Recycled concrete aggregate(RCA)derived from demolition waste has been widely explored for use in civil engineering applications.One of the promising strategies globally is to incorporate RCA into concrete products.Ho...Recycled concrete aggregate(RCA)derived from demolition waste has been widely explored for use in civil engineering applications.One of the promising strategies globally is to incorporate RCA into concrete products.However,the use of RCA in high-performance concrete,such as self-consolidating concrete(SCC),has only been studied in the past decade.This paper summarizes recent publications on the use of coarse and/or fine RCA in SCC.As expected,the high-water absorption and porous structure of RCA have posed challenges in producing a high-fluidity mixture.According to an analysis of published data,a lower strength reduction(within 23%regardless of coarse RCA content)is observed in SCC compared with vibrated concrete,possibly due to the higher paste content in the SCC matrix,which enhances the weak surface layer of RCA and interfacial transition zone.Similarly,SCC tends to become less durable with RCA substitution although the deterioration can be minimized by using treated RCA through removing or strengthening the adhered mortar.To date,the information reported on the role of RCA in the long-term performance of SCC is still limited;thus,a wide range of research is needed to demonstrate the feasibility of RCA–SCC in field applications.展开更多
Due to the presence of old mortar (OM) and interfacial transition zone (ITZ),recycled concrete aggregate (RCA) is inferior to natural aggregate (NA).The purpose of this paper was to study the effect of accelerated car...Due to the presence of old mortar (OM) and interfacial transition zone (ITZ),recycled concrete aggregate (RCA) is inferior to natural aggregate (NA).The purpose of this paper was to study the effect of accelerated carbonation on the macro-properties and micro-properties of RCA under different pressure(0.05,0.15,0.30 MPa).The macro-property tests included colour change,apparent density,water absorption,and crushing value of RCA.The micro-property tests included scanning electron microscopy (SEM),X-ray diffraction (XRD),thermogravimetry-differential scanning calorimetry (TG-DSC),and Vickers micro-hardness(VMH).The results showed that the change trends of apparent density,water absorption,and crushing value of RCA displayed exponential relationships as pressure increasing,with the optimum pressure of 0.30 MPa.SEM images indicated that the calcite caused by the hydration products in RCA and the Ca(OH)_(2) derived from saturated lime water improved the properties of RCA;as the apparent density increased,the water absorption and crushing value decreased.The results of XRD and TG-DSC indicated that,as the pressure increased,the masses of Ca(OH)_(2) in carbonated RCA gradually decreased,while those of CaCO_(3) gradually increased,which demonstrated that the carbonation degree gradually increased.Besides,ITZ-2 was the weakest phase in RCA,but its improvement degree of VMH by accelerated carbonation was higher than that of OM.However,RCA was not completely carbonated,but only carbonated in a certain depth after 24 h accelerated carbonation.展开更多
Due to the existence of the attached mortar,the performance of the recycled concrete aggregate(RCA)is inferior to the natural aggregate,which significantly limits its wide application in industry.In this study,five ki...Due to the existence of the attached mortar,the performance of the recycled concrete aggregate(RCA)is inferior to the natural aggregate,which significantly limits its wide application in industry.In this study,five kinds of modified solutions were used to modify the surface of RCA,and the modification effects were compared.The results showed that sodium silicate,nano-silica(NS),Bacillus pasteurii and soybean powder had relatively good modification effects on RCA,which could reduce the crushing value and water absorption,and increase apparent density.The composite solution(15%sodium silicate and 2%NS)and soybean powder solution had better modification effect.The 28 d compressive strength and splitting tensile strength of recycled aggregate concrete(RAC)prepared by RCA modified by soybean powder solution were 4.6%and 5.2%higher than those prepared by RCA modified by composite solution,respectively.This indicates that among the five kinds of modified solutions,soybean powder solution has the best modification effect on RCA,and the optimal soaking time of soybean powder solution is 8 h.At this time,the crushing value,water absorption and apparent density of RCA are 12.8%,5.3%,and 2653 kg/m^(3),respectively.The research results of this study provide a reference for the modification of RCA and its efficient utilization.展开更多
The substantial generation of building waste had precipitated significant environmental issues in China,attributable to urban expansion.Recycled concrete aggregate(RCA)produced from building waste had been used as an ...The substantial generation of building waste had precipitated significant environmental issues in China,attributable to urban expansion.Recycled concrete aggregate(RCA)produced from building waste had been used as an alternative material in green roofs as substrates.However,the biological dynamics of RCA are unknown.Thus,the column test was performed to investigate the effects of different biochar types and application ratios on the biochemical properties and bacterial communities of RCA.Results show that the nutrients of RCA have been improved by biochar addition regardless of biochar types and application ratios.This led to the increase in plant weight and height by at least 227%and 38%in biochar amended RCA.However,biochar application reduced bacterial richness andα-diversity,while enhancing plant growth.This indicated that biochar application directly improves certain bacterial species that promote plant growth and productivity.Biochar application reduced the Proteobacteria abundance and enhanced the percentage of Acidobacteriota in RCA,indicating that biochar application shaped the bacterial communities into raw soil composition.The outcome of this research suggests that biochar application improved RCA biochemical performance as a substrate for green roofs,which provides a potential method to consume substantial RCA.展开更多
Accurately predicting the compressive strength of recycled aggregate concrete(RAC)incorporating supplementary cementitious materials(SCMs)remains a critical challenge due to the heterogeneous nature of recycled aggreg...Accurately predicting the compressive strength of recycled aggregate concrete(RAC)incorporating supplementary cementitious materials(SCMs)remains a critical challenge due to the heterogeneous nature of recycled aggregates(RA)and the complex interactions among multiple binder constituents.This study advances the field by developing the most extensive and rigorously preprocessed database to date,which comprises 1243 RAC mixtures containing silica fume,fly ash,and ground-granulated blast-furnace slag.A hybrid,domain-informed machine-learning framework was then proposed,coupling optimized Extreme Gradient Boosting(XGBost)with civil engineering expertise to capture the complex chemical and microstructural mechanisms that govern RAC performance.Systematic grid-search optimization(n_estimators=50,learning_rate=0.2,max_depth=7)produced superior predictive accuracy(training R^(2)=0.9923,testing R^(2)=0.937;MAE=2.378 MPa;RMSE=3.591 MPa),which outperformed Extra Trees,Light Gradient Boosting,and traditional regressors.Beyond prediction,model interpretability was achieved using Shapley additive explanations and partial dependence analyses,which revealed curing age as the dominant strength driver,while water-to-binder ratio and recycled aggregate water absorption exhibited strong negative influences.Three-dimensional interaction plots further demonstrated how optimal superplasticizer dosages reduce the strength loss associated with high recycled aggregate content.In summary,this work provides a novel,explainable,and data-driven framework that achieves high predictive accuracy with mechanistic transparency and offers a powerful,interpretable tool for the design and optimization of sustainable RAC mixtures.展开更多
