Initial Elastic Modulus Research Articles

ВЛИЯНИЕ ЦИКЛИЧЕСКОГО ЗАМОРАЖИВАНИЯ И ОТТАИВАНИЯ НА ДИАГРАММУ ДЕФОРМИРОВАНИЯ СЖАТОГО БЕТОНА

Deformation models of materials are widely used in the calculation of reinforced concrete structures. At the same time, concrete compression diagrams are used, taking into account the descending branch. The analytical description of the concrete work diagram is associated with the determination of such concrete parameters as the initial modulus of elasticity; prismatic strength; extreme compression deformations at the top of the diagram; the value of deformations on the descending branch at 85% of the prismatic strength. The influence of alternating temperatures on reinforced concrete structures of buildings and structures leads to the development of destructive processes in concrete. At the same time, the question of the effect of cyclic freezing and thawing on the complete diagrams of concrete, including the ultimate compression deformations, remains poorly investigated. The determining factor affecting the deformation and strength properties of concrete under freezing and thawing conditions is its humidity. In this regard, studies have been conducted on the effect of cyclic freezing and thawing on the diagrams of the state of concrete of various humidity, the results of which are presented in this paper.

Mechanical Properties of Self-Compacting Rubberized Concrete with Different Rubber Types under Triaxial Compression

Different rubber aggregates lead to changes in the effect of stress conditions on the mechanical behavior of concrete, and studies on the triaxial properties of self-compacting rubber concrete (SCRC) are rare. In this study, 35 cylindrical specimens taking lateral stress and rubber type as variables were prepared to study the fresh properties and mechanical behaviors of SCRC under triaxial compression, where the rubber contains two types, i.e., 380 μm rubber powder and 1–4 mm rubber particles, and four contents, i.e., 10%, 20% and 30%. The test results demonstrated that SCRC exhibited a typical oblique shear failure mode under triaxial compression and had a more moderate descending branch compared with self-compacting concrete (SCC). The presence of lateral stress can significantly improve the compression properties, including initial elastic modulus, peak stress and peak strain, with an improvement range of 3%–73% for peak stress. While rubber aggregates mainly targeted the deformation abilities and toughness for improvement, and the peak strain improvement ranges were 0.1–3.1 times and 0.1–1.0 times for SCRC containing rubber powder and SCRC containing rubber particles, respectively, relative to SCC. At a high lateral stress of at least 12 MPa, the loss of strength due to the addition of rubber can be controlled within 10%, in which case the content of rubber powder and rubber particles was recommended to be at most 20% and 30%, respectively. Based on the Mohr-Coulomb theory, the failure criteria of SCRC with different rubber types were established. For analysis and design purposes, an empirical model was proposed to predict the stress-strain behavior under triaxial compression, considering the influence of different rubber content and lateral stress. The results obtained in this study can provide a valuable reference for the design and application of self-compacting rubberized concrete in practical projects, especially those involving three-way compression states and requiring high-quality deformation and energy dissipation.

3D-bioprinted, phototunable hydrogel models for studying adventitial fibroblast activation in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a progressive disease of the lung vasculature, characterized by elevated pulmonary blood pressure, remodeling of the pulmonary arteries, and ultimately right ventricular failure. Therapeutic interventions for PAH are limited in part by the lack of in vitro screening platforms that accurately reproduce dynamic arterial wall mechanical properties. Here we present a 3D-bioprinted model of the pulmonary arterial adventitia comprised of a phototunable poly(ethylene glycol) alpha methacrylate (PEG-αMA)-based hydrogel and primary human pulmonary artery adventitia fibroblasts (HPAAFs). This unique biomaterial emulates PAH pathogenesis in vitro through a two-step polymerization reaction. First, PEG-αMA macromer was crosslinked off-stoichiometry by 3D bioprinting an acidic bioink solution into a basic gelatin support bath initiating a base-catalyzed thiol-ene reaction with synthetic and biodegradable crosslinkers. Then, matrix stiffening was induced by photoinitiated homopolymerization of unreacted αMA end groups. A design of experiments approach produced a hydrogel platform that exhibited an initial elastic modulus (E) within the range of healthy pulmonary arterial tissue (E = 4.7 ± 0.09 kPa) that was stiffened to the pathologic range of hypertensive tissue (E = 12.8 ± 0.47 kPa) and supported cellular proliferation over time. A higher percentage of HPAAFs cultured in stiffened hydrogels expressed the fibrotic marker alpha-smooth muscle actin than cells in soft hydrogels (88 ± 2% versus 65 ± 4%). Likewise, a greater percentage of HPAAFs were positive for the proliferation marker 5-ethynyl-2ʹ-deoxyuridine (EdU) in stiffened models (66 ± 6%) compared to soft (39 ± 6%). These results demonstrate that 3D-bioprinted, phototunable models of pulmonary artery adventitia are a tool that enable investigation of fibrotic pathogenesis in vitro.

