Repair of Second-Generation Recycled Fine Aggregate of Waste Concrete from Freeze-Thaw Environment by Carbonation Treatment

The reuse of waste recycled concrete from harsh environments has become a research hotspot in the field of construction.This study investigated the repair effect of carbonation treatment on second-generation recycled fine aggregate(SRFA)obtained from recycled fine aggregate concrete(RFAC)subjected to freeze-thaw(FT)cycles.Before and after carbonation,the properties of SRFA were evaluated.Carbonated second-generation recycled fine aggregate(CSRFA)at five substitution rates(0%,25%,50%,75%,100%)to replace SRFA was used to prepare carbonated second-generation recycled fine aggregate concrete(CSRFAC).The water absorption,porosity and mechanical properties of CSRFAC were tested,and its frost-resisting durability was evaluated.The results showed after carbonation treatment,the physical properties of SRFA was improved and met the requirements of II aggregate.The micro-hardness of the interfacial transition zone and attached mortar in CSRFA was 50.5%and 31.2%higher than that in SRFA,respectively.With the increase of CSRFA replacement rate,the water absorption and porosity of CSRFAC gradually decreased,and the mechanical properties and frost resistance of CSRFAC were gradually improved.Carbonation treatment effectively repairs the damage of SRFA caused by FT cycles and improves its application potential.

Effect of waste concrete with different strength on mechanical properties of recycled fine aggregate mortar

The influence of source concrete (SC) with different compression strengths on the workability and mechanical properties of recycled mortar made with river sand substituted by 100% fine recycled concrete aggregates (FRCA) is experimentally investigated. The basic physical performance test shows that with the increase in SC strength, FRCA exhibit lower water absorption and crushing index, meanwhile keeping higher densities. Mechanical property tests, including compressive strength, flexural strength and uniaxial compressive stress-strain tests, show that compressive strength,flexural strength and elasticity modulus of recycled sand mortars increase roughly with the increase in SC strength. The proposed mixture design method demonstrates that all of the components can be kept as the same as those in natural mortar mixture design and FRCA must be pre-wetted before making mortar mixture. Meanwhile, the reuse of higher strength SC can ensure that recycled mortar mixtures are able to achieve similar mechanical performance when compared to natural mortar designs.

Controlled low-strength material incorporating recycled fine aggregate from urban red brick based construction waste

Sixteen controlled low-strength material( CLSM)mixtures with various cement-to-sand( C/Sa) ratios and water-to-solid( W/So) ratios were prepared using recycled fine aggregate from urban red brick based construction waste.The fluidity and bleeding of the fresh CLSM mixtures were measured via the modified test methods, and the hardened CLSM mixtures were then molded to evaluate their compressive strength and durability. The results showthat the fluidity of the fresh CLSM mixtures is 105 to 227 mm with the corresponding bleeding rate of 3. 7% to 15. 5%, which increases with the increase in fluidity. After aging for 28 d,the compressive strength of the hardened CLSM mixtures reaches 1. 15 to 13. 96 M Pa, and their strength can be further enhanced with longer curing ages. Additionally, the strength increases with the increase of the C/Sa ratio, and decreases with the increase of the W/So ratio under the same curing age. Based on the obtained compressive strength, a fitting model for accurately predicting the compressive strength of the CLSM mixtures was established, which takes into account the above two independent variables( C/Sa and W/So ratios).M oreover, the durability of the hardened CLSM mixtures is enhanced for samples with higher C/Sa ratios.

Strength and Chloride Diffusion Behaviour of Three Generations of Repeated Recycled Fine Aggregate Concrete

The feasibility of using different generations of recycled fine aggregate（RFA） in structural concrete in a chloride environment was evaluated by studying the performance of the RFA and the corresponding concrete. The different generations of RFA were recycled by following the cycle of ‘concrete-waste concrete-fine aggregate-concrete＇. The properties of three generations of repeatedly recycled fine aggregate（RRFA） were systematically investigated, and we focused on the compressive strength and splitting tensile strength and chloride ion permeability of the related structural concretes with 25%, 75%, and 100% replacement of natural fine aggregates with RFA. The results indicated that the quality of RRFA presents a trend of slow deterioration, but the overall performance of all RRFA still fulfils the quality requirements of recycled fine aggregate for structural concrete. All RRFA concretes achieved the target compressive strength of 40 MPa after 28 days except for the second generation of the recycled aggregate concrete and the third generation of the recycled aggregate concrete with 100% replacement, and all the concrete mixes achieved the target compressive strength after 90 days. The insights obtained in this study demonstrate the feasibility of using at least three generations of RRFA for the production of normal structural concrete with a design service life of 100 years in a chloride environment.