To examine the influences of waste polypropylene fiber(PPF)on the strength and internal pore structure of recycled aggregate concrete incorporating iron ore tailings,both the cubic compressive strength and axial compr...To examine the influences of waste polypropylene fiber(PPF)on the strength and internal pore structure of recycled aggregate concrete incorporating iron ore tailings,both the cubic compressive strength and axial compressive strength of the concrete were measured.Additionally,the microstructure was analyzed using scanning electron microscopy.The evolution of pore structure parameters,including pore size distribution,pore type distribution,and nuclear magnetic resonance spectral area in the concrete,was investigated through nuclear magnetic resonance(NMR)analysis.A model correlating the concrete's pore structure with its macroscopic performance was subsequently developed based on fractal theory.The results demonstrate that an appropriate amount of PPF created a bridging effect that decelerated the progression of macro cracks,enhanced the ductility of the concrete's failure mode,and increased both cubic compressive strength and axial compressive strength,with the most effective dosage being approximately 0.6%.An appropriate amount of PPF(ranging from 0.3%to 0.6%)facilitated the formation of harmless pores and shifted the pore size distribution towards medium and small sizes.Specifically,a fiber content of 0.6%resulted in the most significant reduction in the T2 spectral area.Furthermore,the pore structure of concrete exhibits distinct fractal characteristics.As the PPF content increased,the fractal dimension initially rose and then declined,demonstrating a strong correlation with the mechanical properties.展开更多
The influence of source concrete (SC) with different compression strengths on the workability and mechanical properties of recycled mortar made with river sand substituted by 100% fine recycled concrete aggregates (FR...The influence of source concrete (SC) with different compression strengths on the workability and mechanical properties of recycled mortar made with river sand substituted by 100% fine recycled concrete aggregates (FRCA) is experimentally investigated. The basic physical performance test shows that with the increase in SC strength, FRCA exhibit lower water absorption and crushing index, meanwhile keeping higher densities. Mechanical property tests, including compressive strength, flexural strength and uniaxial compressive stress-strain tests, show that compressive strength,flexural strength and elasticity modulus of recycled sand mortars increase roughly with the increase in SC strength. The proposed mixture design method demonstrates that all of the components can be kept as the same as those in natural mortar mixture design and FRCA must be pre-wetted before making mortar mixture. Meanwhile, the reuse of higher strength SC can ensure that recycled mortar mixtures are able to achieve similar mechanical performance when compared to natural mortar designs.展开更多
Experimental investigations on mechanical property and durability of sulphoaluminate cement concrete with aggregate gradations according to Fuller distribution are presented in this paper. Compressive strength, water ...Experimental investigations on mechanical property and durability of sulphoaluminate cement concrete with aggregate gradations according to Fuller distribution are presented in this paper. Compressive strength, water impermeability and resistance capability to sulfate attack of SACC have the same trend of concrete with fine aggregates of Fuller distribution gradation<concrete with coarse aggregates of Fuller distribution gradation<concrete with total aggregates of Fuller distribution gradation. The relationship between bulk density of aggregate and water penetration depth obeyed the second-order polynomial y=0.002x2-6.863 8x +5 862.3, and had a notable correlation R2=0.979 9. The sulphoaluminate cement concrete with total aggregate gradation with Fuller distribution for h=0.50 had the best resistance capability to sulfate attack. It was a second-order polynomial relationship between bulk density of aggregates and water penetration depth of y=0.002x2-6.863 8x+5 862.3 with R2=0.979 9, which indicated notable correlation. The fitting formula between bulk density of aggregates and sulfate resistance coefficient of SACC was y=0.000 5x+0.370 4 with R2=0.958 5.展开更多
Large quantities of natural aggregates(NA)are required in the manufacturing of asphalt mixtures for pavements.The extraction of NA generates high environmental impacts that negatively affect environmental preservation...Large quantities of natural aggregates(NA)are required in the manufacturing of asphalt mixtures for pavements.The extraction of NA generates high environmental impacts that negatively affect environmental preservation and conservation.Diverse alternative materials obtained in construction and demolition processes in civil construction worked have been studies as possible replacements for NA,with the purpose of reducing these environmental impacts.The foregoing study presents a state of knowledge review referring to the use of recycled concrete aggregates(RCA)when these are used in the manufacturing of asphalt mixtures(mainly hot-mix asphalt HMA type).Environmental aspects are presented,as well as possible benefits and limitations of using RCA as replacements for NA in asphalt mixtures.However,it is not possible to establish a behavior pattern,since the characteristics of the RCA are very heterogeneous and vary from the original source of the concrete.It is highlighted from the literature that RCA has a wide potential for use in the manufacture of asphalt mixtures,since in most studies,the mixtures with these materials comply the quality requirements contemplated by the construction specifications,mainly in low-volume roads.Additionally,based on the literature review,some recommendations and suggestions are presented for future research.展开更多
The concrete aggregate model is considered as a type of weakly discontinuous problem consisting of three phases:aggregates which randomly distributed in different shapes,cement paste and internal transition zone(ITZ)....The concrete aggregate model is considered as a type of weakly discontinuous problem consisting of three phases:aggregates which randomly distributed in different shapes,cement paste and internal transition zone(ITZ).Because of different shapes of aggregate and thin ITZs,a huge number of elements are often used in the finite element(FEM)analysis.In order to ensure the accuracy of the numerical solutions near the interfaces,we need to use higher-order elements.The widely used FEM softwares such as ANSYS and ABAQUS all provide the option of quadratic elements.However,they have much higher computational complexity than the linear elements.The corresponding coefficient matrix of the system of equations is a highly ill-conditioned matrix due to the large difference between three phase materials,and the convergence rate of the commonly used solving methods will deteriorate.In this paper,two types of simple and efficient preconditioners are proposed for the system of equations of the concrete aggregate models on unstructured triangle meshes by using the resulting hierarchical structure and the properties of the diagonal block matrices.The main computational cost of these preconditioners is how to efficiently solve the system of equations by using linear elements,and thus we can provide some efficient and robust solvers by calling the existing geometric-based algebraic multigrid(GAMG)methods.Since the hierarchical basis functions are used,we need not present those algebraic criterions to judge the relationships between the unknown variables and the geometric node types,and the grid transfer operators are also trivial.This makes it easy to find the linear element matrix derived directly from the fine level matrix,and thus the overall efficiency is greatly improved.The numerical results have verified the efficiency of the resulting preconditioned conjugate gradient(PCG)methods which are applied to the solution of several typical aggregate models.展开更多
