外掺MgO水泥净浆和砂浆小尺寸试件的压蒸膨胀变形

为了促进外掺MgO混凝土的推广应用,本文对外掺MgO水泥净浆和砂浆小尺寸试件的压蒸膨胀变形进行了试验研究。结果表明,试件尺寸对外掺MgO水泥净浆和砂浆的压蒸膨胀变形产生明显影响。压蒸膨胀率随着试件尺寸变化的规律是:微试件小试件标准试件。相应地,MgO极限掺量随着试件尺寸变化的规律是:微试件小试件标准试件。粉煤灰对外掺MgO水泥净浆和砂浆小尺寸试件压蒸膨胀变形的抑制作用依然存在,且粉煤灰掺量越大,其抑制膨胀变形的作用越明显。     

干湿循环作用下水泥基复合材料抗氯离子侵蚀性能及其微观结构变化

为了进一步揭示干湿循环条件下氯离子对水泥基复合材料的侵蚀机制,分别研究了两种水灰比水泥混凝土、砂浆及净浆三种层次试件在干湿循环条件下的抗氯离子侵蚀性能。结果发现,干湿循环作用能促进氯离子入侵速度,增加氯离子入侵深度和浓度,并且水胶比越大,干湿循环作用影响效果越明显;掺入矿粉能增加水泥基复合材料抗氯离子入侵性能,并且降低干湿循环作用的影响效果。净浆样品侵蚀前后的微观结构变化表明,干湿循环作用下,经氯盐侵蚀后水泥基复合材料净浆试件表层明显有Friedel盐生成,而矿粉的加入明显增加了样品中Friedel盐的生成量。     

钢纤维-橡胶/混凝土单轴受压全曲线试验及本构模型

将钢纤维(SF)掺入橡胶混凝土中,能够改善由于橡胶颗粒掺入导致的强度降低,并进一步增加延性。为研究SF-橡胶/混凝土的抗压性能,配制得到SF体积分数分别为0vol%、0.5vol%、1.0vol%和1.5vol%及橡胶颗粒等体积替换砂率为0%、10%和20%的10组SF-橡胶/混凝土试件,并进行单轴受压全曲线试验。结果表明:SF的桥联作用及其与橡胶颗粒的协同作用可改善混凝土的抗压性能,试件破坏呈明显延性特征。随SF掺量的增加,SF-橡胶/混凝土试件的抗压强度及弹性模量均明显增大,其相应峰值应力的应变及全曲线峰值后延性也相应增加;随橡胶颗粒掺量的增加,SF-橡胶/混凝土试件相应峰值应力的应变及全曲线峰值后延性增加,而抗压强度及弹性模量有所减小。在已有研究基础上,通过曲线拟合试验数据,提出适用于SF-橡胶/混凝土的单轴受压应力-应变全曲线数学表达式,模型与试验结果吻合较好,为此类混凝土的结构分析设计提供了理论基础。      

硬化水泥净浆基于纳米压痕的相态识别与水化程度计算

采用纳米原位压痕手段测量硬化水泥净浆中单一相态的代表性微观力学性能,并采用纳米点阵压痕研究各相态的含量。研究对象囊括水灰比为0.3、0.4、0.5的纯水泥净浆和水灰比为0.3情况下含50%、70%矿渣掺量复合体系,共5种配比,以表征它们的相态分布和微观力学性质的异同点。掺矿渣的试件中含有明显多的复合相,因此提出三相模型测算复合相中未水化物的体积分数。此外,提出基于纳米压痕技术计算纯水泥和掺矿渣水泥试件水化程度的方法,结果吻合于热重分析的结果,其中纯水泥净浆中复合相较少,计算得到的水化程度优于对掺矿渣水泥试件的计算。     

钢纤维-橡胶/混凝土抗剪性能试验

抗剪强度和剪切韧性是反映构件在复合受力状态下承载能力及耗能能力的重要指标。为研究钢纤维(SF)-橡胶/混凝土的剪切性能,设计了14组SF-橡胶/混凝土试件,通过双面剪切试验,研究了SF体积分数掺量、橡胶掺量和水胶比对SF-橡胶/混凝土试件的抗剪性能及剪切破坏形态的影响。研究表明:SF的桥联作用及其与橡胶颗粒的协同作用可显著改善混凝土的抗剪性能。SF对SF-橡胶/混凝土试件的抗剪性能起主导作用,SF-橡胶/混凝土试件的抗剪强度、峰值变形及剪切韧性相比普通混凝土及橡胶/混凝土试件均显著提高,且增幅随SF掺量的增加而增大,剪切破坏呈现出明显的延性特征。当SF体积分数为1.5vol%时,橡胶掺量(等体积取代砂取代率)为10%的SF-橡胶/混凝土试件的抗剪强度、峰值变形相比橡胶/混凝土分别提高了78%、63%。橡胶对SF-橡胶/混凝土试件的抗剪性能也起到辅助作用,SF-橡胶/混凝土试件的剪切韧性及延性相比SF/混凝土试件进一步增加。采用水胶比优化设计后,随着橡胶掺量的增加,SF-橡胶/混凝土的抗剪强度、峰值变形及峰值前剪切韧性可基本保持不变,而峰值后韧性指标进一步增加,增幅可高达96%。根据试验结果,考虑橡胶及SF掺量的影响提出了SF-橡胶/混凝土的抗剪强度计算式。      

