Mineral admixtures in mortars: Quantification of the physical effects of inert materials on short-term hydration

This work is the second part of an overall project, the aim of which is the development of general mix design rules for concrete containing different kinds of mineral admixtures. The first part presented the separation of the different physical effects responsible for changes in cement hydration when chemically inert quartz powders are used in mortars. This second part describes the development of an empirical model, based on semiadiabatic calorimetry measurements, which leads to the quantification of the enhancement of cement hydration due to the heterogeneous nucleation effect at short hydration times. Experimental results show that not all the admixture particles participate in the heterogeneous nucleation process. Consequently, the concept of efficient surface S_eff is introduced in the model. S_eff is the total admixture surface S (m² of mineral admixture/kg of cement) weighted by a function ξ(p). The efficiency function ξ(p) depends only on the replacement rate p and is independent of time, fineness and type of mineral admixture used. It decreases from 1 to 0: Low replacement rates give an efficiency value near 1, which means that all admixture particles enhance the hydration process. An efficiency value near 0 is obtained for high replacement rates, which indicates that, from the hydration point of view, an excess of inert powder does not lead to an increase in the amount of hydrates compared with the reference mortar without mineral admixture. The empirical model, which is mainly related to the specific surface area of the admixtures, quantifies the variation of the degree of hydration induced by the use of inert mineral admixtures. One application of the model, coupled with Powers' law, is the prediction of the short-term compressive strength of mortars.     

圆锥螺纹中径的测量及不确定度分析

本文叙述了在万能测长仪上用L形斜块和三针组合测量圆锥螺纹中径的方法，并介绍了L型斜块的设计和制作方法及各部分尺寸的确定。最后对该测量方法的不确定度进行了分析。     

On periodic solutions of adaptive systems in the presence of periodic forcing terms

We consider a discrete-time system consisting of a linear plant and a periodically forced feedback controller whose parameters are slowly adapted. Using degree theory, we give sufficient conditions for the existence of periodic solutions. Using linearization methods, we give sufficient conditions for their (in)stability provided the adaptation is slow enough. We then study when the degree theoretic conditions for the existence are satisfied byd-steps-ahead adaptive controllers in the presence of unmodeled dynamics and a persistently exciting periodic reference output.     

Insight into excess pore pressure generation leading to liquefaction of sand with stress history under saturated and unsaturated conditions

Assessments of the liquefaction resistance of clean sand still involve considerable uncertainties, which are a current research topic in the field of soil liquefaction. The factors considered and discussed in this study include the loading history, degree of saturation, and partial drainage. The effects of each of these factors on pore pressure generation and liquefaction resistance have been studied for decades in the laboratory, and empirical relationships have been derived. In this paper, an attempt is made to explain these effects using the unique index of volumetric strain. A pore pressure generation model is developed which is similar to that of Martin et al. (1975), but based on stress-controlled triaxial tests. The model is verified through comparisons of its results with those of laboratory tests. It is confirmed that the plastic volumetric strain that has accumulated in sand, either by drained or undrained cyclic loading, dominates the increase in the liquefaction resistance of the sand. However, the plastic volumetric strain caused by overconsolidation is less effective in reducing the volumetric strain potential for subsequent cyclic shearing, thus enhancing its resistance to liquefaction. The model provides a better understanding of the physical processes leading to the liquefaction of saturated and unsaturated sand with and without stress history.     

3-RRR球面并联机构的工作空间分析及结构优化

根据Kutzbach-Grübler修正公式分析了球面3-RRR并联机构的自由度个数以及性质,采用哈密顿"四元数"对关节矢量进行描述并建立了逆运动学数学模型,找出了动平台运动时连杆之间、连杆与电机主轴相互之间的碰撞干涉条件。根据运动学逆解以及碰撞干涉条件,基于Mathematica软件完成了可达工作空间的分析,绘制了球面并联机构的工作空间形状,给出了工作空间的极限位置。以工作空间的最大化为目标对并联机构进行了结构优化设计,使动平台实现了完整的球面工作空间。     

基于几种常用干摩擦模型的定量比较分析

干摩擦模型常用于水平振动输送机的动力学分析中,不同的摩擦模型会有不同的分析结果。针对干摩擦模型的选择问题,对三种经常在研究中使用的模型,即库伦模型、Stribeck模型和Lu Gre模型进行了定量的比较分析。建立了仿真和实验的单自由度自由振动模型,得到实验曲线和各摩擦模型仿真曲线,通过计算实验曲线和仿真曲线之间所夹面积和欧氏距离的方法,比较三种模型相对实验数据的贴合程度,结果表明,Lu Gre模型的贴合程度最好,为水平振动输送机动力学研究中选择Lu Gre模型,提供了理论依据。     

线型低密度聚乙烯溶液接枝聚甲基丙烯酸甲酯

用溶液接枝聚合的方法在线型低密度聚乙烯(LLDPE)上接枝聚合极性单体甲基丙烯酸甲酯(MMA)制备LLDPE-g-PMMA,研究了反应时间、单体用量和引发剂用量对接枝反应的影响。结果表明,随着引发剂和单体量的增加,聚合物的接枝率增加,当引发剂质量分数为0.48%,单体比率为150%时,接枝率将达到26.1%。利用红外光谱(FTIR)、核磁共振碳波谱仪(13C-NMR)对其进行结构表征,证明PMMA分子链被接枝聚合到LLDPE上。使用X射线衍射仪(XRD)、差示扫描量热仪(DSC)对接枝聚合物的结晶性能进行了分析,发现接枝聚合没有改变晶型,但结晶焓由61.39 J/g降低到47.18 J/g。     

海上平台电气设备的防爆与防护等级分类

海洋石油平台上电气设备的防爆防护工作是非常重要的,这关系着平台操作人员的自身安全,以及平台甚至整个油田的安全生产。中国海油提出并已执行的＂五想五不干＂,也都是基于安全的考虑,最大限度避免人员伤亡、财产损失。因此在平台设计、建造、投产以及后期改造过程中,都要注意电气设备的防爆与防护。     

宅基地置换小区居民满意度评价体系研究

在我国城市化快速推进的过程中,随着我国城市郊区宅基地置换工程的实施和完善,急需一套完整的宅基地置换小区规划效果评价体系。该文针对我国已经成功规划建设的宅基地置换小区,构建了一套宅基地置换小区居民满意度评价体系,该评价体系主要从定量、定性相结合的角度进行综合评价,通过量化、动态检测我国宅基地置换工程的实施成效,填补了规划效果评价领域里的技术空白,具有一定的实际意义与推广价值。