再生混凝土单轴受压下多重界面过渡区的破坏机理研究

为研究再生粗骨料混凝土在单轴受压荷载下的破坏特性,使用数字图像相关技术记录混凝土立方体在破坏过程中的应变变化规律,并结合显微硬度测试、背散射电子成像技术等观测手段对界面过渡区的宽度和孔隙率进行定量表征。试验结果表明,分别经骨料强化和砂浆强化后,再生混凝土的抗压强度较未处理试样提升了17.86%和35.55%,说明砂浆强化更有利于提升再生混凝土的抗压强度。骨料强化可以提升老砂浆显微硬度值,降低老骨料-老砂浆界面过渡区孔隙率;砂浆强化可以提升新砂浆显微硬度值,降低老骨料-新砂浆界面过渡区以及新-老砂浆界面过渡区的孔隙率。经强化处理后的特定界面产生贯通裂缝的概率降低。     

基于细观尺度的再生混凝土性能研究综述

基于当前国内外学者对再生混凝土性能试验的结果,研究骨料、砂浆、界面过渡区等细观参数对再生混凝土宏观性能影响.在骨料方面,骨料分布的均匀程度高、颗粒级配较好、大中型骨料颗粒数越多的再生混凝土在抗拉、抗压、抗折强度及弹性模量等方面都会有所提高;在砂浆方面,随着新、老砂浆的强度提高,再生混凝土力学性能更好,老砂浆厚度越高,再生混凝土内部氯离子浓度越大;在界面过渡区方面,随着新、老界面过渡区弹性模量的提高,再生混凝土的性能表现地越好,界面过渡区厚度越大,氯离子扩散系数越大.     

再生粗骨料附着砂浆含量对再生混凝土抗冻耐久性的影响

选取五种不同附着砂浆含量的商用再生粗骨料在全取代天然粗骨料条件下制备C40再生混凝土,探讨了30%极限拉应力水平下,再生粗骨料附着砂浆含量对再生混凝土抗冻耐久性的影响规律,通过数学拟合确定了基于相对动弹性模量和抗冻耐久性指数要求的砂浆界限含量。结果表明:冻融循环后,再生混凝土抗压强度损失率与附着砂浆含量存在较强线性关系;再生混凝土质量损失率、相对动弹性模量、抗冻耐久性指数均与再生粗骨料附着砂浆含量呈二次函数关系;以冻融循环300次后再生混凝土相对动弹性模量降至60%为限值,获得对应的砂浆界限含量为49.52%(质量分数);以不同设计使用年限及不同冻融工况所规定的抗冻耐久性指数作为界定条件,得到了再生混凝土在拉应力条件下满足相应抗冻耐久性指数的砂浆界限含量。最后,通过搜集文献数据验证了本研究结果的合理性。     

Curing and constitutive relationships for polyester mortar

The compressive and tensile properties of polyester mortar were studied under various curing conditions, temperature, and strain rate. The curing temperature was varied from room temperature to 80°C. The behavior of polyester mortar was studied using a uniform sand with strain rate and temperature varied between 0.01 to 6 percent strain per minute and 22°C and 120°C, respectively. The strength, failure strain, modulus and stress-strain relationships of polyester mortar are influenced by the curing method, testing temperature, and strain rate to varying degrees. The influence of test variables on the mechanical properties of polyester mortar are quantified. Pretreating the aggregates with a silane coupling agent further enhances the compressive and tensile strength of the mortar. The compressive modulus and splitting tensile strength of polyester mortar are related to the compressive strength. A constitutive model is used to predict the compressive stress-strain behavior of polyester mortar.     

Experimental study of the material and bond properties of frost-damaged concrete

In an extensive experimental investigation, several types of tests were conducted on a reference specimen and frost-damaged concrete. Two levels of internal frost damage were quantified by the relative dynamic modulus of elasticity and compressive strength. Test results showed a significant influence of freeze–thaw cycles on the compressive strength and even more influence on the modulus of elasticity and the compressive strain at peak stress. Reduced tensile strength and increased fracture energy were measured. From inverse analysis of wedge splitting test results, a significant effect of frost on the shape of the tensile stress–crack opening relationship was observed: tensile strength was reduced, while the post-peak behaviour was more ductile for the frost-damaged concrete. Pull-out tests showed the influence of freeze–thaw cycles on bond strength and slip. The pull-out test results are compared with similar tests available in the literature and the effect of frost on bond behaviour is discussed.     

