再生粗骨料的随机特性及分级方法研究EI北大核心CSCD

通过96个不同来源废旧混凝土再生粗骨料样本的试验,研究了再生粗骨料吸水率、堆积密度、饱和面干表观密度和压碎指标的概率分布特征。试验结果表明,四个指标均服从正态分布。根据相对方差和极差分析结果,确定了吸水率和压碎指标作为再生粗骨料品质分级的控制指标。然后对不同来源再生粗骨料混凝土进行抗压强度试验,得到了再生粗骨料吸水率和压碎指标的分级范围,从而提出了再生粗骨料品质的分级方法。最后利用其他研究者的试验数据,验证了该方法对再生粗骨料分级的合理性。     

微生物矿化沉积技术强化核壳结构再生粗骨料

为实现废弃混凝土砂粉的高效利用,以废弃砂为内核,废弃粉及少量胶凝材料为壳层原料,采用冷粘成球技术制备核壳结构再生粗骨料,并利用微生物矿化沉积碳酸钙(MlCP)提升骨料品质,本工作研究了矿化时间、钙离子浓度对内核质量增加率、吸水率的影响,及微生物矿化剂对再生粗骨料性能的提升效果,通过SEM、TG-DTG分析再生粗骨料内核及表面形貌、物相含量.结果表明,矿化微生物可加速、诱导碳酸钙沉积,密实结构,显著提升再生粗骨料品质;当微生物矿化剂掺量为1.0％(质量分数)时,再生粗骨料压碎指标、吸水率、微粉含量、质量损失率分别降低50.0％、21.2％、50.0％、15.3％,表观密度增加2.8％,壳层氢氧化钙完全矿化,碳酸钙含量增至24.18％.     

基于需水量比和强度比的再生粗骨料分类方法

我国尚无再生骨料的技术标准,因此,研究再生粗骨料分类方法、制定再生粗骨料的技术标准,对于推动再生混凝土产业化具有重要意义.通过对再生粗骨料与普通粗骨料制备的混凝土工作性和强度的试验研究,首次提出了需水量比和强度比的概念,并给出了相应的试验方法.结果表明,需水量比和强度比两项指标能够很好地反映再生粗骨料与普通粗骨料之间的性能差异.基于上述指标,可将再生粗骨料分成3个等级,以保证再生混凝土满足在不同领域应用的性能要求.     

颗粒整形对再生粗骨料性能的影响

为了有效提高再生粗骨料的性能,必须对再生骨料进行机械强化处理.首次提出了利用高速(线速度≥80m/s)运动的再生骨料之间的反复相互冲击与摩擦作用,有效地打掉较为突出的棱角和除去颗粒表面附着的砂浆和水泥石的一种新技术.研究表明:颗粒整形使再生骨料的颗粒堆积密度平均提高了9.3%、表观密度从2.56g/cm3提高到2.59g/cm3、空隙率从53.3%降至48.5%、吸水率从4.7%降至2.9%、压碎指标值从15.8%降至9.4%,而且堆积密度、密实密度和针片状骨料含量等指标均优于天然碎石骨料,完全能够满足配制普通混凝土的要求.     

再生粗骨料硅烷浸渍处理对混凝土介质传输性能的影响

由于残余砂浆的存在,再生粗骨料的物理力学指标远不及天然骨料,致使再生混凝土力学和耐久性能较差;此外,水分及有害离子侵入混凝土内部是引起混凝土材料性能劣化的主要原因。本试验用质量分数为8wt%的硅烷乳液浸渍强化再生粗骨料,通过抗压强度、毛细吸水和抗氯离子侵蚀试验对硅烷浸渍前后不同骨料质量取代率(0%、30%、50%)的再生混凝土介质传输性能进行了研究,最后利用SEM对再生混凝土内部的微观结构进行分析。试验结果表明,硅烷浸渍处理再生粗骨料的吸水率显著降低,由其制备的混凝土强度稍有所下降;再生混凝土毛细累积吸水量明显减少,且抗氯盐侵蚀性能显著提高,其中骨料质量取代率为50%的再生混凝土浸渍处理后氯离子扩散系数降低了37.5%。研究表明,硅烷浸渍处理再生粗骨料是提高再生混凝土耐久性的有效途径。      

