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Abstract Prestressing techniques are generally used for the construction of large containment shells to overcome tension and to ensure tightness. Thus, the evaluation of the prestressing force variations considering the geometry and mechanical characteristics of tendons becomes an important matter. This paper considers the effects of time dependent deformations of materials on the circumferential prestress force in prestressed concrete cylindrical containments. Numerical studies are performed using creep and relaxation models presented in different codes of practice after checking the numerical model by experimental data reported elsewhere about a prestressed concrete beam. The effects of vertical prestressing and wall thickness are also studied. Obtained results show an amount of 15% to 22% of prestress loss in tendons, specially in those located around wall’s mid-height , on which relaxation of steel has a dominant effect. Furthermore, the minor effect of vertical prestresssing might be mentioned. Besides, the creep model of Iranian concrete code is studied and some suggestions are made for the modification of the coefficient used for the prestress losses due to relaxation effects.

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  Abstract: For theoretical and practical investigation of damage increase on dynamic characteristics of concrete structures can use analytical model to extract dynamic characteristics such as natural frequency and mode shape. In this research, results of experimental and finite element analytical model for various specimens were compared. These specimens include RC beams and pre-stress concrete beams that constructed in laboratory. In this paper, one of the specimens was modeled for showing how modeling cracked concrete beams and specials notes related to nonlinear static analysis and modal analysis. In test case, damages are produced step-by-step applying the static load and modal characteristics of the specimen are measured via modal test immediately after loading step. However, in finite element modeling case is two complicated problems. Firstly, because concrete is a composite material, modeling of cracked concrete is very difficult. Secondly, in RC structures, both the concrete and steel have nonlinear behavior. Results of this research include peculiar notes that can be useful for other similar researches.      

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Deep beams are the members that their behavior is different from conventional beams due to their special geometry and loading condition. Due to the low thickness compared with the height of the beams, the flexural reinforcement’s ratio is usually high and need to be placed in several layers. One of the most effective ways to reduce the ratio of the flexural reinforcement is to use of the prestressed reinforcement instead of conventional reinforcement which more conventional reinforcement can be replaced by a prestressed reinforcement. If that happens, there will be discussion of prestressed deep beams. In recent decades, along with the serious discussion of prestressed deep beams, reinforced concrete members retrofitted with FRP are also considered and in the last years the similar studies have also done on deep beams. The girders are usually prestressed deep beams in the structures such as reinforced concrete bridges, and if the retrofitting of them is considered, it will encounter with prestressed deep beams and it is necessary to have knowledge of the behavior of such members. However, the simultaneous effect of prestressing together with retrofitting has not been studied. For this, the experimental study was carried out in this paper for a better understanding of their behavior and comparing of their behavior with other deep beams. This paper study the behavior of simply supported deep beams experimentally by different conditions and has been examined their behavior compared to conventional deep beams, prestressed deep beams, and deep beams strengthened with CFRP. For this purpose, 10 deep beams with span to depth ratio of 2 are constructed and subjected to single-point failure load. Considering of this span to depth ratio is due to more compatibility with existing codes. The concrete cylindrical strength is considered greater than 400 kg/cm2 because of prestressed specimens. The test indicates that the idea of replacing of the prestressing cable instead of conventional reinforcements is appropriate and can increase the shear strength and initial stiffness of deep beams in addition to their bending strength. The analysis of experimental results shows that the effects of prestressing and strengthening are not the sum of prestressing and strengthening individually. Moreover, if two conventional and prestressed deep beams with equal shear capacity strengthen with the appropriate arrangements of CFRP, ultimate strength of prestressed deep beam will be 7% higher than conventional deep beam. The energy absorption and ductility of prestressed deep beams strengthened with CFRP are higher than strengthened conventional deep beams. Furthermore, the comparison of experimental results with existing codes and relations in the literature shows that none of the relations have the ability to predict the behavior of deep beams tested in this paper. It is necessary to generalize the existing relations to obtain to the accurate prediction.