Mathematical models for properties of mortars with admixtures and recycled fine aggregates from demolished concretes

In order to expand the engineering application of recycle aggregate mortars （RAM） with aggregates from demolished concretes, the models for the properties of RAM and the replacement rate of these recycled fine aggregates were proposed. First, different kinds of mathematical models for the basic properties （compressive strength, water retention rate, and consistency loss） of RAM with two kinds of admixtures, thickening powders （TP） and self-made powdery admixtures （SSCT） designed for RAM, and the replacement rates were established, while the average relative errors and relative standard errors of these models were calculated. Additionally, the models and their error analyses for the curves of drying shrinkage and curing time of RAM ＋ SSCT at different replacement rates were put forward. The results show that polynomial functions should be used to calculate the basic properties of RAM ＋ TP and RAM ＋ SSCT at different replacement rates. In addition, polynonfial functions are the most optimal models for the sharp shrinkage sections in the curves of drying shrinkage-curing time of RAM ＋ SSCT, while exponential functions should be used as the models for the slow shrinkage sections and steady shrinkage sections.

Properties of Cementitious Rendering Mortar Prepared with Recycled Fine Aggregates

The results of an experimental study on investigating the properties of cementitious rendering mortars prepared with a recycled fine aggregate（RA） were presented.The RA was obtained from a recycling plant in which mixed demolition wastes were processed by mechanical crushing,sieving and sorting operations.Two series of rendering mortar mixes were prepared with a constant water/cement and a constant aggregate/cement ratios of 0.55 and 3 respectively.River sand and natural crushed rock fine were originally used in the two series separately,and they were consistently replaced by 25%,50%,75% and 100% by the recycled aggregate.The experimental results showed that mechanical properties,such as compressive strength,flexural strength and modulus of elasticity of the mortars prepared with the RA were lower than the mortars made with the natural aggregates.Nevertheless,the bond strength at the interface between the mortar and masonry bricks determined by the Triplet test was found to be higher for the mortars prepared with the RA.

Properties of Rapid Hardening Controlled Low Strength Material Made of Recycled Fine Aggregate from Urban Red Brick Construction Waste

In some cases of emergency backfill engineering projects, traditional backfill materials cannot meet the requirements of fast construction due to their long curing time. This study presents a new kind of rapid hardening controlled low strength material, which utilizes both rapid hardening sulphoaluminate cement and recycled fine aggregate from urban red brick construction waste. Totally, sixteen mixtures were prepared for the experiment with different cement-to-sand ratios and water-to-solid ratios. The flowability and bleeding rate of fresh mixture were measured to evaluate its workability, and the compressive strength of hardened mixture was tested to evaluate its rapid hardening and mechanical properties. Test results indicate that rapid hardening controlled low strength material containing recycled fine aggregate from urban red brick construction waste can achieve the desirable flowability, but the bleeding rate increases with the increase of flowability. In addition, 2-hour compressive strength can reach 0.08 - 0.12 MPa, and 4-hour compressive strength is 0.32 - 1.54 MPa, which can meet the requirements of emergency backfill construction. At last, based on the derived compressive strength, a fitting model for predicting compressive strength evolution of this new rapid hardening backfill material is developed, which fits accurately with these experimental data.