Experimental Static and Dynamic Characteristics of Recycled Waste Tire Rubber Particle-Cement-Sand Composite Soil.

To investigate the static and dynamic characteristics of rubber-sand composite soil (RS soil) reinforced with cement, a series of triaxial compression tests and resonant column tests was performed by considering the influence of rubber content (10%, 20%, 30%, 40%, and 50%), cement content (0, 1.5, 2.5, 3.5 and 4.0 g/100 mL), and effective consolidation confining pressure (50, 100, and 150 kPa). Compared with the RS soil, the addition of cement significantly improved the shear strength of a cement-rubber-sand composite soil (RCS soil), based on an undrained shear test. The increase in cement content not only makes the elastic modulus and cohesion of the RCS soil increase but also reduces the internal friction angle of the RCS soil. With the increase in rubber content, the failure of the RCS soil samples changes from strain-softening to hardening, and the prediction equation of the initial elastic modulus of the RCS soil is given herein when the recommended cement content is 3.5 g/100 mL. The effects of rubber content, cement content, and effective confining pressure on the dynamic shear modulus and damping ratio of the RCS soil were studied via the resonant column test. The test results show that the increase in rubber content slows down the modulus attenuation of the RCS soil, but increases its damping ratio. The test results also show that the increase in cement content makes the bonding force between particles greater so that the modulus attenuation of the RCS soil becomes slower and the damping ratio is reduced. At the same time, according to the change rule of the maximum dynamic shear modulus of the RCS soil with the rubber content, when the recommended cement content is 3.5 g/100 mL, an empirical formula and recommended value of the shear modulus Gmax of the RCS soil are proposed.

A Unified Nonlinear Elastic Model for Rock Material

Under conditions of low or medium stress, rocks that are in the compression state exhibit perceivable nonlinear elastic characteristics. After a comprehensive review of the existing nonlinear elastic models of rocks and joints, we proposed a new unified nonlinear elastic model. This new model contains two parameters with clear definitions, namely, the initial elastic modulus Eni and the modulus change rate m. This model covers a variety of existing models, including the simple exponential model, BB model and two-part Hooke’s model, etc. Based on the RMT experimental system, a large number of uniaxial compression tests for dolomite, granite, limestone and sandstone were performed, and their nonlinear deformation stress‒strain curves were obtained and fit with the unified nonlinear elastic model. The results show that the rocks have obvious nonlinear elastic characteristics in their initial compression stage, and that the unified nonlinear elastic model is able to describe these phenomena rather well. In addition, an empirical formula for predicting the uniaxial compressive strength of the rock was constructed, corresponding to the unified nonlinear elastic model.

