自密实轻集料混凝土双轴受力力学性能

为探究自密实轻集料混凝土双轴力学性能,采用三轴试验机对其进行双轴压-压和双轴拉-压试验,得到不同工况下的应力-应变曲线及其破坏形态,通过提取应力-应变曲线峰值应力和峰值应变,并与相关文献普通混凝土与轻集料混凝土研究成果对比,分析自密实轻集料混凝土双轴力学性能。研究结果表明:双轴压-压工况下,当侧向压应力较低时,试件主要呈现剪切破坏形态;当侧向应力较高时,试件呈劈裂破坏形态。双轴拉-压工况下,试件主要呈劈拉破坏形态,与侧向应力无关。随着侧向压应力的提高,自密实轻集料混凝土主压应力相对比无侧向应力工况明显提高,峰值应力最大提高均值幅度为68.08%,主拉应力随侧向压应力的提高逐步降低,最大降低幅度为62.35%。应用Kupfer双轴受力破坏准则验证自密实轻集料混凝土受侧向应力影响变化规律较为保守,同时基于Kupfer提出自密实轻集料混凝土双轴力学性能破坏准则,所得到的破坏准则方程具有良好的适用性。      

自密实轻骨料混凝土压-剪复合受力力学性能

为探究自密实轻骨料混凝土压-剪复合受力力学性能,应用液压伺服机和材料压-剪试验机,对自密实轻骨料混凝土进行单轴受压、单轴劈裂抗拉和压-剪复合受力试验研究,通过试验得到不同加载工况下自密实轻骨料混凝土破坏形态和力-变形曲线,引用文献对普通混凝土和轻骨料混凝土压-剪复合受力研究数据,对比分析自密实轻骨料混凝土压-剪复合受力性能。研究结果表明:自密实轻骨料混凝土压-剪复合受力破坏形态与普通混凝土和轻骨料混凝土相类似,随着轴压比的提高,剪切破坏断面摩擦痕迹逐步明显,混凝土碎渣也逐步提高,自密实轻骨料混凝土剪切破坏强度、残余荷载和剪切破坏位移也随之提高;剪切破坏强度提高幅度高于普通混凝土和轻骨料混凝土,残余荷载受轴压比影响提高幅度高于普通混凝土,但略低于轻骨料混凝土。基于主应力空间结合普通混凝土和轻骨料混凝土压-剪试验数据,提出混凝土压-剪复合受力统一破坏准则,同时基于八面体应力空间,提出自密实轻骨料混凝土破坏准则,所提出的破坏准则具有良好的适用性。      

三轴受压粉煤灰陶粒轻骨料混凝土力学性能试验

以粉煤灰陶粒轻骨料浸泡时间、强度等级、侧向围压为变化参数,共设计120个粉煤灰陶粒轻骨料混凝土试件进行常规三轴受压试验,研究其在三轴应力状态下的力学性能。试验观察了轻骨料混凝土的破坏过程及破坏形态,获取了三轴受力状态下的应力-应变全过程曲线,分析了各变化参数对其三轴受压力学性能的影响规律。研究结果表明：随着围压值的增大,试件破坏由竖向劈裂破坏变为斜向剪切破坏,当围压值大于12 MPa时,试件则表现为外表无明显裂缝的鼓起破坏;应力-应变曲线受围压值影响较大,受骨料浸泡时间和强度等级的影响较小,围压值大于12 MPa后,曲线不再出现下降段;峰值应力随骨料浸泡时间、强度等级、侧向围压的增大而增大;峰值应变受陶粒浸泡时间的影响不大,随强度等级的增大而减小,随围压值的增大而增大;弹性模量随强度等级和围压值的增大而增大,受骨料浸泡时间的影响不显著。     

