Deformation Of Concrete Research Articles

Separate modeling technique for deformation monitoring of concrete dams

Deformation monitoring is an important aspect of safety control for concrete dams. Deformation monitoring models (such as statistical models and hybrid models) are extensively applied to predict concrete dam deformation and derive confidence interval of normal deformation for anomaly detection. Deformation monitoring models for concrete dams mainly consist of hydrostatic component, temperature component, and aging component. The optimum parameters of individual components are simultaneously determined by least square method for monitoring data fitting. Thus, significant over-fitting of deformation monitoring models may be induced by mutual compensation among the parameters of model components. In this paper, the Separate Modeling Technique (SMT) is proposed for mitigating the over-fitting problem of deformation monitoring models for concrete dams. Firstly, the Empirical Mode Decomposition (EMD) is adopted to extract and visualize the aging component of displacement sequence. Proper mathematical formulation of the aging component can be established, and the problem of improperly presupposing the mathematical form of aging component in the process of constructing traditional deformation monitoring models is well addressed. In this study, the hydrostatic component is represented by the Hybrid Response Surface (HRS), which is formulated using numerical simulation with varying water levels and material parameters. The displacement variation caused by water level fluctuation is identified in terms of isothermal conditions and is used to calibrate the material parameters in the HRS. The temperature component is separated through subtracting the hydrostatic and aging components from displacement time series and then is expressed with proper mathematical formulations. Finally, hybrid models for displacement monitoring of concrete dams are established by combining the separately formulated components. The Separate modeling technique is applied to formulate crest displacement of the YL concrete gravity dam. The false alarm rate of displacement monitoring and a new model selection criterion (namely over-fitting coefficient) are adopted to compare various deformation monitoring models. It is shown that the over-fitting levels of deformation monitoring models can be effectively reduced using the SMT. The deformation monitoring models constructed with the SMT are of better accuracy in displacement prediction and present no false alarm of displacement monitoring for the tested period.

A Triaxial Viscoplastic Model with Temperature Dependence for Asphalt Concrete Permanent Deformation

Asphalt concrete is a composite material comprised of aggregate skeleton and asphalt cement. Its permanent or viscoplastic (VP) deformation is characterized by time and temperature dependence, memory effect, and pressure sensitivity. To the best knowledge of the authors, there have been no VP models explicitly considering the nonlinear temperature effects in permanent deformation of asphalt concrete. In previous work, the authors developed a triaxial VP model that featured a VP relaxation spectrum and a modulus parameter [Formula: see text],[Formula: see text]. The spectrum was postulated as a material function representing the time dependence and memory effect, while [Formula: see text],[Formula: see text] was only a function of the stress condition to capture the pressure dependence. The objective of this study was to incorporate temperature dependence into the model. It was achieved by introducing the concept of VP shift factor solely to translate the relaxation spectrum. This strategy was inspired by the time–temperature superposition principle in the viscoelastic (VE) theory. The shift factor was identified experimentally at three temperatures different from the model calibration temperature. The VP shift factor function was thus established, and comparison with its VE counterpart showed that the VP response exhibited greater temperature sensitivity. Numerical simulation was conducted at several temperatures to illustrate the nonlinear temperature effects on the VP deformation and relaxation of the internal stress that served as the hardening/softening variable.

Prediction of Concrete Dam Deformation through the Combination of Machine Learning Models

Dam safety monitoring is of vital importance, due to the high number of fatalities and large economic damage that a failure might imply. This, along with the evolution of artificial intelligence, has led to machine learning techniques being increasingly applied in this field. Many researchers have successfully trained models to predict dam behavior, but errors vary depending on the method used, meaning that the optimal model is not always the same over time. The main goal of this paper is to improve model precision by combining different models. Our research focuses on the comparison of two successful integration strategies in other areas: Stacking and Blending. The methodology was applied to the prediction of radial movements of an arch-gravity dam and was divided into two parts. First, we compared the usual method of estimating model errors and their hyperparameters, i.e., Random Cross Validation and Blocked Cross Validation. This aspect is relevant not only for the importance of robust estimates, but also because it is the source of the data sets used to train meta-learners. The second and main research topic of this paper was the comparison of combination strategies, for which two different types of tests were performed. The results obtained suggest that Blocked CV outperforms the random approach in robustness and that Stacking provides better predictions than Blending. The generalized linear meta-learners trained by the Stacking strategy achieved higher accuracy than the individual models in most cases.