Experimental Study on Permeability Evolution of Deep Coal Considering Temperature

With the depletion of shallow mineral resources, the sustainable development and utilization of deep mineral resources will become a normal activity. As a type of clean energy to promote sustainable development, gas in deep coal seams has attracted wide attention. A better understanding of the permeability evolution induced by mining disturbance and the geological environment is of great importance for underground coal exploitation and gas extraction. In order to analyze the evolution of the mechanical properties and permeability of deep coal that are induced by high ground temperature, coal of the Pingdingshan Coal Mine has been investigated, and the seepage tests were carried out by keeping the confining pressure constant and loading and unloading axial stress under different temperature conditions. The effect of temperature on the peak strength and the initial elastic modulus of coal samples is analyzed. The evolution of permeability, which is estimated with the transient pulse method, based on fractional derivative and fracture connectivity, are discussed by establishing the relationship between fracture connectivity and fractional derivative. Meanwhile, the damage variable that is caused by stress and temperature is introduced and the contribution of thermal damage on coal damage accumulation is discussed. A theoretical model is proposed regarding permeability evolution with temperature and stress based on the Cui–Bustin model, which is verified by experimental data. It has been found that the strength and elastic modulus of deep coal decrease nonlinearly with increasing temperature, which demonstrates that temperature has a weakening effect on the mechanical properties of coal. The fracture connectivity and permeability evolution trends with axial strain are consistent under different temperatures, which decrease slowly in the compaction and linear elastic stages, reach the minimum at the volumetric dilation point, gradually increase in the yield stage, then have a sharp increasing trend in the post-peak stage and, finally, become steady in the residual stage. The damage induced by temperature increases with rising temperatures under different external load conditions. When the external load increases gradually, the thermal damage still accumulates, but the thermal damage variable ratio decreases. The proposed permeability model considering temperature and stress can describe the trend of the experimental data. With axial stress increasing, the influence of temperature on permeability decreases, and its leading effect is mainly reflected in the compaction stage and the linear elastic stage of coal.

Study on Influence of Confining Pressure on Strength Characteristics of Pressurised Frozen Sand

In order to reveal the influence law of freezing pressure and confining pressure on the strength characteristics of frozen sand, with the self-developed high-pressure frozen soil triaxial instrument, the triaxial compression tests of frozen sand under different freezing pressures and confining pressures were carried out. The test results show that the freezing pressure will not change the stress–strain curve of the frozen sand. Similar to the confining pressure, the freezing pressure influences the strength of the frozen soil in two ways: strengthening and weakening. The threshold confining pressure resulting from the test was about 45 MPa. Through comparison of the initial elastic modulus with the secant elastic modulus at 0.5 times the strength, it is found that the initial elastic modulus is more appropriate to use in engineering calculations. The internal friction angle is greatly affected by the freezing pressure, and cohesion shows little change with the freezing pressure. Compared with the test results and other constitutive equations, it is found that the constitutive equation established in this paper considering the effect of freezing pressure can better describe the stress–strain relationship of the pressurised frozen sand.

Modal properties of honeycomb wheels: A parametric analysis using response surface method

Three-dimensional finite element (FE) models of a honeycomb non-pneumatic wheel (NPW) with three different spokes configurations of varying cell angles were developed in ABAQUS software in order to investigate its modal properties under a given normal load. The validity of these wheel models was confirmed through comparisons of predicted wheel responses with available data in published studies for similar wheel material properties and components dimensions. The important vibration modes affecting the wheel responses were initially identified together with the important design factors affecting the modal properties. Response surface models relating the natural frequency of each chosen influential vibration mode with its important design factors and the two-factor interactions were subsequently developed utilizing the results of FE simulations corresponding to design points of the screening design and the experimental design based on central composite design (CCD) approach. These regression models showed good fitting accuracy of the natural frequencies obtained in CCDs together with relatively satisfactory prediction ability over the design region considered. The resulting regression models are thus considered to serve as a design guidance for realizing desired wheel modal properties. The results derived from parametric studies showed that natural frequencies of identified in-plane and out-of-plane vibration modes are significantly affected by design parameters of the spokes. These include the initial elastic modulus and thickness of the honeycomb cell-wall and the cell angle. Most of the identified vibration modes are also strongly influenced by the thickness dimensions of the core layer and the tread, while their initial elastic moduli revealed considerable effects only on the wheel Hop and Twist modes.

