Engineering structures of concrete are generally recognized to be rarely fully water-saturated. The water transportation process in unsaturated cementitious materials offers therefore a relevant problem the understanding of which can be crucial in assessing concrete’s degradation and failure mechanisms. In recent years, the number of published papers on this topic is significantly increased. This review presents the latest advancements in determining water permeability of unsaturated cement-based materials by experimental methods and numerical modelling. The effects are summarized of water-cement ratio (w/c), curing age, particle size distribution, interfacial transition zone (ITZ) and supplementary cementitious materials (SCMs) on the permeability of partially saturated cement-based materials. Next, the underlying relationship between relative water permeability and pore structure is presented and discussed. Additionally, an insight into water transport mechanism of unsaturated concrete is proposed. Finally, some evaluative conclusions are drawn that can be instrumental for setting up future studies.

New User-Supplied Subroutines in DIANA have been proposed for the elastic matrix of rubberlike materials in this paper. When the first and second derivatives of the strain energy function are known, other information can be generated automatically in DIANA through the new User-Supplied Subroutines. Knowles’ and Gao’s constitutive laws for rubber-like materials have been implemented in DIANA by this means. Singular problems are described successfully by those constitutive laws. We demonstrate two representative examples: uniaxial and biaxial tension analyses of a rubber cube show that the DIANA finite element package provides data which are consistent with the analytical results. When a new strain energy function is proposed in the future, the new constitutive law could be more conveniently inserted into DIANA than before. The new User-Supplied Subroutines in DIANA render new possibilities to study mechanics of rubber-like materials.

Cauchy paved the way for constructing models in concrete technology, and elsewhere. He determined the (non-flat) surface area in 3D by measuring random total projections. Analogously, he determined the length of a curved line in 2D by way of measuring the total projections. The paper will present the mathematical expressions, because in many branches of concrete technology, modelling is found based on such Cauchy concepts. These branches - fractography in compression, tension or shear, fibre reinforcement and permeability estimation - will briefly be mentioned to demonstrate this. It has been found that, for the discussed fields of engineering relevance, major model parameters for cementitious materials are similar to those developed by Cauchy in the 19th century. In the paper some previous investigations concerning fractography, fibre reinforcement and fracture roughness will be summarized but basically a new development on porosimetry will be presented. Particularly a new achievement of successful implementation of the methodology (also based on Cauchy) for optimizing permeability estimation will be discussed.

The paper discusses a fictitious crack model of concrete in tension proposed by Hillerborg. This model presents a concept that illustrates the mechanism of crack initiation and its propagation in concrete on meso-level. It has proven to be a very useful tool for practical use, for both numerical and experimental research. The model was derived from findings on crack mechanisms on more advanced micro- and macro-scale, as presented in this paper. One of the paramount issues regarding crack analysis is the influence of aggregate size on mechanical and fracture parameters of concrete, and also on micro-crack development and associated macro-crack formation. Although significant progress in recognizing crack mechanisms in concrete has been achieved, there are still some aspects that should be studied in depth, for example the role of aggregate particles on crack development. This problem is analysed in the paper as well.

Research on cementitious materials can provide geometric data on structural topics of interest. Such structural data should be subjected to a process called “geometric averaging” to be coupled on the engineering level with properties like strength, stiffness or permeability. The prime purpose of this work is to demonstrate that the mathematics underlying the process of going from the materials structure to engineering properties is governed by simple stereology-based constants. Particularly, Cauchy concepts are referred to for expressions relating the length of a curved line in a plane or in space, alternatively of a curved surface in space, to extensions of total projections on lines, respectively planes, in random directions. The two parameters due to Cauchy, i.e., 2/π and 1/2 will be demonstrated dominantly governing the geometric averaging process. The stereological methodology to assess the geometric material parameters and the engineering application of the outcomes are also briefly touched upon in the paper.