Research on the bending damage of concrete in areas with large temperature differences based on 3D‐Digital Image Correlation

AbstractDigital Image Correlation (DIC) technique is used to collect the whole‐field strain of concrete subjected to bending loads under different temperature change cycles, cooling rates, and water‐binder ratio. The bending damage behavior of concrete under large‐difference temperature circulation is analyzed with the whole‐field strain cloud map and data. In addition, with the whole‐field strain obtained by 3D‐DIC technology, the strain‐based damage variable is defined. Furthermore, the damage degree of concrete specimen in the bending process is quantified, and the bending damage evolution process of concrete under the action of temperature circulation is discussed. The results show that the damage of concrete in areas with large temperature differences is related to the number of temperature change cycles as well as the cooling rate. Moreover, the increase in the number of temperature change cycles and the cooling rate will aggravate the bending damage of the concrete. Additionally, the higher the water‐binder ratio, the more serious the adverse effect of temperature circulation on the concrete. This paper provide certain reference for the evaluation of concrete bending deformation damage in areas with a large temperature difference.

Effective reinforcement ratio of RC beams: Validation of modelling assumptions with high‐resolution strain data

AbstractConcrete tensile stresses influence the cracking behaviour and the stiffness of reinforced concrete (RC) members. Most design codes account for this tension stiffening effect using an effective reinforcement ratio. Although this ratio has a significant influence on the design of RC structures, its quantification is controversial in many cases, and typically relies on empirical geometry‐based expressions. One main reason for this knowledge gap is that the area of concrete in tension can only be verified indirectly, for example, through crack widths and spacings and using a suitable mechanical model. This indirect validation is subject to considerable uncertainty as it depends on parameters that scatter (e.g., bond stresses and the concrete tensile strength), and further assumptions relating internal stresses to the applied loads are required. This article outlines how refined measurements of the reinforcing steel strains and the concrete deformations in the compression zone, combining distributed fibre optic sensing (DFOS) and digital image correlation (DIC), can be used in order to obtain a more reliable quantification of the parameters affecting tension stiffening and hence, the effective reinforcement ratio. Selected models are validated against experimental data of an RC beam tested under four‐point bending, underlining the potential of DFOS and DIC as valuable tools for a better understanding of RC structures and the enhancement of mechanical models.

Deformation of Stone Matrix Asphalt and Porous Asphalt Concrete under Repeated Load

Deformation of Stone Matrix Asphalt and Porous Asphalt Concrete under Repeated Load

Study on Compression Deformation and Damage Characteristics of Pine Needle Fiber-Reinforced Concrete Using DIC.

Natural fiber-reinforced concrete (NFRC) has the advantages of environmental protection, energy conservation and regeneration. However, studies conducted to improve the macro mechanical properties of concrete by pine needle fiber have achieved good results. In this paper, the deformation and compression damage of pine needle fiber-reinforced concrete (PNFRC) are analyzed by digital image correlation; a fractal dimension is used to quantify the shape of PNFRC after compression damage; and the results of scanning electron microscopy confirm the effect of fiber treatment on deformation and damage of concrete. The results showed that the horizontal strain field of PNFRC has strain concentration zones in the elastic deformation stage, indicating that the fiber enhances the deformation ability of concrete. The defined damage factor can reflect the damage of fiber-reinforced concrete (FRC). The damage curve of natural fiber concrete increases evenly and slowly compared to ordinary concrete.