Effects of aggregate types and softening effect on multiaxial mechanical properties of lightweight aggregate concrete

Softening effect refers to the inherent property of porous media materials of having their strength decrease over time under water saturation compared with that under dry condition. Typical porous media materials, such as ceramsite and ceramsite-based concrete, exhibit a softening effect. This study investigated the softening effect of shale ceramsite (SC) and different types of lightweight aggregate concrete (LWACs). Mix ratios of LC30 all-lightweight shale ceramsite concrete (also known as all-lightweight concrete, ALWC) whose SC and shale pottery (SP) were selectively or simultaneously replaced with limestone gravel (LSG) and river sand (RS) with the same volume fraction, respectively, were prepared for comparison with gravel lightweight concrete (GLWC), sand lightweight concrete (SLWC) and hybrid aggregate lightweight concrete (HALWC) were formed. The four kinds of LWACs (ALWC, GLWC, SLWC and HALWC) were cast into their corresponding specification and size specimens and demoulded after indoor curing for 24 h, with their target ages (7–180 d) maintained under standard and hydrostatic curing. Then, the strength and initial elastic modulus under uniaxial stress, ultimate compressive strength and stress–strain curves under triaxial stress of the specimens were tested to analyse the influence of aggregate types and curing methods on the mechanical properties and softening effect of LWACs. According to the test results, the cube compression, prism compression, splitting tensile, conventional triaxial compression and elastic modulus decreased after 180 d of hydrostatic curing; the experimental age of the conventional triaxial compression test was after 90 d. ALWC demonstrated the most evident decrease, followed by SLWC, HALWC and GLWC. The trend indicates that normal aggregates can significantly improve the softening effect of LWACs. The failure modes of the triaxial compression specimens changed from splitting failure to shear failure, and a plastic platform phenomenon was apparent on the stress–strain curves, with their descending section appearing gentler than those in the uniaxial compression test with the increase in confining pressure. The relationship between ultimate compressive strength and confining pressure of the LWACs can be accurately described by the Mohr–Coulomb single-parameter failure criterion, Hsieh–Ting–Chen four-parameter failure criterion and Willam–Warnke five-parameter failure criterion. However, the three failure criteria demonstrated application limitations based on meridian equation and deviatoric plane analysis. The extension trend of the compression meridian of the LWACs after 90 d of hydrostatic curing was generally the same, and the types of aggregates exerted a minimal effect on the meridian and deviatoric plane limit trace. The softening effect of LWACs was mainly caused by the softening effect of the SC itself and the influence of pore water on the matrix concrete.

Frost resistance and mechanical degradation characteristics of cement-stabilized macadam under the long-term freezing–thawing cycles

• Three different cement contents and removal of two different fine particle sizes of cement-stabilized macadam (CSM) were tested. • The mechanical strength characteristics were degraded by long-term freezing−thawing cycles. • The removal size of larger fine particles had adverse effects on the strength and degradation rate of CSM. • The increase in the cement content led to an increase in the frost resistance and strain energy density of CSM. • The initial elastic modulus determined using unconfined compressive strength tests could be applied to characterize the stiffness of CSM for high-speed railway lines. The high-speed railway subgrade in seasonal frozen soil regions should be able to resist long-term freezing and thawing cycles. In consideration of cement-stabilized macadam (CSM) as the subgrade surface material needs to have resistant to freeze–thaw strength deterioration. This study aimed to evaluate the mechanical degradation characteristics of CSM under the long-term freezing-thawing cycles by the unconfined compressive strength tests. Three different cement contents (3 %, 5 %, and 7 %) and removal of two different fine particle sizes (0.1 and 0.25 mm) of CSM were studied and analyzed. The number of freezing–thawing cycles was set to 0, 1, 3, 5, 7, 10, 15, 20, 30, 40, and 50, respectively. Based on the unified calculation model of strength deterioration, evaluation index of frost resistance, and analysis of strain energy density failure, C 5 P 0.1 could be used in seasonal frozen soil regions. If the subgrade bed had requirements for permeability, fine particles with larger sizes could be removed. The results indicated that, first, the strength of fine particles with larger size removal was smaller and the strength deterioration was faster under the same cement content. Second, when the cement content was higher, CSM showed better resistance to freezing–thawing strength degradation. Further, when the cement content increased, the increase in the amplitude of strain energy density varied significantly; the increasing effect was the best when the cement content increased from 3 % to 5 %. Finally, the initial elastic modulus of the Duncan-Chang hyperbolic model was used to characterize the stiffness of CSM in the high-speed railway subgrade bed. This study revealed the mechanical degradation characteristics of CSM under the long-term freezing–thawing cycles with different cement contents and removal sizes of fine particles. The findings were useful for understanding the mechanical behavior of CSM under the long-term freezing–thawing cycles.

