Effects Of Water-cement Ratio Research Articles

Effect of water-cement ratio on the bond strength of cold joint foam concrete and crack evolution characteristics

Effect of water-cement ratio on the bond strength of cold joint foam concrete and crack evolution characteristics

Effect of the incorporation ratio of recycled concrete aggregate on the properties of self-compacting mortar

This paper investigates the effect of the addition of recycled concrete aggregate (RCA) on the properties of self-compacting cement-based materials at the mortar scale, at the incorporation ratios of 0, 25, 50, and 100 %. The results showed that the yield stress and the plastic viscosity of the mortar increased with the incorporation ratio of RCA. Slump flow and V-funnel time were used to assess these properties. The mechanical strength of the mix also increases with the RCA incorporation ratio. A quantitative analysis of the data demonstrated that the effect of the addition of RCA on the mortar's rheological properties can be attributed to a decrease of the aggregate's packing density (morphology effect), a decrease of the effective water-to-cement ratio of the mix (water-to-cement ratio effect), and an increase of the aggregate's effective volume fraction (volume fraction effect). The latter two effects are caused by the aggregate's high water absorption, whose impact on mechanical strength is stronger than that caused by the morphology effect. In addition, the increase of the mix's strength can also be attributed to the increase of the effective water-to-cement ratio of the mix and to the bonding quality between the aggregate and the cementitious matrix. The increment in the incorporation ratio of RCA causes a reduction of the effective water-cement ratio of the mix and an increase of the bonding quality.

Post-evaluation of frost resistance of cement concrete entities based on pore spacing factors of hardened concrete

In this paper, the effects of water-cement ratio, air-entraining agent type, and air content on the pore characteristic parameters of hardened concrete were investigated through an extensive series of experimental studies. First of all, according to different water-cement ratios, air-entraining agent types, and air contents, the correlation between the concrete pore spacing factor and the frost resistance durability factor was revealed by using the automatic analysis and testing instrument for the pore system of hardened concrete. Then, the decision tree method was utilized to propose specific pore spacing factors for different concrete scenarios, including the non-air-entrained concrete, air-entrained concrete, concrete with unqualified frost resistance, and concrete with qualified frost resistance. In cases where the pore spacing factor measurement fell within an ambiguous range, the classification was further refined according to the water-cement ratio. On that basis, the comprehensive range of pore spacing factors for the concrete with a qualified frost resistance was determined. Finally, the post-evaluation method and standard procedure for assessing the frost resistance of concrete entities based on pore spacing factor and water-cement ratio were proposed to verify whether the frost resistance of concrete buildings meets the life expectancy requirements. This method facilitates the evaluation of the frost resistance quality of newly-built concrete projects, as well as the investigation and monitoring of frost resistance in existing structures during service.

A mix design method for self-compacting recycled aggregate concrete targeting slump-flow and compressive strength

If waste concrete and excavated engineering soil can be recycled into self-compacting concrete, it could not only alleviate the shortage of natural resources but also realize the sustainable development of self-compacting concrete technology. Therefore, a novel method is proposed for designing self-compacting recycled aggregate concrete (SCRAC) mix. The method is developed by target slump-flow and target compressive strength, avoiding tedious adjustments and laboratory re-testing at the concrete level. The proposed method first determines the contents of natural and recycled coarse aggregate based on the target slump-flow and those of natural and recycled sand are then obtained. Then, the required effective water-cement ratio is calculated based on the target compressive strength. The required admixture content is determined based on the paste rheology. Eleven mixtures designed in this manner were cast and tested. The tested variables included the target slump-flow, target compressive strength, replacement ratio of recycled coarse aggregate (RCA) and recycled sand (RS) from the excavated engineering soil, and coarse aggregate size. The results showed that the proposed method was applicable to SCRAC with a 600–800 mm target slump-flow, 40–60 MPa target compressive strength, 0–100% RCA, 0 and 100% RS, and 5–10, 5–16, and 5–20 mm coarse aggregate sizes. Finally, SCRAC using the RCA and punching residues was successfully fabricated to further verify the feasibility of the proposed method. The proposed method provides design guidance for self-compacting concrete containing RCA and RS obtained from excavated engineering soil.

