Effect of De-Icing Chemicals on Concrete Scaling: The Role of Storage Water

This paper deals with the effect of the character of the water used for the water storage of concrete test specimens on the results of tests for resistance to de-icing chemicals. Two experiments were conducted to investigate the effect of the content of free CO₂ in water and leaching of calcium hydroxide from concrete on the test results. In the first experiment, the resistance of mortars to water and de-icing chemicals was investigated. It was found that the character of the water storage, i.e., fresh water vs. previously used water, can significantly affect the test results. The second experiment focused on investigating the effect of the content of free CO₂ in water on the test results. It was found that the content of free CO₂ in the water can statistically significantly influence the test results. In conclusion, the paper shows that the character of the water used for water storage of concrete test specimens and the content of free CO₂ in water are essential factors that can significantly affect the results of concrete resistance tests to de-icing chemicals. Further research is needed to understand these influences and their potential use to improve the resistance of concrete.

Study on the Constitutive Relationship between Ordinary Concrete and Nano-Titanium Dioxide-Modified Concrete at High Temperature

After high-temperature treatment, both nano-titanium dioxide-modified concrete and ordinary concrete exhibit typical splitting failure. High-temperature heating reduces the mechanical properties and brittleness of concrete and improves the ductility of concrete. The stress-strain relationship of the specimens was obtained through the uniaxial compression test of ordinary concrete and nano-titanium dioxide-modified concrete cube specimens under normal temperature and high-temperature conditions. In addition, the relationship between temperature and damage variables was established, and the unified constitutive model containing damage variables after room temperature and high-temperature treatment of ordinary concrete and nano-titanium dioxide-modified concrete were established. It provides a reference for future research on the mechanical properties of high-performance concrete structures after high temperatures (fire).

Interpretable Predictive Modelling of Basalt Fiber Reinforced Concrete Splitting Tensile Strength Using Ensemble Machine Learning Methods and SHAP Approach

Basalt fibers are a type of reinforcing fiber that can be added to concrete to improve its strength, durability, resistance to cracking, and overall performance. The addition of basalt fibers with high tensile strength has a particularly favorable impact on the splitting tensile strength of concrete. The current study presents a data set of experimental results of splitting tests curated from the literature. Some of the best-performing ensemble learning techniques such as Extreme Gradient Boosting (XGBoost), Light Gradient Boosting Machine (LightGBM), Random Forest, and Categorical Boosting (CatBoost) have been applied to the prediction of the splitting tensile strength of concrete reinforced with basalt fibers. State-of-the-art performance metrics such as the root mean squared error, mean absolute error and the coefficient of determination have been used for measuring the accuracy of the prediction. The impact of each input feature on the model prediction has been visualized using the Shapley Additive Explanations (SHAP) algorithm and individual conditional expectation (ICE) plots. A coefficient of determination greater than 0.9 could be achieved by the XGBoost algorithm in the prediction of the splitting tensile strength.

Development and Promotion of Concrete Strength at Initial 24 Hours

Knowing and promoting the strength development of concrete at an earlier age is essential for accelerating formwork circulation of the on-site construction and precast product manufacture. The strength development rate at earlier ages of less than the initial 24 h was investigated. The effect of measures of adding silica fume, calcium sulfoaluminate cement, and early strength agent on the strength development of earlier concrete at ambient temperatures of 10, 15, 20, 25, and 30 °C was studied. The microstructure and long-term properties were further tested. It is shown that the strength increases exponentially first and then logarithmically, different from what is commonly recognized. Increasing cement content exhibited a certain effect only above 25 °C. When the cement content increased from 420 to 460 kg/m³, the strength only increased from 6.2 to 6.7 MPa after 12 h at 25 °C. The early strength agent could increase the strength significantly, the strength could be increased from 6.4 to 10.8 MPa after 20 h at 10 °C and from 7.2 to 20.6 MPa after 14 h at 20 °C. All measures for promoting earlier strength did not have an evident negative effect. The results could be potentially referred for the formwork removal at a suitable moment.

