Influence of Recycled Concrete Fine Powder on Durability of Cement Mortar

In this paper,the durability of cement mortar prepared with a recycled-concrete fine powder(RFP)was examined;including the analysis of a variety of aspects,such as the carbonization,sulfate attack and chloride ion erosion resistance.The results indicate that the influence of RFP on these three aspects is different.The carbonization depth after 30 days and the chloride diffusion coefficient of mortar containing 10%RFP decreased by 13.3%and 28.19%.With a further increase in the RFP content,interconnected pores formed between the RFP particles,leading to an acceleration of the penetration rate of CO_(2)and Cl^(−).When the RFP content was less than 50%,the corrosion resistance coefficient of the compressive strength of the mortar was 0.84-1.05 after 90 days of sulfate attack.But the expansion and cracking of the mortar was effectively alleviated due to decrease of the gypsum production.Scanning electron microscope(SEM)analysis has confirmed that 10%RFP contributes to the formation of a dense microstructure in the cement mortar.

Research and Application of Recycled Concrete Technology in Prefabricated Buildings

The utilization of waste concrete as a raw material for recycled concrete in the domain of prefabricated components is garnering greater interest.This paper delineates and examines the concept,categorization,methodologies of preparation,applicable sectors,and evaluative metrics of recycled concrete technology,highlighting its prospective benefits.Nonetheless,for the successful integration of recycled concrete technology into prefabricated component applications,it is imperative to systematically enhance its physical,mechanical,and attributes,as well as its environmental efficacy.Moreover,to foster the continued advancement of recycled concrete technology,innovative initiatives,standardization,educational programs,demonstration projects,and collaborative efforts are crucial to promote broader adoption and realize improved outcomes within the realm of prefabricated components.In conclusion,recycled concrete technology is poised to play a pivotal role in prefabricated construction,offering robust support for propelling the construction industry towards a sustainable future.

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.

Analysis of a landfill cover without geomembrane using varied particle sizes of recycled concrete

Previous studies have demonstrated the effectiveness of a novel three-layer landfill cover system constructed with recycled concrete aggregates(RCAs)without geomembrane in both laboratory and field.However,no systematic investigation has been carried out to optimize the combination of the particle sizes for fine-grained RCAs(FRC)and coarse-grained RCAs(CRC)that can be used for the three-layer landfill cover system.The aim of this paper is to assist engineers in designing the three-layer landfill cover system under a rainfall of 100-year return period in humid climate conditions using an easily controlled soil parameter D10 of RCAs.The numerical study reveals that when D10 of FRC increases from 0.05 mm to 0.16 mm,its saturated permeability increases by 10 times.As a result,a larger amount of rainwater infiltrates into the cover system,causing a higher lateral diversion in both the top FRC and middle CRC layers.No further changes in the lateral diversion are observed when the D10 value of FRC is larger than 0.16 mm.Both the particle sizes of FRC and CRC layers are shown to have a minor influence on the percolation under the extreme rainfall event.This implies that the selection of particle sizes for the FRC and CRC layers can be based on the availability of materials.Although it is well known that the bottom layer of the cover system should be constructed with very fine-grained soils if possible,this study provides an upper limit to the particle size that can be used in the bottom layer(D10 not larger than 0.02 mm).With this limit,the three-layer system can still minimize the water percolation to meet the design criterion(30 mm/yr)even under a 100-year return period of rainfall in humid climates.

Research Progress on the Influence of Varying Fiber Contents on Mechanical Properties of Recycled Concrete

Applying recycled concrete for engineered projects not only protects the ecological environment but also improves the utilization rate of waste concrete to satisfy sustainable development requirements.However,the mechanical properties of recycled concrete are not as good as those of ordinary concrete.To enhance the former’s performance and increase its popularity and application in engineeringfields,notable advances have been made by using steel,synthetic,plant,and mineralfiber materials.These materials are added to recycled concrete to improve its mechanical properties.Studies have shown that(1)steelfibers have a distinct reinforcing effect and improve the strength,toughness,and elastic modulus of recycled concrete;(2)the addition of syntheticfibers can improve the tension,crack resistance,and durability of concrete,but the size effect needs to be further explored and elaborated;(3)plantfiber concrete is lightweight and environmentally friendly and provides high toughness and good thermal insulation,but thefibers corrode in alkaline environments;in addition,plantfibers have high water absorption capacity,which leads to wet expansion and dry shrinkage phenomena,which need to be further studied;and(4)the cost of basaltfiber,a mineralfiber,is relatively low,and a suitable basalt content can improve the mechanical properties of recycled concrete to a certain extent.