Investigation on the Deformation and Failure Characteristics of Concrete in Dynamic Splitting Tests.

The dynamic response behavior of concrete is constantly concerned because of seismic, impact and explosion events in the service of constructions. As a classic device for testing the dynamic mechanical properties of materials, the splitting Hopkinson pressure bar was used to carry out dynamic splitting tests on concrete in this paper. The variation of the dynamic tensile strength against the stress rate was fitted by the incubation time criterion. The full-field strain distribution on the observed surface of the specimen at the crack initiation stage was obtained by the digital image correlation (DIC) method. Morphological characteristics of the fragmentized process of concrete specimens in splitting processes were obtained by combining the image processing techniques and the FracPaQ. The size distribution of fragments of concrete specimens was obtained by sieving. The results show that the strain concentration zone and crack initiation appear along the loading direction through the center of the specimen. The secondary cracks initiated occurred at the contact end of the specimen, which expanded along the strain concentration zone and then interacted with the main crack. At the early stage of crack extension, the main crack dominates the normalized length of fracture traces in the horizontal direction. The normalized length of the vertical fracture trace increases with the main cracks opening width and the expansion of the secondary crack. The relationship between the length and angle of fracture traces in the dynamic splitting process of concrete conforms to the Gaussian function. Finally, the fragment sizes decrease with the stress rates of impact loads.

Influence of CaO-based expansive agent on chloride penetration resistance of marine concrete

CaO-based expansive agent (CEA) is a high performance expansive agent, which can well compensate for the shrinkage of concrete. However, due to micro-cracks caused by excessive expansion and poor chemical stability and gelling property of additional Ca(OH)2 (expansion product), durability of marine concrete may deteriorate. Therefore, the application of CEA in marine concrete is controversial. In this study, the autogenous deformation, compressive strength, and chloride penetration resistance of marine concretes containing various dosages of CEA were determined, and the influence mechanism of CEA on the chloride penetration resistance was further explored. The results showed that the addition of CEA increased the expansion deformation, chloride diffusion coefficient, and chloride penetration depth and decreased the compressive strength of marine concrete. In addition, as the dosage of CEA increased, porosity increased and chloride binding capacity decreased. Further discussion indicated that the addition of CEA increased the volume of large capillary pore with diameter between 400 and 5000 nm, and decreased the chloride binding capacity due to the increase in the SO3 content, both of which led to the decrease in chloride penetration resistance of marine concrete. However, when the dosage of CEA was not more than 6%, adding CEA can well compensate for the autogenous shrinkage and improve the crack resistance of marine concrete without a relatively small loss of compressive strength (≤8.7%). In addition, for the marine concretes containing CEA, the chloride diffusion coefficients were very small (<1 × 10-12 m2/s). Therefore, adding no more than 6% CEA, marine concrete had good crack resistance and chloride penetration resistance. Hopefully, the outcomes of this research can provide guidance for the application of CEA in marine concrete.

A creep model coupling moisture and mechanical damage for water-bearing concrete

The influence of moisture content and overstress on the creep of concrete was investigated from the perspective of damage rheology theory. The weakening effects of moisture and overstress were characterized by a time-independent and a time-dependent damage functions, respectively, and then combined and applied to a modified creep model that can represent the elasto-visco-plastic creep deformation of concrete. Hence, a damage creep model is proposed to describe the creep behavior of water-bearing concrete. Theoretical and statistical analyses, as well as a comparison with the ACI-209 model were performed to examine the validity and superiority of the proposed model. Further analyses showed that not only the deformation parameters, but also the long-term strength parameters degraded with time and increasing moisture. Experiments were then conducted on saturated cement mortar with different static moisture holding durations to investigate the time correlation of moisture-induced damage on the mechanical properties of the material. The experimental results proved that moisture-induced damage was independent of time.

