Concrete-like Materials Research Articles

Preliminary Study of Stone Sawing Sludges-based Alkali Activated Materials (AAMs) for the Conservation of Archaeological Ceramics

This paper presents research into the feasibility of using stone sawing sludge-based Alkali Activated Materials (AAMs) for conservation of Cultural Heritage. Sawing sludges are a stone processing waste product resulting from the mixing of rock powder with the water used to cool down the cutting blades. The chemical composition of the sawing sludges, when aluminosilicatic, is suitable for acting as a precursor to produce AAMs. AAMs are known for their low environmental impact and versatility since their existence is drawn from recycling waste materials. One of their possible applications is in the conservation of Cultural Heritage objects. This work presents a preliminary investigation into three sawing sludge-based AAMs with different mineralogical compositions and contributes to formulating guidelines for applying them as fillers on modern and archaeological ceramic pottery based on the evaluation of their workability, appearance and physical properties over time from the moment of application and up to 30 days. Dynamic Vapor Sorption and X-Ray Diffraction results provided an overview of the structural and mineralogical changes under high RH conditions, where the tested AAMs showed a type II isotherm curve, as expected for concrete-like materials, as well as disappearance of thermonatrite after one isothermal cycle. Ultrasonic Pulse Velocity test demonstrated the general homogeneity of the AAMs despite the lower velocity exhibited by one of the formulations, probably due to its internal pore distribution and possible presence of microstratification. The Oddy tests, application tests and colourimetric measurements evidenced the advantages and weaknesses of the AAMs, with overall encouraging results ensuing investment in further in-depth studies of these innovative conservation materials in view of their future use in the field of conservation of Cultural Heritage as a result of a circular economy model.

The content of cannabinoids in common hemp plants in the conditions of the Republic of Bashkortostan

Cannabis sativa L. – a herbaceous plant. Recently, interest in this fast-growing plant has increased again due to its multipurpose use: It is indeed a treasure trove of phytochemicals and a rich source of both cellulose and wood fibers. The pharmaceutical and construction sectors are equally interested in this plant, since its metabolites have powerful biological activity for human health, and stem outer and inner tissues can be used to manufacture bioplastics and concrete-like materials, respectively. Cannabinoids are the most studied group of compounds, mainly because of their wide range of pharmaceutical effects on the human body, including psychotropic activity. The content of the main cannabinoids in common hemp plants of the Vera, Nadezhda, Surskaya, Omegadar-1 varieties for 2020-2023 has been studied. The predominant cannabinoid in all varieties during the years of the study is CBD. The studied common hemp varieties in the years of research fully comply with the requirements of the law. The THC content did not exceed 0.1%.

Continuous surface cap model for ultra-high-performance concrete under static and low strain rate loadings: Calibration, experiment, and simulation

The ultra-high compressive strength and ductile tensile behavior of ultra-high-performance concrete (UHPC) make it promising in the application of civil and crashworthy facilities. Although numerous experimental studies have been conducted to investigate the performance of UHPC, studies on the methods for simulating the behavior of UHPC are few. In this paper, an overview of widely used constitutive models of concrete-like materials in hydrocode was presented first, and then a comprehensive calibration of the continuous cap surface (CSC) model was performed to simulate the mechanical behavior of UHPC under static and low strain rate loadings. Four groups of parameters: (1) failure surface parameters; (2) cap and hardening parameters; (c) damage parameters; and (4) strain rate parameters were systematically calibrated via extensive available material test data of UHPC with uniaxial compressive strength ranging from 101 MPa to 238 MPa. A calculation method was proposed to address the inconsistency of existing conclusions regarding the fracture energy of UHPC, which significantly affects the modeling of UHPC damage. Moreover, a method was proposed to convert the engineering uniaxial tensile strain of UHPC to the true uniaxial tensile strain, and so does the tensile fracture energy of UHPC. To validate the accuracy of the calibrated CSC model in simulating the behavior of UHPC under static loading, uniaxial tension and compression tests of UHPC samples and three-point bending tests of steel-UHPC-steel composite beams were conducted and simulated. Existing triaxial compression tests of UHPC cylinders were simulated to validate the accuracy of the calibrated CSC under confinement. Moreover, low-velocity impact tests of UHPC columns were simulated to further validate the reliability of the calibrated CSC model to capture the structural behavior of UHPC members under low strain rate loading. It was found that the calibrated CSC model showed excellence in simulating the material properties of UHPC and the structural behavior of UHPC members under static and low strain rate loadings.