Research on the preparation and diffusion characteristics of coal seam bottom fracture grouting material based on solid waste synergy

Fracture in the top and bottom plates of the coal seams in the Lubei Mining Area of China have developed to communicate with the Austrian gray pressurized water, and the threat of water damage is extremely serious. The rapid development of China's domestic economy has led to an increasing amount of industrial solid waste, which seriously jeopardizes the safety of the ecological environment. Based on the current problem, the slag powder (S) and waste rubber particles (R) were used as the main material, supplemented by cement (C) reinforcement with alkali slag (AS) excitation, and the mine fissure grouting and sealing reinforcement material was developed. The effects of water-cement ratio and (S: R) mass fraction on the physical and chemical properties of the grouted rock mass were analyzed using indoor tests, and the optimal components were determined; secondly, three-dimensional slurry-water-rock flow-solid coupling two-phase seepage simulation was realized using COMSOL, which revealed the diffusion morphology of the solid-waste slurry, preliminarily verified the feasibility of the grouting diffusion inside the rock mass, and determined the diffusion rule of the solid-waste slurry concerning the grouting pressure. The results indicate that at a water-cement ratio of 0.7 and a mass fraction of (S: R) at (70%:30%), the slurry has a flow degree of 275 mm, a water precipitation rate of 7.4%, a 28d compressive strength of 6.9 MPa, and follows the Bingham rheological model, with good injectability. Moreover, the slurry diffuses in simulated fracture grouting in a similar pattern to ultrafine cement slurry, satisfying the reinforcement requirements of field grouting. The research content realizes the combination of mine water damage control and solid waste scale, which is critical to ensuring production safety and the mine's green sustainable development.

Improvement of Computational Efficiency and Accuracy by Firefly Algorithm and Random Forest for Compressive Strength Modeling of Recycled Concrete

It is an important direction for the sustainable development of pavement to mix the discarded concrete blocks with gradation according to a certain proportion after crushing, cleaning and other technological processes, partially or completely replace aggregate, and then add cement, water, and so on to make recycled concrete for pavement paving, but the traditional evaluation model for the compressive strength (CS) of recycled concrete cannot meet the requirements of efficient calculation. To address such issues, the present research proposed to apply the firefly algorithm (FA) to optimize the random forest (RF) model. The results were demonstrated by comparing the consistency of predicted and actual values, and also by analyzing the correlation coefficient (R) and root-mean-square error (RMSE). Higher R values (0.9756 and 0.9328) and lower RMSE values (3.0752 and 6.4369) for the training and test sets present the reliability of the FA and RF hybrid machine learning model. To understand the influence law of input indexes on the output index, the importance and sensitivity of variables are further analyzed. The results displayed that effective water-cement ratio (WC) and nominal maximum recycled concrete aggregate size (NMR) have the greatest impact on the output variable, with importance scores of 2.5947 and 2.4315, respectively, while the change in the recycled concrete aggregate replacement rate (RCA) has a weak influence, with an importance score of 0.4695. Introducing FA to RF for the compressive strength modeling of recycled concrete can significantly improve the computational efficiency and accuracy.

Hydration and material properties of blended cement with ground desert sand

The potential of existing supplementary cementitious materials (SCMs), such as fly ash and slag, has been dramatically decreased after decades of use. For future development, finding new SCMs to address the sustainability issue of cement and concrete is essential. Due to its global distribution and reserves, and the fact that it has not been officially used for construction, desert sand may become a new option for SCM for cement. Since desert sand has a very fine and uniform particle size, regular milling in the field can conveniently transform it into a powder comparable to cement fineness. This study investigated the technical possibility of desert sand powder as an SCM through physical experiments, thermodynamic modeling, and life cycle analysis. The mechanical strength of blended cement with desert sand powder was generally lower than that of pure cement due to the cement reduction and increased effective water-cement ratio. When the dosage of desert sand powder was within 15%, the 28-day strength of blended cement exceeded 75% of that of pure cement, and the strength development of blended cement was better than that of pure cement after 28 days till long curing ages (112 days). Both the initial and renewed stages of early cement hydration were promoted by desert sand powder. The desert sand powder's leading minerals, quartz and calcite, participated in the cement hydration reactions, generating corresponding hydration products, calcium silicate hydrate (C-S-H) and carbonate-based aluminate ferrite mono-sulfate (AFm). The microstructure showed the interactions between the desert sand powder particles and the hydrated cement matrix, including physical and chemical aspects. In the case of high dosage, the hardened matrix's pore structure was apparent with a less dense matrix due to excess water and reduced hydration products. The thermodynamic modeling theoretically quantified the phase assemblage of hydrated cement with desert sand powder, connecting to the observed reactivity of desert sand powder in cement hydration. The life cycle analysis showed that the carbon emission of desert sand powder was comparable to the conventional SCMs (fly ash and limestone powder). However, if it was proposed to be used outside the desert and surrounding regions, the carbon emission may increase due to the extended transportation distance.