Microbially induced carbonate precipitation(MICP)is a promising technique for the autonomous healing of concrete cracks.In this study,the effect of pH on MICP was investigated.The results indicate that the MICP proces...Microbially induced carbonate precipitation(MICP)is a promising technique for the autonomous healing of concrete cracks.In this study,the effect of pH on MICP was investigated.The results indicate that the MICP process was inhibited when the pH was higher than 11.Both vaterite and calcite were produced when the pH was<8,whereas only calcite was produced when the pH was>8.Recycled concrete aggregates(RCA)coated with sodium silicate have been proposed as protective carriers for microbial healing agents.Although the presence of the coated RCA resulted in a loss of the splitting tension strength of the concrete,the loaded healing agents were highly efficient in self-healing cracks.Concrete incorporated with 20%RCA loaded with healing agents exhibited the best self-healing performance.When the initial crack widths were between 0.3 and 0.4 mm,the 7-d mean healing rate was approximately 90%.At 28 d,the crack area filling ratio was 86.4%,while its water tightness recovery ratio was 74.4%and 29.8%,respectively,for rapid and slow absorption.This study suggests that RCA coated with sodium silicate is an effective method for packaging microbial healing agents and has great potential for developing cost-effective self-healing concrete.展开更多
This article presents freeze-thaw(F/T)resistance of concrete containing recycled concrete aggregate(RCA).RCA percentages of 15%,30%,45%,and 60%were replaced by natural aggregate.Air entraining admixture(AEA)with three...This article presents freeze-thaw(F/T)resistance of concrete containing recycled concrete aggregate(RCA).RCA percentages of 15%,30%,45%,and 60%were replaced by natural aggregate.Air entraining admixture(AEA)with three different ratios(0.06%,0.13%,and 0.20%)were used and three different maximum aggregate particle sizes(dmax=16 mm,22.4 mm and 31.5 mm)were selected.F/T resistances of the concretes were determined through the measurement of scaling on the surfaces of the samples placed in 3%NaCl solution in pursuance of TS CEN/TS 12390-9.It was observed that the compressive strength of the concretes decreased by 20%on average compared to the control samples with the increase in the amount of RCA.However,it was determined from the experiments that the use of RCA increases the F/T resistances of the concretes as it provides extra voids in the concrete and decreases capillary permeability.Within the limits of this study,d_(max)=22.4 mm,RCA=45%and AEA=0.13%are recommended as the optimum values for F/T resistance of the concrete.It is also recommended to use RCA in pavement applications including roadways,highways,parking facilities,and other non-structural concrete elements subjected to severe winter conditions.展开更多
The possibility of the use of recycled aggregates from the construction industry in green concrete production is of increasing importance to reduce the negative environmental impact associated with construction and de...The possibility of the use of recycled aggregates from the construction industry in green concrete production is of increasing importance to reduce the negative environmental impact associated with construction and demolition wastes.The objective of this study is to investigate the effect of recycled concrete aggregate(RCA)quality on the properties of hardened concrete properties such as compressive strength,splitting tensile strength,density,water absorption capacity and porosity accessible to water.The RCA used in this study was obtained from the crushing of waste concrete with two different compressive strengths(LRCA obtained from the crushing of waste concrete having compressive strengths below 30 MPa and HRCA obtained from the crushing of waste concrete having compressive strengths above 30 MPa).The natural coarse limestone aggregate was 100%replaced with coarse LRCA and HRCA.As a result of the study,the use of 100%HRCA and%100 LRCA instead of limestone coarse aggregate in the concrete adversely affected its mechanical and physical properties.In addition,HRCA showed better performance in terms of compressive strength,tensile strength,water absorption and porosity compared to the use of LRCA.Furthermore,the percentage of adhered mortar on the surface of LRCA and HRCA was analyzed using a computerized micro tomography device,and it was found that the percentages of attached mortar and aggregates are 61%and 35.5%for LRCA,whilst the attached mortar and aggregate contents for HRCA are 45.9%and 53.7%,respectively.展开更多
The effects of fly ash on the compressive strength, pore size distribution and chloride-ion penetration of recycled aggregate concrete were investigated. Two series of concrete mb：tures were prepared. The concrete mi...The effects of fly ash on the compressive strength, pore size distribution and chloride-ion penetration of recycled aggregate concrete were investigated. Two series of concrete mb：tures were prepared. The concrete mixtures in series I had a water-to-binder ratio and a cement content of 0.55 and 410 kg/ m^3 , respectively. The concrete rnixtures in series II had a water-to-binder ratio and a cement content of 0.45 and 400 kg/ ml respectively. Recycled aggregate was ased as 20% , 50% , and 100% replacements of natural coarse aggregate in the concrete mixtures in both series. In cutdition, fly ash was used as 0% , 25% and 35% by weight replacements of cement. The results show that the compressive strengths of the concrete decreased as the recycled aggregate and the fly ash contents increased. The total porosity and average porosity diameter of the concrete increased us the recycled aggregate content increased. Furtherrruore , an increase in the recycled aggregate content decreased the resistance to chloride ion penetration. Nevertheless, the replacement of cement by 25% fly ush improved the resistance to chloride ion penetration and pore diameters and reduced the total porosity of the recycled aggregate concrete.展开更多
The characteristics of surface appearances,mass loss,relative dynamic modulus of elasticity and strength loss of different recycled aggregate concretes(RAC) exposed to freeze-thaw cycles were analyzed.It was found tha...The characteristics of surface appearances,mass loss,relative dynamic modulus of elasticity and strength loss of different recycled aggregate concretes(RAC) exposed to freeze-thaw cycles were analyzed.It was found that the freeze-thaw resistance of RAC could be determined by the recycled aggregate compositions and admixtures.Both the saturation degree and the air void structure were the key factors influencing the freeze-thaw damage on concrete.Some major proposed freeze-thaw deterioration mechanisms were utilized to interpret the freeze-thaw damage on recycled aggregate concrete.Meanwhile,some potential measures to enhance the freeze-thaw resistance of concrete were summarized and discussed.展开更多
