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Prediction of stress-strain behavior of geotechnical material is one of the major efforts of engineers and researchers in the field of geomechanics. Experimental tests like tri-axial shear strength tests are the most effective apparatus to prepare the mechanical characteristics of gravelly material; but due to difficulties in preparing test samples and costs of the tests, only several tests will be done in a new project. Artificial neural network is a kind of method, in which engineer could judge the results based on numerous data from other similar projects, which enable the engineer to have a good judgment on the material properties. In this research, the behavior of gravelly material was simulated by use of multi-layer perceptron neural network, which is the most useful kind of artificial neural networks in the field of geotechnical engineering. For instance, first exact information was provided from laboratory tests of various barrow areas of embankment dams in the country and effective parameters on shear strength of coarse-grained material were studied. After omitting incorrect or weak data, 95, 20 and 23 sets of data were used for learning, testing and evaluating data, respectively. Input parameters for the model were as follows: particle-size distribution curve, dry density, relative density, Los-angles abrasion percent, confining pressure, axial strain; and outputs were selected as deviator stress. In order to reach a steady state in the model and force the model to behave homogenous to the all inputs, data was normalized to the value between .05 and 0.95. In the simulation, back-propagation algorithm was used for learning or error reduction. The aim of the simulations was defined to reduce error between real data and predicted values; for instance root mean square error (RMS) was used to be minimized through simulation and predicted versus real graphs were used to observe the global error of the model. After modeling the data based on some criteria, it was shown that curves of stress-strain from simulation tests were in good agreement with those from laboratory. These close coherencies were observed in all training, testing and evaluation data, in which the RMS errors were 0.038, 0.037 and 0.026, respectively. To reach this ultimate step, a 10*19*1 multilayer perceptron was used via trial and error. In order to determine quality and quantity of the effect of inputs on outputs, and prove that the results were in good agreement with soil mechanic principles, sensitivity analyses were done on the average data of the inputs. Results show that confine pressure, uniformity coefficient and relative density of the material were the most effective parameters on the stress-strain curves; thus the model has enough capability to predict the stress-strain behavior of gravelly soils.

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The potential use of leca and scoria aggregates available in Iran to produce high strength lightweight concrete has been studied. As a result of various concrete mixes with different lightweight aggregate amount, and using normal techniques, it was possible to obtain high quality lightweight concrete which is suitable for application in reinforced and prestressed concrete structures, with a compressive strength higher than 40MPa at 28 days. In order to investigate the mechanical short term properties of this type of high strength concrete, different tests had done. The properties obtained include unit weight, compressive strength, splitting tensile strength, static modulus of elasticity and poisson's ratio. And, some relationships were given to estimate modulus of elasticity and splitting tensile strength of these concretes based on their compressive strength. The mechanical properties were improved by reducing the amount of lightweight aggregate. The results show that both scoria and leca were effective in improving the mechanical properties, but scoria could reach superior limits. Stress- strain curves of investigated concretes were studied and some recommendations are presented for estimating the curves. Based on results, curves were almost linear in ascending and descending branch.

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The potential use of leca and scoria aggregates available in Iran to produce high strength lightweight concrete has been studied. As a result of various concrete mixes with different lightweight aggregate amount, and using normal techniques, it was possible to obtain high quality lightweight concrete which is suitable for application in reinforced and prestressed concrete structures.In order to investigate the mechanical short term properties of this type of high strength concrete, different tests had done. The properties obtained include unit weight, compressive strength, static modulus of elasticity and poisson's ratio.Among the 28 day aged tested specimens, we could produce HSLWC containing scoria with 55 MPa compressive strength and 2070 kg/m³ weight.In LWC containing leca, a cube compressive strength of about 41 MPa with a unit weight of 1865 kg/m³ was reported.Static modulus of elasticity in concretes containingleca was between 15 to 19 GPa. It was between 17 to 19.5 GPa forconcretescontainingscoria. Some equations were offered to estimate static modulus of elasticity of these concretes based on their compressive strength and unit weight. The mechanical properties were improved by reducing the amount of lightweight aggregate. The results show that both scoria and leca were effective in improving the mechanical properties, but scoria could reach superior limits.Stress- strain curves of investigated concretes were studied and some recommendations are presented for estimating the curves.Stress-strain curves of investigated concretes are almost linear in ascending and descending branch.According to the observed coefficients, withincrease in concrete strength, the slope of the curve is reducedin comparison with modulus of elasticity. For scoria, the slope of curve is lower than modulus in all three mix designs, because of the higher modulus of elasticity of scoria aggregates, in comparison with leca.