Effects of admixture on properties of recycled aggregate mortar

Compared with strengthening the recycled fine aggregate(RFA)from construction and demolition waste,which is time-consuming and complex,adding admixture into the mixtures directly is more efficient and effective to expand the application of recycled aggregate mortar(RAM).The admixture(named as SSC),mixed with sodium hexametaphosphate,sodium ligninsulfonate and citric acid,was directly added into the RAM.First,the compositions and physical properties of the RFA and reference aggregate were studied,respectively.The properties of fresh and hardened mortars were then investigated.The results show that there is a clear difference between the RFA and reference aggregate,and the properties of RAM without SSC are not as good as those of normal mortar.However,the consistency value,water retention rate,compressive strength and setting time values of RAM increase by 5%,7%,66%and 67%,and its consistency loss and density values decrease by 42%and 4%after the SSC is added into the RAM.Therefore,improving the properties of RAM through adding admixture is an effective and efficient approach to expanding its application.

附加用水量对再生砂混凝土工作性和力学性能的影响

再生砂掺量和附加用水量对新拌和硬化再生砂混凝土的性能有重要影响。本工作制备了再生砂掺量(γRFA)为30%、50%、70%、100%(质量分数,下同),附加用水量补偿系数(kwa)为0.6、0.75、0.9、1.0的16组再生砂混凝土,测试了这些再生砂混凝土的工作性(坍落度、扩展度与扩展时间)与力学性能(抗压、劈拉强度与弹性模量),分析了kwa和γRFA对再生砂混凝土性能的影响规律,获得了kwa的合理取值范围。结果表明,随着kwa的降低,再生砂混凝土的工作性下降,力学性能增大;γRFA对混凝土力学性能的影响与附加用水量有关,在中低附加用水量(kwa=0.6、0.75)条件下,随着γRFA的增加,再生砂混凝土的部分力学性能指标增大且其值高于天然砂混凝土,而在高附加用水量(kwa=0.9、1.0)下,再生砂混凝土的力学性能随着γRFA的增加呈线性降低。γRFA低于50%对再生砂混凝土力学性能的影响较小,kwa值在0.75~0.9范围变化对再生砂混凝土工作性能与力学性能的影响均相对较小。

钢纤维对细再生混凝土力学性能的影响研究

为研究细再生集料(FRAP)替代天然细集料(NFA)的可能性。配置1%钢纤维掺量下不同FRAP含量(0%、25%、50%、75%、100%)的水泥混凝土,并测定成型后试件的抗压强度、拉伸特性、应力-应变特性和耐久性能。研究结果表明:以FRAP替代NFA会导致细再生集料水泥混凝的土力学强度降低;加入1%钢纤维,可以显著提高细再生集料水泥混凝土劈裂抗拉强度和抗弯强度,峰值应力增加约15%,峰值应变增加50%,韧性及延展性提高约5倍;以FRAP替代NFA的最佳含量为50%左右。

再生细骨料和砖粉双掺对3D打印混凝土性能的影响

为减少3D打印混凝土中天然骨料及胶凝材料的用量,本文采用再生混凝土细骨料替代部分天然细骨料,砖粉替代部分水泥,首先开展单掺再生细骨料(取代率为0、25%、50%、75%和100%)、单掺砖粉(取代率为0、5%、10%、15%、20%和30%)和两者双掺的现浇混凝土的流动性和抗压强度试验,以获得再生细骨料和砖粉的适宜取代率;然后探究50%再生细骨料和10%砖粉双掺及配合比调整方式(附加水、提高减水剂用量)对3D打印混凝土拌合物性能及硬化后力学性能的影响。试验结果表明,当再生细骨料的掺量不超过50%时,现浇混凝土抗压强度降低幅度在10%以内;随着砖粉掺量从0增加到30%,现浇混凝土抗压强度总体呈现先增后减、再稍有增加的趋势,当砖粉掺量为10%时,混凝土抗压强度最高;相比于单掺50%再生细骨料的混凝土,50%再生细骨料和10%砖粉双掺时混凝土强度有所增加,而流动性基本不变。对于3D打印混凝土,同时掺入50%再生细骨料和10%砖粉,并采用添加附加水的方式保持3D打印混凝土初始扩展度不变,会使得混凝土的可建造性提高,但坍落度、开放时间、抗压和劈裂抗拉强度降低,强度各向异性加剧;而提高减水剂用量不仅能显著提升3D打印混凝土的流动性能、开放时间和抗压强度,还能降低打印试件的强度各向异性。