Development of hybrid SVM-FA, DT-FA and MLR-FA models to predict the flexural strength (FS) of recycled concrete

Recycled concrete from construction waste used as road material is a current sustainable approach. To provide feasible suggestions for civil engineers to prepare recycled concrete with high flexural strength (FS) for the road pavement, the present study proposed three hybrid machine learning models by combining support vector machine (SVM), decision tree (DT) and multiple linear regression (MLR) with the firefly algorithm (FA) for the computational optimization, named as SVM-FA, DT-FA, and MLR-FA, respectively. Effective water-cement ratio (WC), aggregate-cement ratio (AC), recycled concrete aggregate replacement ratio (RCA), nominal maximum recycled concrete aggregate size (NMR), nominal maximum normal aggregate size (NMN), bulk density of recycled concrete aggregate (BDR), bulk density of normal aggregate (BDN), water absorption of RCA (WAR) and water absorption of NA (WAN) were employed as the input variables. To determine the predicting results of varying hybrid models, root mean square error (RMSE) and correlation coefficient (R) were used as performance indexes. The results showed that the SVM-FA demonstrated the highest R values and the lowest RMSE values, and the fitting effect of the predicted values and the actual values of the FS of recycled concrete is the best. All the above analysis proving that the SVM optimized by FA hyperparameters has the highest prediction accuracy and SVM-FA can provide engineers a more accurate and convenient tool to evaluate the FS of recycled concrete. The results of sensitivity analysis showed that WC has the most significant influence on the FS of recycled concrete, while RCA has the weakest influence on the FS, which should be noticed when engineers apply recycled concrete to road design in the future.

Experimental investigation on the flexural performance and damage process of steel fiber reinforced recycled coarse aggregate concrete

The application of recycled concrete requires the reliable information on its flexural performance especially after cracking. In this study, the three-point bending tests were carried out on 40 notched beam specimens to investigate the flexural performance and damage process of steel fiber reinforced recycled coarse aggregate concrete (SFRCAC). The effects of water-cement ratio, recycled coarse aggregate replacement ratio and steel fiber volume fraction on the residual flexural tensile strength and flexural toughness of SFRCAC were analyzed. Moreover, the flexural damage process of the SFRCAC notched beam was explored and analyzed by Acoustic Emission (AE) technology. The results show that the residual flexural tensile strength and flexural toughness of SFRCAC decrease with increased water-cement ratio and recycled coarse aggregate replacement ratio, respectively. Adding steel fibers can effectively improve the residual flexural tensile strength and flexural toughness of SFRCAC. The post-cracking behavior of SFRCAC changes from strain-softening to strain-hardening when steel fiber volume fraction increases from 0.5% to 2%. Finally, the empirical formula of residual flexural tensile strength for SFRCAC has been put forward by analyzing and fitting the test results in this study and related literature.

Effect of water-cement ratio on the electromagnetic shielding performance of C3S paste cured long-term

Effect of water-cement ratio on the electromagnetic shielding performance of C3S paste cured long-term

Experimental study on ultra-early tensile creep of cement paste

Tensile creep releases the internal tensile stress caused by the restriction of internal deformation in cement-based structure at ultra-early age (from initial setting to final setting) and thus reduces the risk of cracking under tension. However, the tensile creep measurement at that age is challenging due to insufficient strength of specimens. Therefore, a test device for measuring the tensile creep of ultra-early age cement paste was designed in this study, which applied multilevel incremental tensile load on the ultra-early age cement paste in a tailored molud. The effect of water-cement ratio and loading age on the ultra-early was studied using that device. The results revealed that the specific creep of ultra-early cement paste decreases exponentially with the increase of loading age, while increases with the increase of water-cement ratio. The ultra-early tensile creep of cement paste was characterized by variable viscosity dashpot element. Moreover, the increase rate of viscosity with time under external force was found equal to the creep modulus of cement paste, and in the logarithmic coordinate system, there is a linear relationship between the initial viscosity of cement paste and the loading age.