Identification of Sensitive Parameters for Deformation of Asphalt Concrete Face Rockfill Dam of Pumped Storage Power Station

Pumped storage power station (PSPS) is an important clean energy project that plays an important role in ensuring the economical, safe, and stable operation of power systems and alleviating the contradiction of peak load regulation. Deformation analysis of the built and under construction PSPS dam was an important process of dam design and operation, which was of great significance to ensure the safe operation of hydraulic structures in the reservoir site. Nevertheless, there were many parameters involved in the model for analyzing dam deformation, which brings a large workload to the inversion and application of model parameters. In this study, the asphalt concrete face rockfill dam (ACFRD) of a PSPS in Ningxia, China, was taken as an example, a dam deformation 3D finite element analysis model based on the Duncan–Chang E-B model was constructed, and the orthogonal test method was used. The model parameters of the main rockfill zone, secondary rockfill zone, and reservoir bottom backfill zone were taken as factors for the sensitivity analysis of horizontal displacement of dam H, vertical displacement u, and asphalt concrete face tensile strain ε. The results showed that initial bulk modulus base Kb, damage ratio Rf, and initial elastic modulus base K had a relatively higher sensitivity and had significant impacts on the calculation results, while internal friction angle φ, fraction angle reduction φ, bulk modulus index m, and elastic modulus index n had a relatively lower sensitivity, which had no significant impact on the calculation results. Therefore, when using the Duncan–Chang E-B model to analyze the deformations of a PSPS dam and asphalt concrete face, Kb, Rf, and K should be the focus. Parameters with a low sensitivity could be determined by engineering analogy so as to achieve the purpose of improving calculation efficiency under the premise of ensuring calculation accuracy. Meanwhile, these parameters should also be strictly controlled during construction. The results of this study could provide a reference for the design and safety assessment of ACFRD in PSPS.

Behavior of pre-damaged normal-strength concrete prisms repaired with basalt FRP sheets under axial compressive monotonic and repeated loading

The damaged concrete columns under earthquake can be retrofitted with fiber-reinforced polymer (FRP) and the axial mechanical properties of the strengthened columns can be greatly enhanced. In the present study, monotonic and repeated axial compressive behavior of damaged concrete prisms repaired by basalt FRP (BFRP) was investigated. Pre-damage tests were performed on plain concrete prismatic specimens under three axial compressive loading levels. Then, the damaged specimens were repaired with 1-, 2-, 4-, and 6-layer BFRP composites, followed by monotonic or repeated compression tests. The test results showed that the compressive strength and axial deformation capacities of the damaged prismatic specimens under monotonic compressive loading were improved remarkably after repairing. However, in the stress–strain curves of BFRP-repaired specimens, the initial elastic modulus, and the strength at the turning and ultimate states decreased with the increase of initial damage. Based on the test results of this study and existing literature, the strength and strain at the turning and ultimate states were estimated, and a monotonic stress–strain relationship was proposed for BFRP-repaired concrete prisms considering the adverse effect of pre-damage level. Further, a modified repeated compressive stress–strain model was proposed for BFRP-repaired damaged concrete prisms. The proposed models predicted well the test results.

