Progress in developing self-consolidating concrete(SCC)constituting recycled concrete aggregates:A review

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.

Microbial-inspired self-healing of concrete cracks by sodium silicate-coated recycled concrete aggregates served as bacterial carrier

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.

Effect of Modification Treatment on Chloride Ions Permeability and Microstructure of Recycled Brick-mixed Aggregate Concrete

The modification methods of pozzolan slurry combined with sodium silicate and silicon-based additive were respectively adopted to treat recycled coarse brick-mixed aggregate(RCBA)in this study.The compressive strength and chloride permeability resistance of recycled aggregate concrete(RAC)before and after modification treatment were tested,and the microstructure of RAC was analyzed by mercury intrusion porosimetry(MIP)and scanning electron microscopy(SEM).The results show that the physical properties of RCBA strengthened by modification treatment are improved,and the compressive strength and chloride permeability resistance of treated RAC are also significantly improved.The modification treatment optimizes the pore size distribution of RAC,which increases the number of gel pores and transition pores,and decreases the number of capillary pores and macro pores.The surface fractal dimension shows a significant correlation with chloride diffusion coefficient,indicating that the variation of chloride permeability of treated RAC is consistent with the microstructure evolution.

Three-Dimensional Multi-Phase Microscopic Simulation of Service Life of Recycled Large Aggregate Self-Compacting Concrete

Recycled large aggregate self-compacting concrete (RLA-SCC) within multiple weak areas. These weak areas have poor resistance to chloride ion erosion, which affects the service life of RLA-SCC in the marine environment. A three-dimensional multi-phase mesoscopic numerical model of RLA-SCC was established to simulate the chloride ions transportation in concrete. Experiments of RLA-SCC immersing in chloride solution were carried out to verify the simulation results. The effects of recycled large aggregate (RLA) content and RLA particle size on the service life of concrete were explored. The results indicate that the mesoscopic numerical simulation results are in good agreement with the experimental results. At the same depth, the closer to the surface of the RLA, the greater the chloride ion concentration. The service life of RLA-SCC in marine environment decreases with the increase of RLA content. Compared with the service life of 20% content, the service life of 25% and 30% content decreased by 20% and 42% respectively. Increasing the particle size of RLA can effectively improve the service life of RLA-SCC in chloride environment. Compared with the service life of 50 mm particle size, the service life of 70 mm and 90 mm increased by 61% and 163%, respectively. .

Use of recycled concrete aggregates in asphalt mixtures for pavements:A review

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.

Multi-Scale Investigation on Concrete Prepared with Recycled Aggregates from Different Parent Concrete

Recycled aggregates(RA)are frequently obtained from various unknown sources,which caused variation in properties among recycled aggregates concrete(RAC).This paper investigated the macro and microscopic properties of RAC prepared with RAs originated from different parent concretes with 90-day strength ranging from 30 MPa to 100 MPa.These parent concretes were prepared in advance and crushed to produce RA of distinct qualities.The attached mortar content can reach up to 69%in the concrete with highest strength grade.The microscopic investigation on different RAC was conducted with the X-ray Micro-Computed Tomography scanning technique and image process.Experimental results showed that the properties of RA derived from various parent concrete varied because of the amount of residual mortar.The development of mechanical properties and chloride-ion penetration of RACs all followed a binomial trend with the strength grades of the parent concrete because of the different quantity and quality of RAs.

The Compression-deformation Behaviour of Concrete with Various Modified Recycled Aggregates

Modified recycled aggregates were prepared with three different cement-admixture grouts. The physical properties, such as water absorption, apparent density, crushing index, slump and compressive strength of the recycled aggregate and the recycled concretes were tested, and the tests for the compression-deformation behavior of the concretes were also performed. The experimental results show that the cement-Kim powder grout is satisfied for enhancing the recycled concrete, and the modification of the recycled aggregate with the grouts can improve the toughness and the deformation ability of the concretes.

Fundamental Issues Towards Unified Design Theory of Recycled and Natural Aggregate Concrete Components

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.

