Effects of binder strength and aggregate size on the compressive strength and void ratio of porous concrete

To test the influence of binder strength, porous concretes with 4 binder strengths between 30.0-135.0 MPa and 5 void ratios between 15%-35% were tested. The results indicated that for the same aggregate, the rates of strength reduction due to the increases in void ratio were the same for binders with different strengths. To study the influence of aggregate size, 3 single size aggregates with nominal sizes of 5.0, 13.0 and 20.0 mm （Nos. 7, 6 and 5 according to JIS A 5001） were used to make porous concrete. The strengths of porous concrete are found to be dependent on aggregate size. The rate of strength reduction of porous concrete with small aggregate size is found to be higher than that with larger aggregate size. At the same void ratio, the strength of porous concrete with large aggregate is larger than that with small aggregate. The general equations for porous concrete are related to compressive strength and void ratio for different binder strengths and aggregate sizes.

Influence of erosion voids and traffic loads on buried large-diameter reinforced concrete pipes

Geotechnical centrifuge tests were conducted to examine the influence of invert voids and surface traffic loads on 1400 mm diameter reinforced concrete pipes buried with a shallow soil cover depth of 700 mm.Void formation beneath the pipe was simulated during centrifuge testing.The test results revealed that before void formation,the surface load directly above the middle of the pipe caused a significant increase in not only the circumferential bending moments but also the longitudinal bending moments,the latter of which was considerable and could not be ignored.Void formation beneath the middle of the pipe led to a reduction in both the circumferential bending moments and longitudinal bending moments at all measuring positions,i.e.,crown,springline,and invert.The most significant reduction occurred at the invert,and there was even a reversal in the sign of the invert longitudinal bending moment.A comparison was made between centrifuge tests with erosion voids and surface loads at different horizontal positions,which had a marked influence even when the positions differed by half a pipe length.Joint rotation played an important role in relieving large bending moments of pipe barrels in a jointed pipeline when the void and surface load were located at the joint.

The Effect of Initial Defects on Overall Mechanical Properties of Concrete Material

Considering the fact that the initial defects,like the imperfect interfacial transition zones(ITZ)and the micro voids in mortar matrix,weaken the mechanical properties of concrete,this study develops corresponding constitutive models for ITZ and matrix,and simulates the concrete failure with finite element methods.Specifically,an elastic-damage traction-separation model for ITZ is constructed,and an anisotropic plastic-damage model distinguishing the strength-difference under tension and compression for mortar matrix is proposed as well.In this anisotropic plastic-damage model,the weakening effect of micro voids is reflected by introducing initial isotropic damage,the distinct characteristic of tension and compression which described by decomposing damage tensor into tensile and compressive components,and the plastic yield surface which established on the effective stress space.Furthermore,by tracking the damage evolution of concrete specimens suffering uniaxial tension and compression,the effects of imperfect status of ITZ and volume fraction of initial voids on the concrete mechanical properties are investigated.

Comparing Design Void Content with Actual Void Content of Laboratory Prepared Pervious Concrete

The goal of this study was to compare the predicted void content with the actual void content of pervious concrete cylinders. All pervious concrete systems are designed with a void content in mind to facilitate a specific permeability;however, due to variable placing techniques and inherent issues with the material, the actual in place void content often varies from designed. This study quantifies this difference and attempts to develop a correction factor, such that design values are more approximate to in place pervious concrete systems. The analysis included multiple mixtures with three design void contents (15%, 25%, 35%), two aggregate types (angular and rounded), and three different water-to-cement ratios (0.33, 0.37, 0.41). These samples were methodically designed to contain a desired void ratio, then casted in the laboratory, in which the compaction of each sample was controlled for consistency. Following casting, the in-place void content was determined using ASTM C1754 and compared to the predicted. The difference was then averaged to create a correction factor requiring more or less cement paste, which was used to redesign the mixtures. The new mixtures were then compared to the predicted void content. The results of this study show that initial designs can vary from 3% - 15% on average from initial designed void content and that a correction factor can be used to obtain within 3% on average of the target void ratios.