物理化学强化对再生混凝土抗氯离子渗透性能的影响

废弃混凝土经简单破碎、一次颗粒整形和二次颗粒整形后分别制得Ⅱ类再生粗骨料、准类再生粗骨Ⅰ料和Ⅰ类再生粗骨料,将三类再生粗骨料在浓度6%的有机硅烷防水剂中进行24h化学浸渍处理,得到三类物理化学强化再生粗骨料。分别研究不同品质物理化学强化再生粗骨料和不同取代率(取代率:0%、25%、50%、75%、100%)取代天然骨料对再生混凝土的抗氯离子渗透性能的影响。结果显示:再生粗骨料混凝土抗氯离子性能为:化学强化二次颗粒整形再生粗骨料混凝土化学强化一次颗粒整形再生粗骨料混凝土化学强化简单破碎再生粗骨料混凝土,且均优于普通混凝土;三类化学强化再生粗骨料混凝土的氯离子迁移系数均随着取代率的增大而减小。     

再生骨料强化方法研究进展

再生骨料混凝土作为新型环保材料,满足节约资源和能源、减少环境污染以及可持续发展战略的要求,是绿色混凝土的主要研究方向和推广方向.再生骨料表面附着老水泥砂浆,表面粗糙、棱角多,并含有大量孔洞以及二次破碎过程产生的微裂纹,与天然骨料相比,再生骨料存在密度低、吸水率大、压碎指标大、坚固性差、离散性大等不足,且再生骨料与其表面附着砂浆间存在薄弱界面区,使再生骨料混凝土的性能受到影响、工程应用受到限制.因此,提高再生骨料性能是再生骨料混凝土研究中的关键问题之一.传统提高再生骨料混凝土性能的方法主要聚焦在混凝土拌和阶段,通过改善拌和方法、加入矿物掺合料、优化混凝土配合比设计,或通过优化再生骨料级配、降低再生骨料取代率,以弱化再生骨料对再生骨料混凝土性能的不利影响,但本质上并未改善再生骨料的缺陷.而对再生骨料进行强化处理可改善再生骨料的性能.现有再生骨料强化方法主要从再生骨料表面附着砂浆着手,通过物理、化学或生物方法去除老砂浆或增强老砂浆来提高再生骨料的性能,如机械研磨、火山灰质浆液浸泡、纳米材料改性、二氧化碳强化法等.此外,通过骨料重组法对再生骨料成分进行分类、重组,也可在一定程度上提高再生骨料性能,进而达到改善再生骨料混凝土性能的目的.本文总结了国内外已有再生骨料强化方法的研究进展,分别介绍了去除老砂浆、增强老砂浆、骨料重组三类再生骨料强化方法及其作用机理,总结分析了再生骨料强化方法的强化效果,提出进一步研究再生骨料强化的方向.     

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。     

型钢再生混凝土组合柱轴压性能试验研究

为了研究型钢再生混凝土(SRRAC)组合柱的轴压性能,设计了23个试件进行轴压试验,考虑了再生粗骨料取代率、箍筋体积配箍率和混凝土强度等级3个变化参数。通过试验观察了试件的破坏形态,获取了试件受力全过程曲线、极限承载力等重要数据,并分析各变化参数对SRRAC柱轴心抗压承载性能的影响,基于试验提出其强度计算公式。研究结果表明:SRRAC柱破坏时型钢受压屈服、再生混凝土压碎,具有良好的承载性能,各变化参数均对其承载性能有显著影响,建议再生粗骨料最优取代率为40%,该文建议强度计算公式计算值与试验结果吻合较好。研究结果可供再生混凝土组合结构的进一步科学研究和工程应用参考。     

砖混再生粗骨料及其在混凝土中的研究与应用进展

随着我国城市化进程的加速以及人们居住水平的不断改善,20世纪60年代至80年代建设的大量砖混结构建筑物面临拆除,这些砖混结构物拆除后经破碎加工而成的粗骨料中含有大量碎砖、碎瓦和低强度的混凝土,简称砖混再生粗骨料（Mixed recycled coarse aggregate,MRCA）。MRCA的高值利用是建筑固废资源化利用的研究热点。MRCA具有吸水性高、压碎指标高、空隙率高以及密度低“三高一低”的物理特性。MRCA具有较大的棱角度、表面粗糙度和分形维数,较小的圆度等颗粒形态特征。MRCA混凝土破坏主要由低强度的碎砖骨料破坏及再生粗骨料与水泥砂浆界面过渡区粘结破坏导致。采用物理和化学方法对MRCA进行强化,并通过适合的配合比设计方法和制备技术可以改善MRCA混凝土性能。再生砖骨料占比是影响MRCA混凝土工作性能、力学强度及耐久性的重要参数之一。相关学者研究了再生砖骨料含量对MRCA混凝土工作性能、力学强度及耐久性能的影响规律,并基于应力应变关系和再生砖骨料的含量提出了MRCA混凝土损伤本构关系模型,探讨了MRCA混凝土的损伤演变规律。另一些学者测试了MRCA混凝土制备的梁、板和柱等构...     