MgO膨胀剂在水工混凝土中的极限掺量研究

为了满足水工大坝对外掺MgO混凝土自生体积膨胀变形的需要和促进外掺MgO混凝土的推广应用,提出采用砂子等量替代混凝土中的粗骨料拌制"模拟砂浆"试件进行压蒸实验。结果表明,采用该法测得的压蒸膨胀率均比采用水泥净浆、水泥砂浆、一级配混凝土作为压蒸试件的测值低,由此确定的混凝土的MgO极限掺量均比现行方法高;若以压蒸膨胀率随MgO掺量变化曲线的拐点对应的MgO掺量作为混凝土的MgO极限掺量,则当粉煤灰掺量为40%时,采用"模拟砂浆"试件确定的MgO极限掺量可达到12%。     

橡胶粒径和掺量对水泥基材料收缩和抗开裂性能的影响

本文考虑了粉末状和颗粒状两种典型粒径的橡胶,定量研究橡胶粒径和掺量对水泥基材料收缩和抗开裂性能影响。采用直接测量长度的方法研究橡胶对水泥基材料自由收缩的影响;采用圆环约束收缩实验方法研究橡胶对水泥基材料约束收缩和抗开裂性能的影响。试验结果表明:橡胶的掺入会加剧水泥基材料的自由收缩。橡胶掺入量相同时,掺入橡胶粉末的水泥基材料比掺入橡胶颗粒的水泥基材料自由收缩率高。橡胶的掺入会抑制水泥基材料的约束收缩和开裂。橡胶掺入量相同时,掺入橡胶粉末的水泥基材料比掺入橡胶颗粒的水泥基材料抗开裂性能好。     

浅谈混凝土外加剂在施工中的应用

混凝土外加剂是在混凝土(包括砂浆、净浆)拌和时或掺入的一种能对混凝土的正常性能按施工要求而改性的精细化工产品,其掺量一般不大于水泥生理的5%。     

改性水稻秸秆对水泥基材料性能影响研究

为了减少农作物纤维废弃造成的污染,研究了利用植物纤维制备建筑材料的工艺,首先采用碱煮法对植物纤维进行改性,然后将其分别与聚丙烯纤维按照不同的比例加入到水泥净浆中进行比较,结果表明:植物纤维在水泥净浆中的分散比加入聚丙烯纤维均匀,经过改性后的秸秆纤维对水泥凝结硬化时间无明显影响,随掺量增加,水泥净浆的抗折强度呈先增后减趋势,且在掺量为4.5%时最高,与未掺纤维的参照组相比抗折强度提高了11%,抗压强度降低了近24%。并将其掺入砂浆中,砂浆抗干缩性能得到改善,植物纤维掺量为6.0%时砂浆干燥收缩值比基准组下降了37%,在本研究掺量条件下,与加入聚丙烯纤维相比,加入秸秆纤维对水泥净浆的抗折强度和抗压强度影响较小,原因在于其分散性更好。     

氧化石墨烯/水泥复合净浆的化学收缩特性及预测模型

为了揭示氧化石墨烯/水泥复合净浆(GO/C)的化学收缩特性,采用体积法对不同水灰比(0.3、0.4、0.5)和不同氧化石墨烯(GO)质量分数(0wt%、0.01wt%、0.02wt%、0.03wt%、0.04wt%、0.05wt%)的水泥复合净浆试件化学收缩进行了测定。试验结果显示:随着水灰比的增加,GO/C化学收缩显著增大。同水灰比条件下掺有氧化石墨烯的GO/C试件前期化学收缩较普通水泥净浆有所下降,当GO质量分数为0.04wt%时,收缩值达到最小;在后期水化中,GO/C试件的化学收缩增长速度明显快于普通水泥净浆。分析表明,GO对水泥净浆孔结构的调控作用和对水化产物氢氧化钙晶体的键合行为是其影响水泥净浆化学收缩性能的主要原因。同时,通过收缩模型与试验值对比发现,现有的普通水泥化学收缩模型无法精准预测GO/C的化学收缩情况,因此,为考虑GO的影响,试验在吴浪模型的基础上,引入K(ξ,t)函数作为影响参数,并通过曲线拟合得到其具体表达式,从而建立起适用于GO/C化学收缩的预测模型。     