Modeling and Simulation of the Hysteretic Behavior of Concrete under Cyclic Tension-Compression Using the Smeared Crack Approach

Concrete structures under wind and earthquake loads will experience tensile and compressive stress reversals. It is very important to accurately reproduce the hysteretic behavior and energy dissipation of concrete materials under cyclic tension-compression for the safety evaluation of concrete structures. A hysteretic model for concrete under cyclic tension-compression is proposed in the framework of smeared crack theory. Based on the crack surface opening-closing mechanism, the relationship between crack surface stress and cracking strain is constructed in a local coordinate system. Linear loading-unloading paths are used and the partial unloading-reloading condition is considered. The hysteretic curves in the model are controlled by two parameters: the initial closing stress and the complete closing stress, which can be determined by the test results. Comparison with several experimental results shows that the model is capable of simulating the cracking process and hysteretic behavior of concrete. In addition, the model is proven to be able to reproduce the damage evolution, energy dissipation, and stiffness recovery caused by crack closure during the cyclic tension-compression. The proposed model can be applied to the nonlinear analysis of real concrete structures under complex cyclic loads.

Alteration of Structure and Characteristics of Concrete with Coconut Shell as a Substitution of a Part of Coarse Aggregate

One of the most promising ways to solve the problem of reducing the rate of depletion of natural non-renewable components of concrete is their complete or partial replacement with renewable plant counterparts that are industrial and agricultural waste. The research significance of this article lies in the determination at the micro- and macro-levels of the principles of the relationship between the composition, the process of structure formation and the formation of properties of concrete based on coconut shells (CSs), as well as the substantiation at the micro- and macro-levels of the effectiveness of such a solution from the point of view of fundamental and applied materials science. The aim of this study was to solve the problem of substantiating the feasibility of concrete consisting of a mineral cement-sand matrix and aggregate in the form of crushed CS, as well as finding a rational combination of components and studying the structure and characteristics of concrete. Test samples were manufactured with a partial substitution of natural coarse aggregate with CS in an amount from 0% to 30% in increments of 5% by volume. The following main characteristics have been studied: density, compressive strength, bending strength and prism strength. The study used regulatory testing and scanning electron microscopy. The density of concrete decreased to 9.1% with increasing the CS content to 30%. The highest values for the strength characteristics and coefficient of construction quality (CCQ) were recorded for concretes containing 5% CS: compressive strength-38.0 MPa, prism strength-28.9 MPa, bending strength-6.1 MPa and CCQ-0.01731 MPa × m³/kg. The increase in compressive strength was 4.1%, prismatic strength-4.0%, bending strength-3.4% and CCQ-6.1% compared with concrete without CS. Increasing the CS content from 10% to 30% inevitably led to a significant drop in the strength characteristics (up to 42%) compared with concrete without CS. Analysis of the microstructure of concrete containing CS instead of part of the natural coarse aggregate revealed that the cement paste penetrates into the pores of the CS, thereby creating good adhesion of this aggregate to the cement-sand matrix.

Development and Preliminary Application of Temperature Stress Test Machine for Cast-in-Place Inner Shaft Lining

Over the past 20 years, as the depth and diameter of shaft lines increased in China, the cracking and water leakage of the inner walls of frozen shafts have become increasingly severe, resulting in significant safety threats and economic losses. Understanding the stress variation patterns of cast-in-place inner walls under the combined effects of temperature and constraint during construction is a prerequisite for evaluating the crack resistance performance of inner walls and preventing water leakage in frozen shafts. The temperature stress testing machine is an important instrument for studying the early-age crack resistance performance of concrete materials under the combined effects of temperature and constraint. However, existing testing machines have shortcomings in terms of applicable specimen cross-sectional shapes, temperature control methods for concrete structures, and axial loading capacity. In this paper, a novel temperature stress testing machine suitable for the inner wall structure shape, capable of simulating the hydration heat of the inner walls, was developed. Then, a reduced-scale model of the inner wall according to similarity criteria was manufactured indoors. Finally, preliminary investigations of the temperature, strain, and stress variations of the inner wall under 100% end constraint conditions were conducted by simulating the actual hydration heating and cooling process of the inner walls. Results show that the hydration heating and cooling process of the inner wall can be accurately simulated. After approximately 69 h of concrete casting, the accumulated relative displacement and strain of the end-constrained inner wall model were -244.2 mm and 187.8 με, respectively. The end constraint force of the model increased to a maximum value of 1.7 MPa and then rapidly unloaded, causing the model concrete to crack in tension. The temperature stress testing method presented in this paper provides a reference for scientifically formulating technical approaches to prevent cracking in cast-in-place concrete inner walls.