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Reactive powder concrete (RPC) represents a new generation of cement-based materials composed of cement, reactive ultrafine powders, siliceous fine aggregates, super plasticizers and fibers. Due to its microstructural properties, this concrete demonstrates specific properties including high compressive and flexural strength, superb durability. Since this is a novel type of concrete, a single design code containing multiple experimental results of high quality, together with reliable stress-strain models for the nonlinear analysis of the structural members made of this concrete type is lacking. Although some experimental equations to predict the strength of the RPC members can be found in the literature, note that there are shortcomings in the information provided specifically regarding the RPC containing synthetic and hybrid fibers. Hence, in this study, ten different mix designs of RPC, containing steel fibers at the volume fractions of 1, 2, and 3%, polyvinyl alcohol fibers at the volume fractions of 0.25, 0.5, and 0.75%, together with hybridizations of the two fiber types at the total fiber volume fraction of 1% were prepared, and then tested to obtain accurate and applicable equations as well as the compressive stress-strain curve with the purpose of estimating the mechanical properties and better predicting the behavior of this type of concrete. Then, the effect of the type and volume fraction of fibers, together with curing regime on the properties of RPC including the compressive strength, strain at peak stress, modulus of elasticity, and the shape of stress-strain curve was investigated. The obtained results indicate that as the volume fraction of steel and polyvinyl alcohol fibers increases, the compressive strength and strain at peak stress of the RPC specimens decreases; a trend which is also observed as the volume fraction of synthetic fibers in the concrete mix containing hybrid fibers increases.. The trend which is observed for the strain at peak stress in the RPC is very close to that for its compressive strength. The secant and tangential modulus of elasticity values of the RPC also demonstrate trends similar to each other, and the tangential modulus of elasticity in all the specimens has values higher than the corresponding secant modulus of elasticity. The RPC containing high volume fractions of steel fibers shows high modulus of elasticity values, due to the crimped shape of fibers as well as the strong cohesion they provide in the concrete. Heat treatment has a positive effect on the compressive strength and strain at peak stress of the RPC specimens, due to the acceleration of the hydration process of cementitious materials at high temperatures as well as the formation of a dense matrix. By using the nonlinear regression analysis of the data, experimental equations were developed for the parameters affecting the stress-strain curve of RPC. Finally, based on the experimental parameters obtained for all the RPC specimens, a model was proposed to predict the compressive stress-strain curve. By comparing the proposed model with the experimental results of the stress-strain curve of RPC, it can be said that the proposed model is capable of predicting the experimental results with a very good accuracy.

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Leed titanate as an ionic Perovskite is ferroelectric at the lower of the below 766 K, which is called the transition temperature (Curie temperature), and at the above of this temperature is in the paraelectric phase. Studying the influence of mechanical parameters on the ferroelectric properties of PbTiO3 is important in the industrial application (such as RAM) of PbTiO3. In this study, using the molecular dynamics simulation method, the stress-strain effects on the polarization of lead titanate in the ferroelectric phase have been investigated. For modeling the atomic potential and interactions between ions in the ferroelectric phase, the short-range Buckingham potential and long-range coulombic potential, and, in addition, the fourth-order potential of oscillatory springs using a shell model (a model for calculating the polarization of a system) has been used. In this study, the effects of mechanical stress-strain action in the ferroelectric phase were investigated in two tensile and compression uniaxial stress-strain. In tensile stress-strain mode, the application of external stress leads to an increase in the polarization of the system, while applying compression stress-strain results in the decrease of the polarization of the system, so that by applying stress-strain, the polarization of the system reaches zero.
 

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Seismic retrofit of concrete columns with FRP composites is a well-known method for enhancing their strength and ductility. Behavior of rectangular concrete columns confined with FRP composites depends on several parameters, including unconfined concrete strength, confinement level, aspect ratio of cross-section (defined as the depth /width of the cross-section), and the sharpness of the section corners. For proper design of rectangular concrete columns confined with FRP composites, a good understanding of the stress–strain behavior of FRP-confined concrete prism under axial monotonic compression is necessary. In recent years many design oriented stress-strain models with simple closed-form expressions have been developed for FRP-confined concrete columns. Also some analysis oriented models are proposed in which the stress-strain behavior of circular columns is generated with an incremental process. But to the best knowledge of authors, there is not an analysis oriented stress-strain model for FRP-confined rectangular columns in the literature. Thus in this paper a base model for actively confined concrete is used to develop a new analysis stress-strain model for rectangular concrete columns confined with FRP. This model considers all parameters that affect behavior of rectangular columns. The procedure for generation of analysis oriented stress–strain curves for FRP–confined concrete based on active confinement model is as follows:
1) For a given axial strain, find the corresponding lateral strain according to the lateral-to-axial strain relationship;
(2) based on force equilibrium and radial displacement compatibility between the concrete core and the FRP jacket, calculate the corresponding lateral confining pressure provided by the FRP jacket;
(3) use the axial strain and the confining pressure obtained from steps (1) and (2) in conjunction with an active-confinement base model to evaluate the corresponding axial stress, leading to the identification of one point on the stress–strain curve of FRP–confined concrete;
(4) Repeat the above steps to generate the entire stress–strain curve.
     It is obvious from above procedure that the main relations in analytical modeling are the lateral-to-axial strain relationship, lateral confining pressure provided by the FRP jacket, peak axial stress on the stress–strain curve of actively confined concrete, axial strain at peak axial stress, and stress–strain equation. Thus in this paper these relations for rectangular sections are presented and when these relations be defined, the stress-strain curve can be generated using above mentioned procedure. In this paper an experimental database containing 167 axial compression test results of externally confined rectangular columns is assembled and used for stress-strain modeling. The proposed model considers different parameters that can affect the behavior of rectangular columns, including aspect ratio, corner radius, confinement ratio, and unconfined concrete strength. Also both the strain hardening and strain softening behavior of rectangular columns can be modelled by the proposed formulation. Comparison between experimental results and those of model predictions indicates that the proposed model provides good predictions for different parts of stress-strain curve such as compressive stress and strain also ultimate stress and strain. Also the shape of predicted stress-strain curve is in a good agreement with the test results.