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.

High-efficiency Carbonation Modification Methods of Recycled Coarse Aggregates

To solve the problem of only surface carbonation and realize high-efficiency carbonation of recycled coarse aggregate,the method of carbonated recycled coarse aggregate with nano materials pre-soaking was first put forward.The carbonation effect of modified recycled coarse aggregate with three different carbonation methods was evaluated,and water absorption,apparent density and crush index of modified recycled coarse aggregate were measured.Combined with XRD,SEM,and MIP microscopic analysis,the high-efficiency carbonation strengthening mechanism of modified recycled coarse aggregate was revealed.The experimental results show that,compared with the non-carbonated recycled coarse aggregate,the physical and microscopic properties of carbonated recycled coarse aggregate are improved.The method of carbonation with nano-SiO_(2) pre-soaking can realize the high-efficiency carbonation of recycled coarse aggregate,for modified recycled coarse aggregate with the method,water absorption is reduced by 23.03%,porosity is reduced by 44.06%,and the average pore diameter is 21.82 nm.The high-efficiency carbonation strengthening mechanism show that the pre-socked nano-SiO_(2) is bound to the hydration product Ca(OH)_(2) of the old mortar with nano-scale C-S-H,which can improve the CO_(2) absorption rate,accelerate the carbonation reaction,generate more stable CaCO_(3) and nano-scale silica gel,and bond to the dense three-dimensional network structure to realize the bidirectional enhancement of nano-materials and pressurized carbonation.It is concluded that the method of carbonation with nano-SiO_(2) pre-soaking is a novel high-efficiency carbonation modification of recycled coarse aggregate.

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.

Prediction Model-based Multi-objective Optimization for Mix-ratio Design of Recycled Aggregate Concrete

The prediction model for mechanical properties of RAC was established through the Bayesian optimization-based Gaussian process regression(BO-GPR)method,where the input variables in BO-GPR model depend on the mix ratio of concrete.Then the compressive strength prediction model,the material cost,and environmental factors were simultaneously considered as objectives,while a multi-objective gray wolf optimization algorithm was developed for finding the optimal mix ratio.A total of 730 RAC datasets were used for training and testing the predication model,while the optimal design method for mix ratio was verified through RAC experiments.The experimental results show that the predicted,testing,and expected compressive strengths are nearly consistent,illustrating the effectiveness of the proposed method.

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.

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.

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.

Effect of CO_(2) Curing on the Physical Properties of Recycled Coarse Aggregate with Different Attached Mortar Contents

The effects of carbon dioxide (CO_(2)) curing conditions (temperature,relative humidity and CO_(2) curing time) on the physical properties of recycled coarse aggregate (RCA) with varying attached mortar (AM) contents were studied.Before and after CO_(2) curing,the physical properties in terms of the apparent density,water absorption and crushing value of RCA were tested and the quality of RCA was determined.Besides,scanning electron microscope was used to observe the microstructure of RCA.Results show that the physical properties variation of RCA with higher AM content are more significant,and the quality of RCA with lower AM content is easier to be upgraded during CO_(2) curing.The physical properties of RCA with 40.8% AM content are earlier stable than that with no less than 44.5% AM content during CO_(2) curing.The optimal temperature and relative humidity are 50 ℃ and 55% for CO_(2) curing,respectively.CO_(2) curing is incapable of upgrading the quality of RCA with AM no less than 50.6%.The quality of RCA with 44.5% AM content can be upgraded only under the optimum CO_(2) curing conditions.Under relative humidity higher than 40% and the CO_(2) curing time more than 12 h,CO_(2) curing upgrades the quality of RCA with 40.8% AM content.