Permeability Coefficient of Concrete under Complex Stress States

Hydraulic structures are typically subjected to long-term hydraulic loading, and concrete-the main material of structures-may suffer from cracking damage and seepage failure, which can threaten the safety of hydraulic structures. In order to assess the safety of hydraulic concrete structures and realize the accurate analysis of the whole failure process of hydraulic concrete structures under the coupling effect of seepage and stress, it is vital to comprehend the variation law of concrete permeability coefficients under complex stress states. In this paper, several concrete samples were prepared, designed for loading conditions of confining pressures and seepage pressures in the first stage, and axial pressures in the later stage, to carry out the permeability experiment of concrete materials under multi-axial loading, followed by the relationships between the permeability coefficients and axial strain, and the confining and seepage pressures were revealed accordingly. In addition, during the application of axial pressure, the whole process of seepage-stress coupling was divided into four stages, describing the permeability variation law of each stage and analyzing the causes of its formation. The exponential relationship between the permeability coefficient and volume strain was established, which can serve as a scientific basis for the determination of permeability coefficients in the analysis of the whole failure process of concrete seepage-stress coupling. Finally, this relationship formula was applied to numerical simulation to verify the applicability of the above experimental results in the numerical simulation analysis of concrete seepage-stress coupling.

Modification of Concrete Composition Doped by Sewage Sludge Fly Ash and Its Effect on Compressive Strength

The sustainable development of construction materials is an essential aspect of current worldwide trends. Reusing post-production waste in the building industry has numerous positive effects on the environment. Since concrete is one of the materials that people manufacture and use the most, it will continue to be an integral element of the surrounding reality. In this study, the relationship between the individual components and parameters of concrete and its compressive strength properties was assessed. In the experimental works, concrete mixes with different contents of sand, gravel, Portland cement CEM II/B-S 42.5 N, water, superplasticizer, air-entraining admixture, and fly ash from the thermal conversion of municipal sewage sludge (SSFA) were designed. According to legal requirements in the European Union, SSFA waste from the sewage sludge incineration process in a fluidized bed furnace should not be stored in landfills but processed in various ways. Unfortunately, its generated amounts are too large, so new management technologies should be sought. During the experimental work, the compressive strength of concrete samples of various classes, namely, C8/10, C12/15, C16/20, C20/25, C25/30, C30/37, and C35/45, were measured. The higher-class concrete samples that were used, the greater the compressive strength obtained, ranging from 13.7 to 55.2 MPa. A correlation analysis was carried out between the mechanical strength of waste-modified concretes and the composition of concrete mixes (the amount of sand and gravel, cement, and FA), as well as the water-to-cement ratio and the sand point. No negative effect of the addition of SSFA on the strength of concrete samples was demonstrated, which translates into economic and environmental benefits.

Numerical Modeling of the Dynamic Elastic Modulus of Concrete

This article introduces simulations of theoretical material with controlled properties for the evaluation of the effect of key parameters, as volumetric fractions, elastic properties of each phase and transition zone on the effective dynamic elastic modulus. The accuracy level of classical homogenization models was checked regarding the prediction of dynamic elastic modulus. Numerical simulations were performed with the finite element method for evaluations of the natural frequencies and their correlation with E_d through frequency equations. An acoustic test validated the numerical results and obtained the elastic modulus of concretes and mortars at 0.3, 0.5 and 0.7 water-cement ratios. Hirsch calibrated according to the numerical simulation (x = 0.27) exhibited a realistic behavior for concretes of w/c = 0.3 and 0.5, with a 5% error. However, when the water-to-cement ratio (w/c) was set to 0.7, Young's modulus displayed a resemblance to the Reuss model, akin to the simulated theoretical triphasic materials, considering matrix, coarse aggregate and a transition zone. Hashin-Shtrikman bounds is not perfectly applied to theoretical biphasic materials under dynamic situations.