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Catastrophic failures due to corrosion are among the most common phenomena in pre-stressed concrete pipeline, which has been reported in Iran, as well. Structural health monitoring, quick assessment and timely detection of corrosion in its early stages with active in-situ sensors is could prove vital in avoiding such hazards. Acoustic emission is a non-destructive technique that can be used to give a better insight on the structural state of such concrete structures. However, the interpretation of the AE measurements is quite challenging and may actually be even more difficult when the concrete is cracked, which would affect the material and structural properties of concrete pipes. The amplitude distribution of the acquired signals is very sensitive to micro-cracking. This paper presents the results of an experiment conducted in the laboratory of Middle East Technical University on pre-stressed concrete pipe for determining the amplitude attenuation and path of acoustic wave propagation and frequency spectrum before and after corrosion using Hsu-Nielsen pencil-lead break source and applying accelerated corrosion. The results from the laboratory tests indicate that since the changing in amplitude and wave propagation path is negligible before and after corrosion, the AE measurements can be used as an accurate method for tackling the problem mentioned above. Then the performed AE measurements are reported and results discussed.

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Corrosion in spiral steel prestressed wires tensioned around core are one of the major weaknesses of prestressed concrete pipes which their untimely detection can cause sudden failure and damages. To date, these kinds of pipes are used and produced in Iran and their abrupt failure due to corrosion has been experienced. In this study acoustic emission monitoring in prestressed concrete was used to investigate the corrosion. An approximately full-scale experimental sample pipe is made in Middle East Technical University laboratory. The pipe is loaded by internal water pressure and accelerated corrosion applied to the sample and the resulted acoustic emission signals are recorded using piezoelectric sensors during corrosion. The sample is tested under wetting and drying cycles frequently for corrosion detection in which during the experiment, pipe inside pressure was fluctuated and Kaiser Effect was studied in different conditions. Experimental results show significant changes in some gained acoustic emission parameters as the pipe work pressure increases to higher amounts. It is shown that the changed AE parameters can be used for damage prediction, condition assessment and corrosion detection of prestressed concrete pipelines.

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Concrete-encased concrete-filled steel tube (CFST) has been presented to integrate reinforced concrete (RC) and CFSTs that have been used increasingly in high-rise buildings and bridges in the world. Concrete-encased CFST exhibits higher confinement of the concrete core, high stiffness and strength, better durability and ductility, small sectional size, higher fire resistance due to the protection of the outer RC encasement compared to CFST, avoidance of local buckling and corrosion of steel tube, and easier connection with steel RC beams. On account of the insufficient research and the unknown behavior of these beam types, in this research, prestressed concrete-encased CFST (PCE-CFST) beams that incorporate CFST in the compression zone to improve the strength of concrete, and prestressed strands in the tension zone to control cracks in reinforced concrete (RC) beams are numerically investigated. The objective of this study is the finite element analysis of parameters that are not feasible to be examined through experimental specimens. Hence, the experimental study has been done to validate the nonlinear finite element modeling and a full-scale model is constructed to explore the flexural behavior of the cross-section. The model is then developed to include parameters such as the longitudinal rebar ratio, prestressed strand ratio, core concrete ratio, and the steel tube ratio indices. Based on findings, a good agreement was observed in the moment-deflection diagrams and the failure modes between the experimental and numerical results. Then the model was developed and 9 PCE-CFST beams is modeled by finite element software of ABAQUS to investigation of longitudinal rebar ratio (0.0257, 0.00856, 0.00286), prestressed strand ratio (0.00228, 0.00912, 0.000569), core concrete ratio (0.0206, 0.07799, 0.0281), and the steel tube ratio (0.01385, 0.00615, 0.000385) indices. The beam specimens were subjected to four-point loading and the parameters of bearing capacity, moment-deflection curve, energy absorption, ductility, failure mode, bending stiffness were investigated. Examination of indices revealed that as the prestressed strand ratio increases, displacement ductility, flexural stiffness and ultimate moment increase by 1.47, 1.06 and 3.22 times, respectively. Further, the elastic and entire absorbed energy of cross-section escalate by 1.04 and 3.22 times respectively, with increasing prestressed strand ratio. Likewise, by increasing the index of longitudinal rebar ratio, flexural stiffness and ultimate moment are 1.18 and 1.22 folded, respectively. In addition, the elastic absorbed energy is increased by 2.85 times as the longitudinal rebar ratio increased. As the ratios of core concrete and steel tube increase, the flexural stiffness is reduced by 5% and 6%, respectively. While, by increasing the core concrete and steel tube ratios, the ultimate moment grow by 1.05 and 1.29 times, respectively. The only effective index on the cross-section ductility and the entire absorbed energy is the prestressed strand ratio. The longitudinal rebar ratio has also the greatest increasing impact on the flexural stiffness and the elastic absorbed energy. Moreover, the core concrete ratio has the least effect (less than 10%) on the flexural stiffness. The prestressed strand and core concrete indices have respectively the highest and lowest escalating effects on the ultimate moment. As a consequence, an increase in the prestressed strand and longitudinal rebar ratios lead to a rise in the flexural stiffness and ultimate moment. On the contrary, an increase in the steel tube and core concrete ratios, decrease the flexural stiffness and gives a marginal increase to the ultimate moment. It was also unveiled that the failure mode of full-scale beams is flexural, and shear crack and shear capacity govern the behavior of PCE-CFST beams with shear span-to-depth ratios of less than 2. As shear span-to-depth ratio increases, that is, the shear failure mode shifts to flexural, flexural stiffness decreases, yet the ultimate bending moment increases. Additionally, a strut-and-tie model was proposed to describe the load transfer mechanism of PCE-CFST beams.