基于振动传递率函数的水泥混凝土铺面脱空识别方法

为提高水泥混凝土铺面脱空识别的准确性和便捷性,提出一种基于振动传递率函数的板底脱空识别方法。首先,建立了九块板全尺寸铺面结构三维有限元模型,获取冲击荷载作用下铺面板的多点加速度响应;其次,在2个振动传递方向上,分别计算相邻2个测点间的振动传递率函数,分析特定频段内传递率函数的差异,并提出传递率损伤指标(transmissibility-damage indicator,TDI)及其矩阵表达式;最后,计算分析11种不同脱空形式、程度下TDI矩阵的差异,并分析测点布设方式、信号噪声水平对TDI矩阵的影响。结果表明:该脱空判定指标能较好识别定位板角和板边局部脱空。所得TDI矩阵在板角脱空的2个传递方向均十分显著,在板边脱空的垂直脱空边方向较为显著;对于板中脱空,2个传递方向上的TDI矩阵均无法有效识别。板中脱空的分布与TDI矩阵的分布一致性较差;对于多区域脱空,其计算得到的TDI矩阵与单区域脱空类似,对于板角和板边局部脱空均能有效识别;测点布设越密,TDI矩阵的分布与脱空分布的一致性越高。然而,该识别方法对噪声水平较为敏感,噪声水平越高,识别精度越低,需要较高精度的传感器或设备予以支撑。

钢壳混凝土复合结构脱空高度与中子响应关系研究

为了揭示钢壳混凝土复合结构间脱空高度与中子的响应关系,建立一种新的脱空高度定量测量方法,本研究基于快中子慢化和中子散射原理,采用室内等效混凝土物理模拟方法构建脱空模型,并开展室内测试,对测试数据进行回归分析,发现热中子计数率与钢壳混凝土复合结构中钢板厚度、混凝土含水量、脱空高度之间均存在显著的相关性。进一步建立钢壳混凝土脱空高度与热中子计数率的标定关系,并开展验证实验,结果表明,通过标定关系计算得到的混凝土脱空值与实际脱空值误差较小,两者具有较好的一致性,快中子穿透钢板后与混凝土作用产生的热中子可用于定量计算钢壳混凝土复合结构脱空高度,可解决当前脱空检测中存在的无法精确定量问题。

论虚实相生与戏曲时空的超越性

本文着重探讨虚实相生法与戏曲时空所包含的审美特质之间的关系,围绕虚实相生法对戏曲艺术的影响,从戏曲时空的多元构成、超越机制,以及时空视角、时空模式等方面,阐释戏曲时空所呈现出的审美意蕴。其中,关于戏曲的多元时空——实形的舞台表演时空、虚拟的叙事时空、空灵的审美意象时空三重时空的叠印与交叉,以戏曲表演为支点,以虚实相生为法门而实现的两次时空超越,戏曲艺术“以‘心’观时空”“以‘道’观时空”的时空视角,戏曲时间推转空间的时空模式等问题,对戏曲艺术的审美意象等美学问题做了比较深入的研究。

基于互信息和MiniRocket网络的CFST脱空识别

为提高钢管混凝土(concrete filled steel tube,CFST)脱空检测的效率和精度,本文提出了一种基于快速傅里叶变换(fast fourier transform,FFT)、互信息(mutual information,MI)和MiniRocket神经网络的智能识别方法。首先,采用FFT将待测CFST敲击声波时域信号转换为频域信号;其次,采用MI建立频域信号与脱空状态的相关性,提取相关性最大的前30个特征建立数据集,避免了复杂的数学运算和冗余信息;建立MiniRocket深度学习网络,通过使用更少的参数量和更小的特征尺寸,提高分类的速度和精度。最后,考察了模型的噪音鲁棒性,并与其他算法、特征提取方法和识别方法进行对比。结果表明,在不同脱空深度和脱空宽度下,所提的方法在100次重复试验中获得了100%的平均预测精度。在高信噪比下,该方法受影响较小。此外,与其他算法、特征提取方法和识别方法相比,本方法具有更好的预测性能。因此,所提出的方法在未来实际CFST结构的智能脱空识别中具有较大的应用潜力。

高原环境引气混凝土研究现状及抗冻耐久性提升措施分析

青藏高原平均海拔4 000多米,海拔高、气压低、正负温交替天数多,冻融破坏是导致高原地区混凝土结构物性能劣化的最主要因素之一。文章对高原环境下引气混凝土的研究现状进行了分析,论述了低气压对引气剂溶液引气性能、混凝土含气量、孔结构与气孔结构的影响规律,提出了高原环境下提高混凝土抗冻耐久性的相关措施。