A Novel MBAS-RF Approach to Predict Mechanical Properties of Geopolymer-Based Compositions

Using gels to replace a certain amount of cement in concrete is conducive to the green concrete industry, while testing the compressive strength (CS) of geopolymer concrete requires a substantial amount of substantial effort and expense. To solve the above issue, a hybrid machine learning model of a modified beetle antennae search (MBAS) algorithm and random forest (RF) algorithm was developed in this study to model the CS of geopolymer concrete, in which MBAS was employed to adjust the hyperparameters of the RF model. The performance of the MBAS was verified by the relationship between 10-fold cross-validation (10-fold CV) and root mean square error (RMSE) value, and the prediction performance of the MBAS and RF hybrid machine learning model was verified by evaluating the correlation coefficient (R) and RMSE values and comparing with other models. The results show that the MBAS can effectively tune the performance of the RF model; the hybrid machine learning model had high R values (training set R = 0.9162 and test set R = 0.9071) and low RMSE values (training set RMSE = 7.111 and test set RMSE = 7.4345) at the same time, which indicated that the prediction accuracy was high; NaOH molarity was confirmed as the most important parameter regarding the CS of geopolymer concrete, with the importance score of 3.7848, and grade 4/10 mm was confirmed as the least important parameter, with the importance score of 0.5667.

Recent Progress of Cement-Based Materials Modified by Graphene and Its Derivatives

Graphene, with its excellent properties and unique structure, has been extensively studied in the context of modifiable cement-based materials. However, a systematic summary of the status of numerous experimental results and applications is lacking. Therefore, this paper reviews the graphene materials that improve the properties of cement-based materials, including workability, mechanical properties, and durability. The influence of graphene material properties, mass ratio, and curing time on the mechanical properties and durability of concrete is discussed. Furthermore, graphene's applications in improving interfacial adhesion, enhancing electrical and thermal conductivity of concrete, absorbing heavy metal ions, and collecting building energy are introduced. Finally, the existing issues in current study are analyzed, and the future development trends are foreseen.

Study on Mass Erosion and Surface Temperature during High-Speed Penetration of Concrete by Projectile Considering Heat Conduction and Thermal Softening

The mass erosion of the kinetic energy of projectiles penetrating concrete targets at high speed is an important reason for the reduction in penetration efficiency. The heat generation and heat conduction in the projectile are important parts of the theoretical calculation of mass loss. In this paper, theoretical models are established to calculate the mass erosion and heat conduction of projectile noses, including models of cutting, melting, the heat conduction of flash temperature, and the conversion of plastic work into heat. The friction cutting model is modified considering the heat softening of metal, and a model of non-adiabatic processes for the nose was established based on the heat conduction theory to calculate the surface temperature. The coupling numerical calculation of the erosion and heat conduction of the projectile nose shows that melting erosion is the main factor of mass loss at high-speed penetration, and the mass erosion ratio of melting and cutting is related to the initial velocity. Critical velocity without melting erosion and a constant ratio of melting and cutting erosion exists, and the critical velocities are closely related to the melting temperature. In the process of penetration, the thickness of the heat affected zone (HAZ) gradually increases, and the entire heat conduction zone (EHZ) is about 5~6 times the thickness of the HAZ.

Study on the Utilization of Waste Thermoset Glass Fiber-Reinforced Polymer in Normal Strength Concrete and Controlled Low Strength Material

Thermoset glass fiber-reinforced polymers (GFRP) have been widely used in manufacturing and construction for nearly half a century, but the large amount of waste produced by this material is difficult to dispose of. In an effort to address this issue, this research investigates the reuse of thermoset GFRP waste in normal strength concrete (NSC) and controlled low-strength materials (CLSM). The mechanical performance and workability of the resulting concrete were also evaluated. To prepare the concrete specimens, the thermoset GFRP waste was first pulverized into granular pieces, which were then mixed with cement, fly ash, and water to form cylindrical concrete specimens. The results showed that when the proportion of thermoset GFRP waste aggregate in the concrete increased, the compressive strengths of NSC and CLSM would decrease. However, when incorporating 5% GFRP waste into CLSM, the compressive strength was 7% higher than concrete without GFRP. However, the workability of CLSM could be improved to meet engineering standards by adding an appropriate amount of superplasticizer. This finding suggests that the use of various combinations of proportions in the mixture during production could allow for the production of CLSM with different compressive strength needs. In addition, the use of recycled thermoset GFRP waste as a new aggregate replacement for traditional aggregates in CLSM was found to be a more sustainable alternative to the current CLSM combinations used in the market.