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The problem of determining the prestressing force in the tendons of prestressed concrete structures and monitoring the non-exceedance of prestressing drops is an issue that has been addressed by many researchers over the past decades and has provided methods in this field. Today, pre-installation sensors are installed in important prestressed concrete structures to monitor prestressing loss. However, due to the unpredictability of such equipment in older structures, monitoring of these forces requires destructive or non-destructive testing but is inaccurate. Therefore, in this paper, a method is presented that without the need for these sensors and destructive tests, only by measuring static displacement, is able to detect the amount of prestressing loss in the cross-sectional tendons of a prestressed concrete beam. In this regard, an algorithm in the Python program environment based on genetic algorithm as well as modeling in the finite element analysis program is provided. The numerical example presented in this research shows that the proposed algorithm detects the values ​​of prestressing loss with good accuracy even in spite of 10% of the intentional error due to measurement. In recent years, the use of prestressing methods has become much simpler and more effective, and its materials have been optimized. Today, a high percentage of structures under construction worldwide are built using this technology, and the advance has found wide applications in the construction of office buildings, residential, commercial, parking lots, sports stadiums, concrete tanks and special structures such as piers. Therefore, in recent years, for long-term monitoring of prefabricated structures, equipment and sensors sensitive to force drop, such as fiber optic sensors and FBG sensors in the construction phase are predicted and installed in the desired locations. [13] However, since the above equipment requires a lot of money and it is not possible to use them in old structures, the need for a technique that shows the amount and location of force reduction in all tendons without using them remains. Therefore, in this paper, a method is presented that, while using the simplest tools, provides the most accurate results only by measuring static displacements under the effect of various loading scenarios and using an artificial intelligence algorithm based on genetic algorithm. The proposed method is based on computer analysis and comparison of the results of two prestressed concrete beams with the same geometry, loading and arrangement of tendons. First, a specific prestressing beam is modeled in the SAP2000 analysis program and the desired prestressing forces are applied to it, and then these forces are reduced in some of the studied tendons. This deliberate change in prestressing values ​​is considered as failure and the technique presented in this mapping tries to discover the extent and location of failure of this beam. In other words, this paper is the determination of the amount of prestressing force in prestressed concrete beams in which force measuring sensors are not predicted without the need for destructive testing and only by measuring the static displacement under load. In the form of a numerical example on a prestressed concrete beam consisting of 6 steel tendons and using a genetic algorithm, it was shown that the displacement is a function of the amount of prestressing and its location and amount of reduction by the technique used. It was correctly detected with 93% accuracy when 10% of the deliberate error due to displacement field measurement was applied. As a suggestion for future work, this research will be able to be developed in the simultaneous diagnosis of prestressing reduction and beam concrete failure.

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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.