Effect of creep on corrosion-induced cracking

Corrosion-induced cracking is the most widely encountered and studied long-term deterioration process in reinforced concrete. Naturally occurring corrosion rates are so low that rust accumulates often over tens of years near the surface of the reinforcement bars before sufficient pressure in the surrounding concrete is generated to induce cracking in the concrete cover. To speed up the process in laboratory tests, corrosion setups with impressed currents have been developed in which the corrosion rate is controlled to be so high that cracking of the concrete cover occurs within a few days. Extrapolating the results of these accelerated tests to those of naturally occurring corrosion requires an understanding of the influence of long-term creep deformations of concrete on the corrosion-induced cracking process. In mathematical models in the literature, creep deformations are often ignored for accelerated but considered for natural corrosion rates in the form of an effective modulus.In this work, three numerical models of increasing complexity are proposed with the aim to investigate the effect of creep on corrosion-induced cracking. The simplest approach is based on an uncracked axis-symmetric thick-walled cylinder combined with a plastic limit on the radial pressure-induced by the accumulation of rust. The model with intermediate complexity comprises a thick-walled cylinder model divided into an inner cracked and an outer uncracked layer. The most comprehensive model consists of a thick-walled cylinder discretised by a three-dimensional lattice approach. Basic creep is predicted in all three approaches by means of the B3 model developed by Bažant and co-workers. Time dependence of strength of concrete is modelled using fib Model Code expressions. It is shown that for the comprehensive lattice model, creep has limited influence on critical corrosion penetration, which indicates that the dependence of the critical corrosion penetration on corrosion rate must have other sources.

Hybrid model for concrete dam deformation in consideration of residual correction by frequency division

Hybrid model for concrete dam deformation in consideration of residual correction by frequency division

Energy Evolution Characteristics and Performance Parameter Degradation of Rubber-Mixed Concrete in Sulfate Attack Environment

In order to study the sulfate resistance of rubber concrete (RC), the compressive strength test of RC with different contents (0%, 5%, 10%, and 15%) was carried out, and the proportion of RC with standard curing for 28 days was optimized. Sulfate attack test was carried out on the selected RC and compared with normal concrete (NC). The degradation degree was measured from the effective porosity, relative dynamic elastic modulus, SO42− concentration, and SEM microstructure observation after different attack times. The energy analysis method is used to study the evolution law of total strain energy, elastic strain energy, and dissipated strain energy of NC and RC in the process of deformation and failure after different attack times, and the influence of sulfate attack on concrete is explored from the perspective of energy. The results show that with the progress of sulfate attack, the effective porosity of NC and RC both decreases first and then increases, and the relative dynamic elastic modulus increases first and then decreases. Rubber is beneficial to improve the sulfate resistance of concrete, delay the attack of SO42− on concrete, and improve the ductile deformation of concrete. This study can provide a theoretical reference for the application of RC in practical engineering.

The validation of elastic modulus models: Code models and their modified versions

AbstractElastic modulus (E) estimation forms an important element in the methods used to predict concrete creep deformation, which is an important structural design consideration. A variety of differentEestimation models for concrete exist but there is uncertainty as to which of the models is the most accurate. This research study assesses the performance of models namely, the ACI 318, ACI 318 Simplified, Mendis et al., Rüsch et al., Carrasquillo et al., ACI 363, AS 3600, Omar et al., AS 3600, BS 8110, Alexander and Davis, SANS 10100, CEB‐FIP, and the EC 2. The included list of models is taken from national codes and includes their superseded and modified versions. TheEvalues of 108 specimens, whose properties differed (in aggregate type, cement type, concrete strength, and curing age), were measured. The actualEvalue was compared to each model's estimations to determine which were most accurate statistically using the coefficient of variation (ωj). The ACI 363 model was the most accurate out of the 16 models assessed, yielding an overallωjof 16%. The CEB‐FIP model was found to be the least accurate showing an overallωjof 31.8%. From the 16 models considered, it was recommended that the ACI 363 model be utilized by creep prediction models. In addition, it was found that not all modified estimate models produced better estimates.