Effect of bacteria on strength properties and toxicity of incinerated biomedical waste ash concrete

A large amount of biomedical waste is generated worldwide, and this waste is hazardous and infectious. The ultimate solution for the issue of disposal of such waste is incineration and then landfill. This incinerated waste is called incinerated biomedical waste ash (IBWA). After incineration, the IBWA is still toxic because of the presence of heavy metals and alkaline metals as they get leached out and have a lethal effect on the environment. This study aims at the use of IBWA in concrete as fine aggregate replacement material. The IBWA was given bacterial treatment to stabilise it against alkalinity and heavy metals leaching. Fine aggregate was replaced with IBWA with ratios having replacement levels from 0%, 5%, 10%, 15%, and 20%. Strength tests performed were compressive strength and splitting tensile strength up to the age of 365 days. United States Environmental Protection Agency's (U.S. EPA) toxicity characteristic leaching procedure, SEM-EDS, and XRD tests were performed. Leachate generated from the concrete mix incorporating IBWA (with and without bacterial treatment) was studied, and the aim was to bind the metals to ensure that the metals leached out are within permissible limits.

Influence of Incorporating Recycled Windshield Glass, PVB-Foil, and Rubber Granulates on the Properties of Geopolymer Composites and Concretes

Waste materials from the automotive industries were re-used as aggregates into metakaolin-based geopolymer (GP), geopolymer mortar (GM), and Bauhaus B20-based concrete composite (C). Specifically, the study evaluates the ability of windshield silica glass (W), PVB-Foils (P), and rubber granulates (G) to impact the mechanical and thermal properties. The addition of the recovered materials into the experimental geopolymers outperformed the commercially available B20. The flexural strength reached values of 7.37 ± 0.51 MPa in concrete with silica glass, 4.06 ± 0.32 in geopolymer malt with PVB-Foils, and 6.99 ± 0.82 MPa in pure geopolymer with rubber granulates; whereas the highest compressive strengths (б_c) were obtained by the addition of PVB-Foils in pure geopolymer, geopolymer malt, and concrete (43.16 ± 0.31 MPa, 46.22 ± 2.06 MPa, and 27.24 ± 1.28 MPa, respectively). As well PVB-Foils were able to increase the impact strength (б_i) at 5.15 ± 0.28 J/cm² in pure geopolymer, 5.48 ± 0.41 J/cm² in geopolymer malt, and 3.19 ± 0.14 J/cm² in concrete, furnishing a significant improvement over the reference materials. Moreover, a correlation between density and thermal conductivity (λ) was also obtained to provide the suitability of these materials in applications such as insulation or energy storage. These findings serve as a basis for further research on the use of waste materials in the creation of new, environmentally friendly composites.

Complete Generalization of the Equations for the Stress-Strain Curves of Concrete under Uniaxial Compression

The existence of more than thirty stress-strain equations, including those proposed by the government regulations in many countries, seems to indicate that additional, unifying, and at the same time generalizing research is necessary for this subject. Many expressions can be found to set or determine the initial modulus of elasticity of concrete, i.e., the modulus of elasticity of concrete when no load has been applied to it. This work proposes a complete generalization of the equations based on scalar damage models, applicable to all types of concrete tested under uniaxial compression with any constant rate of stress or strain, although in no case can it be considered a constitutive model. We prefer to discuss an equation that models the shape of the stress-strain curve. Thus, the shape of this curve is studied here in the same way a forensic scientist would, which is why we could see this work as an autopsy carried out on the test specimen through the trace left in the plane σ-ε by the straining process up until its inevitable outcome. That is to say, we believe in a purely phenomenological approach. The results are compared with the data obtained experimentally by analyzing test specimens made using various mixed portions of cement, water, and aggregates.

Performance of Rubber Concrete Containing Polypropylene and Basalt Fibers under Coupled Sulfate Attack and Freeze-Thaw Conditions: An Experimental Evaluation

Rubber concrete (RC) is a new type of concrete that is currently receiving a lot of attention, solving serious pollution problems by grinding waste tires into granules and adding them to concrete. However, rubber concrete has deficiencies in mechanics and durability, and has been reinforced by adding fibers in many studies. In this study, the mechanical and durability properties of rubber concrete with added polypropylene and basalt fibers (PBRC) were investigated in a series of experiments including apparent morphology, mass, static compressive and tensile tests, ultrasonic non-destructive testing, and scanning electron microscope (SEM) tests under coupled environments of sulfate attack and freeze-thaw. The results showed that the mass loss rate of RC and PBRC gradually increased with the number of freeze-thaw cycles, with more pits and cement paste peeling from the specimen surface. Moreover, the compressive and splitting tensile strengths of RC and PBRC groups exhibited distinct trends, with the former group showing a lower residual strength relative to the latter. The residual compressive strength of the RC group was only 69.4% after 160 freeze-thaw cycles in 5% MgSO₄ solution. However, it is worth noting that the addition of too many fibers also had a negative effect on the strength of the rubber concrete. Additionally, the scanning electron microscopy (SEM) results indicated that the fibers restricted the formation of microcracks in the microstructure and curtailed the brittleness of the concrete. This study can provide a valuable reference for the application of environmentally friendly material fibers in recycled aggregate concrete.

