Effect of Aggregate Gradation on the Properties of 3D Printed Recycled Coarse Aggregate Concrete

A simplex centroid design method was employed to design the gradation of recycled coarse aggregate.The bulk density was measured while the specific surface area and average excess paste thickness were calculated with different gradations.The fluidity,dynamic yield stress,static yield stress,printed width,printed inclination,compressive strength and ultrasonic wave velocity of 3D printed recycled aggregate concrete(3DPRAC)were further studied.The experimental results demonstrate that,with the increase of small-sized aggregate(4.75-7 mm)content,the bulk density initially increases and then decreases,and the specific surface area gradually increases.The average excess paste thickness fluctuates with both bulk density and specific surface area.The workability of 3DPRAC is closely related to the average excess paste thickness.With an increase in average paste thickness,there is a gradual decrease in dynamic yield stress,static yield stress and printed inclination,accompanied by an increase in fluidity and printed width.The mechanical performance of 3DPRAC closely correlates with the bulk density.With an increase in the bulk density,there is an increase in the ultrasonic wave velocity,accompanied by a slight increase in the compressive strength and a significant decrease in the anisotropic coefficient.Furthermore,an index for buildability failure of 3DPRAC based on the average excess paste thickness is proposed.

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.

Performance Degradation of the Repeated Recycled Aggregate Concrete with 70% Replacement of Three-generation Recycled Coarse Aggregate

The feasibility of using different generations recycled coarse aggregate（RCA） on structural concrete was fully evaluated by studying the performance of the recycled coarse aggregates and their corresponding concretes, the different generations of RCA were recycled by following the repeated mode of ‘concrete-waste concrete-coarse aggregate-concrete＇. Moreover, the focus was on ‘three generations＇ of repeated RCAs, the RCA was produced by crushing and regenerating the artificial accelerated degraded concrete, the process was designed to follow the nature degradation of the concrete with a coupling action of accelerated carbonation and bending load. The properties of x-generation（x=1, 2 or 3） of repeated RCA were systematically investigated and the compressive and splitting tensile strengths of relating structural concretes（with 70% replacement of x-generation of RCA） were studied accordingly. The results show a competent compressive and splitting tensile strength of 30 MPa at 28 th day of structural concretes with all generations of repeated RAC. And the gradual degraded performance of the repeated RCAs was observed with an increased numbers of repetition（1〉2〉3 generations）, the overall performances of all repeated RCAs fulfill the Class Ⅲ according to Chinese Standards GB25177-2010. Our gained insight demonstrates a feasibility of using at least 3 generations of repeated RCA for the production of normal structural concrete.

Effect of Recycled Coarse Aggregate on Concrete Compressive Strength

The effect of recycled coarse aggregate on concrete compressive strength was investigated based on the concrete skeleton theory. For this purpose, 30 mix proportions of concrete with target cube compressive strength ranging from 20 to 60 MPa were cast with normal coarse aggregate and recycled coarse aggregate from different strength parent concretes. Results of 28-d test show that the strength of different types of recycled aggregate affects the concrete strength obviously. The coarse aggregate added to mortar matrix plays a skeleton role and improves its compressive strength. The skeleton effect of coarse aggregate increases with the increasing strength of coarse aggregate, and normal coarse aggregate plays the highest, whereas the lowest concrete strength occurs when using the weak recycled coarse aggregate. There is a linear relationship between the concrete strength and the corresponding mortar matrix strength. Coarse aggregate skeleton formula is established, and values from experimental tests match the derived expressions.

Finite Element Analysis on the Uniaxial Compressive Behavior of Concrete with Large-Size Recycled Coarse Aggregate

To model the concrete with complex internal structure of concrete with large sized aggregates the effect of internal structure on uniaxial compression behavior are studied.Large-sized recycled aggregates behave differently in the concrete matrix.To understand the influence on concrete matrix,a finite element model was developed to model recycled aggregate concrete composed of multiple randomly distributed irregular aggregates and cement mortar.The model was used to calculate the effect of large-size recycled coarse aggregate(LRCA)on the strength of recycled aggregate concrete and simulate the compressive strength of cubes and prisms.The factors such as the strength of new concrete,the strength of old concrete,the defective element content,the shape of LRCA,the incorporation ratio of LRCA and the size of LRCA that can affect the strength of concrete are analyzed in this paper.Results showed that the influence of various factors on concrete strength are in the following desend-ing order:(i)strength of newly poured concrete;(ii)original strength of recycled aggregates;and(iii)defects.It can be seen that the cracking of the phase material elements starts along the bonding zones between gravel and mortar or the new and old mortar,then spreads to mortar and finally to LRCA.The cracking tendency is most significant in LRCA,which means that the fracturing is related to the fracture of the LRCA.After evaluating the variations in strength and quality of the recycled concrete,the influences on concrete strength and quality were studied.The results showed that the proposed concrete model with LRCA was successfully applied to studying the uniaxial compressive behavior of concrete with large-size recycled coarse aggregate.

