Prestress Level Research Articles

Characterization and Multiscale Modeling of the Magneto-Elastic Behavior of Galfenol

The properties of giant magnetostrictive materials in actuation, sensing, or energy harvesting applications are very sensitive to pre-stress levels. This work presents the experimental characterization of the magneto-mechanical behavior of Galfenol under static compressive and tensile stress. The results show the high sensitivity of both magnetization and strain to the stress level. A multiscale modeling tool is then used to model the measured magnetic and magnetostrictive responses. The results demonstrate the ability of the proposed multiscale approach to capture the magneto-elastic effects with a very limited number of material parameters.

Endogenous melatonin content confers drought stress tolerance in pepper

Pepper plants are susceptible to drought stress, which occurs when there is a prolonged period of water shortage or inadequate soil moisture. Drought stress can lead to a range of physiological and biochemical changes in plants, which can ultimately result in reduced growth, yield, and overall crop quality. Numerous studies have shown that exogenous application of melatonin can improve drought tolerance in pepper plants by reducing oxidative damage, maintaining better water status, and enhancing photosynthetic efficiency. However, there are few studies that answer the question whether and how the level of endogenous melatonin affects drought stress tolerance of pepper plants. In this paper, the potential relationship between endogenous melatonin content and drought stress tolerance of sixty genotypes of long-fruited pepper (Capsicum annuum L.) subjected to drought stress for twelve days was investigated. Physical (leaf area), physiological (electrical conductivity (EC), visual damage index and water potential) and biochemical parameters (pre-stress and post-stress melatonin, hydrogen peroxide (H2O2), TBARS, proline, chlorophyll, carotenoid levels, as well as catalase-CAT, peroxidase-POX, and superoxide dismutase-SOD activities) were analyzed in the collected leaf samples. Correlation analysis was performed between pre-stress melatonin levels and all other parameters and when significant correlation was found, polynomial regression was carried out to determine the relationship between these components. The correlation analysis showed that pre-stress melatonin contents were in positive relation to the activity of antioxidant enzymes, post-stress melatonin and chlorophyll contents, while in negative relation to H2O2, TBARS, carotenoids, EC, visual injury index, and water potential parameters. The contents of endogenous melatonin in leaves of 60 genotypes ranged from 4.9 to 73.9 μg kg−1 FW. Low melatonin genotypes showed lower water potential, proline and chlorophyll contents and antioxidant enzyme activities (SOD, CAT, and POX), but higher EC, H2O2 and TBARS, suggesting they are much more susceptible to stress. Additionally, weighted scores of the genotypes considered to be susceptible were significantly lower than those of the tolerant ones. All these results suggest that there is a significant relationship between endogenous pre-stress melatonin content and drought stress tolerance. Thus, the possible use of endogenous melatonin content as a physiological marker in selecting drought stress-tolerant varieties of pepper was demonstrated.

Fire behaviour of reinforced concrete slab strips strengthened with prestressed NSM-CFRP laminates

Carbon fibre reinforced polymer (CFRP) composites are now widely used to strengthen reinforced concrete (RC) structures. Among the strengthening systems available, near-surface mounted (NSM)-prestressed CFRP laminates offer several advantages, as they significantly increase the load carrying capacity and also the serviceability performance of RC structures. However, as for other CFRP systems, there is a concern about their behaviour at elevated temperature and under fire exposure due to the glass transition process undergone by their polymeric components. Nevertheless, the fire performance of NSM-prestressed-CFRP strengthening systems has not yet been investigated. This paper presents an experimental study about the fire resistance behaviour of RC slab strips strengthened with prestressed NSM-CFRP laminates; the slabs were simultaneously subjected to a mechanical (fire) load and the ISO 834 fire curve, and the influence of the following parameters on their fire resistance behaviour was assessed: (i) the prestress level (0 %, 25 % and 50 % of the CFRP tensile strength), and (ii) the presence of passive fire protection, comprising up to 48 mm thick calcium silicate (CS) boards. The results obtained showed that: (i) without fire protection, the strengthening system remained effective during a very low period of fire exposure, which significantly decreased with the prestress level, from 16 min (0 %) to less than 5 min (50 %); (ii) with fire protection, even with 50 % of prestress, the strengthening system remained effective for more than 120 min; and (iii) the “critical” temperatures in the anchorages of the strengthening systems were found to be about 2.5Tg, 2.0Tg and 1.5Tg, for the prestress levels of 0 %, 25 % and 50 %, respectively, with Tg being the glass transition temperature of the adhesive.

