Properties and microstructures of full graded concrete containing varied impurity aggregate

We investigated mechanical properties of concretes made with impurity aggregates of different combinations. Besides the mechanisms were explored by EDS, CT, and hardness testing. The results showed that fully rust-stained and surface rust-stained sandstone aggregate had significant adverse impact on the compressive strength of concrete while sandstone aggregate had a much more obvious impact on the ultimate tension of concrete. Concrete crack was more prone to expand along surfaces and the micro-hardness of interfacial transition zone of different aggregates was ranked in decreasing trend as sandstone, slate, SR sandstone, marble, and FR sandstone. The cluster growth of long needle-like ettringite crystal and strong preferential growth trend of Ca(OH)₂ crystals would result in wider interfacial transition zone range of concretes made with fully rust-stained sandstone and marble aggregate, respectively. Therefore, the impurity aggregate content should be strictly controlled during aggregate selection.     

Effect of Al in slag blended cement paste under sulfate attack

Unilateral sulfate attack of cementitious materials containing 40% slag with different water to binder ratios was investigated. The results showed that the degradation of slag blended cement pastes was nearly from the corners of paste surface with cracking and spallings, water-to-binder(w/b) ratio made a significant sense to the damage that low w/b ratio led to little weight loss, less cracking and spalling damage and vice versa. Microstructural experimental results demonstrated that in the three different stages of sulfate attack, degradation of pastes was primarily associated with the migration behavior and bonding configuration of aluminum, in the early ages Al was mostly present in C-(A)-S-H, and thus, the damage of pastes hardly appeared while at later ones Al had been largely transferred from C-(A)-S-H into AFt, leading to expansive damage.     

硅粉增强混凝土抗冲磨性能的微观机理

采用Fourier红外光谱(FTIR)和29Si固体核磁共振谱(29SiNMR)结合去卷积技术研究了硅粉增强混凝土抗冲磨性能的微观机理。结果表明硅粉具有较高的水化活性,硅粉掺量为10%的水泥体系水化3d时,大约有41%的硅粉参与反应,水化120d时大约有96%的硅粉参与反应。此外,硅粉的火山灰反应改变了C-S-H结构中Qn的分布和C-S-H的聚合度,在这个过程中,硅粉结构中的Si-O-Si共价键在碱性环境中断裂后形成的水化硅酸根单体与C-S-H二聚体以及Ca2+、OH-结合,形成高聚C-S-H。在分子尺度(几个纳米),硅粉优化了C-S-H结构,使得C-S-H结构有序性增强,在微米尺度,硅粉的火山灰反应使得C-S-H堆积结构更加密实,胶凝性更强,从而改善了混凝土中浆体的性质。这样就从C-S-H层面解释了硅粉改善混凝土强度以及抗冲磨性能的机理。     

新型CFRP-UHPC组合管混凝土圆柱轴压性能

为研究超高性能混凝土(UHPC)管替代碳纤维增强聚合物(CFRP)-钢管混凝土组合柱中钢管的可行性,提出一种外部缠绕CFRP的UHPC预制管、内部现浇填充普通混凝土的新型CFRP-UHPC组合管混凝土(Concrete-filled CFRP-UHPC tube,CFFUT)柱。对10个CFFUT圆柱(包含2个对比柱)进行了单调轴压试验,研究了UHPC管壁厚度、CFRP环向包裹层数和核心混凝土强度等的影响规律。结果表明：CFRP-UHPC管可以有效提高组合柱的承载力、变形能力和延性;CFFUT圆柱破坏形态为核心混凝土压溃、UHPC管开裂和CFRP拉断,破坏后整体性较好,属延性破坏模式;CFFUT圆柱的极限承载力与UHPC管壁厚度、CFRP层数和核心混凝土强度呈正相关;延性系数随UHPC管壁厚度、CFRP层数增加而提高,随核心混凝土强度增加先提高后降低。揭示了CFFUT柱的界面增强作用机制,CFFUT柱极限承载力与同等截面普通混凝土柱相比提高93.9%～203.5%,且CFFUT柱极限承载力一定程度上与CFRP-钢管混凝土柱相当。建立了CFFUT圆柱轴压极限承载力理论计算模型,并通过有限...     