Structure characteristics of three kinds of ceramsite with different water absorption and the influence on microstructure of interfacial zone as well as performance of chloride permeabil-ity and frost resistance of co...Structure characteristics of three kinds of ceramsite with different water absorption and the influence on microstructure of interfacial zone as well as performance of chloride permeabil-ity and frost resistance of combined aggregate concrete were investigated. The results show that, dense shell and closed internal pore have sharp effects on lowering water absorption of ceramsite. However, the ceramsite with high water absorption has obvious effect on the densification of interfa-cial paste which would develop a structure with lower porosity, finer aperture and higher microhard-ness. Furthermore, the impermeability and frost-resistance of concrete can be improved due to the ef-fect of water absorption and releasing by ceramsite with higher water absorption.展开更多
Lightweight aggregate concrete cube specimens (100 mm×100 mm×100 mm) and plate specimens (100 mm×100 mm×50 mm) were tested under biaxial compression-compression (CC) and compression-tension (CT) lo...Lightweight aggregate concrete cube specimens (100 mm×100 mm×100 mm) and plate specimens (100 mm×100 mm×50 mm) were tested under biaxial compression-compression (CC) and compression-tension (CT) load combinations. For comparison, normal concrete plate specimens (100 mm×100 mm×50 mm) were tested under the same load combinations. Based on the test results, a two-level strength criterion of lightweight aggregate concrete in both octahedral stress coordinate and principal stress coordinate was suggested. The lightweight aggregate concrete cube specimens (100 mm×100 mm×100 mm) were then tested under triaxial compression-compression-compression (CCC) load combination with corresponding tests on normal concrete cube specimens (100 mm×100 mm×100 mm). The effect of intermediate principal stress on triaxial compressive strength is further examined. A "plastic flow plateau" area was apparent in principal compressive stress-strain relationships of lightweight aggregate concrete but not in normal concrete. A quadratic formula was suggested for the expression of strength criterion under triaxial compression.展开更多
Through the flexural behavior test of coral aggregate reinforced concrete beams(CARCB) and ordinary Portland reinforced concrete beams(OPRCB), and based on the parameters of concrete types, concrete strength grades an...Through the flexural behavior test of coral aggregate reinforced concrete beams(CARCB) and ordinary Portland reinforced concrete beams(OPRCB), and based on the parameters of concrete types, concrete strength grades and reinforcement ratios, the crack development, failure mode, midspan deflection and flexural capacity were studied, the relationships of bending moment-midspan deflection, load-longitudinal tensile reinforcement strain, load-maximum crack width were established, and a calculation model for the flexural capacity of CARCB was suggested. The results showed that with the increase in the reinforcement ratio and concrete strength grade, the crack bending moment(Mcr)and ultimate bending moment(Mu) of CARCB gradually increased. The characteristics of CARCB and OPRCB are basically the same. Furthermore, through increasing the concrete strength grade and reinforcement ratio, Mcr/Mu could be increased to delay the cracking of CARCB. As the load increased, crack width(w) would also increase. At the beginning of the loading, w increased slowly. And then it increased rapidly when the load reached to the ultimate load, which then led to beam failure. Meanwhile, with a comprehensive consideration of the effects of steel corrosion on the loss of steel section and the decrease of steel yield strength, a more reasonable calculation model for the flexural capacity of CARCB was proposed.展开更多
To study the flexural behavior and calculation model,8 coral aggregate concrete(CAC)beams with different types of steel were designed.The flexural behavior of CAC beam was tested.The failure mode,bearing capacity,the ...To study the flexural behavior and calculation model,8 coral aggregate concrete(CAC)beams with different types of steel were designed.The flexural behavior of CAC beam was tested.The failure mode,bearing capacity,the maximum crack width(ws)and average crack spacing(lm)were studied.A calculation model for the bearing capacity of CAC beam was proposed.The results indicated that with the steel strength increased,the cracking moment(Mcr)and ultimate moment(Mu)of CAC beam increased,and the development of the ws gradually slowed,which effectively inhibited the formation of cracks and improved the flexural behavior of CAC beam.For CAC structures in the ocean engineering,it is recommended to use organic new coated steel to extend its effective service life.In addition,considering the influence of steel corrosion,a calculation model for the Mcr,Mu,lm and ws of CAC beam was established.展开更多
In the past 20 years,recycled aggregate concrete(RAC),as a type of low-carbon concrete,has become a worldwide focus of research.However,the design methodology for RAC structural components remains a challenge.Conseque...In the past 20 years,recycled aggregate concrete(RAC),as a type of low-carbon concrete,has become a worldwide focus of research.However,the design methodology for RAC structural components remains a challenge.Consequently,demands for a unified design of natural aggregate concrete(NAC)and RAC components have been presented.Accordingly,this study analyses the necessity of a unified design theory and provides an in-depth demonstration of the strength determination,compressive constitutive relationship,and design method of concrete components.The coefficient of variation of RAC strength is found to be generally higher than that of NAC strength.The compressive and tensile strengths of RAC can be defined and determined using the same method as that used for NAC.The uniaxial compressive constitutive relationship between NAC and RAC has a unified mathematical expression.However,the elastic modulus of RAC decreases,and its brittleness exhibits an increasing trend compared with that of NAC.Finally,to unify the design formulae of RAC and NAC components for bearing capacity,modification factors for RAC components are proposed considering safety and reliability.Additionally,the feasibility of the proposed unified time-dependent design theory is demonstrated in terms of conceptual design and structural measures considering the effects of strength degradation and reinforcement corrosion.It is believed that this study enriches and develops the basic theory of concrete structures.展开更多
基金Financial support from the National Natural Science Foundation of China(Contract No.897201143)。
文摘Recycled concrete aggregate(RCA)derived from demolition waste has been widely explored for use in civil engineering applications.One of the promising strategies globally is to incorporate RCA into concrete products.However,the use of RCA in high-performance concrete,such as self-consolidating concrete(SCC),has only been studied in the past decade.This paper summarizes recent publications on the use of coarse and/or fine RCA in SCC.As expected,the high-water absorption and porous structure of RCA have posed challenges in producing a high-fluidity mixture.According to an analysis of published data,a lower strength reduction(within 23%regardless of coarse RCA content)is observed in SCC compared with vibrated concrete,possibly due to the higher paste content in the SCC matrix,which enhances the weak surface layer of RCA and interfacial transition zone.Similarly,SCC tends to become less durable with RCA substitution although the deterioration can be minimized by using treated RCA through removing or strengthening the adhered mortar.To date,the information reported on the role of RCA in the long-term performance of SCC is still limited;thus,a wide range of research is needed to demonstrate the feasibility of RCA–SCC in field applications.