A review on flexural behavior of reinforced concrete beams strengthened with prestressed carbon fiber reinforced polymer plates

In recent years, with the rapid development of China’s economy, reinforced concrete structure as an economical and good mechanical properties material, is widely used in buildings. In bridge engineering, Carbon Fiber Reinforced Polymer materials is mostly used as externally bonded reinforcement for flexural strengthening of reinforced concrete (RC) beams. However, this method makes the strength of CFRP plates cannot be fully utilized, while the prestressed technology can significantly improve the efficiency of CFRP materials and the flexural strength of the RC beams. This paper compares different methods of prestressed CFRP plates strengthening RC beams and discuss the development of the technology. The results shows that RC beams strengthened with prestressed CFRP plates have better flexural performance. The cracking, yield and ultimate loads increase significantly compared with ordinary beams or non-prestressed strengthened beams. The effectiveness of strengthening process are related to many factors ranging from anchorage methods, prestress levels, ratio of corrosion to bond length. Future research suggestions are also given. At present, most of the experiments are operated in ordinary environment, which is different from the project in real life. More testing environment like chlorine corrosion must be considered. The trials about prestress loss during strengthening process are not enough. There also need more comparative trials in prestressed CFRP plates strengthening concerning prestress loss and non-prestressed CFRP plates.

Study on the effect of pre-stressed level on the force performance of wedge steel plate jacket and steel wire mesh-PCM strengthened RC beams

In this study, the shear behavior of reinforced concrete (RC) beams strengthened with wedge steel plate jacket (WSPJ) and high-strength steel wire mesh-polymer cement mortar (PCM) was investigated. The total contained 6 strengthened beams and 3 control beams, the test variables were the level of prestressing applied to the steel wire and the shear-span ratio of the RC beam. Our findings indicate that shear failure and bending failure were two types of strengthened beams failure, and the WSPJ and steel wire mesh-PCM strengthening technology can effectively improve the shear bearing capacity and ductility of RC beams. Furthermore, it delayed and inhibited the emergence and development of oblique cracks. Under the same shear-to-span ratio, the characteristic load as well as the shear bearing capacity of strengthened RC beams were significantly increased with the increase of prestressing level. The ductility is increased, and the damage mode of strengthened RC beams was changed. Finally, a modified model based on Truss-Arch model was proposed to forecast the shear capacity of strengthened beams. This model considers the impacts of the prestressing level and the shear-to-span ratio and then validated by the experimental data.

Nonlinear stability and design of prestressed stayed columns in fire considering non-uniform temperature distribution

Prestressed stayed columns are vulnerable to fire due to material weakening. The fire behaviour of the stayed columns is investigated with non-uniform temperature distributions considered using steady state analysis within ABAQUS. This study aims to reveal the load-carrying capacity reduction of stayed columns in fire and propose a design method. Results show that ultimate loads decrease with temperature increasing, and the stayed columns approximate failure at the temperatures higher than 600°C. The failure deflected mode maintains similar to the dominant linear buckling mode for the uniform temperature distribution, and the sensitivity of ultimate loads to prestressing levels becomes less significant at higher temperatures. Moreover, compared to buckling and ultimate behaviour, initial post-buckling behaviour shows more significant sensitivity to temperature distributions. The non-uniform distribution can lead to asymmetric deflected mode due to stiffness re-distribution from the material weakening at elevated temperatures, and the post-buckling behaviour of the cases with rotational symmetric mode dominance is sensitive to prestressing levels. Finally, a fire design method is proposed based on the ultimate load reduction in fire.