A hybrid algorithm of particle swarm optimization and finite element method to identify local mesoscopic damage of concrete-like materials

Meso-scale material modeling and parameter identification are still unsettled issues in the current multi-scale methods for the field of computational mechanics. In this investigation, a hybrid algorithm of particle swarm optimization and finite element method coupled with continuum damage mechanics is proposed to forecast local mesoscopic elasticity modulus and damage of concrete-like materials based on the macroscopic stress-strain relationship data curve. The algorithm establishes a bridge between mesoscopic mechanical analysis and macroscopic mechanical analysis, which can also guarantee the consistency of analysis in different scales. A representative numerical case is conducted to support the algorithm. Additionally, the effect of different divisions of representative volume element is analyzed based on the algorithm, and the recommended size of representative volume element is 5 mm. The prediction results match well with the corresponding macroscopic experimental results and mesoscopic experimental results based on the chosen recommended size of representative volume element. The effectiveness of the algorithm is verified, which can support an effective numerical tool to optimize and identify mesoscopic mechanical parameters and damage for multi-scale material modeling and analysis.

Investigating the fracture behavior of structural concrete shear key in prefabricated walls by discrete modeling

In this study, the cracking behavior of concrete-like materials in keyed shear joints of a structural member under the push-off test is investigated by employing discrete and continuum models. Initially, the behavior of the reinforced concrete member is simulated by a novel discrete element previously developed by the authors and referred to as the Rigid Body Coupled Spring model (RBCS). Shear key connections with different failure modes are modeled and compared to experimental observations. Following that, the numerical outcomes are discussed and compared with the outcome of Finite Element Method (FEM) models employing a continuum plasticity-based damage model. According to the obtained results, with the applied continuum method and constitutive model, unlike the experimental evidence, the modification of the shear key design did not have a significant effect on the failure mode of the modeled connection. On the other hand, the employed discrete element technique was able to represent the failure modes of the shear key connections. This study demonstrates the possibility of the application of discrete methods in studying the behavior of concrete in structural elements.

A modified p∼α equation of state for concrete-like materials

The p∼α equation of state has been widely used in dynamic constitutive models for concrete-like materials. However, the latest study (J Build Eng 2022; 61: 105262) has shown that its accuracy at low pressures is much compromised. In this note, a modified p∼α equation of state for concrete-like materials is presented herein on the basis of the analysis of recently obtained plate impact test data (J Build Eng 2022; 61: 105262) for cement mortar. Comparison is then made between the plate impact test data for the cement mortar and the predictions from the two equations of state (i.e. the modified p∼α equations of state and the p∼α equations of state) in terms of αp′∼p relationship, α∼p relationship and pressure-volumetric strain relationship. Furthermore, the plate impact test data available for normal concrete is also compared with the predictions from the two equations of state. It is found that the modified p∼α equation of state is in good agreement with the plate impact test data for concrete-like materials and is advantageous over the p∼α equation of state below full compaction pressure.

Rigid-Block DEM Modeling of Mesoscale Fracture Behavior of Concrete with Random Aggregates

The mesoscale structure generated in concrete plays a significant role in the mechanical properties and local failure behavior of mesoscale concrete. This work proposes a novel rigid-block discrete-element method (RB-DEM) for concrete modeling with random mesoscale structure. The results of the uniaxial compression using the RB-DEM demonstrated satisfactory agreement with experimental data. The RB-DEM does not only show satisfactory performance in simulation but also it is simple to use. The RB-DEM model can be built from any finite-element mesh generator, either open source codes or commercial software, which are readily available. The interfacial transition zone (ITZ) is modeled directly by assigning the weakened contact model or parameter at the interface between aggregate and mortar. The effects of ITZ parameters, aggregate volume fraction, and geometric shape on the stress–strain curve and crack propagation are discussed. This work provides a novel and efficient tool for the mesoscale fracture simulation of concretelike material considering the relatively large time-step of the DEM.

Size effect on flexural strength of notched and un-notched concrete and rock specimens by Finite Fracture Mechanics

Size effect in quasi-brittle material cannot be described simply by Linear Elastic Fracture Mechanics, which moreover is limited to cracked geometries. In the present paper we apply Finite Fracture Mechanics to determine the size effect on the flexural strength of notched and un-notched quasi-brittle material specimens under quasi-static mode I loadings. Although the framework is general, special attention is paid on the Three Point Bending and Semi-Circular Bending test geometries. The scaling of flexural strength is of particular interest for concrete and concrete-like materials because of the large variation from laboratory to real structural sizes. Theoretical predictions are discussed and compared with several experimental data from the literature, related to both concrete and rock specimens. The excellent matching with the experimental data proves the soundness of the Finite Fracture Mechanics approach, even more valuable because of its simplicity: just two shape functions are needed to determine the scaling of the flexural strength (or of the nominal fracture toughness) for a given geometrical shape.