Effect of High Temperature Curing on the Frost Resistance of Recycled Aggregate Concrete and the Physical Properties of Second-Generation Recycled Coarse Aggregate under Freeze-Thaw Cycles

With the emphasis on environmental issues,the recycling of waste concrete,even recycled concrete,has become a hot spot in the field of architecture.But the repeated recycling of waste concrete used in harsh environments is still a complex problem.This paper discusses the durability and recyclability of recycled aggregate concrete(RAC)as a prefabricated material in the harsh environment,the effect of high-temperature curing(60℃,80℃,and 100℃)on the frost resistance of RAC and physical properties of the second generation recycled coarse aggregate(RCA_(2))of RAC after 300 freeze-thaw cycles were studied.The frost resistance of RAC was characterized by compressive strength,relative dynamic elastic modulus,and mass loss.As the physical properties of RCA_(2),the apparent density,water absorption,and crushing value were measured.And the SEM images of RAC after 300 freeze-thaw cycles were shown.The results indicated that the frost resistance of RAC cured at 80℃ for 7 days was comparable to that cured in the standard condition(cured for 28 days at 20℃±2℃ and 95%humidity),and the RAC cured at 100℃ was slightly worse.However,the frost resistance of RAC cured at 60℃ deteriorated seriously.The RAC cured at 80℃ for 7 days is the best.Whether after the freeze-thaw cycle or not,the RCA that curd at 60℃,80℃,and 100℃ for 7 days can also meet the requirements of Grade III RCA and be used as the aggregate of non-bearing part of prefabricated concrete components.RCA_(2) which is cured at 80℃ for 7 days had the best physical properties.

Quality Improvement of Recycled Concrete Aggregate by Accelerated Carbonation under Different Pressure

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.

Evaluation of Corrosion Degradation Law of Recycled Reinforced Concrete in Saline Soil Under Electrified Environment

In order to investigate the corrosion mechanism of recycled reinforced concrete (RRC) under harsh environments,four recycled coarse aggregate (RCA) contents were selected,and saline soil was used as an electrolyte to perform electrified accelerated corrosion experiments.The relative dynamic elastic modulus and relative corrosion current density were considered to describe the deterioration law of the RRC in saline soil.The results indicated that as the energization time increased,the corrosion current density,corrosion potential,and polarization resistance of the steel bar decreased gradually.Compared with ordinary reinforced concrete,when the RCA content was 30%,the ability of the RRC to resist corrosion was improved slightly;however,when the RCA content exceeded 30%,the corrosion resistance of the RRC deteriorated rapidly.Scanning electron microscopy revealed that for a dense RRC,less corrosion products were generated in the pores inside the concrete and on the surface of the steel bar.X-ray diffraction results indicated that SO_(4)^(2-) can generate ettringite and other corrosion products,along with volume expansion.The main corrosion products generated on the surface of the steel bars included Fe_(2)O_(3),Fe_(3)O_(4) and FeO(OH),which were the corrosion products generated by steel bars under natural environments.Therefore,using saline soil as an electrolyte is more consistent with the actual service environments of RRC.Both the relative dynamic mode and relative corrosion current density of the degradation parameters conform to the Weibull distribution;furthermore,the relative dynamic mode is more sensitive and the corresponding reliability curve can better describe the degradation law of RRC under saline soil environments.

Dynamic Impact Compressive Behavior Investigation of Sisal Fiber-reinforced Coral Seawater Concrete

Split Hopkinson pressure bar(SHPB)was used to investigate the dynamic compressive properties of sisal fiber reinforced coral aggregate concrete(SFCAC).The results showed that,with the increase of strain rate,the dynamic compressive strength,peak strain and toughness index of SFCAC are all greater than its static properties,indicating that SFCAC is a kind of rate-sensitive material.When the sisal fiber was blended,the failure mode showed obvious ductility.At high strain rates,the SFCAC without sisal fiber specimen was comminuted,and the SFCAC showed a"cracked without breaking"state.The results indicated that the sisal fiber played a significant role in reinforcing and strengthening the properties of concrete.The finite element software LS-DYNA was used to simulate two working conditions with strain rates of 78 and 101 s-1.The stressstrain curves and failure patterns obtained were in good agreement with the experimental results.