Multifactorial Chloride Ingress Model for Reinforced Concrete Structures Subjected to Unsaturated Conditions

The attack of chloride ions is one of the most important factors affecting reinforced concrete (RC) durability. Chloride ingress into concrete is usually studied by assuming constant diffusivity and constant surface chloride concentration. However, these two approximations could badly estimate the chloride concentration in RC structures and then the lifetime assessment. Several factors influence the chloride concentration and ingress mechanisms in the convection area. In this paper, a new multifactorial and multiphase model to account for some effects on chloride surface concentrations in the convection zone is proposed. 136 values have been collected to identify the position and the chloride concentration in the border between the diffusion and convection zones. In addition, a time-dependent multifactorial diffusivity is considered. Diffusivity, which is the key parameter of the mechanical diffusion accounts in this paper for the water/cement ratio, chloride binding, temperature, concrete age, internal humidity, concrete deformation, and damage. The surface chloride model considers environment humidity, temperature, superficial concrete irregularities, and convection area of concrete. Advanced numerical solutions have been carried out to consider space and time dependencies in the model. Results show that the error function-based solutions could underestimate the chloride concentration C for periods &lt; 10 years and for concrete depths &gt; 4.0 cm in comparison with the proposed model.

Seismic performance evaluation of recycled aggregate concrete-filled steel tubular columns with field strain detected via a novel mark-free vision method

In this study, low cycle reciprocating load tests were conducted on nine recycled aggregate concrete-filled steel tube columns (RACSTCs) and a solitary ordinary concrete-filled steel tube contrast column (OCSTC). The axial compression ratio and steel-tube wall thickness were considered as the parameters of interest. Accordingly, the failure mechanism, hysteresis performance, skeleton curve, ductility coefficient, energy consumption performance, and stiffness degradation of the two types of columns were compared. A vision measurement system was developed to track the full field deformations of specimens, and a quadocular vision system was constructed to determine the concrete column deformations. The deformations were compared with those obtained from laser measurements using an image and point cloud stitching algorithm. The results show that the hysteresis curves of RACSTCs are non-pinched with large ductility coefficients for all the specimens and the equivalent viscous damping coefficients are in the range of 0.462–0.975. RACSTC specimens exhibit good seismic and deformation performance. A three-fold restoring force model was established for RACSTC specimens based on skeleton curve test data. The theoretical hysteresis curve obtained using the skeleton curve model and hysteresis rule shows good agreement with the hysteresis curve obtained from experiment. This can be used as a reference for research on the restoring force characteristics of RACSTCs. The results obtained using the vision measurement system show that the measured mean relative error values are less than 0.504%. This confirms that the multiple vision measurement method can be used to accurately measure curved surface deformations as well as the full field strain values of RACSTCs. Therefore, the proposed method can be used to evaluate the asymmetric deformation of structures subjected to vibrations, impact, and other load types, thereby providing early warning decision data.

Effects of reinforcement on autogenous deformation of early-age concrete containing CaO-based expansion agent

High-performance concrete (HPC) is a common building material in practical engineering, but HPC cracks easily due to the large autogenous shrinkage (AS). The CaO-based expansive agent (CEA) addition and reinforcement are effective ways to reduce the cracking risk of concrete. However, few studies were addressing the autogenous deformation (AD) of early-age concrete with the combination of CEA and reinforcement. This paper focuses on the effects of three different reinforcement ratios on the AD of early-age concrete with CEA addition. The experimental results showed that reinforcement could effectively reduce the concrete AS, and the reduction increased with age and reinforcement ratio. The AD of concrete incorporating CEA exhibited two stages: expansion and net AS. The reduction of reinforcement on AD decreased with CEA content. The equivalent degree of restraint increased with reinforcement ratio and decreased with CEA content. Based on the test data, a two-stage AD prediction model for concrete with reinforcement and CEA was established. The model was in good consonance with the experimental data.