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Considerable age of numerous concrete structures and due to some reasons like changes in design philosophy, increase in applied loads, etc., have made strengthening and maintenance compulsory. Shear failure in reinforced concrete beams is frequently sudden and brittle. For this reason, efforts are made to avoid this type of failure by strengthening them, especially in structures that were made with engineering mistakes and damaged structures. Steel, fiber-reinforced polymers, and carbon fiber-reinforced polymers are used as conventional solutions, but these methods have some drawbacks. For instance, prestressing them is hardly applicable, and the prestressing force decreases over time. Therefore, nowadays, as an alternative, shape memory alloys (SMAs) are investigated as new strengthening methods owing to their unique features. Shape memory alloys are novel and smart material groups that have been considered in civil engineering for many purposes, including active and passive control of structures, dampers, and strengthening of structures like reinforced concrete structures and bridges, etc., due to unique features such as pseudo-elasticity and shape memory effect. They have the particular property of returning to their initial shape by heating which is called the shape memory effect. If the SMAs prevented from returning to their initial shape by using mechanical fixation, a prestress force develops owning to the shape memory effect property. NiTi or Nitinol has been used for damping applications in civil engineering, and it has been investigated in the literature. Iron-based shape memory alloys (Fe-SMAs) have attracted much attention in civil engineering applications due to their shape memory effect. Particularly for strengthening applications, iron-based shape memory alloys have some benefits such as wide transformation hysteresis, high elastic modulus, and lower cost compared to conventional NiTi alloys. The advantage of shape memory alloys over fiber-reinforced polymer is that they can be prestressed more easily than FRP, and the prestressing force will not reduce over time. In addition, it does not require any mechanical and hydraulic jacks. Prestressing these materials has some advantages in strengthening. For example, cracks and deformations can be reduced or at least prevented from further growing, and the stresses in internal stirrups are reduced. The usage of prestressing for shear strengthening is rare because it is very complex from a practical standpoint. This study aims to assess the behavior of RC beams strengthened in shear with iron-based shape memory alloys. For this purpose, based on experiments in the literature, T-beams with 5.2-meter long are investigated numerically by using finite-element analysis software, ABAQUS. Three-dimensional finite element models were developed using the concrete damage plasticity and were verified with experimental results. Comparison between the results from the FE models and experimental test results confirmed the accuracy of the proposed models. Furthermore, the effects of parameters such as shape memory alloy diameters, prestressing force, and shotcrete thickness on beams' shear behavior are also investigated. The results of the analysis indicate a notable increase in the final shear strength of the strengthened beams and a reduction in stirrups' stresses. The prestressing ability of shape memory alloys delays the yielding of stirrups and the appearance of shear cracks and reduces the thickness of the cracks.
 

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Accelerated construction methods are extensively used worldwide to reduce the negative impacts of bridge construction on urban traffic. These methods usually require prefabricating parts of the bridge off-site, which reduces on-site construction time and improves the quality and safety of construction. While the use of precast elements for bridge decks is relatively common, using precast elements for bridge piers is a recent development, especially in high-seismicity regions. Prefabrication of bridge piers can further expedite the construction of bridges. Moreover, the use of precast elements can be combined with a self-centering capability, through which the earthquake-induced damage and cost of post-earthquake repairs are greatly reduced. Despite a number of previous numerical and experimental studies on the behavior of precast, self-centering bridge piers, limited information is available on the selection of design parameters for such piers, and important decisions such as the prestressing force needed to achieve suitable seismic behavior remains to a large extent uncertain. This study aims to investigate the seismic behavior of concrete bridges consisting of precast self-centering piers, in which unbonded, post-tensioned tendons are used for self-centering and reinforcing steel is used to dissipate earthquake energy. A two-dimensional numerical model was developed in OpenSees to simulate the behavior of concrete bridges consisting of precast self-centering piers. The model consisted of fiber elements to model concrete and mild steel, as well as truss elements to model unbonded post-tensioning steel. The model also involved the use of zero-length sections to model the bond-slip behavior of mild steel bars. The modeling approach was validated based on experimental results available in the literature on cyclic loading of four bridge piers. To evaluate the effects of various design parameters on the behavior of precast segmental bridge piers, 9 segmental piers with different percentages of prestressing force and reinforcing steel were designed according to 2017 AASHTO LRFD Bridge Design Specifications. All piers were designed to possess similar nominal flexural capacities. The piers were then subjected to monotonic, cyclic, and dynamic time history analyses. The results showed the positive effects of prestressing in delaying cracking and reducing the residual drifts of precast bridge piers. Increasing the prestressing force ratio up to 10 percent of compressive strength of the pier cross section was observed to improve the overall seismic behavior of the structure, above which a further increase in the prestressing level may result in a diminished performance. The optimal value for the prestressing force ratio, which resulted in the most desirable behavior for cyclic and dynamic loadings was therefore found between 0.1 and 0.15. In piers with a prestressing ratio above 0.15, a decrease was observed in the area of hysteresis loops, which was accompanied by negative stiffness of the base shear versus drift curve. Moreover, the residual drift of the pier increased when prestressing ratios greater than 0.15 were used. The maximum drift of the structure was found to be insensitive to the prestressing force ratio. The results of this study are of great value for optimal design of precast, self-centering bridge piers in high-seismicity regions.