Effect of Recycled Aggregate and Slag as Substitutes for Natural Aggregate and Cement on the Properties of Concrete:A Review

Using recycled aggregate(RA)and slag instead of natural aggregate(NA)and cement can reduce greenhouse gas emissions(GHGE)and achieve effective waste recovery.In recent years,RA has been widely used to replace NA in concrete.Every year,several researchers conduct investigations on the mechanical performance and durability of recycled aggregate concrete(RAC).Due to the loose and porous material properties of RA,the mechanical properties and durability of RAC,such as strength,carbonation resistance,permeability resistance and chloride ion penetration resistance,are greatly reduced compared with natural aggregate concrete.In contrast,concrete containing slag instead of NA and cement generally improved the strength of concrete and reduced the internal porosity of materials.Herein,we discuss the effects of RA and slag on the workability,compressive strength,splitting tensile strength,ultrasonic pulse velocity(UPV)value,and elastic modulus of concrete.The relationships between the compressive strength and the splitting tensile strength,UPV value,and elastic modulus are discussed,and the optimal substitution method is proposed.In addition,various equations for calculating the compressive strength of concrete based on performance factors related to the compressive strength are summarized.

Experimental Investigation on Compressive Properties of Fiber Recycled Aggregate Concrete

This paper presents an experimental study to explore the compressive properties of fiber recycled aggregate concrete.A total of 75 specimens with the replacement rate of recycled coarse aggregate and fiber type were conducted under a uniaxial compressive test.The failure modes,stress-strain whole curves,peak stress,peak strain,and energy dissipation capacity were systematically observed and revealed.Test results indicate that steel fiber has the best modification effect on energy dissipation capacity and the toughness index of recycled concrete,corresponding to the enhancement of 81.75% and 22.90% on average.The addition of polyvinyl alcohol fiber can effectively improve the compressive strength and energy dissipation capacity of recycled aggregate concrete by 28.49% and 29.43% on average,respectively.The compressive strength and energy dissipation capacity of recycled aggregate concrete is increased by an average of 16.5% and 24.4% by incorporating carbon fiber.The energy dissipation capacity of recycled aggregate concrete is increased by an average of 13.5% with the incorporation of polypropylene fiber.However,the addition of carbon fiber results in a slight reduction of toughness by 16.97%,and the effect of polyvinyl alcohol fiber on the energy dissipation capacity is limited.Besides,with the increase in replacement rate,the compressive strength and the energy dissipation capacity of recycled coarse aggregate concrete with fiber decreased,and toughness first decreased and then increased.Finally,based on the analysis of test data,a segment-based stress-strain model of fiber recycled aggregate concrete was proposed,which shows good agreement with the test results.

Bond of Seawater Scoria Aggregate Concrete to Stainless Reinforcement

This study investigates the bond between seawater scoria aggregate concrete(SSAC)and stainless reinforcement(SR)through a series of pull-out tests.A total of 39 specimens,considering five experimental parameters—con-crete type(SSAC,ordinary concrete(OC)and seawater coral aggregate concrete(SCAC)),reinforcement type(SR,ordinary reinforcement(OR)),bond length(3,5 and 8 times bar diameter),concrete strength(C25 and C30)and concrete cover thickness(42 and 67 mm)—were prepared.The typical bond properties(failure pattern,bond strength,bond-slip curves and bond stress distribution,etc.)of seawater scoria aggregate concrete-stainless rein-forcement(SSAC-SR)specimen were systematically studied.Generally,the failure pattern changed with the con-crete type used,and the failure surface of SSAC specimen was different from that of OC specimen.SSAC enhanced the bond strength of specimen,while its effect on the deformation of SSAC-SR was negative.On aver-age,the peak slip of SSAC specimens was 20%lower while the bond strength was 6.7%higher compared to OC specimens under the similar conditions.The effects of variables on the bond strength of SSAC–SR in increasing order are concrete type,bond length,concrete strength and cover thickness.The bond-slip curve of SSAC-SR specimen consisted of micro-slipping,slipping and declining stages.It can be obtained that SSAC reduced the curve curvature of bond-slip,and the decline of curve became steep after adopting SR.The typical distribution of bond stress along bond length changed with the types of concrete and reinforcement used.Finally,a specific expression of the bond stress-slip curve considering the effects of various variables was established,which could provide a basis for the practical application of reinforced SSAC.