硬沥青改性沥青混凝土黏结层路用性能研究

选择硬沥青作为一种天然沥青添加剂,通过在沥青混凝土黏结层中添加6%,7%,8%的硬沥青,与未添加硬沥青的混合物进行比较,评估硬沥青对沥青混凝土性能的影响。结果表明:添加硬沥青的混合物稳定性提高的同时,流动性增加,导致混合物具有更大的变形性;当硬沥青添加比例为6%时,混合物具有最优路用性能,表明在沥青混凝土黏结层中添加适量的硬沥青可有效提升道路铺装的质量和耐用性。

不同压实程度的大空隙沥青混凝土空隙细观特征

为探究不同压实状态下的大空隙沥青混凝土空隙特性,揭示沥青混凝土内部空隙演化行为,基于CT和数字图像处理技术,对沥青混合料的欠压状态、标准压实状态和超压状态的空隙长宽比、空隙率、空隙数量、等效直径等参数进行研究。结果表明:空隙长宽比和等效直径可以较好地表征空隙形状;截面面积、空隙率和空隙数量对空隙的空间分布有较好的描述。随着压实程度的不断增强,其内部的空隙面积和长宽比逐渐减小,空隙数量逐渐增大,空隙等效直径逐渐减小,空隙更加细化,分布更加均匀。

混凝土脱空修复对角钢连接件力学性能影响

为探索钢壳混凝土角钢连接件根部混凝土脱空对连接件抗剪性能的不利影响,提出合理的脱空修复方法,基于深中通道沉管隧道角钢连接件的局部构造,以脱空形状、脱空修复和混凝土强度为变化参数,设计8组足尺推出试件,并开展抗剪性能试验和有限元模拟分析。分析显示:1)普通混凝土试件先在角钢内部发生混凝土压溃破坏,试件破坏模式受脱空形式及脱空修复影响不明显;2)脱空对角钢连接件的抗剪承载力和抗剪刚度均有削弱影响,三棱柱整体脱空时角钢连接件的抗剪承载力和抗剪刚度可折减32.8%和76.9%;3)脱空修复对角钢连接件的抗剪承载力和抗剪刚度有提升作用,但由于修复料与脱空界面间难以协同受力,性能提升效果并不显著;4)考虑脱空处未修复,用超高性能混凝土替换普通混凝土时,推出试件的抗剪承载力较未脱空时提高77%,抗剪刚度较脱空时折减8.9%;5)超高性能混凝土的优良力学性能使角钢连接件的抗剪承载力得到明显提高,但由于脱空导致角钢连接件出现较大变形,因此无法有效提升脱空时的抗剪刚度;6)提高修复料力学性能对改善三棱柱脱空修复后的承载性能影响不明显。

关于C100混凝土强度不足的优化探讨

本文从原材料、试验细节、配合比等方面着手分析现有配合比强度不足的原因。通过对混凝土骨料空隙率的系统优化、胶材用量比例的对比筛选,以更低的砂率、更少的胶材用量,更低的成本,确保该项目C100混凝土满足布料施工性能的同时满足强度要求且有富余,配合比方向性的变化也更有益于混凝土的长期性能。

基于废旧塑料纤维的透水混凝土配合比设计研究

废旧塑料纤维具有较好的力学性能,能够减少混凝土的收缩裂缝。基于此,本文选择8组级配,掺配3%废旧塑料纤维,通过测定透水混凝土试件的力学性能及空隙率,确定透水混凝土的最优水灰比、灰集比及级配。试验结果表明,当水灰比为0.28时,混凝土的抗折和抗压强度均为最大;当灰集比为0.24时,混凝土的抗折和抗压强度达到最大;3#级配混凝土抗压,抗折强度和空隙率均为最优。

混凝土路面脱空高效压浆及提高补强性能施工技术研究

文章介绍了工程项目中混凝土路面脱空高效压浆及提高补强性能的施工技术,通过研发一种定向压浆装置,可使压浆方向更为集中,满足脱空空间压浆充盈度要求,实现了脱空路面高效压浆施工,并对压浆材料配比、压浆工艺进行了改进和优化,提高了压浆对路面的补强效果,道路的使用性能显著提高,可为类似混凝土路面脱空整治提供参考和借鉴。

水泥混凝土路面板底脱空无损检测技术研究

为了进一步明确落锤式弯沉仪(FWD)和探地雷达(GPR)进行水泥混凝土路面板底脱空的无损检测方法和判断标准,基于2种方法的测试原理分析,分别提出了弯沉比值法和雷达信号畸变特征分析法进行路面脱空判定,并对两种脱空检测技术进行了工程应用。结果表明,两种检测技术判定结果具有较好的一致性,由于GPR检测速度快且为连续性检测,更适合运营道路水泥混凝土路面板底脱空检测。