基金Funded by Joint Funds of the National Natural Science Foundation of China (No.U1904188)Key R&D and Promotion Projects in Henan Province,China (No.212102310288)the Key Science and Technology Program of Henan Province,China (No.202102310253)。
文摘Due to the presence of old mortar (OM) and interfacial transition zone (ITZ),recycled concrete aggregate (RCA) is inferior to natural aggregate (NA).The purpose of this paper was to study the effect of accelerated carbonation on the macro-properties and micro-properties of RCA under different pressure(0.05,0.15,0.30 MPa).The macro-property tests included colour change,apparent density,water absorption,and crushing value of RCA.The micro-property tests included scanning electron microscopy (SEM),X-ray diffraction (XRD),thermogravimetry-differential scanning calorimetry (TG-DSC),and Vickers micro-hardness(VMH).The results showed that the change trends of apparent density,water absorption,and crushing value of RCA displayed exponential relationships as pressure increasing,with the optimum pressure of 0.30 MPa.SEM images indicated that the calcite caused by the hydration products in RCA and the Ca(OH)_(2) derived from saturated lime water improved the properties of RCA;as the apparent density increased,the water absorption and crushing value decreased.The results of XRD and TG-DSC indicated that,as the pressure increased,the masses of Ca(OH)_(2) in carbonated RCA gradually decreased,while those of CaCO_(3) gradually increased,which demonstrated that the carbonation degree gradually increased.Besides,ITZ-2 was the weakest phase in RCA,but its improvement degree of VMH by accelerated carbonation was higher than that of OM.However,RCA was not completely carbonated,but only carbonated in a certain depth after 24 h accelerated carbonation.
基金the financial support of National Key R&D Program of China(No.2019YFC1906200)the National Natural Science Foundation of China(Nos.51879093,51009057 and 52108206)+1 种基金Jiangsu Science and Technology Department of China(No.BE2015706)Science and Technology Project of Nanjing Water Authority(No.201802).
文摘Due to the existence of the attached mortar,the performance of the recycled concrete aggregate(RCA)is inferior to the natural aggregate,which significantly limits its wide application in industry.In this study,five kinds of modified solutions were used to modify the surface of RCA,and the modification effects were compared.The results showed that sodium silicate,nano-silica(NS),Bacillus pasteurii and soybean powder had relatively good modification effects on RCA,which could reduce the crushing value and water absorption,and increase apparent density.The composite solution(15%sodium silicate and 2%NS)and soybean powder solution had better modification effect.The 28 d compressive strength and splitting tensile strength of recycled aggregate concrete(RAC)prepared by RCA modified by soybean powder solution were 4.6%and 5.2%higher than those prepared by RCA modified by composite solution,respectively.This indicates that among the five kinds of modified solutions,soybean powder solution has the best modification effect on RCA,and the optimal soaking time of soybean powder solution is 8 h.At this time,the crushing value,water absorption and apparent density of RCA are 12.8%,5.3%,and 2653 kg/m^(3),respectively.The research results of this study provide a reference for the modification of RCA and its efficient utilization.
基金the National Key R&D Program of China(No.2023YFC3905800)the State Key Laboratory of Subtropical Building and Urban Science in South China University of Technology(No.2022ZC01).
文摘The substantial generation of building waste had precipitated significant environmental issues in China,attributable to urban expansion.Recycled concrete aggregate(RCA)produced from building waste had been used as an alternative material in green roofs as substrates.However,the biological dynamics of RCA are unknown.Thus,the column test was performed to investigate the effects of different biochar types and application ratios on the biochemical properties and bacterial communities of RCA.Results show that the nutrients of RCA have been improved by biochar addition regardless of biochar types and application ratios.This led to the increase in plant weight and height by at least 227%and 38%in biochar amended RCA.However,biochar application reduced bacterial richness andα-diversity,while enhancing plant growth.This indicated that biochar application directly improves certain bacterial species that promote plant growth and productivity.Biochar application reduced the Proteobacteria abundance and enhanced the percentage of Acidobacteriota in RCA,indicating that biochar application shaped the bacterial communities into raw soil composition.The outcome of this research suggests that biochar application improved RCA biochemical performance as a substrate for green roofs,which provides a potential method to consume substantial RCA.
基金supported by the Ongoing Research Funding Program(Grant No.ORFT-2025-025-6)at King Saud University,Riyadh,Saudi Arabia.