纤维增韧轻骨料混凝土单轴受压应力-应变全曲线试验研究

为探究纤维增韧后轻骨料混凝土应力-应变全曲线,完成了不同混凝土强度等级、纤维种类及掺量下的9组棱柱体单轴受压试验,分析了破坏过程和破坏特征,系统研究了各因素对峰值应力、峰值应变和弹性模量的影响,并结合已有研究给出了各曲线特征点计算模型,考虑纤维轻骨料混凝土自身特征,建立了分段式纤维轻骨料混凝土应力-应变全曲线模型。研究表明:轻骨料混凝土破坏特征与普通混凝土显著不同,掺入纤维有效抑制了其内部微裂缝的开展,起到阻裂、增韧的效果,且随混凝土强度等级、纤维种类及掺量变化差异明显;纤维增韧后试件的应力-应变曲线下降段坡度趋于平缓,脆性得到有效改善;建议的应力-应变全曲线模型与试验结果吻合良好,能够准确描述纤维增韧轻骨料混凝土在单轴受压作用下的受力变形特征。     

橡胶自密实混凝土疲劳断裂全过程声发射特征辨识参量试验研究

废旧轮胎日益增多带来严重的环境问题,将废旧轮胎橡胶颗粒作为混凝土骨料能够减少污染和减少天然骨料的使用。采用液压闭环伺服材料试验机MTS322试验装置(material testing system,MTS)对橡胶自密实混凝土梁进行了静态弯拉试验和不同加载工况下的疲劳弯拉试验。对橡胶自密实混凝土梁在三点弯作用下的断裂性能进行了系统地研究,并考虑了橡胶自密实混凝土梁在不同加载应力水平和应力比下的疲劳破坏规律。讨论了应力场强度因子与断裂韧度的比值与疲劳次数的对数之间的数学关系,建立了橡胶自密实混凝土的疲劳损伤预测模型。试验过程中还采用声发射技术实时测量了循环荷载作用下橡胶自密实混凝土梁内部的声发射信号,对比了不同加载应力水平和应力比下橡胶自密实混凝土梁内部裂缝发展的特征,发现了橡胶自密实混凝土梁内部裂缝的生长分为三个阶段,应力水平越低的情况下,橡胶自密实混凝土梁内部疲劳损伤程度越严重。     

钢纤维高性能轻骨料混凝土多轴强度和变形特性研究

为了研究钢纤维掺量和三轴应力比对高性能轻骨料混凝土破坏准则和本构关系的影响规律,进行了钢纤维全轻混凝土和钢纤维次轻混凝土多轴强度和变形特性的试验研究,考虑到试验机加载能力和新拌高性能轻骨料混凝土的工作性能,选取的钢纤维体积掺量为0、0.5%、1.0%和1.5%,试验加载路径有单轴拉、压,双轴等压和真三轴压。结果发现在单轴应力和低应力比条件下,钢纤维能够明显地发挥增强阻裂作用,随着钢纤维掺量的增加,中间主应力对极限抗压强度和峰值应变的影响越来越大,且钢纤维体积掺量对两种轻骨料混凝土应力-应变曲线下降段有一定的影响;在高应力比条件下,钢纤维体积掺量对峰值强度、峰值应变和应力-应变曲线下降段无明显影响,但对高应力比下轻骨料混凝土应力-应变曲线上特有的应力平台区域有较大的影响。考虑钢纤维含量特征参数的影响,对普通骨料混凝土的Kotsovos破坏准则进行了相应的修正,得出了适合钢纤维增强轻骨料混凝土的破坏准则表达式。     

再生混凝土三轴受压力学性能试验研究

为了研究再生混凝土在三轴应力状态下的力学性能,以再生骨料取代率和围压值为变化参数,设计了36个圆柱体试件,采用位移控制的加载方式进行常规三轴试验。通过试验观察了试件的破坏形态,获取试件受力破坏全过程应力-应变曲线以及峰值应力、峰值应变等特征参数,分析了再生骨料取代率和围压值对再生混凝土三轴受压力学性能的影响,提出了侧向围压值与初始弹性模量、竖向峰值应力、峰值应变之间的关系公式。研究结果表明:取代率对三轴受力再生混凝土的峰值应力和峰值应变影响不大,与天然混凝土相比,其变化分别在5%和2%的范围内波动,而取代率对再生混凝土的初始弹性模量有较大的影响,与天然混凝土相比,三轴受力再生混凝土的弹性模量提高了20%左右;侧向围压值对三轴受力再生混凝土的各项性能指标影响显著,增大围压值能有效提高再生混凝土的弹性模量、承载能力和变形性能。研究结果为再生混凝土的进一步科学研究和推广应用提供参考。     