Numerical and experimental investigation on the flexural stiffness of RC beams strengthened with prestressed CFRP plate exposed to wetting/drying cycles

The load-deflection relationship is critical for the serviceability and structural durability of reinforced concrete (RC) structures strengthened with prestressed carbon fiber-reinforced polymer (CFRP) plates. This study aims to investigate the impact of prestressing levels and wetting/drying cycles (WDCs) on the flexural stiffness of prestressed CFRP-strengthened RC beams. A modified flexural stiffness prediction model was proposed by incorporating prestressing levels, interfacial bond slip, and softening after WDCs. Subsequently, four-point bending tests were conducted on 11 strengthened beams with different prestressing levels and WDCs to estimate flexural behavior. The results suggest that WDCs can adversely affect the flexural stiffness of the strengthened beams, particularly for the post-cracking stiffness of the non-prestressed specimens. However, prestressed strengthening can mitigate the stiffness degradation caused by WDCs. Moreover, a finite element (FE) model utilizing a traction separation model can accurately predict flexural stiffness. Finally, the flexural stiffness was computed using both the existing and proposed models and the proposed model provides the closest prediction of the flexural stiffness for CFRP-strengthened beams under WDC and prestressing.

A Study of Anchor Cable and C-Shaped Tube Support for the Roadway of Shuangliu Coal Mine

Active support using highly prestressed cable bolts and anchor cables has become a mainstream support technology for coal mine roadways. However, the ability of bolts and anchor cables to withstand transverse shear decreases with the prestress level, jeopardizing mining safety. This study proposed a technical solution to this problem featuring anchor cables enclosed in an axisymmetrical tube with a C-shaped cross-section (ACC), which are highly prestressed and can withstand high transverse shear. The ACC mechanical performance was tested in the #318 gas extraction roadway of the Shuangliu Coal Mine, China, characterized by extensive deformation under original support conditions. Theoretical analysis, laboratory tests, numerical simulation, and field tests were performed to analyze the shear mechanical properties of the ACC and anchor cables alone. The double shear test results revealed that the proposed ACC scheme increased the transverse shear resistance and stiffness by 10–25% and 20–40%, respectively. The FLAC3D numerical simulation showed that the roof-and-floor and rib-to-rib convergences decreased by 9.53 and 25.11%, respectively. The area of the stress concentration zone also decreased. Field monitoring showed that the ACC achieved good support performance. During the monitoring period, the maximum roof-and-floor and rib-to-rib displacements were 40 and 49 mm, respectively. The ACC scheme offered adequate shear resistance and effectively controlled surrounding rock deformation in the gas extraction roadway under study, making it applicable to similar engineering scenarios.

Shear performance degradation of basalt fiber‐reinforced polymer bars in seawater environments: Coupled effects of seawater sea‐sand geopolymer mortar coatings and sustained loading

AbstractGeopolymers, as a promising substitute for ordinary cement, exhibit different coating effects with regard to the durability of basalt fiber‐reinforced polymer (BFRP) bars. In this study, a comparative investigation was carried out on the shear performance of prestressed BFRP bars coated with seawater sea‐sand geopolymer mortar (SSGM). The alkaline content of the SSGM was 4% or 6%. The prestress level was 20% of the ultimate tensile strength at room temperature. The immersion temperatures were room temperature (RT), 40, or 60°C. Interlaminar shear tests and transverse shear tests were conducted at each period (30, 60, 120, and 240 days). The results showed that BFRP bar coated with 6% alkaline content exhibited a slightly higher degradation of shear strength. Furthermore, the prestress developed microcracks in the bars and weakened the interface between the fiber and resin matrix, particular at the elevated temperature. This led to further degradation of the BFRP bars. Additionally, a comparison was made between the shear strength results of BFRP bars coated with SSGM and those coated with ordinary portland concrete or seawater sea‐sand concrete, revealing that the long‐term shear performance of BFRP bars was less negatively impacted by the SSGM coating.Highlights Coupled effects of SSGM coatings and sustained loading on the degradation of BFRP bars are investigated. SSGM provides better protection for BFRP bars than cement‐base concrete. Alkaline content of activator in SSGM has insignificant impact on the BFRP degradation.

Prediction of prestress level in steel structures strengthened by bonded Fe‐SMA strips

AbstractPrevious studies have demonstrated a great potential of prestressed strengthening of steel structures employing iron‐based shape memory alloys (Fe‐SMAs). A bonded Fe‐SMA strengthening solution has been proposed with partial activation, in which the prestrained Fe‐SMA strip is fully bonded onto the steel structure, and the middle region of the Fe‐SMA strip is heated to generate prestress. The two ends remain unheated and transfer the prestress to the parent structure. In this study, a column model and a beam model are developed to analyze the prestress level in the steel structures strengthened by bonded Fe‐SMA. The predicted strains in the parent structure are in excellent agreement with the experimental measurement from the literature.