A dimensional analysis based thermal–mechanical damage model for crack growth simulation of concrete-like materials at elevated temperatures

A dimensional analysis based thermal–mechanical damage model is established to describe softening behaviors of concrete-like materials at elevated temperatures. An iterative procedure for damage updating is coupled in the modified continuum damage mechanics (CDM) based finite element method (FEM) to simulate the thermal cracking process of concrete-like materials. Here two numericalexamples including ablation and thermalhot expansionof concrete specimens were utilized to support the developed thermal–mechanical damage model and thermal cracking simulation method by comparing the corresponding experimental results. The comparison results verify that the model and method can be used to study thermal–mechanical behaviors of concrete-like materials at elevated temperatures with high computational efficiency.

Investigation of the strength of concrete-like material with waste rock and aeolian sand as aggregate by machine learning

Abstract Solid waste filling is an important development direction for filling mining technology. This paper proposes to use waste rock and aeolian sand as aggregates to fill the underground extraction area in order to reduce the environmental damage of related waste solid. The experiments are optimized by response surface methodology and multivariate nonlinear response models are constructed to investigate the effects of different factors on uniaxial compressive strength (UCS) of concrete-like material (CLM). The performance of different swarm intelligence optimization algorithms is analyzed and combined with support vector regression model (SVR) to construct an intelligent prediction model for UCS. The results show that the packing density has a maximum value of 0.74 when the proportion of waste rock is around 0.6. The response model constructed in this paper has a P-value < 0.01 and R2 > 0.8, which indicates its high significance and goodness of fit. The UCS of CLM increases with the increase of cement content and slurry mass fraction, while it also increases and then decreases with the increase of proportion of waste rock. The ratio of waste rock and aeolian sand will affect the compactness of cemented structure. The better the ratio, the higher the average grey value of the cemented structure, the more compactness the cemented structure. The whale optimization algorithm-SVR model constructed in this paper has a prediction accuracy of more than 99% for UCS of CLM, which achieves high accuracy and fast prediction of UCS under multifactor conditions.

On the equation of state for concrete-like materials

This paper presents a systematic study on the equation of state for concrete-like materials. A new method is first proposed to analyze plate impact test data by considering the effects of both wave structures and strength surface; plate impact tests are then performed on cement mortar with impact velocities ranging from 45 m/s to 3050 m/s and experimental results are analyzed by using the newly proposed method; Subsequently, the accuracy of the existing equations of state (i.e. the p∼α equation of state and the equation of state of HJC) is evaluated and similarities and differences are also discussed. Finally, a new equation of state is proposed based on the previous work and the newly acquired test data. The numerical results using the three equations of state are compared with plate impact test data in terms of particle velocity-time history, longitudinal wave velocity and HEL. It is found that the newly proposed equation of state is in good agreement with plate impact test data of concrete-like materials and is advantageous over the existing equations of state.

Dynamic increase factors of concrete-like materials at very high strain rates

Strain rate effects on the compressive and tensile strengths of a concrete-like material play an important part in the construction of its dynamic constitutive model. So far no data for the same material covering a wide range of strain rates were reported in the literature, not to mention the data for compression at strain rates greater than 103 s−1. This paper presents a combined theoretical and experimental study on the dynamic increase factors of concrete-like materials at very high strain rates. First, a method is proposed to determine dynamic increase factor for a concrete-like material in compression due to strain rate effect only (that is to say to eliminate the inertial (containment) effect) at very high strain rates (>103 s−1) by in-depth analysis of its Hugoniot elastic limit (HEL). Then, various material tests (i.e. quasi-static uniaxial compression and tensile tests, SHPB and SHTB tests, plate impact test and dynamic compression-shear test) were performed on a certain type of cement mortar in a wider range of strain rates (i.e. 10-5 s−1 ∼ 106 s−1). Finally, the test data are compared with the previously proposed semi-empirical equations for dynamic increase factors. Furthermore, the plate impact test results for ultra high performance concrete (UHPC) available in the literature are also analyzed using the proposed method. Dynamic increase factor for tension at strain rate of 103 s−1 is obtained indirectly by the relationship between dynamic uniaxial tensile strength and dynamic shear strength through the shear wave tracking (SWT) technique which is based on dynamic compression-shear test. It is found that dynamic increase factor for concrete-like materials in compression due to strain rate effect only can be regarded as a constant at very high strain rates. It is also found that the previously proposed semi-empirical equations can accurately describe the strain rate sensitive behavior of concrete-like materials in both tension and compression.