钢纤维再生粗骨料混凝土受压声发射特性与损伤演化

为了探究钢纤维掺量(体积率0、0.8%、1.5%)对C30再生粗骨料混凝土(质量分数取代率0、100%)基本力学性能的影响，结合声发射特征参数和应力-应变曲线来描述钢纤维再生粗骨料混凝土在轴心受压过程中的损伤演化规律。试验结果表明，通过对声发射损伤定位、撞击计数与能量计数分析，可实现钢纤维再生粗骨料混凝土轴心受压破坏从累计损伤到微裂缝演变，再到宏观裂缝扩展的全过程动态监测，不同钢纤维掺量再生粗骨料混凝土与普通混凝土试件在加载过程中损伤点的密集集中位置与试件最终破坏位置相符；基于声发射累计撞击计数建立的混凝土损伤模型可用于分析钢纤维再生粗骨料混凝土的损伤演化规律。     

围压对Ⅱ型CA砂浆力学性能的影响

对不同围压下的CRTS-Ⅱ型CA砂浆进行了三轴压缩试验,并测定CA砂浆的应力应变曲线。分析了峰值应力、峰值应变、残余强度及弹性模量随围压的变化规律,并对其进行线性拟合。结果表明,随着围压的增大,应力应变曲线峰值处逐渐平缓;峰值应力、峰值应变及残余强度逐渐变大,弹性模量基本不变;CA砂浆试件破坏时的裂缝与轴线的夹角逐渐增大。     

自密实钢纤维轻骨料混凝土的早期性能与损伤分析

在自密实轻骨料混凝土基础之上掺入钢纤维配制出自密实钢纤维轻骨料混凝土,分析了自密实钢纤维轻骨料混凝土的抗压强度、抗拉强度等主要力学性能以及收缩、抗碳化等耐久性能,并与普通骨料自密实混凝土进行对比分析。探讨了钢纤维对于改善自密实轻骨料混凝土损伤所起的作用及其机理。结果表明:掺入钢纤维后自密实轻骨料混凝土的抗压强度增大,劈拉强度明显提高,收缩及抗碳化能力也有明显改善。与普通骨料混凝土相比,自密实钢纤维轻骨料混凝土初始裂缝的产生与发展得到有效抑制。     

Acoustic emission waveform characterization of crack origin and mode in fractured and ASR damaged concrete

Different constituents of concrete can have cracking behavior that varies in terms of the acoustic waveform that is generated. Understanding the waveform may provide insight into the source and behavior of a crack that occurs in a cementitious composite. In this study, passive acoustic emission (AE) was used to investigate the waveform properties of the individual components of concrete (i.e., aggregate, paste, and interfacial transition zone (ITZ)). First, acoustic events produced by cracks generated using mechanical loading in a wedge splitting test were detected. It was observed that cracks that occurred through the aggregate have an AE frequency range between 300 kHz and 400 kHz, while cracks that propagated through the matrix (paste and ITZ) have a frequency range between 100 kHz and 300 kHz. Second, tests were performed using samples that were susceptible to alkali silica reaction; and AE and X-ray computed tomography were used to detect cracking. AE events with a frequency range between 300 kHz and 400 kHz were detected at early ages, suggesting the initiation of cracks within reactive aggregate. At later ages, AE events were detected with frequency ranges of 100–300 kHz, indicating crack development and propagation in the matrix.     

Freeze-thaw crack determination in cementitious materials using 3D X-ray computed tomography and acoustic emission

As concrete freezes and thaws cracks may develop. These cracks can provide a path for water and ionic species to penetrate the concrete. This may reduce the service-life of the concrete element. In this study, X-ray computed tomography (CT) was used as a non-destructive technique to characterize the microstructure of mortar samples that were exposed to different levels of freeze-thaw damage by varying degree of saturation in the samples (75, 90, 95, and 100% degrees of saturation). Acoustic emission (AE) experiments were performed during freezing and thawing to investigate sample cracking behavior. The volume of cracks present within the mortar samples after freezing and thawing were determined using X-ray CT and compared to passive acoustic emission data. The location/source of cracks was also determined using X-ray CT. The crack sources (i.e., void, aggregate, interfacial transition zone, or paste) were determined using X-ray CT and were related to AE activities during cracking. Crack volumes were found to increase with increased levels of saturation, and visual observations of cracking were found to correlate with AE signatures of various crack sources.     