Experimental study on flexural behaviours of fresh or aged hollow reinforced concrete girders strengthened by prestressed CFRP plates

The paper presents a well-rounded experimental study on the flexural performance of Reinforced Concrete (RC) box girders strengthened with prestressed carbon fibre reinforced polymer (CFRP) plates. The motivation behind the study was twofold: the rising need for structural reinforcement of existing aged and heavily utilised hollow RC box girders, and the absence of prior attempts to integrate prestressed CFRP plate strengthening for those hollow girders. Previous experimental studies are scarce and fewer studies are focused on the combined prestress and thin-wall effects, such as prestress-related stress condensation and shear lag. However, experimental results are important in directing further analytical studies for hollow sections with more complex behaviours than solid sections since there is a need to predict the behaviour of the prestress-strengthened hollow RC structures for routine design. This pivotal experimental study aims to quantify the structural interactions initiated by prestress in hollow sections and evaluate the impact of age while promoting further analytical initiatives. In this study, two types of CFRP plates, ordinary CFRP and steel-wire-CFRP (SW-CFRP), were used on different specimen beams with varying prestressing levels, sizes of the CFRP plates, and pre-damaged states representing aged and over-used members. Their performance indexes, including cracking load, yield load, ultimate load, structural stiffness, ductility, and crack resistance, were tested and summarised in this paper. The CFRP plates of the eight specimen beams were prestressed to different levels (non-prestressed, and 30% and 40% of the CFRP plate's ultimate strength). The test results suggest that the crack load increased by 86% and 134%, when the specimens were enhanced with the combinations of 30% prestress level for the same CFRP cross-section, and 40% prestress level with a thicker CFRP plate, respectively. The flexural capacity also increased by 42% and 72%, and flexural stiffness increased by 3% and 63%, respectively. The experimental results proved that the proposed prestressed CFRP plate technology effectively strengthens the new or aged RC box girders, but the ductility is sacrificed. These first-hand test results provide an excellent target dataset for further development in the analysis and design of prestressed CFRP plate-strengthened RC box girders.

Flexural Response of Prestressed FRP Strengthened Reinforced Concrete Beam under Static Load

The motive of building sustainable structures and strengthening the structural members is important to overcome the problems associated with poor initial design and/or construction, accidental events, and degradation related to the environment. This paper aims to assess the flexural behavior of a prestressed basaltic fiber-reinforced polymer- (FRP-) strengthened reinforced concrete (RC) beam under static load through the finite-element method using ABAQUS CAE that is validated with experimental results from relevant literature. To achieve the desired objective, 18 models were prepared based on prestressing level, thickness of basalt FRP (BFRP), width of BFRP, length of BFRP, bond between concrete and BFRP, and type of FRP as study parameters. The results indicate that the use of prestressed BFRP is efficient in strengthening the flexural strength of the RC beam. Compared to nonstrengthened specimens, the flexural capacity of the specimens strengthened with BFRP at prestressing levels of 0%, 15%, 35%, and 45% show enhancement of 6.09%, 9.17%, 13.89%, and 17.57%, respectively. While increasing the width, length, and thickness of the prestressed BFRP section, increased yielding and ultimate load capacity of the specimen have been obtained; however, the ductility has been reduced up to 13.87% with increasing the BFRP thickness. The result also shows that the specimen strengthened with prestressed BFRP has better ductility than the prestressed carbon FRP (CFRP)-strengthened specimen; however, the model strengthened with prestressed CFRP has shown higher load-carrying capacity. It is also noticed that a cohesive bond better suited the experimental specimen than the fixed interference of the concrete surface, and relative to the cohesive bond, the fixed interference has shown a 17.4% higher ultimate load carrying capacity.