Determination of Tensile Strength at Crack Initiation in Dynamic Brazilian Disc Test for Concrete-like Materials

Concrete is a brittle material whose tensile strength is about one-tenth of its compressive strength. Tensile strength is a key parameter for concrete under impact loading. When a turning point occurs before peak load in the load–time curve from the dynamic Brazilian disc test, there is no basis for choosing the turning point or the peak load to calculate the tensile strength. The objective of this study is determining the crack initiation tensile stress at the turning point or the peak. The method contrasts the time duration from Digital image correlation (DIC) and the load–time curve from a split Hopkinson pressure bar (SHPB) system in order to obtain the load value when cracking first appears. The crack initiation tensile strength is less than the peak strength for concrete specimens with a turning point in the load–time curve. The crack initiation tensile strength is equal to the peak strength for concrete specimens without a turning point in the load–time curve. The proposed method is successfully applied to determine crack initiation of concrete specimens and obtain tensile strength at crack initiation of concrete specimens.

Numerical investigation of the failure mechanism of concrete specimens under tri-axial dynamic loads

The dynamic properties of concrete-like materials under true tri-axial loads were seldom studied because of the lack of proper testing facilities for conducting multi-axial impact tests. Although a three-dimensional Split Hopkinson pressure bar (3D-SHPB) device has developed in Tianjin University which could provide synchronized tri-axial dynamic loads with the same amplitude, conducting such tests is expensive and time consuming. As a supplement to physical tests, high fidelity numerical modelling is becoming more and more common to study the material and structural performances under different loading conditions. In this study, a mesoscale concrete model with the consideration of ITZ (interfacial transition zone) is developed and verified by the testing data of the 3D-SHPB tests. The dynamic properties of concrete material under different true tri-axial loading paths which the 3D-SHPB testing system could not provide are numerically investigated using the verified model. The results show that the strain rate effect on concrete compressive strength becomes less significant under tri-axial loading than that under uni-axial loading owing to the tri-axial loads acting on the specimen which limits the distribution and propagation of cracks. The change of the failure modes with strain rate under tri-axial loading is also less prominent compared to that under uni-axial loading. Based on the numerical results, an empirical relation is proposed to modify the DIF obtained from uni-axial impact loads to that under tri-axial dynamic loads for concrete material. The proposed empirical formulas can be used to more accurately model the dynamic strength increment of concrete material under tri-axial compressive loads.

Recent progress in buckling restrained braces: A review on material development and selection

Passive energy dissipation devices are a widely accepted solution used to improve the response of the structures under seismic conditions. While several types of passive energy dissipation elements have been proposed for replaceable elements, buckling restrained braces (BRB) was one of the most effective replaceable elements which has been used to retrofit frame structures in the past few years. A traditional BRB consists of a steel brace core with a cruciform cross section that is surrounded by a stiff steel tube (restraining unit) and unbonded materials to ensure the brace to move freely. The region between the tube and brace is filled with concrete-like materials. In recent years, a significant amount of research has been carried out on new types of BRBs which are more efficient and inexpensive. This paper presents an overview of recent advances in the development of BRB materials and buckling mechanism, with an emphasis on the material development for the restraining unit. The application of fibre-reinforced polymer (FRP) as restraining unit in BRB is highlighted.

Machine learning method to predict dynamic compressive response of concrete-like material at high strain rates

Machine learning (ML) methods with good applicability to complex and highly nonlinear sequences have been attracting much attention in recent years for predictions of complicated mechanical properties of various materials. As one of the widely known ML methods, back-propagation (BP) neural networks with and without optimization by genetic algorithm (GA) are also established for comparisons of time cost and prediction error. With the aim to further increase the prediction accuracy and efficiency, this paper proposes a long short-term memory (LSTM) networks model to predict the dynamic compressive performance of concrete-like materials at high strain rates. Dynamic explicit analysis is performed in the finite element (FE) software ABAQUS to simulate various waveforms in the split Hopkinson pressure bar (SHPB) experiments by applying different stress waves in the incident bar. The FE simulation accuracy is validated against SHPB experimental results from the viewpoint of dynamic increase factor. In order to cover more extensive loading scenarios, 60 sets of FE simulations are conducted in this paper to generate three kinds of waveforms in the incident and transmission bars of SHPB experiments. By training the proposed three networks, the nonlinear mapping relations can be reasonably established between incident, reflect, and transmission waves. Statistical measures are used to quantify the network prediction accuracy, confirming that the predicted stress-strain curves of concrete-like materials at high strain rates by the proposed networks agree sufficiently with those by FE simulations. It is found that compared with BP network, the GA-BP network can effectively stabilize the network structure, indicating that the GA optimization improves the prediction accuracy of the SHPB dynamic responses by performing the crossover and mutation operations of weights and thresholds in the original BP network. By eliminating the long-time dependencies, the proposed LSTM network achieves better results than the BP and GA-BP networks, since smaller mean square error (MSE) and higher correlation coefficient are achieved. More importantly, the proposed LSTM algorithm, after the training process with a limited number of FE simulations, could replace the time-consuming and laborious FE pre- and post-processing and modelling.