Experimental Study on Mechanical Properties of Self-Compacting Recycled Concrete

The application of self-compacting recycled concrete can solve the problem of environmental pollution caused by construction waste but its mechanical properties have not been unified and need further study.The strength of recycled concrete is unstable,and its performance still needs further study.The combination of fixed sand and stone volume method and free water cement ratio method is used to determine the mix ratio of self-compacting recycled concrete.24 sets of slump expansion tests and 24 sets of cube axial compression tests were carried out to study the effect of recycled aggregate replacement rate on the flow performance and axial compressive strength of self-compacting recycled concrete,and the performance conversion formula of self-compacting recycled concrete was given.The results show that with the increase of the regenerated coarse aggregate substitution rate,the fluidity and filling property of the self-compacting regenerated concrete mix decreased.The failure of self-compacting recycled concrete is mainly due to the failure of strength between old mortar and new mixture.As the substitution rate increases from 0 to 100%,the axial compressive strength decreases by 15.2%.

Influence of Recycled Construction Materials Aggregate on Mechanical and Physical Properties of Concrete

The main purpose of this research is to study the properties of re-use different types of construction materials such as PVC （polyvinylchloride） scraps, clay brick and recycled concrete as a partial replacement of coarse aggregate. Different proportions （1%, 3%, 5% and 7%） by weight were used for PVC. scrap, （10%, 20%, 30%, and 40%） by weight were used for recycled concrete and （5%, 10%, 15%, and 20%） by weight were used for clay brick. Mechanical tests such as compressive and tensile strength tests and physical tests such as ultrasonic pulse velocity, bulk density, porosity, specific gravity and water absorption tests were done to the samples after curing in normal water for 28 days. Test results showed slightly degradation in mechanical and physical engineering properties of concrete specimens that used partial replacement of recycled concrete coarse aggregate, degradation increased with increasing of replacement but test results still closely to reference samples. Use of polyvinyl chloride in proportions not more than 5% as a partial replacement of coarse aggregates given acceptable results in comparison with reference samples but all test results degraded at 7% replacements. Test results of partial replacement of crushed brick coarse aggregates unacceptable and the range of degradation are wide because of increased （water： cement） ratio to improve the concrete workability.

Punching shear behavior of steel fiber reinforced recycled coarse aggregate concrete two-way slab without shear reinforcement

In this paper,the punching shear performance of 8 steel fiber reinforced recycled coarse aggregate concrete(SFRCAC)two-way slabs with a size of 1800 mm×1800 mm×150 mm was studied under local concentric load.The effects of RCA replacement ratio(rg)and SF volume fraction(Vf)on the punching shear performance of SFRCAC two-way slabs were investigated.Digital Image Correlation(DIC)measurement and Acoustic Emission(AE)technique were introduced to collect pictures and relevant data during the punching shear test.The test results show that the SFRCAC two-way slab mainly exhibits punching shear failure and flexure failure under local concentric load.The punching shear failure space area of SFRCAC two-way slab has no obvious change with increasing rg,however,show a gradual increase trend with increasing Vf.Both of the punching shear ultimate bearing capacity(Pu)and its deflection of SFRCAC two-way slab decrease with increasing rg and increase with increasing Vf,respectively.Finally,through the regression analysis of the results from this study and the data collected from related literature,the influence of rg and Vf on the Pu of two-way slabs were obtained,and the equations in GB 50010-2010,ACI 318-19,and Eurocode 2 Codes were amended,respectively.Furthermore,the amended equations were all applicable to predicted the ultimate bearing capacity of the ordinary concrete two-way slab,RCAC two-way slab,SFRC two-way slab,and SFRCAC two-way slab.

Chloride diffusion in concrete with carbonated recycled coarse aggregates under biaxial compression

Chloride attack on concrete structures is affected by the complex stress state inside concrete,and the effect of recycled aggregates renders this process more complex.Enhancing the chloride resistance of recycled concrete in a complex environment via carbonization facilitates the popularization and application of recycled concrete and alleviates the greenhouse effect.In this study,the chloride ion diffusion and deformation properties of recycled concrete after carbonization are investigated using a chloride salt load-coupling device.The results obtained demonstrate that the chloride ion diffusivity of recycled concrete first decreases and then increases as the compressive load increases,which is consistent with the behavior of concrete,in that it first undergoes compressive deformation,followed by crack propagation.Carbonation enhances the performance of the recycled aggregates and reduces their porosity,thereby reducing the chloride diffusion coefficient of the recycled concrete under different compressive load combinations.The variation in the chloride ion diffusivity of the carbonized recycled aggregate concrete with the load is consistent with a theoretical formula.