Effect of wetting/drying cycling on flexural behavior of RC beams strengthened by prestressed CFRP plate: Experimental and analytical investigations

Using prestressed carbon fiber reinforced polymers (CFRPs) is a promising technique for strengthening reinforced concrete (RC) structures. This study aimed to investigate the prestress losses and flexural behavior of strengthened RC beams under wetting/drying cycling (WDC) in a chloride-containing environment. Two prestress levels (20% and 40%) and two WDC durations (90 and 180 cycles) were considered. The time-dependent prestress loss under WDC was first monitored by fiber bragg grating (FBG) for 90 days, and the composition of the prestress loss was theoretically analyzed. A four-point bending test was conducted on the strengthened beams after the WDC exposure, and the intermediate crack (IC) debonding strain and the ultimate moment of the strengthened beams were analytically evaluated. The results show that although WDC exposure aggravated the time-dependent prestress losses, the maximum time-dependent prestress losses after 90 WDCs were only 1.75%, indicating the effectiveness of the end anchorage system. After WDC exposure, the failure mode of the strengthened beams shifted from CFRP rupture to IC debonding, accompanied by the reduction of cracking and the ultimate load, CFRP utilization, and the increase of the maximum crack width. Besides, the model of Lu et al. and Said and Wu could be applied to the IC debonding strain prediction after the WDC. When the environmental reduction coefficient was incorporated, ACI 440.2 R and CECS-146 resulted in a precise IC debonding strain. The proposed method could accurately predict the ultimate moment of the prestressed FRP-strengthened RC beams with different failure modes after WDC.

Flexural Strengthening of Concrete Structures Using Externally Bonded and Unbonded Prestressed CFRP Laminates: A Literature Review

Carbon fiber-reinforced polymer (CFRP) materials have been widely used in prestressed and non-prestressed systems for the flexural strengthening of concrete structures in many construction projects worldwide. Strengthening with non-prestressed CFRP materials, which is known as passive repair, could lead to an increase in the ultimate capacity of the concrete member. However, it has little effect on its serviceability performance (e.g., cracking, yielding, and deflection properties). Flexural strengthening with prestressed CFRPs provides an active repair technique that positively effects the strength and serviceability of structures. In addition, strengthening with prestressed fiber-reinforced composites efficiently uses the material’s high tensile strength. Although different fiber-reinforced polymer (FRP) materials and forms have been used in prestressing applications, this paper focused on the use of CFRP laminates (e.g., sheets, strips, or plates). These laminates have been widely used due to their excellent mechanical properties, ease of application in various geometrical shapes, good conforming surface area of contact with structures, and low installation costs. A comprehensive literature review was presented on strengthening using prestressed CFRP laminates in conventionally and internally prestressed concrete structures (PCSs). The externally bonded and unbonded prestressed CFRP laminate applications that used different anchorage systems were presented. The flexural behavior, failure modes, and serviceability performance of the strengthened concrete members were discussed. The effects of the prestressing level on ductility and energy absorption and a summary of the recommended optimum prestressing level discussions were presented in this paper. Furthermore, a review of the theoretical studies that were conducted for prestressed CFRP applications was provided. The knowledge gaps in the research area and future research recommendations were presented.

Cracking behavior of prestressed CFRP strengthened RC beams subjected to wetting/drying cycling under flexural load

The cracking pattern and spacing are essential for the structural durability of RC structures strengthened by prestressed CFRP plates. In this study, the effects of prestressing levels and wetting/drying cycling (WDC) on the cracking behavior of RC beams strengthened by prestressed CFRP plates were investigated. A modified model for calculating the average crack spacing was developed by incorporating the interfacial stress between adjacent cracks, the prestressing levels, and the WDCs. The effects of interfacial properties and prestressing levels on the average crack spacing were analytically studied. Besides, four-point bending tests were conducted on seventeen strengthened beams subjected to WDC. The results show that cracking propagation was restrained by prestressing forces but facilitated by WDCs. Moreover, WDC and prestressing force lead to an increase in the average cracking spacing of the strengthened beams. The average cracking spacings calculated by different equations were compared to the experimental results, and it can be found that the modified modelprovides the closer prediction of the average crack spacing for CFRP-strengthened beams under WDC and prestressing.