文摘Accurately predicting the compressive strength of recycled aggregate concrete(RAC)incorporating supplementary cementitious materials(SCMs)remains a critical challenge due to the heterogeneous nature of recycled aggregates(RA)and the complex interactions among multiple binder constituents.This study advances the field by developing the most extensive and rigorously preprocessed database to date,which comprises 1243 RAC mixtures containing silica fume,fly ash,and ground-granulated blast-furnace slag.A hybrid,domain-informed machine-learning framework was then proposed,coupling optimized Extreme Gradient Boosting(XGBost)with civil engineering expertise to capture the complex chemical and microstructural mechanisms that govern RAC performance.Systematic grid-search optimization(n_estimators=50,learning_rate=0.2,max_depth=7)produced superior predictive accuracy(training R^(2)=0.9923,testing R^(2)=0.937;MAE=2.378 MPa;RMSE=3.591 MPa),which outperformed Extra Trees,Light Gradient Boosting,and traditional regressors.Beyond prediction,model interpretability was achieved using Shapley additive explanations and partial dependence analyses,which revealed curing age as the dominant strength driver,while water-to-binder ratio and recycled aggregate water absorption exhibited strong negative influences.Three-dimensional interaction plots further demonstrated how optimal superplasticizer dosages reduce the strength loss associated with high recycled aggregate content.In summary,this work provides a novel,explainable,and data-driven framework that achieves high predictive accuracy with mechanistic transparency and offers a powerful,interpretable tool for the design and optimization of sustainable RAC mixtures.
基金Funded by the Natural Science Foundation of Shaanxi Province(No.2023-JC-QN-0553)。
文摘To examine the influences of waste polypropylene fiber(PPF)on the strength and internal pore structure of recycled aggregate concrete incorporating iron ore tailings,both the cubic compressive strength and axial compressive strength of the concrete were measured.Additionally,the microstructure was analyzed using scanning electron microscopy.The evolution of pore structure parameters,including pore size distribution,pore type distribution,and nuclear magnetic resonance spectral area in the concrete,was investigated through nuclear magnetic resonance(NMR)analysis.A model correlating the concrete's pore structure with its macroscopic performance was subsequently developed based on fractal theory.The results demonstrate that an appropriate amount of PPF created a bridging effect that decelerated the progression of macro cracks,enhanced the ductility of the concrete's failure mode,and increased both cubic compressive strength and axial compressive strength,with the most effective dosage being approximately 0.6%.An appropriate amount of PPF(ranging from 0.3%to 0.6%)facilitated the formation of harmless pores and shifted the pore size distribution towards medium and small sizes.Specifically,a fiber content of 0.6%resulted in the most significant reduction in the T2 spectral area.Furthermore,the pore structure of concrete exhibits distinct fractal characteristics.As the PPF content increased,the fractal dimension initially rose and then declined,demonstrating a strong correlation with the mechanical properties.
基金The National Key Research and Development Programm of China(No.2018YFD1100402-05)the National Natural Science Foundation of China(No.6505000184)
文摘The influence of source concrete (SC) with different compression strengths on the workability and mechanical properties of recycled mortar made with river sand substituted by 100% fine recycled concrete aggregates (FRCA) is experimentally investigated. The basic physical performance test shows that with the increase in SC strength, FRCA exhibit lower water absorption and crushing index, meanwhile keeping higher densities. Mechanical property tests, including compressive strength, flexural strength and uniaxial compressive stress-strain tests, show that compressive strength,flexural strength and elasticity modulus of recycled sand mortars increase roughly with the increase in SC strength. The proposed mixture design method demonstrates that all of the components can be kept as the same as those in natural mortar mixture design and FRCA must be pre-wetted before making mortar mixture. Meanwhile, the reuse of higher strength SC can ensure that recycled mortar mixtures are able to achieve similar mechanical performance when compared to natural mortar designs.
基金Funded by the National Natural Science Foundation of China(No.51302104)Research Award Fund for Outstanding Young Scientists of Shandong Province(No.BS2012NJ003)Foundation of University of Jinan(No.XBS1328)
文摘Experimental investigations on mechanical property and durability of sulphoaluminate cement concrete with aggregate gradations according to Fuller distribution are presented in this paper. Compressive strength, water impermeability and resistance capability to sulfate attack of SACC have the same trend of concrete with fine aggregates of Fuller distribution gradation<concrete with coarse aggregates of Fuller distribution gradation<concrete with total aggregates of Fuller distribution gradation. The relationship between bulk density of aggregate and water penetration depth obeyed the second-order polynomial y=0.002x2-6.863 8x +5 862.3, and had a notable correlation R2=0.979 9. The sulphoaluminate cement concrete with total aggregate gradation with Fuller distribution for h=0.50 had the best resistance capability to sulfate attack. It was a second-order polynomial relationship between bulk density of aggregates and water penetration depth of y=0.002x2-6.863 8x+5 862.3 with R2=0.979 9, which indicated notable correlation. The fitting formula between bulk density of aggregates and sulfate resistance coefficient of SACC was y=0.000 5x+0.370 4 with R2=0.958 5.
文摘Large quantities of natural aggregates(NA)are required in the manufacturing of asphalt mixtures for pavements.The extraction of NA generates high environmental impacts that negatively affect environmental preservation and conservation.Diverse alternative materials obtained in construction and demolition processes in civil construction worked have been studies as possible replacements for NA,with the purpose of reducing these environmental impacts.The foregoing study presents a state of knowledge review referring to the use of recycled concrete aggregates(RCA)when these are used in the manufacturing of asphalt mixtures(mainly hot-mix asphalt HMA type).Environmental aspects are presented,as well as possible benefits and limitations of using RCA as replacements for NA in asphalt mixtures.However,it is not possible to establish a behavior pattern,since the characteristics of the RCA are very heterogeneous and vary from the original source of the concrete.It is highlighted from the literature that RCA has a wide potential for use in the manufacture of asphalt mixtures,since in most studies,the mixtures with these materials comply the quality requirements contemplated by the construction specifications,mainly in low-volume roads.Additionally,based on the literature review,some recommendations and suggestions are presented for future research.
基金This work was supported in part by the National Natural Science Foundation of China(Grant No.11601462)the Hunan Provincial Natural Science Foundation of China(Grant No.14JJ2063)the Scientific Research Fund of Hunan Provincial Education Department(Grant No.15A183).
