混凝土层裂强度测量的新方法

提出了利用Hopkionson压杆测量混凝土层裂强度的新实验方法:用高聚物材料取代传统的金属材料透射杆,混凝土试件为细长杆,由于高聚物波阻抗比混凝土小,试件中压缩波在试件/吸收杆界面反射后形成拉伸波使试件产生层裂破坏,通过吸收杆上透射波形可以确定混凝土层裂强度。由于波在粘弹性材料中的弥散效应,吸收杆中透射波形会发生变化,但三维有限元分析表明,在利用吸收杆上透射波确定混凝土层裂强度时弥散产生的影响可以忽略。按照一维特征线理论,可以由吸收杆上的应变波形确定出混凝土材料的层裂强度。     

Modeling of Thermal Conductivity of Concrete with Vermiculite by Using Artificial Neural Networks Approaches

In this article, the thermal conductivity of concrete with vermiculite is determined and also predicted by using artificial neural networks approaches, namely the radial basis neural network and multi-layer perceptron. In these models, 20 datasets were used. For the training set, 12 datasets (60%) were randomly selected, and the residual datasets (8 datasets, 40%) were selected as the test set. The root mean square error, the mean absolute error, and determination coefficient statistics are used as evaluation criteria of the models, and the experimental results are compared with these models. It is found that the radial basis neural network model is superior to the other models.     

Using origami boxes to explore concepts of geometry and calculus

The purpose of this classroom note is to provide an example of how a simple origami box can be used to explore important concepts of geometry and calculus. This article describes how an origami box can be folded, then it goes on to describe how its volume and surface area can be calculated. Finally, it describes how the box could be folded to maximize the surface area and the volume.     

Study of γ‐Fast Neutron Attenuation and Mechanical Characteristics of Modified Concretes for Shielding and Sheltering Purposes

The study of γ‐neutrons attenuation and mechanical characteristics of modified concrete are vital and crucial parameters for the construction of civilian radiological, nuclear shielding, and/or shelters. In this work, fifteen samples of ordinary concretes with five different additives; steel fibers, polypropylene, silica fume, and fly ash, with variation of cement percentages, were prepared and used for performing the mechanical and radiation attenuation investigations. The compressive strength, tensile strength, slump test, bulk density, and water permeability were also carried out for the prepared concrete mixes. Collimated coherent beams from ⁶⁰Co and Pu‐Be fast neutron sources were used to check the radiation penetrability through the syntheized mixed concrete‐additives. Very sensitive and well calibrated gamma‐neutron pulse shape discriminating spectrometer with its electronic componenets and stilbene organic detector and 3′′ × 3′′ NaI scintillation crystal was used to measure the radiation before and after attenuation and transmission. The integrated fast neutron removal macroscopic cross section (Σ_r) and linear attenuation coefficient of total gamma rays (μ) were calculated for all the analysed concrete mixes. The results of measurements, tests, analyses and calculations are given and explained. The investigated modified concrete mixes show good workability and properties from the view point of mechanical loads and γ‐fast neutrons penetrability and resistance. These results can be used for shielding and sheltering design.     

含孔隙混凝土二维细观建模方法研究

根据混凝土的细观组成和结构特点,对传统二维建模方法加以继承与改进,提出了一种高效的分步入侵判定算法.将孔隙直观地反映在模型中,建立了不同的含孔隙混凝土细观模型.对含圆形、椭圆形、多边形骨料与圆形、椭圆形孔隙的混凝土标准试件分别进行了建模研究,结果表明本文的算法具有较强适用性.同时,通过对不同面积率与多种形状骨料/孔隙混凝土的大量建模进一步验证了该算法的效率.模拟了混凝土试件在单轴压缩下的准静态力学性能,分析了混凝土内部孔隙对其裂纹扩展的主要路径、破坏模式以及宏观力学性能的显著影响.     

超声动载荷下三维随机骨料混凝土的损伤

混凝土是一种由粗骨料与水泥砂浆组成的非均质复合材料。本研究利用APDL语言程序编写三维水泥混凝土骨料随机投放程序并导入ABAQUS中,同时赋予各相材料塑性损伤本构关系来研究混凝土动态加载下的破坏规律,运用超声波在混凝土破碎中的作用机理对混凝土动态损伤破坏过程进行模拟研究。结果表明:随着超声动态载荷的增大,粗骨料体积分数为40%的混凝土始终能够承受最大应力载荷;随着超声应力波幅值增大,混凝土在动载荷下的损伤值逐渐增大,且粗骨料体积分数为40%时,其抗损伤能力最优;当粗骨料最大粒径逐渐增大,或者当粗骨料最小粒径增大,混凝土级配不合理导致性能不稳定,更易损伤破坏。     

高强混凝土单轴压缩声发射频率特征试验研究

为研究高强混凝土破裂前声发射信号的频率特征,对C60、C70、C80高强混凝土试件进行单轴压缩下的高、低频双通道声发射试验,得到破裂过程的力学参数和声发射参数,探求高强混凝土不同加载阶段声发射信号频率的分布特征。研究表明,三种高强混凝土在峰值应力前,高、低频通道声发射信号均集中在特定的频段内;临近峰值应力时,高、低频通道的声发射信号频率向低频段移动,同时优势频段内的频率趋于分散,这可作为预测高强混凝土破坏的前兆信息。     

混凝土材料裂纹尖端损伤带尺寸特性*

本文采用作者研究各向异性材料裂纹尖端塑性区特性的分析方法,研究了混凝土材料裂纹尖端损伤带尺寸特性.基于单轴应力假定,考虑了混凝土材料的非线性软化特性等影响,提出用两种分析模型:即能量模型和断裂模型来分别讨论.最后,还对这些模型作了比较.     

钢筋腐蚀监测的光波导传感方法原理探索

本文提出了一种用于混凝土结构钢筋腐蚀监测的光波导传感方法,该方法利用金属膜层局部取代光波导的介质包层,构成腐蚀敏感膜,从而获取金属腐蚀信息.本文依据波导理论讨论了该方法的原理,利用电化学方法在721比色皿的石英玻璃上证实了平面光波导腐蚀传感方案的可行性,并进一步用电镀方法制备出了光纤腐蚀传感器(FOCS)实验样品,取得了预期实验结果.     

预应力混凝土空心板梁的温度应力分析

以南京新机场高速公路施工预制场预制的预应力空心板梁由于温度应力产生的裂缝为例,进行了温度应力的分析和计算,并提出了裂缝的预防和弥补的措施．