Comportement mécanique des modèles contenant des granulats légers

Résumé Nous avons schématisé le béton léger par un modèle élémentaire plan constituté d'un granulat léger dans une matrice. Nous avons étudié expérimentalement le comportement mécanique de ce modèle sous des sollicitations de compression monoaxiale et biaxiale.

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Summary An elementary plane model consisting of a circular lightweight aggregate bonded in a matrix is used to represent lightweight concrete. The mechanical behaviour of this model is examined in uniaxial and biaxial compression, then compared with that of models where the singularity is first a hole and then an aggregate whose rigidity is practically infinite compared with that of the matrix. The localization of cracks and the cracking and failure fields in models in the R₂ space of applied stresses show that the mechanical behaviour of models containing lightweight aggregates is very close to that of models where singularities consist of holes. Research reveals that the crack and failure limit states of models in biaxial compression are not independent of the value of the principal intermediate stress. Criteria of the Tresca and Caquot type cannot be applied to the overall model and consequently to lightweight aggregate concrete.

     

冻融循环后引气混凝土双轴破坏准则研究

对经过0次、100次、200次、300次、400次冻融循环作用后的引气混凝土试件进行了双轴压、双轴拉-压荷载作用下的强度试验研究。观察了试件的破坏形态和表面裂缝的走向特征,根据试验结果分析了极限抗压、抗拉强度随冻融循环次数和应力比的变化规律。在主应力空间,建立了不同冻融循环次数后考虑应力比影响的双轴破坏准则;对于双轴压,建立了同时考虑应力比及冻融循环次数影响的破坏准则,对于双轴拉-压,建立了八面体应力空间的破坏准则;为寒冷地区处于双轴荷载作用下的引气混凝土结构的强度分析提供试验和理论依据。     

Fracture process and reliability of concrete made from high grade recycled aggregate using acoustic emission technique under compression

The increasing amount of waste concrete makes desirable collection of high quality of recycled aggregate from waste concrete to be reused for construction. This research used high grade recycled coarse aggregate (RCA) created using pulsed power technology to make concrete specimens. Concrete created from natural aggregate was also prepared to compare the properties of concrete made using pulsed power recycled aggregate. Established acoustic emission (AE) parameter analyses which are AE hit, relationship between RA value and average frequency, and b-value of AE amplitude distribution were applied to analyze the concrete fracture behavior. In addition, AE Weibull analysis was also proposed to evaluate the reliability of the concrete. A set of AE measurement testing was applied to the concrete specimens during compression loading. At the age of 28 days, compressive strength reaches 35.4 MPa and Young’s modulus is 23.6 GPa. The results indicate that the fracture process and reliability of concrete made using pulsed power RCA is similar to that of natural coarse aggregate concrete suggesting that both concrete have equivalent characteristic under compression. Furthermore, the good agreement results shared by AE Weibull analysis with those of other analyses suggesting this method can also be employed as one parameter to determine the condition of concrete.     

虚拟裂缝尖端拉应力大小的确定

针对单边切口的混凝土轴心受拉构件,基于虚拟裂缝模型提出一种计算极限承载力的解析模型,并在此基础上确立了虚拟裂缝尖端拉应力与混凝土轴心抗拉强度之间的关系。结果表明:二者的比值随初始缝高比的增大呈线性增加,但对混凝土强度等级的变化不敏感。其原因是由于所有的混凝土试件都存在初始缺陷,导致截面上存在明显的应力梯度,因而得到的混凝土轴心抗拉强度值是截面应力的平均值,而虚拟裂缝尖端拉应力为截面上的最大应力。很显然,轴心受拉构件的初始缺陷越长,截面的应力梯度越大,虚拟裂缝尖端拉应力与平均应力的比就越大。通常情况下,虚拟裂缝尖端的拉应力大小约为混凝土轴心抗拉强度值的1.22倍,约等于混凝土的抗折强度。