/d, on the behavior of RC beams under shear. The results show that the shear capacity of the beam cannot be increased beyond an optimum amount of transverse steel ratio. This paper will try to provide a precise definition of this optimum transverse steel ratio. Another finding of the present study is that increasing tensile longitudinal steel ratio increases the amount of the optimum transverse steel ratio, while increasing a/d decreases the optimum transverse steel ratio.