Properties of fresh and hardened concrete

The present paper reviews the literature related to the properties of fresh and hardened concrete published after the previous (12th) International Congress on the Chemistry of Cement held in Montreal in 2007.Workability and fundamental rheological properties, reversible and non-reversible evolution, thixotropy, slump loss, setting time, bleeding, segregation and practical issues related to formwork filling and pressure, are addressed among the properties of fresh concrete.Among hardened concrete properties compressive strength and other mechanical and physical properties of hardened concrete, such as tensile strength, elastic properties, shrinkage, creep, cracking resistance, electrical, thermal, transport and other properties are covered. Testing, interpretation, modeling and prediction of properties are addressed, as well as correlation with properties of fresh concrete and durability, effects of special binders, recycled and natural aggregates, fiber reinforcement, mineral and chemical admixtures. Special attention is given to the properties of hardened lightweight and self-compacting concrete.     

Effect of strain rate and temperature on the performance of epoxy mortar

The behavior of epoxy mortar was studied under various curing conditions, temperature and strain rate. The effect of aggregate size and distribution on the mechanical properties of epoxy mortar was also studied. Epoxy mortar with a uniform fine sand was cured at various temperatures to determine the optimum curing condition. The strain rate was varied between 0.01 to 6 percent strain per minute and the testing temperature between 22°C and 80°C. The strength, modulus, and compressive strain-strain relationship of polymer mortar are influenced by the curing method, testing temperature, and strain rate to varying degrees. The influence of test variables on the mechanical properties of epoxy mortar are quantified. Compared to the uniformly graded fine aggregate fillers the gap-graded aggregates produced polymer mortar with better mechanical properties. The compressive modulus and splitting tensile strength of epoxy mortar are related to their compressive strength. A new nonlinear constitutive model is proposed to predict the complete compressive stress-strain behavior of epoxy mortar. The constitutive relationship parameters are also related to the testing temperature and logarithmic strain rate.     

Post-fire residual mechanical properties of concrete made with recycled concrete coarse aggregates

This paper presents results of an experimental study on the residual mechanical performance of concrete produced with recycled coarse aggregates, after being subjected to high temperatures. Four different concrete compositions were prepared: a reference concrete made with natural coarse aggregates and three concrete mixes with replacement rates of 20%, 50% and 100% of natural coarse aggregates by recycled concrete coarse aggregates. Specimens were exposed for a period of 1 h to temperatures of 400 °C, 600 °C and 800 °C, after being heated in accordance with ISO 834 time–temperature curve. After cooling down to ambient temperature, the following basic mechanical properties were then evaluated and compared with reference values obtained prior to thermal exposure: (i) compressive strength; (ii) tensile splitting strength; and (iii) elasticity modulus. Results obtained show that there are no significant differences in the thermal response and post-fire mechanical behaviour of concrete made with recycled coarse aggregates, when compared to conventional concrete.     

再生骨料和橡胶颗粒对透水混凝土性能的影响

以C20透水混凝土为设计基准,再生骨料和橡胶颗粒掺量为影响因素,通过测定透水混凝土的连通孔隙率、透水系数、28 d抗压强度、劈裂抗拉强度及弹性模量等性能指标,研究再生骨料和橡胶颗粒对透水混凝土基本性能的影响规律;同时,采用拟合回归和综合对比的方法建立透水混凝土连通孔隙率与透水系数、劈裂抗拉强度及弹性模量与28 d抗压强度之间的关系模型.     

The effect of compressive prestressing on the mechanical properties of some nuclear graphites

The effects of prestressing in compression at room temperature on the Young's modulus, the tensile strength, and the compressive strength of some nuclear graphites were examined. Also the effects of strain rate in the range of 10⁻⁵ ~ 10⁻² sec⁻¹ on the room temperature tensile and compressive strengths were investigated. The Young's modulus and the tensile strength of the graphites decreased as the compressive prestress increased above a particular prestress level. However, the compressive strength of the graphites did not change appreciably for the above range of strain rates. The decrease in Young's modulus and tensile strength with increasing prestress is considered to be due to two factors: an increase in the dislocation density and an increase in the size and number of cracks produced. It is inferred that the decrease in the strength at higher prestress levels is due to the formation and propagation of cracks with increasing prestress, since some growth of cracks was observed in the higher prestressed graphites.     