Study on vibration characteristics of fine breccia soil subgrade reinforced with a new prestressed structure under train cyclic loading

Train cyclic loading generally results in significant dynamic responses of the breccia soil subgrade, resulting in a series of subgrade problems. A new prestressed reinforcement structure (PRS) is proposed to reinforce subgrade structures. However, characteristics of the prestressed subgrade under dynamic loading have not been investigated. This study investigates the vibration characteristics of a new prestressed subgrade structure under train cyclic loading. The acceleration responses of the subgrade surface under different train axle loads and prestress levels are measured and discussed by developing a 1:5 scale dynamic model test on a prestressed fine breccia soil subgrade structure. Subsequently, a three-dimensional dynamic finite element model of the prestressed subgrade is constructed for an in-depth analysis of the internal acceleration of the prestressed subgrade. The results showed that a periodic acceleration response of the subgrade should be observed, and its mean crest and mean trough are used to characterize the long-term vibration degree of the subgrade. The peak vertical acceleration of the subgrade is significantly affected by the prestress level, and the dynamic response of the subgrade around the lateral pressure plate (LPP) is reduced more obviously by the prestress than that of the subgrade’s inner region. The peak vertical acceleration at the subgrade shoulder presents linear increase and exponential attenuation with the rise in the train axle load and prestress level, respectively. In addition, an empirical model describing the relationships among the peak acceleration, train axle load, and prestress is proposed. The vertical dynamic displacement and dynamic stress decrease with the increase of the prestress. The average elastic deformation and peak dynamic stress of the subgrade under a prestress level of 100 kPa are found to be 95.8% and 87.22–90.07% of that with the absence of prestress, respectively. The vertical vibration of the prestressed subgrade structure can be reduced by taking measures such as adjusting the prestress magnitude and increasing the subgrade slope ratio, thereby achieving an effective prestress reinforcement effect.

Numerical dynamic characterisation of concrete bridge stays

AbstractFollowing the recent collapse of a cable‐stayed bridge in Genoa, Italy, an interest has arisen to understand if it would have been possible for unknown localised material deterioration and/or decrease in prestress levels to introduce noticeable changes in the dynamic behaviour of RC stays. As such, in this study we start by reviewing past research work, experimental and analytical, on the effects that prestress level may or may not have on the dynamic response of RC beam elements. We then review also available analytical formulations used to determine the natural frequency of vibration of prestressed beams, and, subsequently, complete the preliminary investigation on the effects of prestress levels, and local damage, through the analysis of finite element models of prestressed beams and stays. The comparison of analytical and numerical estimations with results obtained from an in‐situ dynamic characterisation campaign on a RC bridge stay is also undertaken. In the second part of the study, we explicitly consider the Morandi bridge case‐study, for which finite element models of the stays alone, as well as of the full bridge, are developed and analysed. The obtained results are then also compared with the observations made in pre‐collapse in situ dynamic characterisation endeavours. All results obtained and discussed lead to the conclusion that, most regrettably, dynamic characterisation endeavours do not have the capability of providing insight on possible localised material deterioration or partial reduction of the average state of compression in this type of structural elements.