Potential of Repurposing Recycled Concrete for Road Paving: Flexural Strength (FS) Modeling by a Novel Systematic and Evolved RF-FA Model

Concrete can be recycled after certain processing technologies for use in pavement engineering but the flexural strength (FS) is difficult to predict accurately in the design process. This study proposes a novel systematic and evolved approach to estimate the FS of recycled concrete. The proposed methods are conducted based on the random forest (RF) model as well as the firefly algorithm (FA), where the latter is employed to tune the hyperparameters of the RF model. For this purpose, data sets were collected from previously published literature for the training and verification of the model, and the accuracy of the model was verified by the fitting effect of the predicted and actual values. The results showed that the proposed hybrid machine learning model has a good fitting effect on the predicted and actual values; the calculation and evaluation process demonstrated fast convergence and significantly lower values of RMSE for the proposed model to determine the FS of the recycling concrete. In addition, the study analyzed the sensitivity of the FS of recycled concrete to input variables, and the results showed that effective water-cement ratio (WC), water absorption of recycling concrete (WAR), and water absorption of natural aggregate (WAN) show more obvious influences on FS, so these factors should be paid more attention in future pavement design using the recycling of concrete.

Effects of Water-cement Ratio and Hydration Heat Regulating Materials on Hydration Process of Early-age Cementitious Materials

Effects of Water-cement Ratio and Hydration Heat Regulating Materials on Hydration Process of Early-age Cementitious Materials

Effect of Water-Cement Ratio on Brittleness Characteristics of Concrete under Uniaxial Compression

Effect of Water-Cement Ratio on Brittleness Characteristics of Concrete under Uniaxial Compression

Effects of water-cement ratio on bond strength of concrete

It is vital that reinforced concrete structures develop and maintain a high level of composite action to achieve the purpose for which it is designed. This involves transfer of loads or stresses between concrete and reinforcing steel otherwise known as bond. Studies have shown that bond strength is affected amongst other factors, by grades of composite materials. There is paucity of information on influence of water-to-cement (w/c) ratio on the bond strength of concrete. Water is often arbitrarily added to concrete mix to achieve a higher workability at the construction site, it is therefore pertinent to determine the effect of w/c on bond strength of concrete and this is the focus of this study. Sieve analysis was carried out on aggregates and for tests on concrete, cubes and cylinders with reinforcing bars inserted were cast with w/c ratios of 0.4, 0.45, 0.5, 0.55 and 0.6 using mix ratio of 1:2:4. Slump and compaction factor tests were performed on the fresh concrete to determine the workability followed by the determination of compressive strengths of the concrete cubes at 7, 14, 21 and 28th day while pull-out test on the hardened concrete cylinders was carried out at 28th day only. Sieve analysis results showed that fine and coarse aggregates were uniformly graded; w/c ratios of 0.4, 0.45 and 0.5 gave low (stiff mix) workability while those of 0.55 and 0.6 gave medium (plastic mix) workability for slump and compacting factor tests. The compressive strength and the pull-out (bond) strength in N/mm2 at 28 day were (23.24, 5.11), (20.31, 5.34), (18.22, 5.87), (16.76, 4.53) and (16.09, 3.81), respectively for w/c ratios of 0.4, 0.45, 0.5, 0.55 and 0.6. The results clearly showed that increase in w/c ratio increases workability and reduces compressive strength and bond strength of concrete within the range of w/c ratios considered.

Ultrasound Non-Destructive Characterization of Early Hydration of Cement Pastes: The Effects of Water-Cement Ratio and Curing Temperature

A newly developed ultrasound method that acquires at the same time both reflected and transmitted P-waves, at different angles, using two immersion transducers of 0.5 MHz central frequency, is described in this present study. This non-destructive method allows calculating the longitudinal and transverse velocities, and thus, the Young modulus, so that its evolutions is followed in time domain. The closely correlation between the evolution in time domain of those calculated parameters and hydration properties of cement based materials was used to characterize the effects of different water-cement ratio and curing temperatures on early age hydration behaviour of cement pastes. To do so, cement samples were prepared by mixing Portland cement and freshwater. Results indicates that lower water to cement ratio reduces the workability and increases the Young modulus of resulted cement medium. Also, both ultrasound velocities and Young modulus values increases linearly with increasing curing temperature.

Steel rebar corrosion and corrosion-induced cracking in reinforced foamed concrete

This study investigated the corrosion behavior of steel rebar in foamed concrete and the effects of water-cement ratio, fly ash, and fiber on reinforced foamed concrete’s corrosion rate and crack width. It was found that the corrosion rate of steel rebar and crack width of foamed concrete increased slowly with time and then developed rapidly, finally growing slowly. The higher the density of foamed concrete, the stronger the corrosion resistance of steel rebar. The best corrosion resistance of steel rebar in foamed concrete is achieved when the water-cement ratio is 0.5, fly ash and fiber are 20% and 0.6%, respectively.