不同类型再生细骨料对保温混凝土力学性能的影响

为了研究不同类型再生细骨料对玻化微珠保温混凝土力学性能的影响,将废弃混凝土再生细骨料和废弃黏土砖再生细骨料分别以25%、50%、75%和100%(体积分数)取代率取代保温混凝土中的天然河砂,并测试了再生保温混凝土的抗压强度、劈裂抗拉强度和弹性模量。结果表明: 25%是废弃混凝土再生细骨料在保温混凝土中的最优取代率;75%是废弃黏土砖再生细骨料在保温混凝土中的最优取代率。再生细骨料的压碎值和再生胶砂强度比在实际工程中可以有效区分不同类型再生细骨料的品质。废弃黏土砖再生细骨料中的火山灰材料可以有效提高再生保温混凝土的密实度,而废弃混凝土再生细骨料中残余的氢氧化钙却降低了再生保温混凝土的力学性能,因而废弃黏土砖再生细骨料具有更高的回收利用价值。     

玄武岩纤维混凝土耐高温性能分析

对不同玄武岩纤维体积率混凝土进行室内高温试验,总结与分析了温度和纤维体积率对混凝土立方体抗压强度、劈裂抗拉强度和静弹性模量的影响规律。研究结果表明:玄武岩混凝土的抗压强度、抗拉强度和弹性模量均在200℃高温出现拐点,200℃高温后玄武岩纤维混凝土的力学性能均出现不同程度的降低;混凝土的力学性能随玄武岩纤维体积率的增大而呈现出先增大后减小的趋势,最优的玄武岩纤维体积率为0.15%;玄武岩再生混凝土的力学性能随再生骨料取代率的增大而减弱,再生骨料取代率不宜大于30%。     

再生粗骨料混凝土抗冻性能研究进展

将再生粗骨料应用于混凝土制备可有效减少原材料生产对生态环境的破坏,还可实现建筑废弃物的再生利用。但由于存在老旧砂浆包裹、原始缺陷较多等问题,再生粗骨料混凝土吸水率较高且薄弱界面较多,抗冻性能较差,对寒冷环境下的混凝土结构产生了极为不利的影响。本文综合分析国内外对再生粗骨料混凝土抗冻性能的研究现状,发现复杂脆弱的界面过渡区是再生粗骨料混凝土承受冻害时的薄弱环节,微裂缝和孔隙的发育使其在冻融循环过程中的性能不断降低。再生混凝土的抗冻性能随着再生粗骨料掺量的增加而加速劣化,再生粗骨料质量掺量应控制在50%以内。质量损失、相对动弹性模量和抗压强度是评价再生混凝土冻融损伤水平的常用指标,质量损失在试验初期会出现负增长情况,相对动弹性模量在有限冻融循环次数下的反应较不敏感,相比之下抗压强度能更好地体现再生粗骨料混凝土的冻融损伤水平。未来,还应对再生混凝土冻融损伤机理进行更加系统深入的研究,探究效果良好的非破坏性试验指标,以及在标准冻融试验的基础上结合特定工程环境对再生混凝土抗冻性能进行具体评价,推动再生粗骨料混凝土更加广泛的应用。     

Thermal and residual mechanical profile of recycled aggregate concrete prepared with carbonated concrete aggregates after exposure to elevated temperatures

Thermal and residual mechanical performance of recycled aggregate concrete (RAC) prepared with recycled concrete aggregates (RCAs) after exposure to high temperatures has so far received less attention than that of conventional concrete prepared with natural aggregates (NAs). This study experimentally investigated thermal and residual mechanical performance of RAC prepared with different replacement percentages of non‐carbonated and carbonated RCAs after exposure to high temperatures. The residual mechanical properties, including compressive strength, modulus of elasticity, and peak strain at the maximum strength, were measured for evaluating the fire resistance of RAC. The experimental results showed that although the fire‐resistant ability of natural granite aggregates was high, thermal deterioration of the conventional concrete after exposure to 600°C, presented by thermal induced mesocracks, was more serious than that of RAC due to thermal incompatibility between NAs and mortar. Using the carbonated RCAs can reduce the width of thermal mesocrack in RAC. The residual mechanical properties of RAC after exposure to 600°C can be obviously improved by incorporating 20% to 40% of the carbonated RCAs. For the RAC made with the 100% carbonated RCAs, the ratio of residual to initial compressive strength after exposure to above 500°C was even higher than that of the conventional concrete.     

水泥细度对早龄期碾压混凝土综合抗裂性的影响(英文)

水泥水化热与比表面积和化学组成有关,但是相对于调整水泥的化学组成来说,通过减小水泥的比表面积来降低水泥水化热要容易得多。为了探索水泥比表面积与碾压混凝土抗裂性能的关系,采用相同熟料磨制了3种细度的水泥,研究了水泥细度对水化热、胶砂强度的影响,以及对混凝土的工作性、力学性能(抗压强度、抗拉强度和抗拉弹性模量)、极限拉伸值、绝热温升等性能的影响;同时,采用温度–应力试验机,评估了在100%约束和近似绝热条件下水泥细度对早龄期碾压混凝土综合抗裂性能的影响。结果表明:水化热与比表面积成线性关系,降低水泥比表面积是降低混凝土温升的有效、便捷的措施;粗磨水泥提高了碾压混凝土的工作性,降低了混凝土的抗压强度和弹性模量,但混凝土极限拉伸值没有明显变化;温度–应力试验表明,随着水泥比表面积的降低,混凝土第二零应力温度更低,粗磨水泥碾压混凝土综合抗裂风险更低。     