Na+和K+对水泥基材料早期收缩性能的影响

利用压汞、纳米压痕以及27Al固体核磁共振方法,系统研究了不同碱金属离子(Na~+和K~+)与含量对水泥基材料早期自收缩和干燥收缩性能的影响规律。结果表明,超量的Na~+或K~+均促进了水泥基材料早期自收缩和干燥收缩,但K~+的促进作用明显大于Na~+,其机制在于:(1)K~+比Na~+更易促进水泥基材料中小于50纳米孔数量的增加,这使得超量K~+水泥基材料在干燥时具有更大的收缩驱动力;(2)虽然K~+比Na~+更易促进水泥基材料生成高密度水化硅酸钙(HD C-S-H),但同时超量K~+水泥基材料中C-S-H总量也较高,大量的C-S-H是导致较高早期收缩的根源;(3)与Na~+相比,K~+更明显地阻碍了钙矾石(AFt)晶体的形成,削弱了AFt的骨架作用,且K~+可促使AFt向单硫型水化硫铝酸钙(AFm)转变,因而导致超量K~+水泥基材料更易发生收缩变形。     

变形翼微系统阵列的协调跟踪控制研究

研究了变形翼的微系统阵列的协调跟踪控制问题.为了增强微系统阵列的协调运动能力,设计了虚拟模型跟踪控制方法.针对在外部干扰作用下的微系统阵列,研究了分布式鲁棒跟踪控制律设计方法,得到线性矩阵不等式形式的控制律参数求解条件,解算方便.理论分析与仿真结果表明:所设计的控制律能够在有外部干扰的情况下,使系统渐近跟踪期望的形状,并具有协调控制能力,使变形翼在跟踪控制的过程中保持形状平滑.     

一种新的非最小相位系统的控制方法

非最小相位系统的控制中，需要抑制由不稳定零点引起的负调并同时缩短系统的调节时间．针对非最小相位系统负调与调节时间的相互影响及负调与不稳定零点的相互关系，提出将控制过程分为抑制负调阶段和跟踪输入阶段，并适时改变系统不稳定零点数，用遗传算法统一优化各阶段的控制器参数．仿真结果表明该控制方法大大减弱了负调，并同时缩短了调节时间，达到了良好的控制效果．     

三峡工程永久船闸输水洞衬砌混凝土热学与变形性能试验研究

对三峡永久船闸输水洞衬砌混凝土的力学性能，热物理学笥能和变形性能进行了试验研究，为该混凝土温度应力计算提供了参数，为现场混凝土温度应力控制提供了依据，从而肯定了输水洞衬砌混凝土进行温度应力控制的意义。     

高速转轴多阶临界转速的精确计算

针对工程上常用方法计算高速转轴临界转速精度不高，且不便计算多阶临界转速的缺陷，本文应用克雷洛夫函数精确解作为传递矩阵，推导了求解高速转轴系统临界转速的特征方程，以Matlab软件为数值计算工具给出了具体的算例。结果表明：此法能方便快捷地精确计算出高速转轴的多阶临界转速。     

Characterization of pozzolanic reaction and its effect on the C-S-H Gel in fly Ash-cement paste

High resolution solid-state ²⁹Si MAS NMR, combined with XRD, SEM and FTIR were used to characterize the pozzolanic activity of FA, type of main pozzolanic reaction products, and the effect of pozzolanic reaction on the C-S-H microstructure in fly ash-cement (FC) paste. The experimental results indicate that in the hydrated FC paste with 30% dosage of FA at 3 d, FA partially participated in the pozzolanic reaction, while, at 120 d, FA largely reacts. During the hydration of FC paste at laboratory temperature, the pozzolanic reaction products are C-S-H gel rather than zeolitic gel. Moreover, after the covalent bonds of Si-O-Si, Si-O-Al and Al-O-Al in the structure of FA are broken, monosilicates Si-OH and Al-OH groups form, these chemical species can connect C-S-H dimers, thus producing more Al-free C-S-H and aluminous C-S-H than in the plain cement paste. The increased content of Al for Si substitution in the bridging tetrahedra of C-S-H may decrease the stability of C-S-H, which results in a rather obvious loss in the mechanical strength of hardened FC paste.