Shaking table experimental study of recycled concrete frame-shear wall structures

In this study, four 1/5 scaled shaking table tests were conducted to investigate the seismic performance of recycled concrete frame-shear wall structures with different recycled aggregates replacement rates and concealed bracing detail. The four tested structures included one normal concrete model, one recycled coarse aggregate concrete model, and two recycled coarse and fi ne aggregate concrete models with or without concealed bracings inside the shear walls. The dynamic characteristics, dynamic response and failure mode of each model were compared and analyzed. Finite element models were also developed and nonlinear time-history response analysis was conducted. The test and analysis results show that the seismic performance of the recycled coarse aggregate concrete frame-shear wall structure is slightly worse than the normal concrete structure. The seismic resistance capacity of the recycled concrete frame-shear wall structure can be greatly improved by setting up concealed bracings inside the walls. With appropriate design, the recycled coarse aggregate concrete frame-shear wall structure and recycled concrete structure with concealed bracings inside the walls can be applied in buildings.

Damage assessment for seismic response of recycled concrete filled steel tube columns

A model for evaluating structural damage of recycled aggregate concrete filled steel tube （RCFST） columns under seismic effects is proposed in this paper. The proposed model takes the lateral deformation and the effect of repeated cyclic loading into account. Available test results were collected and utilized to calibrate the parameters of the proposed model. A seismic test for six RCFST columns was also performed to validate the proposed damage assessment model. The main test parameters were the recycled coarse aggregate （RCA） replacement percentage and the bond-slip property. The test results indicated that the seismic performance of the RCFST member depends on the RCA contents and their damage index increases as the RCA replacement percentage increases. It is also indicated that the damage degree of RCFST changes with the variation of the RCA replacement percentage. Finally, comparisons between the RCA contents, lateral deformation ratio and damage degree were implemented. It is suggested that an improvement procedure should be implemented in order to compensate for the performance difference between the RCFST and normal concrete filled steel tubes （CFST）.

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.

Study on Carbonation Damage Constitutive Curve and Microscopic Damage Mechanism of Tailing Recycled Concrete

To improve the resource utilization of recycled aggregate concrete(RAC)and make use of the unique pozzolanic activation characteristics of iron ore tailing(IOT),the constitutive curves of tailing recycled concrete(TRC)before and after carbonization were analyzed theoretically,experimentally and microscopically.Firstly,according to the experimental data,the damage constitutive and related damage parameters of TRC were theoretically established by Weibull probability distribution function.Secondly,the comprehensive damage parameter b under different working conditions was studied.Finally,the damage mechanism was formed by EDS and SEM.The results showed that the damage constitutive model based on Weibull probability distribution function was in good agreement with the experimental results.Under each carbonization period,the b first decreased and then rose with the increase of tailings content.When its content was 30%,the b values of TRC were minimized,which were 22.14%,20.99%,25.39%lower than those of NAC,and 41.09%,34.89%,35.44%lower than those of RAC,indicating that IOT had a relatively good optimization effect on the constitutive curve of RAC.The microscopic analysis results also proved that the IOT addition with a proper amount would improve the matrix structure of RAC and increased its compactness,but when the content was higher,it would also cause harmful cracks in its matrix structure and reduced its density.Therefore,the optimal tailing content was about 30%.This paper provided a new method for damage constitutive calculation and analysis of TRC before and after carbonization.

Basic Mechanical Properties and Microstructural Analysis of Recycled Concrete

The regeneration aggregate, natural aggregate, POO42.5 R Portland cement, coal fly ash, and slag powders S95 graining blast furnace, homemade P（AA-co-MA）/PEG carboxylic acid water reducing agent, were used together with recycled concrete aggregate in different regeneration rates to prepare recycled concrete （RC）. The influences of different renewable aggregate ratios on the basic RC replacement mechanical properties, uniaxial compression stress and strain curve, and the elastic modulus and rebound value were investigated.The results show that RC mechanical properties decreases with renewable aggregate replacement rate increasing. The prolongation can reduce the reduced span.