磁化水钢纤维再生混凝土早期强度研究

为研究磁化水和钢纤维对再生混凝土早期强度的影响,以C40强度为基准,研究分析不同磁场强度(0 mT、200 mT、260 mT、320 mT)的磁化水和不同体积掺量的钢纤维(0%、0.6%、1.2%)对再生混凝土立方体早期抗压强度和劈裂抗拉强度的影响,并对其微观结构进行观察分析。试验结果表明:钢纤维能够显著提高再生混凝土早期抗压强度和劈裂抗拉强度;磁化水对于再生混凝土抗压强度和劈裂抗拉强度均有不同程度的提升,其中劈裂抗拉强度增幅较小;在0.6%钢纤维掺量和260 mT磁场强度下再生混凝土早期抗压强度增幅较为明显。     

掺级配良好再生PVC骨料混凝土的力学和吸能性能

将级配良好的再生聚氯乙烯(PVC)颗粒以不同掺量等体积替代天然细骨料后加入混凝土中制成试件,进行力学性能试验和冲击试验,得到立方体压缩强度、劈裂抗拉强度、卸载弹性模量、能量吸收率和微观结构图,用来探究不同掺量的再生PVC骨料混凝土的力学和吸能性能。结果表明,当PVC骨料掺量为0%,5%,10%,15%,20%,30%时,其混凝土压缩强度分别为49.34,52.44,45.79,46.41,45.6,43.46 MPa,劈裂抗拉强度为2.73,3.58,3.2,2.92,2.69,2.44 MPa,混凝土的压缩强度和抗拉强度均随掺量增加呈先增加后缓慢降低趋势,并且用级配良好的再生PVC颗粒替代天然细骨料加入混凝土的力学性能比单一粒径塑料颗粒优良;随PVC骨料掺量增加,混凝土脆性得到改善且延性增强;混凝土的能量吸收能力随再生PVC细骨料掺量增加呈直线增加趋势。     

Mechanical properties of recycled aggregate concrete under uniaxial loading

In this paper, the compressive strength and the stress-strain curve (SSC) of recycled aggregate concrete (RAC) with different replacement percentages of recycled coarse aggregate (RCA) are investigated experimentally. Concrete specimens were fabricated and tested with different RCA replacement percentages of 0%, 30%, 50%, 70% and 100%, respectively. Uniaxial compression loading is applied in the experiments. Special attention of the analysis is devoted to the failure behaviour and the influences of the RCA contents on the compressive strength, the elastic modulus, the peak and the ultimate strains of RAC. Analytical expressions for the peak strain and the stress-strain relationship of RAC are given, which can be directly used in theoretical and numerical analysis as well as practical engineering design of RAC structures.     

Properties of lightweight expanded polystyrene aggregate concretes containing fly ash

Lightweight concretes can be produced by replacing the normal aggregates in concrete or mortar either partially or fully, depending upon the requirements of density and strength levels. The present study covers the use of expanded polystyrene (EPS) beads as lightweight aggregate, both in concrete and mortar. The main aim of this programme is to study the mechanical properties of EPS concretes containing fly ash and compare the results with these in literature on concretes containing OPC alone as the binder. The effects of EPS aggregate on the green and hardened state characteristics of concretes containing fly ash were evaluated. The compressive strength of the EPS concretes containing fly ash show a continuous gain even up to 90 days, unlike that reported for OPC in literature. It was also found that the failure of these concretes both in compression and split tension was gradual as was observed earlier for the concretes containing plastic shredded aggregates. The stress-strain relations and the corresponding elastic modulus were also investigated.     

温度对碱激发再生混凝土微粉砂浆抗压强度的影响

以再生微粉砂浆的强度、微观孔隙率为指标,结合部分再生微粉砂浆的XRD衍射图谱,对比研究在不同温度下,碱激发和不用碱激发再生微粉活性制备砂浆的抗压强度大小.通过比较研究发现碱激发反应温度对再生微粉砂浆抗压强度影响很大;温度过低,碱激发反应慢,一定时间内生成的胶凝性物质少,砂浆内孔隙率高,抗压强度低;温度过高,碱激发反应快,体系内生成胶凝土物凝结太快,砂浆孔隙率高,抗压强度低;因此碱激发再生微粉活性存在合适温度.