Bond-Slip Behavior of Steel Bar and Recycled Steel Fibre-Reinforced Concrete

Recycled steel fiber reinforced concrete is an innovative construction material that offers exceptional mechanical properties and durability.It is considered a sustainable material due to its low carbon footprint and environmental friendly characteristics.This study examines the key influencing factors that affect the behavior of this material,such as the steel fiber volume ratio,recycled aggregate replacement rate,concrete strength grade,anchorage length,and stirrup constraint.The study investigates the bond failure morphology,bond-slip,and bond strength constitutive relationship of steel fiber recycled concrete.The results show that the addition of steel fibers at 0.5%,1.0%,and 1.5%volume ratios can improve the ultimate bond strength of pull-out specimens by 9.05%,6.94%,and 5.52%,respectively.The replacement rate of recycled aggregate has minimal effect on the typical bond strength of pull-out specimens.However,the ultimate bond strengths of pull-out specimens with concrete strength grades C45 and C60 have improved compared to those with C30 grade.The specimens with longer anchorage lengths exhibit lower ultimate bond strength,with a reduction of 33.19%and 46.37%for anchorage lengths of 5D and 7D,respectively,compared to those without stirrups.Stirrup restraint of 1φ8 and 2φ8 improves the ultimate bond strength by 5.29%and 6.90%,respectively.Steel fibers have a significant effect on the behavior of concrete after it cracks,especially during the stable expansion stage,crack instability expansion stage,and failure stage.

Experimental Study on Compressive Strength of Recycled Aggregate Concrete under High Temperatur

This research aims to study the effect of elevated temperature on the compressive strength evolution of concrete made with recycled aggregate.Demolished building concrete samples were collected from four different sites in Saudi Arabia,namely from Tabuk,Madina,Yanbu,and Riyadh.These concretes were crushed and recycled into aggregates to be used to make new concrete samples.These samples were tested for axial compressive strength at ages 3,7,14,and 28 days at ambient temperature.Samples of the same concrete mixes were subjected to the elevated temperature of 300°C and tested for compressive strength again.The experimental result reveals that the recycled aggregate concrete samples have good quality at ambient and elevated temperatures and are considered fairly close to the concrete made with natural aggregate.However,recycled aggregate concrete at high temperatures showed higher strength degradation than natural aggregate concrete,but with differences that do not exceed 5%to 10%.The concrete samples made from recycled coarse aggregates also reached the design strength.It can be considered acceptable,considering the high variation in the concrete’s thermal response found in the literature.

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.

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.

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.

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。

不同粗骨料取代率下粉煤灰再生混凝土的力学性能研究

为探讨不同再生粗骨料取代率下粉煤灰对再生混凝土力学性能的影响,以及再生混凝土抗折强度与再生粗骨料取代率、水胶比之间的关系,以RC30设计强度等级为基础设计了16组配合比,制作了96块再生混凝土试块,主要进行了再生混凝土的抗压强度试验和抗折强度试验。得出如下结论:随粉煤灰掺量的增加,不同再生粗骨料取代率,抗压强度变化规律有所不同;再生粗骨料取代率为0、30%和40%时,抗压强度随粉煤灰掺量的增加整体呈下降趋势;而当再生粗骨料取代率为50%时,随粉煤灰掺量的增加,抗压强度先减小后增大。粉煤灰掺量大于20%时,天然骨料混凝土的抗压强度急剧下降,而再生骨料混凝土的抗压强度降低幅度均较小,大掺量粉煤灰在再生混凝土中或将有更大的应用空间。通过观察抗折强度试件破坏断面,分析再生混凝土变形破坏特征。粉煤灰的掺入提高了天然骨料混凝土的抗折强度,且掺量为20%强度最高。再生粗骨料取代率为30%时,随粉煤灰掺量变化抗折强度整体呈下降趋势,但下降幅度很小;再生粗骨料取代率为40%和50%时,呈先减后增再减的趋势。天然骨料混凝土的折压比随粉煤灰掺量的增加而提高;再生混凝土折压比随粉煤灰掺量的变化趋势与抗折强度一致,但变化幅度略小。粉煤灰对再生混凝土抗折强度的影响大于抗压强度;再生粗骨料取代率为30%,粉煤灰掺量为10%为较优掺量。提出再生混凝土抗折强度预测式。