再生细骨料混凝土的性能研究进展

合理使用再生细骨料(以下简称RFA)可同时解决建筑垃圾的资源化利用问题和天然砂、石过度开采引起的环境问题。介绍了RFA的颗粒形状、空隙率、吸水率、有害物质含量等性能,总结了RFA的性能强化措施,综述了再生细骨料混凝土(以下简称RFAC)的工作性、力学性能和耐久性能,并对其机理进行了分析,可为RFA的进一步推广应用提供参考。

Recycled glass replacement as fine aggregate in self-compacting concrete

With increasing environmental pressure to reduce solid waste and to recycle as much as possible,the concrete industry has adopted a number of methods to achieve this goal by replacement of waste glass with concrete composition materials.Due to differences in mixture design,placement and consolidation techniques,the strength and durability of Self Compacting Concrete(SCC)may be different than those of conventional concrete.Therefore,replacement of waste glass with fine aggregate in SCC should deeply be investigated compared to conventional concretes.The aim of the present study is to investigate the effect of glass replacement with fine aggregate on the SCC properties.In present study,fine aggregate has been replaced with waste glass in six different weight ratios ranging from 0%to 50%.Fresh results indicate that the flow-ability characteristics have been increased as the waste glass incorporated to paste volume.Nevertheless,compressive,flexural and splitting strengths of concrete containing waste glass have been shown to decrease when the content of waste glass is increased.The strength reduction of concrete in different glass replacement ratios is not remarkable,thus it can be produced SCC with waste glass as fine aggregate in a standard manner.

再生细骨料残余浆体对混凝土流变性和强度的影响

文章开展再生细骨料残余浆体的定量表征实验、混凝土常规工作性能试验、流变性能试验和力学性能试验,研究不同残余浆体质量分数对再生细骨料混凝土流变性能和力学性能的影响。研究发现,再生骨料表面残余浆体的去除可以明显改善其基本性能,再生细骨料的使用会使新拌混凝土的坍落度和扩展度损失加快,屈服应力增大,塑性黏度降低。随着再生细骨料残余浆体质量分数的减少,用其制备出的再生混凝土的抗压强度和劈裂抗拉随之增大。当浆体质量分数为14.9%时,骨料水泥界面过渡区粘结紧密,再生细骨料混凝土的28 d抗压强度和劈裂抗拉强度分别为28.50、2.30 MPa,约为普通混凝土的89%、77%,表明较低残余浆体质量分数的再生细骨料可取代天然砂应用于混凝土中。

再生细骨料对高延性水泥基复合材料力学性能及碳化耐久性的影响

为促进再生细骨料(RFA)在高延性水泥基复合材料(HDCC)中的应用,研究了RFA取代率(0%、25%、50%、75%、100%,质量分数)对HDCC抗压强度、拉伸性能、四点弯曲强度和碳化耐久性的影响。结果表明,随着RFA取代率提高,HDCC的抗压强度逐渐下降,但拉伸性能与弯曲强度显著提高。100%RFA取代率下HDCC峰值应变为3.06%,弯曲强度为7.9 MPa,较天然细骨料HDCC分别提升了31.33%和11.27%,表现出良好的延展特性。当RFA取代率高于50%时,有助于提高HDCC的碳化耐久性。基于测试结果进行拟合,建立了关于RFA取代率的HDCC碳化深度预测模型。

全再生细骨料混凝土的制备及性能试验研究

为提高再生细骨料(regenerate fine aggregates,RFA)的回收使用率,本研究采用一种含砖RFA进行全再生细骨料混凝土(fully recycled fine aggregate concrete,FRFAC)的试验制备.设计吸水率试验拟合公式计算RFA的主要组成及含量;通过设计正交试验研究水料比、减水剂和RFA粒径级配对不同养护天数下FRFAC的强度与热工性能的影响.研究结果表明,该RFA中主要包含废砖、废混凝土和泥块,其含量分别为13.58%、52.98%和33.44%,泥块含量较多,所以在制备前需对RFA进行筛分处理,该RFA平均吸水率较大,所以需采用饱和面干法进行预处理;对FRFAC强度影响最大的是RFA粒径级配,其次是水料比和减水剂,强度随着养护天数呈缓慢上升趋势;FRFAC的导热系数低,通过抗压导热交互分析得到配比最优方案为A3B3C2,制备的FRFAC强度达到25.8 MPa,导热系数为0.4636 W/(m·K),可用于建筑工程中.