文摘The concrete aggregate model is considered as a type of weakly discontinuous problem consisting of three phases:aggregates which randomly distributed in different shapes,cement paste and internal transition zone(ITZ).Because of different shapes of aggregate and thin ITZs,a huge number of elements are often used in the finite element(FEM)analysis.In order to ensure the accuracy of the numerical solutions near the interfaces,we need to use higher-order elements.The widely used FEM softwares such as ANSYS and ABAQUS all provide the option of quadratic elements.However,they have much higher computational complexity than the linear elements.The corresponding coefficient matrix of the system of equations is a highly ill-conditioned matrix due to the large difference between three phase materials,and the convergence rate of the commonly used solving methods will deteriorate.In this paper,two types of simple and efficient preconditioners are proposed for the system of equations of the concrete aggregate models on unstructured triangle meshes by using the resulting hierarchical structure and the properties of the diagonal block matrices.The main computational cost of these preconditioners is how to efficiently solve the system of equations by using linear elements,and thus we can provide some efficient and robust solvers by calling the existing geometric-based algebraic multigrid(GAMG)methods.Since the hierarchical basis functions are used,we need not present those algebraic criterions to judge the relationships between the unknown variables and the geometric node types,and the grid transfer operators are also trivial.This makes it easy to find the linear element matrix derived directly from the fine level matrix,and thus the overall efficiency is greatly improved.The numerical results have verified the efficiency of the resulting preconditioned conjugate gradient(PCG)methods which are applied to the solution of several typical aggregate models.
基金financial support provided by the National Key Research and Development Program of China(2019YFC1906203).
文摘Microbially induced carbonate precipitation(MICP)is a promising technique for the autonomous healing of concrete cracks.In this study,the effect of pH on MICP was investigated.The results indicate that the MICP process was inhibited when the pH was higher than 11.Both vaterite and calcite were produced when the pH was<8,whereas only calcite was produced when the pH was>8.Recycled concrete aggregates(RCA)coated with sodium silicate have been proposed as protective carriers for microbial healing agents.Although the presence of the coated RCA resulted in a loss of the splitting tension strength of the concrete,the loaded healing agents were highly efficient in self-healing cracks.Concrete incorporated with 20%RCA loaded with healing agents exhibited the best self-healing performance.When the initial crack widths were between 0.3 and 0.4 mm,the 7-d mean healing rate was approximately 90%.At 28 d,the crack area filling ratio was 86.4%,while its water tightness recovery ratio was 74.4%and 29.8%,respectively,for rapid and slow absorption.This study suggests that RCA coated with sodium silicate is an effective method for packaging microbial healing agents and has great potential for developing cost-effective self-healing concrete.
文摘This article presents freeze-thaw(F/T)resistance of concrete containing recycled concrete aggregate(RCA).RCA percentages of 15%,30%,45%,and 60%were replaced by natural aggregate.Air entraining admixture(AEA)with three different ratios(0.06%,0.13%,and 0.20%)were used and three different maximum aggregate particle sizes(dmax=16 mm,22.4 mm and 31.5 mm)were selected.F/T resistances of the concretes were determined through the measurement of scaling on the surfaces of the samples placed in 3%NaCl solution in pursuance of TS CEN/TS 12390-9.It was observed that the compressive strength of the concretes decreased by 20%on average compared to the control samples with the increase in the amount of RCA.However,it was determined from the experiments that the use of RCA increases the F/T resistances of the concretes as it provides extra voids in the concrete and decreases capillary permeability.Within the limits of this study,d_(max)=22.4 mm,RCA=45%and AEA=0.13%are recommended as the optimum values for F/T resistance of the concrete.It is also recommended to use RCA in pavement applications including roadways,highways,parking facilities,and other non-structural concrete elements subjected to severe winter conditions.
文摘The possibility of the use of recycled aggregates from the construction industry in green concrete production is of increasing importance to reduce the negative environmental impact associated with construction and demolition wastes.The objective of this study is to investigate the effect of recycled concrete aggregate(RCA)quality on the properties of hardened concrete properties such as compressive strength,splitting tensile strength,density,water absorption capacity and porosity accessible to water.The RCA used in this study was obtained from the crushing of waste concrete with two different compressive strengths(LRCA obtained from the crushing of waste concrete having compressive strengths below 30 MPa and HRCA obtained from the crushing of waste concrete having compressive strengths above 30 MPa).The natural coarse limestone aggregate was 100%replaced with coarse LRCA and HRCA.As a result of the study,the use of 100%HRCA and%100 LRCA instead of limestone coarse aggregate in the concrete adversely affected its mechanical and physical properties.In addition,HRCA showed better performance in terms of compressive strength,tensile strength,water absorption and porosity compared to the use of LRCA.Furthermore,the percentage of adhered mortar on the surface of LRCA and HRCA was analyzed using a computerized micro tomography device,and it was found that the percentages of attached mortar and aggregates are 61%and 35.5%for LRCA,whilst the attached mortar and aggregate contents for HRCA are 45.9%and 53.7%,respectively.
基金Funded by the Environment and Conservation Fund, the WooWheelock Green Fund andthe Hong Kong Polytechnic University
文摘The effects of fly ash on the compressive strength, pore size distribution and chloride-ion penetration of recycled aggregate concrete were investigated. Two series of concrete mb：tures were prepared. The concrete mixtures in series I had a water-to-binder ratio and a cement content of 0.55 and 410 kg/ m^3 , respectively. The concrete rnixtures in series II had a water-to-binder ratio and a cement content of 0.45 and 400 kg/ ml respectively. Recycled aggregate was ased as 20% , 50% , and 100% replacements of natural coarse aggregate in the concrete mixtures in both series. In cutdition, fly ash was used as 0% , 25% and 35% by weight replacements of cement. The results show that the compressive strengths of the concrete decreased as the recycled aggregate and the fly ash contents increased. The total porosity and average porosity diameter of the concrete increased us the recycled aggregate content increased. Furtherrruore , an increase in the recycled aggregate content decreased the resistance to chloride ion penetration. Nevertheless, the replacement of cement by 25% fly ush improved the resistance to chloride ion penetration and pore diameters and reduced the total porosity of the recycled aggregate concrete.