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 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.

Long term performance of recycled concrete beams with different water-cement ratio and recycled aggregate replacement rate

The purpose of this study is to reveal the service performance of recycled aggregate concrete(RAC)components for different values of water-cement ratio and replacement rate of recycled coarse aggregate(RCA).Generally,the concrete strength decreases with the increase of the replacement rate of RCA,in order to meet the strength requirements when changing the replacement rate of RCA,it is necessary to change the water-cement ratio at the same time.Therefore,the axial compressive strengths of prism with 25 mix proportions,the short-term mechanical properties and long-term deformation properties of reinforced concrete beams were tested respectively by changing water-cement ratio and RCA replacement rate.The bearing capacity and the strain nephogram of samples under different loads were obtained using the Digital Image Correlation(DIC)method,and a self-made gravity loading experimental device was used for long-term deformation investigation.Results showed that the damage pattern of RAC was the same as that of natural aggregate concrete(NAC),but the brittleness was more pronounced.The brittleness of concrete before failure can be reduced more effectively by adjusting the replacement rate of RCA than by adjusting the water-cement ratio.The water-cement ratio has an evident influence on the axial compressive strength and early creep of concrete,while the replacement rate of RCA has a remarkable effect on the long-term deformation of the concrete beams.

Effects of coarse and fine aggregates on long-term mechanical properties of sea sand recycled aggregate concrete

Typical effects of coarse and fine aggregates on the long-term properties of sea sand recycled aggregate concrete(SSRAC)are analyzed by a series of axial compression tests.Two different types of fine(coarse)aggregates are considered:sea sand and river sand(natural and recycled coarse aggregates).Variations in SSRAC properties at different ages are investigated.A novel test system is developed via axial compression experiments and the digital image correlation method to obtain the deformation field and crack development of concrete.Supportive results show that the compressive strength of SSRAC increase with decreasing recycled coarse aggregate replacement percentage and increasing sea sand chloride ion content.The elastic modulus of SSRAC increases with age.However,the Poisson’s ratio reduces after 2 years.Typical axial stress-strain curves of SSRAC vary with age.Generally,the effect of coarse aggregates on the axial deformation of SSRAC is clear;however,the deformation differences between coarse aggregate and cement mortar reduce by adopting sea sand.The aggregate type changes the crack characteristics and propagation of SSRAC.Finally,an analytical expression is suggested to construct the long-term stress-strain curve of SSRAC.

Recent studies on mechanical properties of recycled aggregate concrete in China-A review

Numerous experimental and theoretical studies on recycled aggregate concrete have been carried out in China in the past 10 years.This paper provides a comprehensive review of the related findings of research on the mechanical properties of RAC in China.The influences of the RCA on the strength and deformation characteristics of concrete,the statistical characteristics for the strength of RAC,fracture energy,stress-strain relationships under uniaxial compression,uniaxial tension as well as pure shear,and the residual strength of RAC after exposure to high temperatures,the bond between RAC and different kinds of steel rebar were also reviewed.Furthermore,some recent studies on the numerical simulation of the failure mechanism for RAC at the meso-structure level were discussed.

On carbonation behavior of recycled aggregate concrete

This study tries to reveal the carbonation behavior when recycled coarse aggregates(RCAs) are used to mix new concrete.Six series of tests including 22 groups of recycled aggregate concrete(RAC) specimens were carried out,in which the effects of the water/binder(w/b) ratios,the binder content,the types of mineral admixture,the quality of RCAs,the RCAs replacement and the loading levels on the carbonation behavior of RAC were evaluated,respectively.The results showed that the carbonation behavior of RAC was not only influenced by the quality of new mortar but also by the properties of RCAs.The admixture of mineral admixture influenced the carbonation,and the loading stress level might have a significant impact on the evolution of the carbonation.

Punching shear behavior of recycled aggregate concrete slabs with and without steel fibres

A study on the punching shear behavior of 8 slabs with recycled aggregate concrete(RAC)was carried out.The two main factors considered were the recycled coarse aggregate(RCA)replacement percentage and the steel fibre volumetric ratio.The failure pattern,load-displacement curves,energy consumption,and the punching shear capacity of the slabs were intensively investigated.It was concluded that the punching shear capacity,ductility and energy consumption decreased with the increase of RCA replacement percentage.Research findings indicated that the incorporation of steel fibres could not only improve the energy dissipation capacity and the punching shear capacity of the slab,but also effectively improve the integrity of the slab tension surface and thereby changing the trend from typical punching failure pattern to bending-punching failure pattern.On the basis of the test,the punching shear capacity formula of RAC slabs with and without steel fibres was proposed and discussed.

