Surface-Diffusion Theory for Drying Creep Effect in Portland Cement Paste and Concrete

An attempt is made to introduce quantitative analysis into the complex problems of the mechanism of drying creep in cement paste. The main source of creep is assumed to be the change in thickness of hindered adsorbed water layers caused by diffusion of water, coupled with diffusion of solids dissolved from the confining adsorbent particles. The basic equations of equilibrium and irreversible thermodynamics governing the problem are formulated, reduced to an initial-boundary value problem, and solved numerically. Computer studies have indicated that the drying creep effect is very likely caused by nonlinear coupling between the diffusion of water and that of solids in the hindered adsorbed layers.     

Scaling of quasibrittle fracture: asymptotic analysis

Fracture of quasibrittle materials such as concrete, rock, ice, tough ceramics and various fibrous or particulate composites, exhibits complex size effects. An asymptotic theory of scaling governing these size effects is presented, while its extension to fractal cracks is left to a companion paper [1] which follows. The energy release from the structure is assumed to depend on its size D, on the crack length, and on the material length c _f governing the fracture process zone size. Based on the condition of energy balance during fracture propagation and the condition of stability limit under load control, the large-size and small-size asymptotic expansions of the size effect on the nominal strength of structure containing large cracks or notches are derived. It is shown that the form of the approximate size effect law previously deduced [2] by other arguments can be obtained from these expansions by asymptotic matching. This law represents a smooth transition from the case of no size effect, corresponding to plasticity, to the power law size effect of linear elastic fracture mechanics. The analysis is further extended to deduce the asymptotic expansion of the size effect for crack initiation in the boundary layer from a smooth surface of structure. Finally, a universal size effect law which approximately describes both failures at large cracks (or notches) and failures at crack initiation from a smooth surface is derived by matching the aforementioned three asymptotic expansions. Walter P. Murphy Professor of This revised version was published online in July 2006 with corrections to the Cover Date.     

SO CLOSE,YET SO FAR AWAY? THE EFFECTS OF CITY SIZE,DENSITY AND GROWTH ON LOCAL CIVIC PARTICIPATION

ABSTRACT: Recent studies in the U.S. context have suggested that political participation is a function of the size and concentration of a city's population. Most of this research focuses on the idea that there is an optimal size and concentration of population that favors active political participation in terms of a higher propensity to vote in local elections, contact local officials, and attend community meetings. The conventional argument suggests a negative relationship between city size and political participation that is mitigated to some extent by the deeper social interactions generated by increased population density. We extend this research by also investigating the influence of population growth on the broader concept of civic participation. Civic participation is a multidimensional concept that requires the use of a broad set of indicators. We expand the number of measures to gauge civic participation at the local level by including data on the formation of volunteer associations, volunteer fire brigades and not‐for‐profit organizations as well as voter turnout. We test the hypotheses derived from extant research using aggregate data collected from Portuguese cities and discuss the implications of our findings for the literature on local civic participation.     

Mathematical Model for Freeze-Thaw Durability of Concrete

Although the equations governing the individual basic physical processes involved in freezing and thawing of concrete are known, a mathematical model for this complex phenomenon is unavailable. Its formulation is attempted in the present study. Desorption and absorption isotherms for concrete below 0°C are constructed on the basis of isotherms for concrete above 0°C, using pore size distribution functions. Water movement during freezing or thawing is described as a double diffusion process, involving both macroscopic diffusion through concrete and local diffusion of water into or out of air-entrained bubbles. Heat conduction is formulated taking into account the latent heat of freezing. Pore pressures are used in a two-phase material model, which makes it possible to predict the stress in the solid structure of concrete caused simultaneously by freezing and applied loads. This in principle reduces the freeze-thaw durability problem to the calculation of stresses and strains. However, development of the model to full application would require various new types of tests for calibration of the model, as well as development of a finite element code to solve the governing differential equations. Such a mathematical model could be used to assess the effect of cross-section size and shape, the effect of cooling rate, the delays due to diffusion of water and of heat, the effect of superimposed stresses due to applied loads, the role of pore size distribution, the role of permeability, and other factors which cannot be evaluated at present in a rational manner.     

NUMERICAL SMEARED FRACTURE ANALYSIS: NONLOCAL MICROCRACK INTERACTION APPROACH

A recently proposed new nonlocal concept based on microcrack interactions is discussed, its implementation in a smeared cracking finite element code for concrete is presented, numerical studies are reported, and comparisons with experimental results are made. The nonlocality is not merely a mathematical device to prevent excessive spurious localization into a zone of zero volume but is a necessary physical consequence of microcrack interactions. Since the constitutive law itself is strictly local, the new nonlocal concept can be combined with any type of constitutive law for strain-softening nonlocal damage, which is here chosen to be the microplane model. A simple method is formulated to approximately identify the material parameters in the model from the basic characteristics of concrete such as the tensile strength, fracture energy and maximum aggregate size. The results of finite element analysis are shown to be mesh insensitive, and good convergence is obtained. Cracking damage is found to localize into a volume whose size and shape depend on the macroscopic concrete properties as well as the current stress–strain state. Although the damage is considered to be tensile on the microlevel, due solely to mode I microcracks, the new nonlocal model can describe well not only mode I fracture tests but also complex shear-dominated and mixed-mode types of failure such a diagonal shear, and can do so for the same values of material parameters (which was not the case for previous nonlocal models). Most importantly, the new nonlocal model can correctly capture the size effect of quasibrittle fracture, in approximate agreement with Bažant's size effect law.     

Chemical composition and mineral content of goat milk from four indigenous Portuguese breeds in relation to one foreign breed

Raw goat milk samples from Saanen and indigenous Portuguese breeds were analysed for gross composition, minerals and trace elements. Mean results revealed that some distinction could be seen for the Serrana breed. A significant geographical difference (P < 0.05) between Serrana herds was also observed. Generally, milk from Saanen did not differ significantly (P > 0.05) from that of the indigenous Portuguese breeds. Multivariate analysis showed that goat breeds could be divided into two groups, the ones reared mainly inland and those reared towards the coast. Potentially toxic elements were not detected.