等强条件下橡胶粉对碾压混凝土强度与变形性能的影响

采用橡胶粉等体积代砂的方法,选择橡胶粉体积含量为5%、10%和12 %,通过试验探讨了等抗压强度(40 MPa)条件下橡胶粉用量对碾压混凝土力学强度、抗裂性能和变形性能的影响及其变化规律。结果表明：橡胶粉的掺入使碾压混凝土的韧性得到改善,压缩破坏表现出明显的韧性破坏特征,碾压橡胶混凝土的立方体抗压强度、轴心抗压强度、抗拉强度和抗弯强度之间具有与普通混凝土相近的对应关系;等抗压强度条件下,碾压橡胶混凝土的抗弯强度、抗拉强度和极限拉伸值较不掺橡胶粉的碾压混凝土有所增大;橡胶粉的添加不会改善碾压混凝土的干缩性能,反而会使干缩变形速率和干缩变形量有所增大。     

Dynamic compressive and splitting tensile properties of concrete containing recycled tyre rubber under high strain rates

In order to raise the efficiency of resource utilization, recycling waste rubber particles into concrete as aggregate has been widely accepted. When the size and content of the rubber particles are appropriate, rubberized concrete can achieve many excellent properties. This study investigated the impact of rubber replacement on dynamic compressive and splitting tensile properties of concrete. The split Hopkinson pressure bar tests of rubberized concrete containing 5%, 10%, 15% and 20% volume replacement for sand were completed. The failure modes, stress curves and dynamic strength values of rubberized concrete under high strain rates were recorded. The results reveal that the dynamic compressive and splitting tensile strength of rubberized concrete decrease with increasing rubber content. Meanwhile, peak strain increases with increasing rubber content. Dynamic increase factors (DIFs) of compressive and splitting tensile strength also were calculated, where rubberized concrete shows a stronger strain rate sensitivity. The analysis of specific energy absorption illustrates that rubberized concrete with 15% rubber replacement has the best impact toughness. In addition, ratios of dynamic compressive–tensile strength of rubberized concrete were calculated, which are between 3.82 and 5.39.     

再生橡胶颗粒对自流平砂浆力学性能影响的试验研究

分别研究了再生橡胶颗粒掺量、颗粒粒径及乳化沥青复合对自流平砂浆力学强度、折压比等力学性能参数的影响规律;提出了再生橡胶颗粒在水泥基材料体系中的三大作用效应,并分析了相应的影响机理。结果表明,再生橡胶颗粒掺量、颗粒粒径对自流平砂浆的抗折强度、抗压强度以及折压比存在较为显著的影响,当橡胶颗粒等量取代骨料体积15%时,体系的折压比达到最大,且橡胶颗粒与乳化沥青复掺后可进一步显著提高试件的折压比;与普通振捣密实成型砂浆体系相比,橡胶颗粒对自流平砂浆力学强度的影响未有明显的不同。     

Strength and shrinkage behaviors of roller-compacted concrete with rubber additives

The strength and shrinkage behaviors of roller-compacted concrete (RCR) with used tire-rubber additive were experimentally investigated by keeping the compressive strengths of all specimens at the level of 40 MPa. Four rubber contents were used: 50, 80, 100, and 120 kg/m³. Test results show that the rubber particles homogeneously distributed in RCR, rubber particle emersion was not observed during vibrating for compaction. The workability of RCR was slightly influenced by replacing portion of sand with the same volume of tire rubber particles. RCR specimen exhibited a ductile failure under compression. In comparison with the control concrete, when the compressive strength was kept constant for RCR, the tensile strength, flexural strength, and ultimate tension elongation increased with the increase of rubber content. The incorporation of tire rubber did not help in reducing the drying shrinkage, on the contrary, a little more drying shrinkage developed in RCR than that in the control concrete.     

钢纤维橡胶再生混凝土的抗冻性试验

为使废弃混凝土和再生橡胶在北方地区混凝土工程中得以应用,采用正交试验法研究再生粗骨料掺量、再生粗骨料强化方式、钢纤维掺量与橡胶掺量对钢纤维橡胶再生混凝土(C45)立方体抗压强度和抗冻性的影响规律。利用扫描电镜和螺旋CT扫描技术研究了钢纤维橡胶再生混凝土的宏观和细观结构及其对抗冻性能的影响机理。结果表明:橡胶颗粒掺量是影响再生混凝土含气量、抗压强度和相对动弹模量的重要因素,再生粗骨料掺量是影响相对动弹模量和强度损失率的次要因素,钢纤维掺量对混凝土抗压强度增强作用较小,粗骨料强化方式对混凝土性能影响不大;橡胶颗粒与砂浆界面的裂缝宽度在5~55μm之间,二者之间的相容性较差;当橡胶颗粒掺量(与砂的体积比)大于20%后,随橡胶颗粒掺量增大,混凝土内部孔洞数目增多,钢纤维橡胶再生混凝土抗压强度降低、抗冻性减弱。     