МОДЕЛЮВАННЯ РОБОТИ ПІДСИЛЕНОГО ПОПЕРЕДНЬО НАПРУЖЕНОГО ЗГИНАЛЬНОГО ДЕРЕВ’ЯНОГО ЕЛЕМЕНТУ

An undeniable disadvantage of wood when using it in construction is its excessive flexibility. One of the ways to increase the stiffness of wooden elements is to use prestressing and reinforcement with stiffer elements. The manufacturing process of pre-stressed bending elements proposed by us is simple. However, determining the necessary effort, the necessary bending of the beam to ensure reliable operation, preventing the destruction of the element is quite difficult. One of the methods of prestressing is described, namely, the method by which the beam receives internal stresses due to the release of the bending element after the action of the external load by gluing reinforced elements. The principle of such tension can be described in the following sequence: 1. We create a bend in the bending element by applying an external load to the element. 2. We reinforce the lower zone of the bending element. At the same time, we need to know under which stress-strained state the reinforced element was installed, so that its operation can be predicted. 3. We remove the applied force with which we created the bend in the flexible wooden element, while the element wants to acquire its original shape, but this will be prevented by the armature, which will absorb part of the load and leave a small bend. It was established that the level of prestressing, namely the curvature acquired by the bending element after prestressing, depends on the initial curvature of the wooden element, as well as on the area and physical and mechanical characteristics of the materials reinforcing the beam zones. To determine the prestress, it is necessary to establish the stress-deformed states of the bending element, which occur after gluing and release of the external force. At the same time, 3 levels of the stress-strain state can be distinguished. 1. At the first stage, the compressed zone is more than the stretched zone. The relative deformations of the compressed lower zone, where the material is to be attached, is the initial start for the work of the reinforced element. 2. At the second stage, the external bending moment decreases, the internal redistribution of forces takes place, in addition to the moment perceived by the compressed and stretched zone, a moment also occurs in the stretched reinforced element. 3. The third stage is characterized by the absence of an external load, and this causes a balance between the internal forces of the bending prestressed element. The equilibrium equation for three stages was obtained. When using the element as a load-bearing structure, the cross-section in the element can be with both positive and negative curvature. The peculiarities of these two stress-strain states are manifested in the change in the position of the compressed and stretched zones. In the first case, the compressed zone is located in the lower part of the element and the upper part is occupied by the stretched zone. After the curvature changes its sign from negative to positive, the upper part becomes compressed, and the lower part becomes stretched. As a result of the simulation, it is possible to conclude that the cross-section of a bending wooden element undergoes 3 main stress-deformed states during prestressing and two during its operation

Influence of prestressing force on performance of concrete plates under impact loading

An experimental and numerical study has been performed to explore the performance of one-way pretensioned concrete plates against impact loading. The impact resistance, experimental results and damage within the pretensioned concrete have been compared with the non-pretensioned concrete. The plate specimens of concrete grades M40 and M60 have been pretensioned to prestress level 10 and 20% of the compressive strength of the concrete. While, all the tendons employed in the non-pretensioned concrete were kept unstressed. The plates were struck at the mid-span by a steel mass (242.85 kg) dropped from 0.5 to 1.0 m heights. The numerical simulations have been executed using explicit finite element code considering the Holmquist–Johnson–Cook (HJC) and the metal plasticity model for concrete and steel, correspondingly. The performance of the plates is governed by the grade of concrete, impact energy and level of the prestress within the concrete. The induced prestress within the concrete enhanced the stiffness and, consequently, the impact resistance of the pretensioned concrete plates. The pretensioned concrete hence witnessed increased impact force and reduced deflection by 18.1% and 11.0%, correspondingly, compared to the non-pretensioned concrete. The splitting and punching crack within the plates became pronounced once the drop height increased from 0.5 m to 1.0 m. The simulations have estimated the peak impact force and reaction within 19.7% and 15.5% deviation, respectively. The displacement and energy absorption have been calculated using an analytical methodology closely correlated with the actual results within 18% and 14% deviation, respectively. Further, the simulations performed on two-way pretensioned concrete have shown improved performance of the plates witnessing no splitting crack and uniform crack distribution compared to one-way pretensioned concrete.

Study on propagation mechanism and attenuation law of acoustic emission waves for damage of prestressed steel strands

The study of acoustic emission waves propagation mechanism and attenuation laws for damage of prestressed steel strands is the theoretical basis for acoustic emission health monitoring of prestressed steel strands such as stay cables, slings, suspenders, and concrete structures. Firstly, the propagation mechanism of acoustic emission waves in prestressed steel strands is studied using numerical simulation methods. Secondly, the amplitude and energy attenuation laws of acoustic emission waves caused by damage of prestressed steel strands are analyzed by experimental research, and the amplitude and energy attenuation models of the influence of prestress level are constructed. Finally, the constructed acoustic emission attenuation models are analyzed in combination with practical engineering. The results show that numerical simulation visualizes the propagation path of acoustic emission waves in prestressed steel strands; With the increase of prestress levels, the acoustic emission frequency distribution of prestressed steel strands damage ranges from 100 to 200 kHz, with a dominant frequency of 160 kHz, the acoustic emission amplitude and energy attenuation coefficients decrease, and the amplitude attenuation is slower than the energy attenuation; The acoustic emission amplitude attenuation of prestressed steel strands damage in prestressed hollow slab beams is 5 times faster than that of steel strands without surrounding media.