Stress-strain relationship of recycled self-compacting concrete filled steel tubular column subjected to eccentric compression

As a typical compression member,the concrete-filled steel tube has been widely used in civil engineering structures.However,little research on recycled self-compacting concrete flled circular steel tubular(RSCCFCST)columns subjected to eccentric load was reported.In this study,21 specimens were designed and experimental studies on the stress-strain relationship of were carried out to study the mechanical behaviors.Recycled coarse aggregate replacement ratio,concrete strength grade,length to diameter ratio and eccentric distance of specimens were considered as the main experimental parameters to carry out eccentric compression tests.The corresponding stress-strain relationship curves were used to analyze the influence of concerned parameters on ecentric load-bearing capacity of RSCCFCST columns.The experimental results show that the strain of the eccentric compression stress-strain curves increase with the increase of recycled coarse aggregate replacement ratio and concrete strength grade.With increase of eccentric distance,the ductility of specimens increases while the bearing capacity decreases.Moreover,a phenomenological model of RSCCFCST columns is proposed,which exhibits versatile ability to capture the process during loading.The present study is expected to further understanding the behaviors and to provide guidance of RSCCFCST columns in design and engineering applications.

再生粗骨料混凝土应力-应变关系

再生粗骨料混凝土应力-应变关系是实现其材料到结构力学分析的桥梁纽带,成为再生粗骨料混凝土结构基础理论的基石。介绍了作者团队多年来在再生粗骨料混凝土应力-应变关系方面取得的研究进展:采用模型化再生粗骨料方法,研究了复杂界面过渡区对再生粗骨料混凝土破坏行为的影响,揭示了再生粗骨料混凝土细观损伤本质与演化机理;从静力作用到动力作用,系统地开展了不同工况下再生粗骨料混凝土应力-应变行为试验研究,探明了载荷条件对再生粗骨料混凝土应力与变形的影响规律并建立了相适应的力学与数学模型;进一步考虑再生粗骨料性能时空变异性,发现了再生粗骨料混凝土力学响应的概率分布特征,提出了再生粗骨料混凝土随机损伤本构关系;基于获得的本构模型,完成了再生粗骨料混凝土构件时变可靠度分析和结构动力非线性分析,为再生粗骨料混凝土在实际工程中的安全应用提供了理论支撑;提炼了相关研究结论并对未来研究工作进行了展望。

MMA强化再生粗骨料混凝土力学性能试验研究

目的为了提高再生粗骨料混凝土的性能,扩大再生粗骨料混凝土在工程中的使用范围。方法采用聚合物-甲基丙烯酸甲酯(MMA)单体浸渍,然后聚合的方法对再生粗骨料进行了强化研究。对聚合温度、聚合时间、再生粗骨料含水率等因素对MMA强化再生粗骨料效果的影响及强化后的再生粗骨料对再生粗骨料混凝土力学性能的影响进行了分析。结果研究结果表明:MMA强化再生粗骨料时的最佳聚合温度为60℃,最佳聚合时间为24 h,MMA强化可以有效改善再生粗骨料的吸水率和压碎指标等物理性能,对再生粗骨料进行预干燥处理可以使再生粗骨料的吸水率和压碎指标等物理性能得到进一步提升;MMA强化处理后的再生粗骨料混凝土3d、7d、14d和28d立方体抗压强度分别比普通再生粗骨料混凝土提高了26.9%,20.3%,21.4%,23.9%,轴心抗压强度、劈裂抗拉强度、抗折强度和弹性模量等力学性能分别比普通再生粗骨料混凝土提高了20.9%、14.7%、12.2%和8.3%,同时再生粗骨料混凝土的延性也得到了显著改善。结论MMA单体浸渍,然后进行聚合的方法可以有效改善再生粗骨料性能,从而提高再生粗骨料混凝土的力学性能。

GFRP筋再生混凝土梁受弯承载力研究

为研究GFRP筋再生混凝土梁(GFRP-RAC)的受弯承载力,完成了5根配GFRP筋、1根配BFRP筋和1根配钢筋的再生混凝土梁抗弯试验,分析了不同再生骨料取代率、纵筋类型以及纵筋配筋率等因素对各试验梁开裂荷载和极限承载力的影响。结果表明:再生骨料取代率对开裂荷载的影响较大,而纵筋类型及纵筋配筋率对其影响不明显;当再生骨料取代率由0增加至50%和100%时,试验梁开裂荷载分别减小了7.9%和17.3%;极限承载力受再生骨料取代率影响较小而受纵筋配筋率影响较大,当GFRP-RAC梁纵筋配筋率由0.38%提高至0.6%和1.17%时,极限荷载分别提升了17.9%和52.5%。另外,结合国内外规范对GFRP-RAC梁的受弯承载力进行对比分析,并基于试验数据和相关试验结果的回归分析,得到开裂荷载修正系数αcr,开裂荷载修正后的计算值与试验值吻合较好。