Investigation on Dynamic Mechanical Properties of Recycled Concrete Aggregate under Split-Hopkinson Pressure Bar Impact Test

The dynamic mechanical properties of recycled concrete (RC) and natural concrete (NC) were studied by impact tests and numerical simulation. The quasi-static tests were conducted by a servo-hydraulic machine, while the impact test used a 50 mm diameter split-Hopkinson pressure bar (SHPB). The ANSYS/LS-DYNA software simulation was selected to validate the experimental results. The recycled coarse aggregates (RCAs) came from the housing demolition and were conducted with the microwave-assisted beneficiation method. The stress–strain curves, compressive strength, dynamic increase factor (DIF), initial elastic modulus and failure modes were analyzed and discussed. The results showed that the quasi-static compressive strengths of the RC were lower than that of the NC by 5.0%. The maximum dynamic compressive strengths of the NC increased by 105.9% when the strain rates varied from 46–108, while the RC increased by 102.2% when the strain rates varied from 42 to 103. The stress–strain curves of the RC and NC demonstrated a similar pattern. The DIF showed an increasing tendency with the increasing of strain rates, while the initial elastic modulus showed a decreasing tendency. The failure modes first initiated from the edge of specimens and then propagated to the center of specimens. An empirical equation was proposed for the estimation of the DIF of the RC which was obtained from the microwave-assisted beneficiation. The simulation results for the prediction of stress–strain curves of the RC showed good agreement with the experimental results. In addition, these results suggested that the RCAs were obtained by the microwave-assisted beneficiation can be recycled and may be used in some actual engineering.

Improved fatigue properties, bone microstructure and blood glucose in type 2 diabetic rats with verapamil treatment

BackgroundType 2 diabetes mellitus is a global epidemic disease, which leads to a severe complication named increased bone fracture risk. This study aimed to explore if verapamil treatment could improve bone quality of type 2 diabetes mellitus. MethodsRat models of control, diabetes and verapamil treatment with 4/12/24/48 mg/kg/d were established, respectively. Blood glucose was monitored during 12-week treatment, and bilateral tibiae were collected. Microstructural images of bilateral metaphyseal cancellous bone and high-resolution images of cortical bone of left tibial shafts were obtained by micro-computed tomography. Fatigue properties of bone were evaluated via cyclic compressive tests of right tibial shafts. FindingsVerapamil treatment had no significant effect on blood glucose, but blood glucose tended to decline with the increase of verapamil-treated time and dose. Compared with controls, osteocyte lacunar and canal porosities in diabetes and verapamil-treated groups were significantly decreased (P < 0.05), trabecular separation and degree of anisotropy were significantly increased (P < 0.05), while trabecular tissue mineral density, trabecular bone volume fraction and trabecular number in verapamil-treated (48 mg/kg/d) group were significantly higher than those in diabetes (P < 0.05). Compared with diabetes, initial compressive elastic moduli in verapamil-treated (12/24/48 mg/kg/d) groups were significantly increased (P < 0.05), while secant modulus degradations in verapamil-treated (24/48 mg/kg/d) groups were significantly decreased (P < 0.05). InterpretationVerapamil could improve bone microstructure and fatigue properties in type 2 diabetic rats; and high-dose verapamil presented a significant effect on improving bone quality. These findings provided a new possibility for preventing the high bone fracture risk of type 2 diabetes mellitus in clinics.

Experimental Study on Triaxial Compressive Mechanical Properties of Polypropylene Fiber Coral Seawater Concrete.

In order to study the mechanical properties of polypropylene fiber all-coral seawater concrete in triaxial compression, 36 specimens were developed and constructed for triaxial compression load testing employing confining pressure value (0, 6, 12, 18 MPa) and polypropylene fiber admixture (1 kg·m−3, 2 kg·m−3, 3 kg·m−3) as variation parameters. The test observed the failure mode of the specimen and obtained the stress–strain curve of the whole process of its force damage failure. An in-depth analysis of polypropylene fiber all-coral seawater concrete’s peak stress, peak strain, initial elastic modulus, axial deflection, energy dissipation, ductility, and damage evolution process was carried out based on the experimental data. The test findings indicated that the best effect on the deformation properties of polypropylene fiber all-coral seawater concrete is obtained when 3 kg·m−3 of polypropylene fiber is blended. Under triaxial compression, the correct number of polypropylene fibers may significantly enhance the peak stress, peak strain, ductility, and elastic modulus of polypropylene fiber all-coral seawater concrete, therefore enhancing the brittle characteristics of coral concrete. During the triaxial surround pressure test, the confining pressure value and polypropylene fiber coupling effect delayed the appearance of initial damage in polypropylene fiber complete coral seawater concrete specimens, slowed the development of damage, and reduced the degree of damage to the specimens.