Experimental Study of Post-heated Steel Reinforced Recycled Concrete Columns Repaired with CFRP

The mechanical and thermal properties of steel reinforced concrete columns with CFRP reinforcement were examined after exposure to a high temperature of 500℃. The concrete made with normal and recycled coarse aggregate（RCA） was fabricated and three different RCA replacement ratios（0, 50%, and 100%） were investigated. The fatigue properties of steel reinforced concrete with RCA and CFRP reinforcement were tested for two million cycles at a frequency of 2.5 Hz. The test results show that the failure of strengthened specimens is mainly caused by rupture of CFRP jacket and buckling of inner section steel reinforcement. However, for the unstrengthened specimen, both of inner steel buckling and core concrete cracking are the main contributors to the damage. The load-bearing capacity, deformation and energy dissipation of the specimens during the fatigue test could be strengthened greatly by CFRP reinforcement. However, the CFRP reinforcement has little influence on the improvement of the stiffness of the specimens, which may be caused by a plastic damage accumulation during the early cycles of fatigue tests. Finally, a static test was conducted on the postfatigue specimens, the results showed that a large decrease in stiffness was observed for the specimens subjected to high temperature and fatigue, and the fatigue loading had a higher influence on the specimens than the high temperature.

Utilization of Recycled Concrete Powder in Cement Composite:Strength,Microstructure and Hydration Characteristics

Recycled concrete powder(RCP)is used more and more in cement-based materials,but its influence on the hydration process is still unclear.Therefore,this paper studied the influence of recycled concrete powder(RCP)on the hydration process of cement and provides a theoretical basis for the hydration mechanism of cement composite materials.The hydration heat method was used to systematically analyze the thermal evolution process of cement paste with or without RCP.Hydration products were identified using X-ray diffraction(XRD)and thermal analysis(TG–DSC).The pore structure change of cement pastes was analyzed by mercury intrusion porosimetry(MIP)method.The mechanical properties of mortar were also evaluated.Four recycled concrete powder(RCP)dosages,such as 10%,20%,30%and 40%are considered.The results indicate that with the increase of RCP content,the hydration heat release rate and total heat release amount of paste decreased,but the second heat release peak of hydration reaction advanced;the proportion of harmful pores and more harmful pores increases,the total porosity and the most probable pore size also increase;the fluidity and mechanical strength of mortar decrease,but the crystal type of hydration products does not change.When the content of RCP is less than 20%,it has little effect on the mechanical strength of mortar.When fly ash and silica fume are mixed,the fluidity difference of mortar decreases,and when the content of fly ash is the highest,the fluidity of mortar is the highest,which is 15mm higher than that of the control group.When RCP content is 15%,fly ash and silica fume content is 15%(FA:SF=3:2),the hydration heat of the clean pulp is the highest among all the compounding ratios,and the hydration reaction is the most complete;the proportion of harmless pores increased by 9.672%,the proportion of harmful pores and more harmful pores decreased,and the compactness of material structure increased;the compressive strength and flexural strength of mortar reached 50.6 MPa and 9 MPa respectively,both exceeding those of control mortar.

Evaluation of Various Modification Methods for Enhancing the Performance of Recycled Concrete Aggregate

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.

Developing Sustainable High Strength Concrete Mixtures Using Local Materials and Recycled Concrete

This study presents the development of high strength concrete (HSC) that has been made more sustainable by using both local materials from central Texas and recycled concrete aggregate (RCA), which has also been obtained locally. The developed mixtures were proportioned with local constituents to increase the sustainable impact of the material by reducing emissions due to shipping as well as to make HSC more affordable to a wider variety of applications. The specific constituents were: limestone, dolomite, manufactured sand (limestone), locally available Type I/II cement, silica fume, and recycled concrete aggregate, which was obtained from a local recycler which obtains their product from local demolition. Multiple variables were investigated, such as the aggregate type and size, concrete age (7, 14, and 28-days), the curing regimen, and the water-to-cement ratio (w/c) to optimize a HSC mixture that used local materials. This systematic development revealed that heat curing the specimens in a water bath at 50℃ (122oF) after demolding and then dry curing at 200℃ (392oF) two days before testing with a w/c of 0.28 at 28-days produced the highest compressive strengths. Once an optimum HSC mixture was identified a partial replacement of the coarse aggregate with RCA was completed at 10%, 20%, and 30%. The results showed a loss in compressive strength with an increase in RCA replacement percentages, with the highest strength being approximately 93.0 MPa (13,484 psi) at 28-days for the 10% RCA replacement. The lowest strength obtained from an RCA-HSC mixture was approximately 72.9 (MPa) (10,576 psi) at 7-days. The compressive strengths obtained from the HSC mixtures containing RCA developed in this study are comparable to HSC strengths presented in the literature. Developing this innovative material with local materials and RCA ultimately produces a novel sustainable construction material, reduces the costs, and produces mechanical performance similar to prepackaged, commercially, available construction building materials.