On the classification of mixed construction and demolition waste aggregate by porosity and its impact on the mechanical performance of concrete

The properties of recycled aggregate produced from mixed (masonry and concrete) construction and demolition (C&D) waste are highly variable, and this restricts the use of such aggregate in structural concrete production. The development of classification techniques capable of reducing this variability is instrumental for quality control purposes and the production of high quality C&D aggregate. This paper investigates how the classification of C&D mixed coarse aggregate according to porosity influences the mechanical performance of concrete. Concretes using a variety of C&D aggregate porosity classes and different water/cement ratios were produced and the mechanical properties measured. For concretes produced with constant volume fractions of water, cement, natural sand and coarse aggregate from recycled mixed C&D waste, the compressive strength and Young modulus are direct exponential functions of the aggregate porosity. Sink and float technique is a simple laboratory density separation tool that facilitates the separation of cement particles with lower porosity, a difficult task when done only by visual sorting. For this experiment, separation using a 2.2 kg/dm3 suspension produced recycled aggregate (porosity less than 17%) which yielded good performance in concrete production. Industrial gravity separators may lead to the production of high quality recycled aggregate from mixed C&D waste for structural concrete applications.     

Effect of natural coarse aggregate type on the physical and mechanical properties of recycled coarse aggregates

Many environmental problems caused by the large volumes of construction and demolition waste (C&DW), the lack of adequate deposition sites and the shortage of natural resources have led to the use of C&DW as replacement of natural aggregates in the production of new concrete. As in the case of natural aggregates, when recycled aggregates are used to manufacture structural concrete, the assessment of their physical, mechanical and durable characteristics is a key issue. The different physical and mechanical properties of the recycled coarse aggregate (RCA) are evaluated. RCA was obtained by crushing conventional concretes with different strength levels (different w/c ratios) containing four different types of natural coarse aggregates (three crushed stones and a siliceous gravel), which differ in shape, composition and surface texture. There is a significant influence of the natural coarse aggregate (NCA) on the properties of RCA, which in many cases is greater than that of the w/c ratio of the source concrete.     

Compressive stress–strain behavior of steel fiber reinforced-recycled aggregate concrete

The use of recycled aggregate from construction and demolition waste (CDW) as replacement of fine and coarse natural aggregate has increased in recent years in order to reduce the high consumption of natural resources by the civil construction sector. In this work, an experimental investigation was carried out to investigate the influence of steel fiber reinforcement on the stress–strain behavior of concrete made with CDW aggregates. In addition, the flexural strength and splitting tensile strength of the mixtures were also determined. Natural coarse and fine aggregates were replaced by recycled coarse aggregate (RCA) and recycled fine aggregate (RFA) at two levels, 0% and 25%, by volume. Hooked end steel fibers with 35 mm of length and aspect ratio of 65 were used as reinforcement in a volume fraction of 0.75%. The research results show that the addition of steel fiber and recycled aggregate increased the mechanical strength and modified the fracture process relative to that of the reference concrete. The stress–strain behavior of recycled aggregate concrete was affected by the recycled aggregate and presented a more brittle behavior than the reference one. With the addition of steel fiber the toughness, measured by the slope of the descending branch of the stress–strain curve, of the recycled concretes was increased and their behavior under compression becomes similar to that of the fiber-reinforced natural aggregate concrete.     

Performance of mortars containing recycled fine aggregate from construction and demolition waste

Waste from construction and demolition accumulates in large quantities in the modern world. Recycled coarse aggregates derived from this waste can replace virgin aggregates used in the production of new concretes but the studies on the effect of using the fine fraction of this waste on the properties of new concrete have not yet led to clear conclusions. The present study evaluated the properties of recycled fine aggregates derived from two recycling plants using two different waste treatment procedures, as well as their effects on the properties of fresh and hardened mortars prepared using these aggregates at two water-to-cement ratios and three replacement ratios. It was found that the recycled aggregates were more porous than the natural aggregates and may have contained some organic matter. Setting times were longer when recycled aggregates replaced natural aggregates and strength and durability were reduced as well. Partial replacement of the fine aggregate is possible if an appropriate compensation of the water to cement ratio is applied.     

Influence of recycled aggregate on slump and bleeding of fresh concrete

The recycling of construction and demolition (C&;D) waste as a source of aggregates for the production of new concrete has attracted increasing interests from the construction industry. While the environmental benefits of using recycled aggregates are well accepted, some unsolved problems prevent this type of material from wide application in structural concrete. One of the major problems with the use of recycled aggregates in structural concrete is their high water absorption capacity which leads to difficulties in controlling the properties of fresh concrete and consequently influences the strength and durability of hardened concrete. This paper presents an experimental study on the properties of fresh concrete prepared with recycled aggregates. Concrete mixes with a target compressive strength of 35 MPa are prepared with the use of recycled aggregates at the levels from 0 to 100% of the total coarse aggregate. The influence of recycled aggregate on the slump and bleeding are investigated. The effect of delaying the starting time of bleeding tests and the effect of using fly ash on the bleeding of concrete are explored.     