基金Funded by the National Key Research and Development Program of China during the“13th Five-Year Plan”(No.2018 YFD1101001)。
文摘The characteristics of surface appearances,mass loss,relative dynamic modulus of elasticity and strength loss of different recycled aggregate concretes(RAC) exposed to freeze-thaw cycles were analyzed.It was found that the freeze-thaw resistance of RAC could be determined by the recycled aggregate compositions and admixtures.Both the saturation degree and the air void structure were the key factors influencing the freeze-thaw damage on concrete.Some major proposed freeze-thaw deterioration mechanisms were utilized to interpret the freeze-thaw damage on recycled aggregate concrete.Meanwhile,some potential measures to enhance the freeze-thaw resistance of concrete were summarized and discussed.
基金Funded by the Western Transportation Construction of Communication Ministry (No. 200331882008)
文摘Structure characteristics of three kinds of ceramsite with different water absorption and the influence on microstructure of interfacial zone as well as performance of chloride permeabil-ity and frost resistance of combined aggregate concrete were investigated. The results show that, dense shell and closed internal pore have sharp effects on lowering water absorption of ceramsite. However, the ceramsite with high water absorption has obvious effect on the densification of interfa-cial paste which would develop a structure with lower porosity, finer aperture and higher microhard-ness. Furthermore, the impermeability and frost-resistance of concrete can be improved due to the ef-fect of water absorption and releasing by ceramsite with higher water absorption.
基金Project (No. 50679007) supported by the National Natural Science Foundation of China
文摘Lightweight aggregate concrete cube specimens (100 mm×100 mm×100 mm) and plate specimens (100 mm×100 mm×50 mm) were tested under biaxial compression-compression (CC) and compression-tension (CT) load combinations. For comparison, normal concrete plate specimens (100 mm×100 mm×50 mm) were tested under the same load combinations. Based on the test results, a two-level strength criterion of lightweight aggregate concrete in both octahedral stress coordinate and principal stress coordinate was suggested. The lightweight aggregate concrete cube specimens (100 mm×100 mm×100 mm) were then tested under triaxial compression-compression-compression (CCC) load combination with corresponding tests on normal concrete cube specimens (100 mm×100 mm×100 mm). The effect of intermediate principal stress on triaxial compressive strength is further examined. A "plastic flow plateau" area was apparent in principal compressive stress-strain relationships of lightweight aggregate concrete but not in normal concrete. A quadratic formula was suggested for the expression of strength criterion under triaxial compression.
基金financially supported by the National Key Basic Research Development Plan of China(973 Program,Grant No.2015CB655102)the National Natural Science Foundation of China(Grant Nos.51508272 and 51678304)+2 种基金the Natural Science Foundation of Jiangsu Province(Grant No.BK20180433)the Project funded by China Postdoctoral Science Foundation(Grant No.2018M630558)the Open Research Funds for State Key Laboratory of High Performance Civil Engineering Materials(Grant No.2015CEM001)
文摘Through the flexural behavior test of coral aggregate reinforced concrete beams(CARCB) and ordinary Portland reinforced concrete beams(OPRCB), and based on the parameters of concrete types, concrete strength grades and reinforcement ratios, the crack development, failure mode, midspan deflection and flexural capacity were studied, the relationships of bending moment-midspan deflection, load-longitudinal tensile reinforcement strain, load-maximum crack width were established, and a calculation model for the flexural capacity of CARCB was suggested. The results showed that with the increase in the reinforcement ratio and concrete strength grade, the crack bending moment(Mcr)and ultimate bending moment(Mu) of CARCB gradually increased. The characteristics of CARCB and OPRCB are basically the same. Furthermore, through increasing the concrete strength grade and reinforcement ratio, Mcr/Mu could be increased to delay the cracking of CARCB. As the load increased, crack width(w) would also increase. At the beginning of the loading, w increased slowly. And then it increased rapidly when the load reached to the ultimate load, which then led to beam failure. Meanwhile, with a comprehensive consideration of the effects of steel corrosion on the loss of steel section and the decrease of steel yield strength, a more reasonable calculation model for the flexural capacity of CARCB was proposed.
基金Projects(11832013,51878350)supported by the National Natural Science Foundation of ChinaProject(B200201063)supported by the Fundamental Research Funds for the Central Universities,ChinaProject(BK20180433)supported by the Natural Science Foundation of Jiangsu Province,China。
文摘To study the flexural behavior and calculation model,8 coral aggregate concrete(CAC)beams with different types of steel were designed.The flexural behavior of CAC beam was tested.The failure mode,bearing capacity,the maximum crack width(ws)and average crack spacing(lm)were studied.A calculation model for the bearing capacity of CAC beam was proposed.The results indicated that with the steel strength increased,the cracking moment(Mcr)and ultimate moment(Mu)of CAC beam increased,and the development of the ws gradually slowed,which effectively inhibited the formation of cracks and improved the flexural behavior of CAC beam.For CAC structures in the ocean engineering,it is recommended to use organic new coated steel to extend its effective service life.In addition,considering the influence of steel corrosion,a calculation model for the Mcr,Mu,lm and ws of CAC beam was established.
基金the financial support from the Distinguished Young Scholars of China by the National Natural Science Foundation of China(51325802)the National Natural Science Foundation of China(51178340,52078358,and 52008304)。
文摘In the past 20 years,recycled aggregate concrete(RAC),as a type of low-carbon concrete,has become a worldwide focus of research.However,the design methodology for RAC structural components remains a challenge.Consequently,demands for a unified design of natural aggregate concrete(NAC)and RAC components have been presented.Accordingly,this study analyses the necessity of a unified design theory and provides an in-depth demonstration of the strength determination,compressive constitutive relationship,and design method of concrete components.The coefficient of variation of RAC strength is found to be generally higher than that of NAC strength.The compressive and tensile strengths of RAC can be defined and determined using the same method as that used for NAC.The uniaxial compressive constitutive relationship between NAC and RAC has a unified mathematical expression.However,the elastic modulus of RAC decreases,and its brittleness exhibits an increasing trend compared with that of NAC.Finally,to unify the design formulae of RAC and NAC components for bearing capacity,modification factors for RAC components are proposed considering safety and reliability.Additionally,the feasibility of the proposed unified time-dependent design theory is demonstrated in terms of conceptual design and structural measures considering the effects of strength degradation and reinforcement corrosion.It is believed that this study enriches and develops the basic theory of concrete structures.