Compressive Performance of Fiber Reinforced Recycled Aggregate Concrete by Basalt Fiber Reinforced Polymer-Polyvinyl Chloride Composite Jackets

The development of recycled aggregate concrete(RAC)provides a new approach to limiting the waste of natural resources.In the present study,the mechanical properties and deformability of RACs were improved by adding basalt fibers(BFs)and using external restraints,such as a fiber-reinforced polymer(FRP)jacket or a PVC pipe.Samples were tested under axial compression.The results showed that RAC(50%replacement of aggregate)containing 0.2%BFs had the best mechanical properties.Using either BFs or PVC reinforcement had a slight effect on the loadbearing capacity and mode of failure.With different levels of BFs,the compressive strengths of the specimens reinforced with 1-layer and 3-layer basalt fiber reinforced polymer(BFRP)increased by 6.7%–10.5%and 16.5%–23.7%,respectively,and the ultimate strains increased by 48.5%–80.7%and 97.1%–141.1%,respectively.The peak stress of the 3-layer BFRP-PVC increased by 42.2%,and the ultimate strain improved by 131.3%,relative to the control.This reinforcement combined the high tensile strength of BFRP,which improved the post-peak behavior,and PVC,which enhanced the structural durability.In addition,to investigate the influence of the various constraints on compressive behavior,the stress-strain response was analyzed.Based on the analysis of experimental results,a peak stress-strain model and an amended ultimate stress-strain model were proposed.The models were verified as well;the result showed that the predictions from calculations are generally consistent with the experimental data(error within 10%).The results of this study provide a theoretical basis and reference for future applications of fiber-reinforced recycled concrete.

Experimental Behavior of Recycled Aggregate Concrete Filled Steel Tubular Columns

During the modernization or rehabilitation activity,the demolished structural waste causes large soil pollution and unavailability of natural aggregate is the big concern for the construction industry.Therefore,this manuscript deals with the Composite Steel Circular Column(CSCC)with Recycled Aggregate concrete(RAC)as infill is partly used,with the replacement of 25%and 50%in M30 grade of Concrete.And internal reinforcement steel is fully replaced by rolled steel tubes(circular and square)with varied thickness,ISA-unequal angle.Around 14 specimens are cast and examined under axial load for analysis of the deflection characteristics,the load-bearing capacity along with its buckling behavior.The experimental values are estimated through LVDT(linear variable differential transducer)at 3-phase.The curve of load-deflection is drawn with the load pattern.From the date interpretation,it is found column made of 50%-RAC has more than 25%-RAC.

Mechanical and Permeability Analysis and Optimization of Recycled Aggregate Pervious Concrete Based on Response Surface Method

In this paper,the effects of different influencing factors and factor interaction on the compressive strength and permeability of recycled aggregate pervious concrete(RAPC)were studied based on the response surface method(RSM).By selecting the maximum aggregate size,water cement ratio and target porosity as design variables,combined with laboratory tests and numerical analysis,the influences of three factors on the compressive strength and permeability coefficient of RAPC were revealed.The regression equation of compressive strength and permeability coefficient of recycled aggregate pervious concrete were established based on RSM,and the response surface model was optimized to determine the optimal ratio of RAPC under the conditions of meeting the mechanical and permeability properties.The results show that the mismatch item of the model is not significant,the model is credible,and the accuracy and reliability of the test are high,but the degree of uncorrelation between the test data and the model is not obvious.The sensitivity of the three factors to the compressive strength is water cement ratio>maximum coarse aggregate particle size>target porosity,and the sensitivity to the permeability coefficient is target porosity>maximum coarse aggregate particle size>water cement ratio.The absolute errors of the model prediction results and the model optimization results are 1.28 MPa and 0.19 mm/s,and the relative errors are 5.06%and 4.19%,respectively.With high accuracy,RSM can match the measured results of compressive strength and permeability coefficient of RAPC.

Compressive Strength, Pore Size Distribution and Chloride-ion Penetration of Recycled Aggregate Concrete Incorporating Class-F Fly Ash

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.

Research on Flexural Behavior of Coral Aggregate Reinforced Concrete Beams

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.