Dynamic Increase Factors of Concrete-Like Materials at Very High Strain Rates

Dynamic Increase Factors of Concrete-Like Materials at Very High Strain Rates

Study of the rheology of lunar regolith simulant and water slurries for geopolymer applications on the Moon

To reduce transportation costs for building a lunar base station, it is important to utilise in-situ resources like lunar regolith. Lunar regolith can be used to create a concrete-like material by means of geopolymerization. For application of geopolymerizing mixtures, the rheology of the slurry must be known in advance. This article investigates the rheology of mixtures of lunar dust simulants with water, NaOH (as alkaline agent) and urea (as superplasticizer). The rheology of all the mixtures can be approximated with the Bingham Model. Aging has a dramatic effect on the rheology of the water + simulant mixture and, after 2 days, the yield stress decreases by six times. The presence of NaOH, on the contrary, increases both the apparent viscosity and the yield stress as it promotes geopolymerization. The addition of urea reduces the viscosity by 25%, but it has a limited effect on the yield stress. These findings can enable the design of construction 3D printers on the moon.

Peridynamic modeling and simulations on concrete dynamic failure and penetration subjected to impact loadings

In this paper, we simulate the dynamic fracture and failure of three-dimensional concrete structures using the peridynamic theory. Considering the heterogeneities and mesoscopic characteristics of concrete material, the dynamic failure of concrete is investigated by employing an intermediately homogenized peridynamic (IH-PD) model, in which information from the composition at the mesoscale is linked to the macroscale fracture behavior, and cement mortar, aggregates, and interfacial transition zone (ITZ) are taken into account. Moreover, a plastic constitutive model under the framework of peridynamics, taking the effect of volume change and hydrostatic pressure on the concrete mechanical behavior into account, is formulated to depict the complex dynamic mechanical response of concrete under the high-velocity impact. Several typical numerical examples, including elastic wave propagation in a concrete plate, fracture in a notched concrete beam, and penetration of concrete targets subjected to high-velocity impacting are investigated. The numerical results are compared with corresponding experimental observations to validate the capabilities of the proposed model and approach. Further comparison with the fully-homogenized model shows the advantages of the present model on studying impact failure problems in concrete-like materials and structures.

The numerical simulation of large diameter split Hopkinson pressure bar and Hopkinson bundle bar of concrete based on mesoscopic model

In the comparison of Hopkinson bundle bar (HBB) system and the large diameter split Hopkinson pressure bar (SHPB) system, the influence of bar diameter on the dispersion effect of the one-dimensional longitudinal wave is analyzed theoretically firstly. Three trapezoidal loads with different peak values are applied to the elastic bars with two types of sections to investigate the stress wave propagation. Then, based on the fractal characteristics of concrete aggregate, the 3D realistic mesoscopic model is constructed as numerical models. And the dynamic responses of concrete under two kinds of test systems have been compared and analyzed. The results show that, the HBB system can more effectively reduce the lateral dispersion in the stress wave propagation and is beneficial to maintain the waveform than the large diameter SHPB system. The large diameter SHPB simulated results based on the 3D realistic mesoscopic model are consistent with the experimental results, which verify the feasibility of the mesoscopic model. The HBB system can obtain the similar simulated results as the large diameter SHPB system, such as the similar waveform with the double peak phenomenon of the reflection wave. However, there is a significant difference in the failure pattern of the specimens under two test systems, which is caused by the boundary effect of the hexahedron specimen in the HBB system. In conclusion, the HBB system can also carry out the dynamic mechanical properties of concrete-like materials. Moreover, HBB system can obtain more detailed information, reflecting the difference of local mechanical properties of concrete. Although there are still some deficiencies, it can still provide additional ideas for related researchers.