Experimental evaluation of concrete confined with hand laid carbon FRP grids

In many cases structures serve in highly corrosive environments and as a result the service life of the elements is significantly reduced. An alternative could be the use of fiber reinforced composites. The skepticism behind using this material has to do with the lack of ductility. A different approach must be followed in the design of such structures to achieve a behavior similar to regular reinforced concrete. In this case failure should initiate and end in concrete with confinement providing ductility. This paper investigates concrete confinement through experimental testing of concrete specimens utilizing embedded custom made, hand laid carbon FRP (CFRP) grid. The grid was formulated into the shape of a tube and embedded in concrete specimens to evaluate the grid tube effect on concrete properties. Typically, the FRP material is applied externally using an adhesive. In this case the hand laid CFRP grid was pre-manufactured and was embedded into concrete. Standard sized cylindrical specimens were manufactured using hand laid CFRP grid. Some of the specimens were made using plain concrete and served as reference specimens. The specimens were loaded axially in compression. This allowed the evaluation of the extent to which the hand laid tubular shaped grid positively affected the ultimate capacity and deformation of concrete. This application is unique and its confinement effectiveness needs to be investigated and evaluated. Experimental results show a very significant change in the maximum capacity and deformability for confined concrete. Depending on the amount of CFRP reinforcement in the grid strands the enhancement in strength was between 42 % and 58 %. The deformability of the specimens with the hand laid CFRP grid was between 134 % and 196 % higher compared to the reference specimens, with higher values associated with higher amounts of the CFRP grid.

КОЭФФИЦИЕНТ СЕКУЩЕГО МОДУЛЯ ДЛЯ РЕКОНСТРУИРУЕМОГО ЖЕЛЕЗОБЕТОНА

During the reconstruction, or upon expiration of the service life, as well as after external impact, reinforced concrete structures require examination and verification calculations. The most modern and accurate calculations are based on diagram methods. Existing diagrams of concrete deformation are focused on designing new structures and are not adapted to the concretes that have changed their strength and deformation properties over time or because of external influences. The main parameter describing the deformability of concrete is the secant modulus ratio. A technique for determining the secant modulus ratio is proposed in this arctical. The technique is based on using the exponential concrete deformation model. The obtained secant modulus ratio can be used for the widely adopted description of the concrete deformation diagram. The technique is illustrated on a specific example. The proposed technique significantly expands the application range of diagram methods and extend them to concrete after long-term operation or external impact.

СИЛОВОЕ СОПРОТИВЛЕНИЕ ЖЕЛЕЗОБЕТОННЫХ КАРКАСОВ МНОГОЭТАЖНЫХ ЗДАНИЙ С КОСВЕННЫМ АРМИРОВАНИЕМ В ЗАПРЕДЕЛЬНЫХ СОСТОЯНИЯХ

The results of studies of monolithic reinforced concrete frames of multi-storey buildings and reinforced concrete frames simulating fragments of such frames in over-extreme limit states caused by special actions are presented. Two options for reinforcing the beams of the frames are considered: with double reinforcement, which ensures the operation of the beam when the force flows in the frame and, accordingly, the sign of the moment change; option with double reinforcement and installation of additional indirect reinforcement in the support zones of the beams for the entire height of the section. Primary and secondary design models were built using volumetric finite elements for concrete and reinforcement rods. The obtained design parameters of deformations, crack patterns and frame failure patterns for all options are compared with each other and with the results of testing physical models of these structures. To assess the effect of mixed reinforcement on the limiting deformations of compressed concrete in over-extreme limit states, based on the theory of plasticity of concrete and reinforced concrete G.A. Geniyeva, deformation dependencies are constructed for a typical reinforced concrete element reinforced with rods in one direction and meshes in the other two under uniaxial compression and volumetric deformation. It has been established that the use of indirect reinforcement in combination with double longitudinal reinforcement in bending elements under static-dynamic loading conditions significantly increases the ultimate deformation of the compressed zone. This reinforcement option can working way to protect monolithic reinforced concrete frames of multi-storey buildings from progressive collapse under special actions.