Kriging, Polynomial Chaos Expansion, and Low-Rank Approximations in Material Science and Big Data Analytics

In material science and engineering, the estimation of material properties and their failure modes is associated with physical experiments followed by modeling and optimization. However, proper optimization is challenging and computationally expensive. The main reason is the highly nonlinear behavior of brittle materials such as concrete. In this study, the application of surrogate models to predict the mechanical characteristics of concrete is investigated. Specifically, meta-models such as polynomial chaos expansion, Kriging, and canonical low-rank approximation are used for predicting the compressive strength of two different types of concrete (collected from experimental data in the literature). Various assumptions in surrogate models are examined, and the accuracy of each one is evaluated for the problem at hand. Finally, the optimal solution is provided. This study paves the road for other applications of surrogate models in material science and engineering.

Nanomaterials as Promising Additives for High-Performance 3D-Printed Concrete: A Critical Review

Three-dimensional (3D) printed concrete (3DPC), as one of the subset of digital fabrication, has provided a revolution in the construction industry. Accordingly, scientists, experts, and researchers in both academic and industry communities are trying to improve the performance of 3DPC. The mix design of all kinds of concrete has always been the most crucial property to reach the best efficiency. Recently, many studies have been performed to incorporate nano- and micro-scale additives to ameliorate the properties of 3DPC. The current study aims to present the main design properties of 3DPC and completely cover both fresh and hardened state characteristics of 3DPC containing different nano- and micro-additives. Our observations illustrate that nanomaterials can be mainly utilized as a thickener to ameliorate the thixotropic behavior and the structural build-up of 3DPC, resulting in higher yield stress and better viscosity recovery. Furthermore, each nanomaterial, through its unique impact, can provide lower porosity and permeability as well as better mechanical strengths for 3DPC. Although much research investigate the fresh properties of 3DPC containing nano and micro additives, future studies are needed to provide better insight into the impact of these kinds of additives on the hardened characteristics of 3DPC. In addition, researchers may devote more research to address the effects of the additives discussed herein on the performance of other kinds of 3DPC such as lightweight, self-compacting, etc. It should be noted that the effect mechanism of nanomaterials on the inter-layer bond strength of 3DPC is another crucial issue that should be investigated in future studies. Furthermore, nano-scale fillers from source of waste and biomass can be attractive additives for future research to achieve high performance of sustainable 3D-printed concrete.

Compressive Strength Prediction of Rice Husk Ash Concrete Using a Hybrid Artificial Neural Network Model

The combination of rice husk ash and common concrete both reduces carbon dioxide emission and solves the problem of agricultural waste disposal. However, the measurement of the compressive strength of rice husk ash concrete has become a new challenge. This paper proposes a novel hybrid artificial neural network model, optimized using a reptile search algorithm with circle mapping, to predict the compressive strength of RHA concrete. A total of 192 concrete data with 6 input parameters (age, cement, rice husk ash, super plasticizer, aggregate, and water) were utilized to train proposed model and compare its predictive performance with that of five other models. Four statistical indices were adopted to evaluate the predictive performance of all the developed models. The performance evaluation indicates that the proposed hybrid artificial neural network model achieved the most satisfactory prediction accuracy regarding R² (0.9709), VAF (97.0911%), RMSE (3.4489), and MAE (2.6451). The proposed model also had better predictive accuracy than that of previously developed models on the same data. The sensitivity results show that age is the most important parameter for predicting the compressive strength of RHA concrete.