The Behavior of Recycled Concrete through the Application of an Isotropic Damage Model

The main goal of this paper is to describe the mechanical behavior of the CDW recycled concrete in compression, using an isotropic damage model adapted to the variation of the replacement rate of natural aggregates by recycled ones. The isotropic model by Mazars was used as a constitutive equation for the CDW concrete and its adjustment parameters, A and B, were written as quadratic polynomials according to the aggregates replacement rate. The model was evaluated for conventional and recycled concretes. For the latter ones, the aggregates replacement ratios evaluated were 50% and 100%. The results show good approximation between the analytical and numerical values obtained with the adapted isotropic damage model and experimental concrete results for both compressive and flexural strength.

Bond Behavior between Recycled Concrete Containing CDW and Different Types of Steel Bars

The operation of reinforced concrete structures is directly associated with the adhesion between the steel bar and the concrete,which allows the inter­nal forces to be transferred to the reinforcement during the process of load­ing the structural elements.The modification of the concrete composition,with the introduction of recycled aggregate from construction and demoli­tion waste(CDW),affects the steel-concrete interface and can modify the bonding stress,which is also influenced by the type and diameter of the bar used.In this work,the influence of the recycled fine aggregate(RFA)and types of steel bar on the steel-concrete bond was experimentally evaluated using the pullout test.Conventional concrete and recycled concrete,with RFA replacement level of 25%,were produced.Two types of steel rebars(i.e.,plain and deformed)with diameters of 10.0 and 16.0 mm were con­sidered in this paper.The results indicate a reduction in the adhesion stress with the introduction of recycled aggregate,but this trend is influenced by the diameter of the bar used.The use of ribbed bars modifies the stress bon-slip behavior,with an increase in the average bond strength,which is also observed with the reduction of the diameter of the bar.

Experimental and Theoretical Study on the Flexural Behavior of Recycled Concrete Beams Reinforced with GFRP Bars

This paper experimentally investigated the flexural behavior of reinforced recycled aggregate concrete(RAC)beams reinforced with glass fiber-reinforced polymer(GFRP)bars.A total of twelve beams were built and tested up to failure under four-point bending.The main parameters were reinforcement ratio(0.38%,0.60%,and 1.17%),recycled aggregate replacement ratio(R=0,50%,and 100%)and longitudinal reinforcement types(GFRP and steel).The flexural capacity,failure modes,flexibility deformation,reinforcement strains and crack distribution of the tested beams were investigated and compared with the calculation models of American code ACI 440.1-R-15,Canadian code CSA S806-12 and ISIS-M03-07.The tested results indicated that the reinforcement ratio has great influence on the ultimate load,crack width and deflection of GFRP-RAC beams,the recycled aggregate replacement ratio has little influence on it.However,it was found that the reinforcement ratio has no obvious influence on the cracking load which was only related to the recycled aggregate replacement ratio.The average cracking load decreased by 5%and 15%as the recycled aggregate replacement ratio increased from 0 to 50%and 100%.For the steel-RAC beams,the ultimate load was found to be about 1/2 of the ultimate load of GFRP-RAC beam under the same condition and the trend of strain,deflection and crack width were different from GFRP-RAC beams.This is due to the different material properties of GFRP bars and steel rebar.On the other hand,the calculation results showed that ACI 440.1-R-15 and CSA S806-12 underestimated the ultimate load of GFRP-RAC beams.Moreover,the deflection prediction of GFRP-RAC beams by CSA S806-12 is relatively accurate compared with ACI 440.1-R-15 and ISIS-M03-07.As for the prediction of crack width,the results of ACI 440.1-R-15 prediction were in good agreement with the experimental results at the ultimate load,with the average value of 1.09±0.28.

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. .