Compressive Behavior of Predamaged Concrete Cylinders Repaired with Jute and Basalt FRP Composites: A Comparative Study

Natural fibers have become a research hotspot in composite materials recently. This study investigated the axial compressive behavior of predamaged concrete repaired by jute fiber–reinforced polymer (JFRP) composites. Basalt FRP (BFRP) was also utilized to repair the predamaged concrete for comparison. Thirty FRP-jacketed concrete cylinders with a diameter of 150 mm and a height of 300 mm were designed and fabricated. Plain concrete cylinders were axially preloaded to three predamage levels, then repaired by JFRP and BFRP composites, and reloaded to evaluate the compressive behavior after repair. The results showed that the predamage of concrete had a noticeable adverse effect on the compressive behavior of JFRP- and BFRP-repaired concrete. Compared with the JFRP-jacketed intact concrete, the compressive strength and initial elastic modulus of the JFRP-repaired predamaged concrete decreased by 6%–18% and 16%–54%, respectively. However, the compressive strength and ultimate strain of the predamaged concrete were still improved by 5%–41% and 82%–291% after JFRP repair, respectively, when compared with those of the plain concrete. On the contrary, the ultimate axial strain and lateral strain capacities of JFRP- and BFRP-repaired concrete were insignificantly affected by the concrete predamage. Under similar confinement ratios, the compressive strengths of JFRP- and BFRP-jacketed intact concrete were close to each other, whereas the axial strain–lateral strain relationship and the variations of the Poisson’s ratios were highly dependent on the FRP types and the number of FRP layers. Based on the test results and data collected from the open literature, an ultimate strength model, an ultimate axial strain model, and a stress–strain relationship model were newly developed for JFRP-repaired predamaged concrete.

Numerical study on the deformation and failure of soft rock roadway induced by humidity diffusion

In underground space engineering, the humidity in the environment has a great influence on the structural performance of soft rock roadway. It not only changes the stress state of rock mass, but also weakens the physical and mechanical properties of surrounding rock, resulting in many accidents such as swelling deformation and even collapse of the roadway. In this paper, the deformation and failure process of soft rock roadway in high humidity environment are simulated by using the humidity diffusion-deformation-fracture coupling model based on the finite-discrete element method (FDEM). Firstly, the coupling model is verified through two simple examples with analytical solutions. Then, the humidity diffusion and floor heave of soft rock roadway after excavation are studied by using the coupling model. The results show that only the humidity of the surrounding rock surface changes greatly at the beginning of excavation and the roadway floor heave caused by humidity is small but with a large change rate. However, with further diffusion of humidity, the displacement of the floor increases slowly at a constant rate. Besides, several main factors affecting floor heave are studied, including humidity swelling coefficient, initial elastic modulus, and initial humidity of floor. Finally, the coupling model is used to simulate the roadway failure process induced by humidity diffusion. The numerical results provide a better understanding of the deformation and failure behavior of soft rock roadway in high humidity environment.

