Pozzolanic cements

Natural and artificial pozzolanas have been used to obtain hydraulic binders for over a thousand years. Hardening of pozzolanic cement pastes can result from the reaction between pozzolana and the lime that is added to the mix as hydrated lime or is produced following hydration of portland cement silicates. The pozzolanic reaction does not alter cement clinker hydration; it complements and integrates the hydration process because it results in a lower portlandite content and an increase in calcium silicate hydrates.

Besides reviewing the most recent investigations on pozzolana-containing cements, this paper shows that the behaviour of different types of pozzolana can be quite similar when they are blended and become hydrated along with portland cement clinker. Portland cement properties may undergo several qualitative modifications the extent of which substantially depends on the pozzolana/clinker ratio. So, a maximum is reached in pozzolanic cements.

As in the case of pozzolanic cements, for which the current pozzolana content is about one third by weight of cement, the most outstanding variations induced in the behaviour of portland cement can be summarised as follows. Heat of hydration decreases whilst the rate of clinker hydration increases, paste porosity increases and permeability decreases, both portlandite content and Ca/Si ratio in C-S-H decrease and the C-S-H content increases.

Chemical and physical properties of pozzolanic cements eventually affect engineering ones. Early strength of both pastes and concretes decreases while ultimate strength is often found to exceed that of the reference portland cement.

If cements contain small amounts of very active pozzolana (silica fume, for example), both early and ultimate strengths may be higher than those of the substituted cement.

Creep is found to increase definitely with increasing pozzolana content whereas shrinkage remains practically unaffected.

Chemical and microstructural variations in the paste also influence resistance of concretes to environmental attacks.

The low basicity and permeability resulting from the presence of pozzolana increase the concrete's resistance to lime leaching, sulphate and sea water attacks, and chloride penetration. Carbonation depth is practically unaffected. Pozzolana containing cements can help avoid expansion induced by alkali-silica reaction. Concrete resistance to freezing is not affected by the use of pozzolanic cement since it basically depends on the entrained air content.

The results of a variety of studies introducing a comparison between pozzolana-containing cements and corresponding portland cements can be summarised as follows: cements with appreciable pozzolana contents perform better in the long term rather than at an early age.

In most cases, however, the differences between the two types of cements are not so marked and as a consequence both cements are interchangeable especially for the most common building types.     

Hydrodynamic coefficients for water-wave diffraction by spherical structures

Evaluation of hydrodynamic coefficients and loads on submerged or floating bodies is of great significance in designing these structures. Some special regular-shaped geometries such as those of cylindrical (circular, elliptic) and spherical (hemisphere, sphere, spheroid) structures are usually considered to obtain analytical solutions to wave diffraction and radiation problems. The work presented here is the result of water-wave interaction with submerged spheres. Analytical expressions for various hydrodynamic coefficients and loads due to the diffraction of water waves by a submerged sphere are obtained. The exciting force components due to surge and heave motions are derived by solving the diffraction problem. Theory of multipole expansions is used to express the velocity potentials in terms of an infinite series of associated Legendre polynomials with unknown coefficients and the orthogonality of the polynomials is utilized to simplify the expressions. Since the infinite series appearing in various expressions have excellent truncation properties, they are evaluated by considering only a finite number of terms. Gaussian quadrature is used to evaluate the integrals. Numerical estimates for the analytical expressions for the hydrodynamic coefficients and loads are presented for various depth to radius ratios. Consideration of more values for depth makes it easy to compare the results with those available. The results obtained match closely with those obtained earlier by Wang and Wu and their coworkers     

透水路面路基性能的实验研究

目的研究透水路面土基的渗透、承载力性能及饱水状态下基层的抗冻性能.方法调整黏土和砂之间的比例,采用变水头法测出黏土中的砂含量和渗透系数之间的关系,模仿落球仪快速测定土基现场CBR值实验原理测出含砂量与土基承载力的关系;模拟外界冻融环境测试饱水状态下基层抗冻性能.结果土基的渗透系数随含砂量增加而增强,当含砂量大于25%时,土基的渗透系数均满足规范要求;饱水状态下,土基承载力随土的含砂量的降低呈缓慢下降,当土中含砂量为62.5%时,土基承载力曲线呈明显下降趋势;高空隙率级配碎石在饱水状态及负温下满足抗冻性要求,细集料在饱水状态及负温下有微小膨胀.结论土基的渗透性随含砂量增大而增强.土基承载力随含砂量的减小而降低;基层粗集料在饱水及负温下不膨胀,细集料在饱水及负温下有微小膨胀.     