机制砂再生粗骨料混凝土抗压强度试验研究

为探究机制砂原料种类和机制砂级配对机制砂再生混凝土抗压强度的影响,以细骨料类型(天然砂、石灰石机制砂、卵石机制砂)细骨料级配、水灰比为变量制作了30组机制砂再生混凝土立方体试块和15组天然砂再生混凝土立方体试块。结果表明:在石粉掺量10%时,机制砂再生混凝土抗压强度高于天然砂再生混凝土,且机制砂原料为石灰石的抗压强度高于卵石;机制砂各筛孔量占比分别为5%、7%、18%、25%、26%和14%时,机制砂再生粗骨料混凝土的抗压强度最高。研究表明:严格控制水灰比、颗粒级配可使机制砂再生混凝土达到设计强度、满足生产应用要求,采用机制砂替代天然砂制备再生骨料混凝土更有优势,是缓解天然砂资源短缺的可行途径之一。

低强度再生混凝土微观形态与力学性能的相关性研究

建筑垃圾来源复杂,为提高再生混凝土的资源利用,并满足基本工程应用要求,在仅考虑再生骨料密度、吸水和含水率、压碎指标的条件下,采用再生粗骨料混凝土配合比设计法配置低强度再生混凝土,结合切片法和SEM扫描电镜对再生混凝土的孔隙结构变化与微观形貌进行观察,分析水胶比、减水剂、粉煤灰对低强度再生混凝土内部结构的影响,发现当水胶比降低时,再生混凝土内部结构的密实度提升,絮状C-S-H之间更为密实,多害孔的比例降低,抗压和劈裂强度提高;当减水剂掺量提升时,钙钒石(AFt)的含量增加,内部结构更为致密;粉煤灰对再生混凝土的抗压和劈裂强度呈负面影响,降低C-S-H与再生骨料之间的粘结性,内部孔隙率变大,坍落度升高,可降低经济成本。

预浸碳化再生骨料混凝土的动态力学性能研究

为研究再生骨料的预浸碳化处理方式及碳化循环次数对再生混凝土动态力学性能的影响,将强化后的再生骨料100%取代天然骨料制备再生混凝土试件,分别开展静、动态压缩试验。研究结果表明,静态加载时,预浸碳化组的抗压强度随碳化循环次数的增加而增大,而直接碳化组的抗压强度仅在第1次碳化循环后有所提升,继续进行碳化循环对再生混凝土强度的提升效果不明显。动态加载时,预浸碳化组再生混凝土呈现出明显的应变率强化效应,动态抗压强度与动态增加因子均随应变率的增大而提高,相较于普通再生混凝土,再生骨料预浸碳化循环3次后制备的再生混凝土试件动态抗压强度提升了23.8%,而直接碳化组仅提升了5.7%,且动态增加因子与应变率的对数存在良好的线性关系,随应变率的lg10线性增加;当应变率相同时,随着碳化循环次数的增加,动态抗压强度也增大。从能量的角度看,预浸碳化组在高应变率下的比能量吸收值大于低应变率下的比能量吸收值,再次说明了预浸碳化再生骨料混凝土仍具有应变率效应,相同应变率条件下,预浸碳化组的比能量吸收值大于直接碳化组,说明预浸碳化再生骨料提升了再生混凝土的抗冲击性能,且效果优于传统的直接碳化处理。上述研究也为预浸碳化强化再生骨料及再生混凝土在工程结构中的应用提供了可靠的试验结果和理论依据。

再生粗骨料取代率及粒径对混凝土抗压性能影响试验研究

为研究单轴受压情况下再生粗骨料粒径范围、取代率对再生混凝土抗压性能的影响规律,分别以粒径范围5~10、10~20和20~31.5 mm,以及取代率分别为0%、10%、30%和50%为参量设计制作再生混凝土棱柱体试件,并对其开展单轴抗压性能试验研究。结果表明:在取代率相同时,粒径范围为5~10 mm时再生混凝土的抗压强度及峰值应变均比粒径范围为10~20、20~31.5 mm时的大;在粒径范围相同情况下,随取代率增大,再生混凝土抗压强度呈现出逐渐减小的变化趋势,而峰值应变则呈现出逐渐增大的变化趋势。根据试验数据,得出了再生混凝土抗压强度和峰值应变的函数关系式。