Impact response of lightweight mortars containing expanded perlite

This paper describes the mechanical response of lightweight mortars subjected to impact loading in flexure. Expanded perlite aggregate with a bulk density of 64 kg/m³ was used at between 0 and 8 times by volume of Portland cement to yield a range of mortars with density between 1000 and 2000 kg/m³. Some specimens were reinforced with a polypropylene microfibre at 0.1% volume fraction and the dynamic fracture toughness was evaluated by means of an instrumented drop-weight impact system. Companion tests were carried out in compression under quasi-static loading to standardise the mixes. The compressive strength and elastic modulus scale as the cube of the relative density, defined as the ratio of the density of the mortar to that of Portland cement paste. Whereas the flexural strength and fracture toughness were both linearly proportional to the relative density of the mortar under quasi-static loading, there was an increase in their sensitivity to relative density at higher loading rates. Contrary to what is seen in regular concrete, fibre reinforcement led to an increase in the stress-rate sensitivity of flexural strength in lightweight mortars. For the same impact velocity, the stress-rates experienced by a specimen was strongly influenced by its density. While the stress-rate sensitivity of flexural strength dropped with a decrease in the mix density, that of the fracture toughness was consistently higher for the lighter mixes.     

Rubber-mortar composites: Effect of composition on properties

The Weibull modulus of mortar specimens containing 35% (w/w of cement content) of NaOH-treated rubber particles was calculated showing a value of 9.1 for the control and 9.4 for the specimens with rubber, indicating that the incorporation of a high amount of rubber does not change the casting reproducibility. Since the flexural strength of these rubber-containing specimens was reduced by 43%, new composites were prepared using 10% of rubber as addition or aggregate. Water sorption by immersion, resistance to acid attack, flexural strength and freeze-thaw experiments were performed. Transport properties were improved for the addition-rubber-containing composites; the best results were obtained with the aggregate-rubber-containing composites. A reduction of 15% in flexural strength was observed for addition-rubber-containing composites and 25% for the aggregate-rubber-containing composites, roughly as expected if the flexural strength varies linearly with the rubber content. Furthermore, after 60 freezing and thawing cycles, a reduction of 75% in flexural strength was observed for the control specimens and only 20% for the addition-rubber-containing specimens.     

FRP tube encased rubberized concrete cylinders

In this study, FRP tube encased waste tire rubber modified concrete cylinders were investigated. Four batches of confined and unconfined concrete cylinders with a diameter of 101.6 mm and a height of 304.8 mm were prepared. Each batch contained three confined cylinders and three unconfined cylinders. The total number of effective cylinders prepared was 24. Batches 1–3 were made of rubberized concrete. In Batch 1, 15% by volume of coarse aggregate was replaced by waste tire fibers or stripes; in Batches 2 and 3, 15% by volume of sand and 30% by volume of sand were replaced by crumb rubbers, respectively. Batch 4 was a control batch with conventional plain concrete. Uniaxial compression tests were conducted on all the cylinders per ASTM C39. Strain gages were installed on the encased rubberized concrete cylinders to obtain local strain distributions. The compressive strength and strain at the peak load were compared with the predictions by available design-oriented confinement models. It is found that the FRP tube encased rubberized concrete cylinders have higher confinement effectiveness, ductility, and elastic regions than FRP tube encased conventional concrete cylinders. Waste tire fiber modified concrete performs better than crumb rubber modified concrete with a lower cost. Instead of volume contraction, FRP encased rubberized concrete cylinders experience volumetric expansion. The current design-oriented confinement models cannot consistently predict the compressive strength and strain of the encased cylinders. An 1-D coupon test cannot uniquely determine the hoop tensile strength of the FRP tubes which are subjected to a 2-D stress condition.     

橡胶对水泥基材料干燥收缩性能的影响

为了考察橡胶增加水泥基材料干燥收缩量的机制,以橡胶水泥砂浆作为研究对象,采用毛细管张力理论分析了造成水泥砂浆干燥收缩的因素。使用压汞试验研究橡胶/水泥砂浆的孔结构,并进行了弹性模量和干燥收缩试验。研究结果表明,橡胶掺入会降低水泥砂浆的弹性模量,增加其孔隙率和干燥收缩量,且相同掺量条件下,小粒径橡胶的作用效果更明显。基于试验数据,考虑橡胶掺入对砂浆弹性模量的折减系数KE和橡胶掺入对毛细孔(孔径50nm的孔隙)数量的增加系数K_h,拟合了橡胶对水泥砂浆干燥收缩的影响参数δ_(mr)。