Enhancement of Workability of Cement-Poor Concrete by Optimizing Paste Content

This study describes the performance of concrete in fresh state, intended for sealing deep bore-holes in the host rock of radioactive repositories. Set of different paste volumes, combinations of water-to-powder ratios and fine aggregate contents have been performed within the frame of this study. The main objective was to search for tendencies, logical connections and phenomena that occur for different combination of materials regarding the fluidity and segregation and mainly the effect from the (paste) or fine aggregate content. It shall be pointed out that this investigation is a suggestion on how concrete can be optimized using two simple test methods based on changing the paste content. The results highlighted the importance of having sufficient amounts of filler and cement paste for separate and carry larger particles, which gives the concrete good workability and fluidity at casting. It was concluded that the slump behaviors can be optimized based on the adjustments of the superplastisizer dosage.

Influence of Hydrostatic Pressure on Compressive Strength of Self-consolidating Concrete

The design of unique chamber, in which the SCUWC （self-consolidating underwater concrete） can be tested under the impact of the hydrostatic pressure from 0.1 MPa to 0.5 MPa, is presented in the paper. The results of the preliminary tests of the effect of the hydrostatic pressure on the compressive strength of SCUWC were shown. The impact of the hydrostatic pressure on the compressive strength values of test specimens has been confirmed. There has been an increase in the strength of the specimens taken from the upper parts of the concrete samples. As it can be seen from the preliminary research, the differences in compressive strength are related to the differences that occur in the size and distribution of air voids in the samples taken from upper and lower parts of the test specimens. On the basis of the carried out investigations of the compressive strength, it can be concluded that the hydrostatic pressure has a favorable effect on the compressive strength of the tested specimens of SCUWC. Increase of the compressive strength is observed mostly in the upper layers of the samples. Preliminary analysis of the quantity and distribution of air pores in the samples of concrete subjected to pressure 0.5 MPa confirms the positive impact of the hydrostatic pressure on the layers close to the surface indicated by the absence of large air voids above 1,500μm and by reducing the quantity of air pores of size above 300μm.

基于声频阈值估计的混凝土空洞快速识别方法研究

传统的声振技术在实际工程应用中普遍存在识别参数模糊、操作过程复杂等诸多问题,针对该问题本文提出了基于声频阈值估计的混凝土空洞快速识别方法。使用comsol软件建立声固耦合有限元模型,对多种工况的空洞模型进行了数值模拟,得到了声压、频率等相关参数的变化规律,提出了基于空气耦合冲击回波法的混凝土空洞识别参数,并结合物理模型试验进一步进行探索验证。研究表明,相比于空洞深度,空洞宽度对声波基频的影响显著提高,当空洞宽度C>0.35 m时,一阶模态成为主导模态,声波基频随空洞宽度增加而减小,变化比较明显。基于空气耦合技术的检测方法可以有效识别宽度大于0.35 m的空洞病害,利用本研究中的识别参数可以对混凝土结构中深度0.4 m以内的空洞情况进行快速有效的识别。该研究成果将有利于隧道、桥梁等混凝土结构的损伤检测与状态评估。

基于导纳法的地铁板式无砟轨道SCC层无损检测

导纳法作为一种机械阻抗分析方法,当无砟轨道充填层出现脱空时,缺陷在一定程度上会对轨道结构局部或整体的力学状态产生影响,引起轨道板支撑条件的变化,从而造成轨道板相应位置处机械阻抗的改变。为研究导纳法在地铁减振板式无砟轨道自密实混凝土充填层脱空检测的适用性,采用ANSYS建立减振板式无砟轨道有限元模型,通过对比分析正常状态和脱空状态下对应的速度时程曲线和速度导纳频谱,构建了无砟轨道充填层脱空的识别参数,并进行了现场足尺模拟脱空试验和揭板验证。研究结果表明:当自密实混凝土充填层出现脱空时,速度时域曲线表现为速度振幅增大,振幅衰减减缓,导纳频谱表现为各阶频峰向低频处迁移,对应的峰值增大,但1阶导纳峰值对于充填层脱空变化并不明显;导纳均值初始频率宜取100 Hz,当截止频率取1150 Hz以上时,导纳均值与脱空程度具有良好的对应关系,随着脱空程度增大,导纳均值增大;无砟轨道的边角导纳对脱空更为敏感,出现脱空时,边角位置的导纳率大于内部位置,基于导纳法可有效识别减振板式无砟轨道充填层脱空。