Effect of Hydration Temperature Rise Inhibitor on the Temperature Rise of Concrete and Its Mechanism

The rapid drop in internal temperature of mass concrete can readily lead to temperature cracks. Hydration heat inhibitors reduce the risk of concrete cracking by reducing the temperature during the hydration heating phase of cement-based material but may reduce the early strength of the cement-based material. Therefore, in this paper, the influence of commercially available hydration temperature rise inhibitors on concrete temperature rise is studied from the aspects of macroscopic performance and microstructure characteristics, and their mechanism of action is analyzed. A fixed mix ratio of 64% cement, 20% fly ash, 8% mineral powder and 8% magnesium oxide was used. The variable was different admixtures of hydration temperature rise inhibitors at 0%, 0.5%, 1.0% and 1.5% of the total cement-based materials. The results showed that the hydration temperature rise inhibitors significantly reduced the early compressive strength of concrete at 3 d, and the greater the amount of hydration temperature rise inhibitors, the more obvious the decrease in concrete strength. With the increase in age, the influence of hydration temperature rise inhibitor on the compressive strength of concrete gradually decreased, and the decrease in compressive strength at 7 d was less than that at 3 d. At 28 d, the compressive strength of the hydration temperature rise inhibitor was about 90% in the blank group. XRD and TG confirmed that hydration temperature rise inhibitors delay early hydration of cement. SEM showed that hydration temperature rise inhibitors delayed the hydration of Mg(OH)₂.

On the Chloride Distribution in Concrete and Mortar Samples after an RCM Test

It is of essential need to face the challenges of CO₂ reduction in industrial cement and concrete production reliable test methods in order to evaluate the performance of concretes, especially with regard to the service life of our infrastructure. The rapid chloride migration test (RCM test) is a standard method to assess the resistance against chloride ingress of concrete. However, during our study, certain critical questions arose with regard to the chloride distribution. The sharp chloride ingress front based on the model assumptions contradicted the shallow gradient of the experimental data. For this reason, investigations on chloride distribution in concrete and mortar samples after RCM tests were performed. The focus was on the factors influencing the extraction, e.g., time after RCM test and the location on sample. Furthermore, differences between concrete and mortar samples were investigated. The investigations showed that no sharp gradient on concrete samples was found due to the extremely uneven chloride front. In contrast, the theoretical profile shape was instead demonstrated on mortar specimens. The prerequisite for this result is that the drill powder must be taken directly after the completion of the RCM test from very uniform penetration areas. Thus, the model assumptions on the chloride distribution via the RCM test could be confirmed.

Exploring Exact Effects of Various Factors on Chloride Diffusion in Cracked Concrete: ABAQUS-Based Mesoscale Simulations

Chloride ion attack is a major cause of concrete durability problems, and existing studies have rarely addressed the effects of damage zones. In this paper, an improved mesoscale model including five phases was constructed using the finite element software ABAQUS to study the diffusivity of chloride ions in cracked concrete. It was found that the damage zone is negligible when the crack width is less than 50 μm, while the width and depth of the damage zone are about 15 times the crack width and 15% of the crack depth when the crack is greater than 50 μm. The results show that the diffusion of chloride is greatly influenced by the crack width, while it is little-influenced by the crack shape. Low water-cement ratio and adequate hydration of the concrete are also key factors affecting chloride diffusion. In contrast, regular rounded aggregates have a positive effect on reducing chloride diffusion compared to irregularly shaped aggregates, and this effect becomes weaker with increasing service time. In addition, the protective layer can effectively prevent the diffusion of chloride in concrete. Therefore, when designing marine concrete, efforts should be made to ensure that the concrete has a low water-cement ratio, adequate hydration, less cracking and a protective layer.

Simplified Method of Estimating the A300 Micropore Content in Air-Entrained Concrete

It is well known that the most important parameters for predicting the frost resistance of pavement concrete are the air pore spacing factor, L, and the micropore content, A₃₀₀. The A₃₀₀ parameter requires complex calculations with the estimation of the air-void size in a 3D space. The procedure is based only on one-dimensional chord lengths. The air-void distribution is used only to determine the content of micropores and has no other practical application. Based on the results of the analysis, it was found that there is a simpler way to estimate the A₃₀₀ parameter without the tedious calculations described in the EN 480-11 Standard. The presented approach is based on the existing linear correlation between the A₃₀₀ parameter and the number of chords in 28 length classes. The developed function includes only a few coefficients (eight classes) because only chord lengths of 10-350 µm are statistically significant. This fact is important not only for the simplification of calculations but may also have consequences for the methodology of testing parameters characterizing the structure of air-entrained concrete using the 2D method. The presented function allows the estimation of A₃₀₀ with a standard error not exceeding 0.02%, so it is useful for practical use.