A nonlinear visco-hyperelastic model for spray polyurea and applications

Spray polyurea (SPUA) is widely used as a protective coating material for engineering structures subjected to the intensive impulsive loadings attributed to its large deformation and viscoelastic properties. Both experimental and theoretical studies of the hyperelastic and viscoelastic behaviors of SPUA, as well as the applications in the numerical simulations of the SPUA retrofitted structures under impact and blast loadings were conducted. Firstly, the reasonable geometries and dimensions of specimens are designed, and the complete compressive stress-strain curves of SPUA with the strain rate ranging from 10−3-103 s−1 were experimentally derived. The dependences of the dynamic initial elastic modulus, stress and fracture strain on the strain rate are formulated. It indicates that the initial elastic modulus and stress increase as the strain rate increases, while the fracture strain is linearly negatively correlated with the logarithm of strain rates. Then, a twenty-four-parameter nonlinear visco-hyperelastic constitutive model composed of a hyperelastic model and seven Prony series terms is established and validated, in which the model parameters are determined through a multi-objective optimization based on genetic algorithm. Finally, by evaluating the existing sixteen constitutive models embedded in finite element program LS-DYNA, the Simplified rubber/foam model is selected for describing the mechanical behavior of SPUA. The Simplified rubber/foam model parameters are further calibrated by the established nonlinear visco-hyperelastic model and comprehensively validated by comparisons with six series of impact and explosion tests on the SPUA, SPUA retrofitted steel plate and masonry wall. The present work could provide a helpful reference for the design and evaluation of the SPUA retrofitted structures.

Testing and Modeling the Gradually Applying Compressive Stress to Measuring the Strain of Self-Compacted Cement Paste Using Vipulanandan p-q Model

Abstract This study tested and quantified the stress-strain behavior, elastic modulus, and toughness of self-compacted cement paste. The characteristics of self-compacted flowable cement paste modified with two types of polymer up to 0.06 % (dry weight of cement) were investigated. Stress-strain behavior of modified cement with polymer was investigated for different curing times 1, 3, 7, and 28 days. Adding polymers improved the flowability of cement by 7 to 26 %, but it decreased the water/cement ratio (w/c) from 0.5 to 0.29, depending on the polymeric structure and its content. The nonlinear Vipulanandan p-q equation was tested to predict the stress-strain relationship of the modified cement with polymers and was compared with the β model. When 0.06 % polymers were added to cement, the compressive strength increased by 107 to 257 %. According to polymer types, curing age (t), w/c, and polymer content, the initial elastic modulus (E) of cement also increased by 23 to 240 % with additional polymers. Based on the statistical evaluation tools, the Vipulanandan p-q model predicted the stress-strain relationships better than β model. Linear and nonlinear relationships were used to predict the compressive strength, modulus of elasticity, and toughness of the cement paste as a function of w/c, curing time, and polymer contents.

Experiment and mesoscopic modelling on the dynamic compressive behaviors of a new carbon fiber-reinforced cement-based composite

This paper presents an experimental and numerical study on the compressive behaviors of a new carbon fiber-reinforced cement-based composite (CFRCC) that is composed of magnesium oxysulfate cement, carbon fiber and some chemical admixtures. Parametric studies on the effects of carbon fiber content and strain rate on the compressive behaviors of CFRCC, are conducted using a series of quasi-static and dynamic compressive tests. The research item includes the failure morphology, stress-strain curve, dynamic increase factor (DIF), etc. Besides, a three-dimensional (3D) mesoscale modelling approach is proposed to simulate and analyze the mechanical responses and the failure mechanisms of CFRCC subjected to dynamic compressive loading, where the random dispersion of chopped fibers are realistically considered. The results indicate that the cracking and failure modes of CFRCC are significantly affected by the bridging effect of carbon fibers randomly dispersed in cement matrix. With the increase of fiber content, the dynamic peak stress and peak strain of CFRCC at similar strain rates show an increasing and decreasing trend, respectively. An evident strain-rate hardening effect could be found on the initial elastic modulus and the peak stress of CFRCC as well. While the relationship between the DIF of CFRCC and the logarithm of strain rate could be described using a quadratic function. Moreover, the mesoscale modelling approach has been validated by a good agreement between the numerical and tested results, demonstrating the feasibility of the developed mesoscale model in simulating and investigating the dynamic behaviors of CFRCC. All in all, the CFRCC has proven to be of great significance in resisting to impact loads, and more insights should be provided into its dynamic properties.