喷射混凝土回弹料再生混凝土基本力学性能研究

为研究喷射混凝土回弹料再生混凝土(SRM-RFAC)的基本力学性能,以喷射混凝土回弹料再生细骨料(SRM-RFA)取代率和粉煤灰(FA)取代率为变量,进行了立方体抗压强度、棱柱体抗压强度、立方体劈裂抗拉强度和静弹性模量试验。结果表明:SRM-RFAC的立方体抗压破坏、轴心抗压破坏和劈裂抗拉破坏形态与普通混凝土(NC)基本相似,且混凝土的抗压强度与劈拉强度随着SRM-RFA和FA取代率的增加而逐渐降低,基于规范式,提出棱柱体抗压强度、立方体劈裂抗拉强度和静弹性模量的换算式,计算值与试验结果吻合度较好。

再生砖细骨料形态对混凝土流变行为与力学性能的影响

评价了再生砖细骨料(RFCB)的几何形状和表面形态,研究了RFCB颗粒级配和初始饱水度对混凝土流变行为与力学性能的影响.结果表明:相比天然河砂,RFCB几何形状不规则且形状参数较低,表面形态呈凹陷凸起状且粗糙度较大;RFCB复杂的物理特性导致新拌混凝土的屈服应力与塑性黏度增大,劈裂抗拉强度有所提高且对抗压强度无较大不利影响;适当降低RFCB的最小粒径和初始饱水度,虽会使混凝土的流变性能变差,但有助于在混凝土中形成以再生砖细骨料为软核、界面为硬壳的结构,从而提高了混凝土的力学性能.

再生细骨料对超高性能混凝土的力学与自收缩性能影响

将再生细骨料应用于超高性能混凝土(UHPC)不仅能降低成本,还可实现建筑垃圾资源化利用。采用预湿状态再生细骨料制备UHPC,探究再生细骨料的粒径(0.6~<1.18 mm和1.18~2.36 mm)和不同掺量(10%、20%、30%和40%,均为体积分数)对UHPC的工作性能、力学性能、体积稳定性、抗氯离子渗透性能的影响,利用热重分析(TG-DTG)、显微硬度、压汞仪(MIP)和扫描电子显微镜(SEM)等测试方法揭示其影响机理。结果表明,再生细骨料的粒径和掺量是影响UHPC力学性能和体积稳定性的关键因素。随着预湿再生细骨料掺量增大,UHPC的流动性和自收缩值逐渐减小,但其抗压强度表现出先增大后降低的趋势,其中掺入20%预湿再生细骨料(粒径为1.18~2.36 mm)制备的UHPC性能更优,与未掺再生细骨料UHPC相比,其28 d抗压强度提升7.0%,7 d自收缩值降低了78.8%;预湿再生细骨料内养护效应有助于降低UHPC内部自干燥进程、提高水化反应程度、优化界面过度区组成结构、减少有害孔和多害孔数量。

饱和面干处理方式对再生细骨料UHPC自收缩性能的影响

采用预湿饱和面干和外加水饱和面干处理再生细骨料(RFA),并以RFA替代50%的河砂,制备超高性能混凝土(UHPC).研究不同处理方式获得的饱和面干RFA对UHPC抗压强度、内部相对湿度和自收缩性能的影响,并利用压汞分析进行机理解释.研究结果表明,当总水胶比为0.24时,与采用预湿饱和面干RFA的UHPC相比,采用外加水饱和面干RFA的UHPC中孔径大于50 nm的孔体积增多,抗压强度降低;内部相对湿度减少,自收缩增大;其28.0 d抗压强度仅降低3.2%,且7.0 d的自收缩仅提高10.5%.因此,在实际工程的UHPC中可采用外加水饱和面干RFA,以获得更好的施工效益.