水平冻结加固冻胀融沉模拟试验

为填补当今学界在冻胀防治技术的应用研究方面的空白,文章结合上海地铁建设工程实际,采用模拟试验的方法,分析研究了预留管拔除法和热盐水循环法抑制冻胀的机理以及抑制冻胀的实际效果和相关的影响因素,为上海软土冻结加固冻胀融沉的防治提供了理论依据和有关的参考数据.     

用能量法确定考虑冻胀力和冻土抗力作用时桩基的临界荷载

借鉴经典欧拉弹性稳定理论,建立了冻土地区自由桩稳定理论的能量方程,再用能量法求解出在冻胀力和冻土抗力作用下桩屈曲临界荷载计算的通用公式。在ANSYS中用离散弹簧单元模拟桩侧冻土的弹性抗力作用,以昆仑山多年冻土地区桩基试验场试验桩为例,进行有限元法求解。两种方法的计算结果吻合较好,最后通过参数分析得出冻土地区影响桩基稳定性的主要因素。     

冻干机用捕水器结霜特性研究中存在的问题

真空冷冻干燥过程中，从物料中升华出来的水分在真空泵的抽吸作用下，不断地在捕水器（也即冷阱）表面结霜，随着霜在捕水器表面不断凝结，致使其捕水效率不断降低，对真空系统形成不利的影响。但目前，对于低温、低压下的结霜机理研究极少，本文介绍了国内外有关低温冷壁面结霜的研究现状，并分析研究了真空冷冻干燥系统中冷壁面结霜过程中存在的问题。     

Growth Rate of Frost Heave in Helium and Mass Transport in Solid 4He

Frost heave phenomena have been studied in ⁴He on porous vycor glass, in which ⁴He in the pores remained supercooled fluid below the bulk melting temperature, T _m . When we cool a bulk solid at T below T _m on the vycor, the bulk solid sucks the supercooled liquid in the pores and grows. We measured the maximum frost heave pressure over bulk melting pressure, P _m , as a function of ΔT=T _m −T. When temperature was suddenly lowered, the frost heave pressure increased in time to a next equilibrium pressure and we measured the time constant and derived the frost heave rate. The frost heave rate was measured as a function of temperature and decreased very rapidly as temperature was lowered. We propose models to explain the mass transport in solid either by vacancy or by amorphous solid between bulk solid ⁴He and vycor. From measured temperature dependence of the rate in comparison with our model, we conclude the frost heave rate is determined by mass flow in solid ⁴He due to thermally-activated vacancy diffusion.      

压实土地基防冻害技术在涵闸工程中的应用

介绍季节冻土区压实土地基的抗冻机理和设计方法，为寒区中小型涵闸等板基建筑物的冻害防治提供一个简便实用的方法。     

不同引发剂对应马来酸酐改性橡胶混凝土抗冻性能研究

通过不同引发剂对马来酸酐改性橡胶并制备橡胶混凝土,对不同引发剂、橡胶粒径和橡胶掺量对抗冻性能影响进行研究,并通过SEM对冻融前后橡胶混凝土界面进行分析.试验结果表明,以DCP作为引发剂抗冻效果最佳,且橡胶粉粒径为60目、掺量为15%时,橡胶混凝土抗冻性能最佳.经200次冻融循环后,橡胶混凝土内部结构相对松散,结构发生破坏.     

道面结构不均匀冻胀模型试验及数值模拟

为更好地揭示道面结构内部温度场、水分场和冻胀量的基本规律,为防治道面结构不均匀冻胀病害提供有效的理论依据,自主设计了道面结构不均匀冻胀模型试验箱,开展了外部水分入渗道肩和道面结构不均匀冻胀模型试验,基于ABAQUS的二次开发实现了道面结构内部水热力三场耦合数值模拟,并将数值模拟结果与模型试验结果进行了对比分析.结果表明:模型试验直观地模拟了跑道与道肩冻胀错台现象,并阐明了错台的原因是跑道与道肩的不均匀冻胀;客观地揭示了道面结构内部温度场、水分场及冻胀量的发展规律;水分通过道肩渗入道面结构引发水分场的重分布;道面结构不均匀冻胀是水热耦合作用的产物,温度梯度和降温速率影响着冻结过程中水分的迁移和积聚,水分场的重分布反过来也影响着温度的传递.数值模拟结果与模型试验结果吻合良好,且符合实际,数值模型的正确性和适用性得到了验证.结合模型试验的客观性优势和数值模拟的全面性优势有助于深入研究更多寒区构筑物地基的不均匀冻胀病害问题.