Influence of field recycled coarse aggregate on properties of concrete

This paper investigates the influence of different amounts of recycled coarse aggregates obtained from a demolished RCC culvert 15 years old on the properties of recycled aggregate concrete (RAC). A new term called “coarse aggregate replacement ratio (CRR)” is introduced and is defined as the ratio of weight of recycled coarse aggregate to the total weight of coarse aggregate in a concrete mix. To analyze the behaviour of concrete in both the fresh and hardened state, a coarse aggregate replacement ratio of 0, 0.25, 0.50 and 1.0 are adopted in the concrete mixes. The properties namely compressive and indirect tensile strengths, modulus of elasticity, water absorption, volume of voids, density of hardened concrete and depth of chloride penetration are studied. From the experimental results it is observed that the concrete cured in air after 7 days of wet curing shows better strength than concrete cured completely under water for 28 days for all coarse aggregate replacement ratios. The volume of voids and water absorption of recycled aggregate concrete are 2.61 and 1.82% higher than those of normal concrete due to the high absorption capacity of old mortar adhered to recycled aggregates. The relationships among compressive strength, tensile strengths and modulus of elasticity are developed and verified with the models reported in the literature for both normal and recycled aggregate concrete. In addition, the non-destructive testing parameters such as rebound number and UPV (Ultrasonic pulse velocity) are reported. The study demonstrates the potential use of field recycled coarse aggregates (RCA) in concrete.     

Bond behavior between steel reinforcement and recycled concrete

In this paper the bond behavior of recycled aggregate concrete was characterized by replacing different percentages of natural coarse aggregate with recycled coarse aggregate (20, 50 and 100 %). The results made it possible to establish the differences between the conventional concrete bond strength and the recycled concrete bond strength depending on the replacement percentage. It was thus found that bond stress decreases with the increase of the percentage of recycled coarse aggregate used. In order to define the influence of recycled aggregate content on bond behavior, normalized bond strength was calculated taking into account the reduced compressive strength of the recycled concretes. Finally, using the experimental results, a modified expression for maximum bond stress (bond strength) prediction was developed, taking into account replacement percentage and compressive strength. The obtained results show that the equation proposed provides an experimental value to theoretical prediction ratio similar to that of conventional concrete.     

The role of 0–2 mm fine recycled concrete aggregate on the compressive and splitting tensile strengths of recycled concrete aggregate concrete

The aim of this study is to investigate the role of 0–2 mm fine aggregate on the compressive and splitting tensile strengths of recycled concrete aggregate (RCA) concrete with normal and high strengths. Normal coarse and fine aggregates were substituted with the same grading of RCAs in two normal and high strength concrete mixtures. In addition, to keep the same slump value for all mixes, additional water or superplasticizer were used in the RCA concretes. The compressive and splitting tensile strengths were measured at 3, 7 and 28 days. Test results show that coarse and fine RCAs, which were achieved from a parent concrete with 30 MPa compressive strength, have about 11.5 and 3.5 times higher water absorption than normal coarse and fine aggregates, respectively. The density of RCAs was about 20% less than normal aggregates, and, hence, the density of RCA concrete was about 8–13.5% less than normal aggregate concrete. The use of RCA instead of normal aggregates reduced the compressive and splitting tensile strengths in both normal and high strength concrete. The reduction in the splitting tensile strength was more pronounced than for the compressive strength. However, both strengths could be improved by incorporating silica fume and/or normal fine aggregates of 0–2 mm size in the RCA concrete mixture. The positive effect of the contribution of normal sand of 0–2 mm in RCA concrete is more pronounced in the compressive strength of a normal strength concrete and in the splitting tensile strength of high strength concrete. In addition, some equation predictions of the splitting tensile strength from compressive strength are recommended for both normal and RCA concretes.     

Caractérisation de la durabilité des bétons recyclés à base de gros et fins granulats de briques et de béton concassés

Among the transport phenomena, water absorption, water permeability and shrinkage prove to be of primary and great importance for the evaluation of durability of recycled concrete with coarse and fine recycled aggregates. Either coarse aggregates, fine aggregates or both coarse and fine aggregates were partially replaced (25, 50, 75 and 100%) with crushed concrete and brick aggregates. The results indicate that water absorption is high and water permeability can be double that of concrete made with 100% natural aggregate concrete. This study also showed that recycled concrete mix having the highest water absorption and water permeably corresponds always to the mix with the highest shrinkage. The physical and mechanical properties of recycled concretes seem to be acceptable.