Effect of water-cement ratio and size on tensile damage in hardened cement paste: Insight from peridynamic simulations

This paper uses the improved bond-based peridynamics (BB PD) method to predict the crack evolution and macroscopic mechanical properties of the hardened cement paste at the micro-scale. Specifically, the prototype micro-elastic brittle (PMB) model is improved by considering an attenuation kernel function and the surface effect correction. Subsequently, the uniaxial tension simulations of hardened cement microstructures generated by μic software are systemically conducted to investigate the influences of the heterogeneity of microstructure, size effect, and water-cement ratio (w/c) on crack growth and mechanical properties, i.e., tensile strength and Young's modulus. The results show that the heterogeneity of the microstructure inevitably will cause the discreteness of the macroscopic properties of the numerical cement paste. Therefore, a statistical method is considered to analyze the mechanical properties of the hardened cement. Moreover, the tensile strength has a remarkable size effect, with the size increase of microstructure, the tensile strength decreases; the simulation results fit well with the existing analytical size effect model, i.e., Weibull size effect model, multifractal scaling law, and Bažant's size effect law. Besides, the higher ratio of w/c is likely to result in lower tensile strength, Young's modulus and premature damage to the cement paste. The predicted macroscopic mechanical properties are in good agreement with the previous simulation and experimental data, which demonstrates the reliability of the improved bond-based peridynamic method in studying the crack initiation and propagation of hardened cement paste.

Developing a model for chloride transport through concrete considering the key factors

The corrosion of chloride-induced on concrete is affected by many factors. In this research, an innovative empirical model was developed to predict the long-term chloride migration behavior in concrete, considering the effects of water-cement ratio, time, bonding effect, temperature, relative humidity, and concrete deterioration. The reliability and validity of the results evaluated by the empirical prediction model were verified by the chloride concentration data in concrete specimens exposed to the marine environment for 3, 5, and 10 years reported in the literature. Combined with the established empirical prediction model, the concrete model was regarded as a three-phase composite material composed of mortar, coarse aggregate, and interfacial transition zone. The effects of key factors on chloride migration were further analyzed by using the mesoscopic finite element numerical simulation method. It was observed that when temperature increases from 5 ℃ to 65 ℃, chloride diffusion depth rises by 3.3 times. When relative humidity increases from 20% to 100%, chloride diffusion depth rises by 4.3 times. Also, the water-cement ratio, concrete deterioration, and chloride binding effect have a non-negligible impact on chloride migration.

Monotonic and cyclic bond responses of steel bar with steel-polypropylene hybrid fiber reinforced recycled aggregate concrete

The bond failure between steel bars and concrete is the main reason for the strength degradation and deformation increase of structures in service. In this study, the bond behavior of steel bars with steel-polypropylene hybrid fiber reinforced recycled aggregate concrete (HFRAC) under monotonic and cyclic loadings was investigated. The acoustic emission (AE) technique was applied to monitor the bond response and the inner damage evolution of specimens during the loading process, and the microstructure of HFRAC was analyzed through the scanning electron microscope (SEM). The hysteresis curves, failure modes, bond strength degradation rate, and energy dissipation capacity, were systematically investigated by considering the effects of water-cement ratio, recycled aggregate replacement ratio, steel fiber volume fraction, and polypropylene fiber volume fraction. Experimental results demonstrated that there was more obvious degradation in the bond strength for the HFRAC specimens under cyclic loading, compared with that under monotonic loading. In the case of cyclic loading, the bond strength was degraded with the increase of the number of cycles, and the degradation rate increased quickly with the increase of the displacement amplitude. Moreover, the analysis of AE parameters measured on steel bar bonded recycled aggregate concrete (RAC) and HFRAC specimens, including the AE hits, AE energy, localization of AE events, and average frequency, was performed. Results showed that the AE technique could effectively monitor the bond damage of HFRAC specimens at different stages. A good correlation between the results of AE parameters analysis and the cracking process of specimens was confirmed. Based on the Weibull cumulative distribution function, the bond-slip models of steel bars with HFRAC under monotonic and cyclic loadings were proposed by considering the coupling effect of hybrid fibers and recycled coarse aggregates.