Four topics of high engineering relevance in which we were involved are introduced herein. Aggregate packing in concrete is of obvious relevance: denser packings lead to reduced cement demands, while modern developments in the (super) high performance range of cementitious materials are based on particle packing. Fiber reinforcement efficiency in concrete that we have studied extensively offers a relatively simple stereological problem for which Cauchy laid down the fundament. In the third problem of damage analysis the dispersion of small cracks in concrete is at issue. Insight into damage characteristics would be relevant in all (fracture) mechanical experiments. This topic can equally be linked to Cauchy. In both cases, the data acquisition by stereological methods is indicated. The fourth topic is of high actual relevance. It involves porosimetry in computer-simulated (virtual) cementitious materials, ultimately aiming permeability estimation. The stereological problems involved are indicated, and - again – Cauchy can be linked to finding solutions. Finally, new, mostly yet unpublished developments are indicated aiming for more economic procedures as well as improving reliability of permeability estimates by nano-packing of globules, so that ultimately this methodology could replace the laborious and expensive (and biased) experimental route.

A survey of a methodology for estimating material properties of cementitious systems on the basis of computer-made concrete/cement paste (compucrete) is presented. This computer model of the cementitious material is based on a sequence of modules that are shortly reviewed in this paper. Each one is important because of fulfilling a different role. However, the most crucial one is the underlying foundation of the particle packing system. Contrary to general practice in concrete technology, the discrete element model (DEM) should be selected for this purpose. The other modules, which are built on experiences, are either updated (like the hydration simulation module) or they are selected from other fields of materials science (DRaMuTS from robotics and SVM from life science experimental practice) and upgraded for the present purpose. Relevant references are provided.

Various systems for simulating particulate matter are developed and used in concrete technology for producing virtual cementitious materials on the different levels of the microstructure. Basically, the systems can be classified as two distinct families, namely random sequential addition systems (RSAs) and discrete element methods (DEMs). The first type is hardly being used for this purpose outside concrete technology, but became popular among concrete technologists. Hence, it is of utmost relevance to compare the two families in their capabilities, so that the reliability of produced data can be estimated. This paper pursues to do this on the basis of earlier published material of work performed by a succession of PhD students in the group of the second author. Limited references will be given to external sources.

To assess the pore size of virtual cementitious materials, the star volume method (SVM) can be considered an effective tool. Unfortunately, the SVM requires a large number of plane sections in each of the very large number of random points, resulting in a time-consuming and expensive operation. As a more economical alternative, this paper presents a stereology-based contracted method, which uses a well-known theoretical concept proposed by Cauchy. This method completed pore size measurements in a shorter period of time (reductions as high as 85%) while demonstrating reliability to be maintained at the same level.

The interfacial transition zones (ITZs) are supposed to promote fluid transport through concrete. As a consequence, one would expect an increase in permeability with an increasing aggregate fraction. This has been shown in some experiments, however, the opposite effect is observed as well. The permeability ratio of ITZ to matrix seems to be a key parameter in interpreting this controversial phenomenon. A higher ratio favors the flow of water through the interface zone. This work aims at studying this ratio at various conditions (i.e., hydration degree, water/cement ratio, particle size range and water saturation degree) using a numerical model, so that the influence of the ITZ on the permeability of cementitious composites can be better understood. The findings presented in this paper can provide a new perspective on controversial experimental results as to the effect of the ITZ on transport capacity.

Realistically simulating fresh and hardening cementitious materials renders possible understanding controversial issues existing in the field of concrete technology. The experimental studies on the impact of the interfacial transition zone (ITZ) on permeability of concrete reveal two controversial results. The first involves the concept of promoting the permeability by increasing the aggregate fraction of concrete that will lead to (more) ITZ percolation. This is supported by some experiments reported in the literature. However, contradictory data are also published by other researchers. This paper aims at explaining by an advanced modelling technique why these conflictive observations are experimentally obtained.

In this study, a numerical approach developed in our group is used to assess the permeability of partially saturated cement paste based on a discrete element modelling (DEM) technique. The relationship between saturation degree and permeability is found to be in good agreement with experimental observations. Also, outcomes of a systematic study of the effects of technological parameters (i.e., hydration period, water/cement ratio and cement particle size range) on the permeability of partially saturated specimens follow expected trends. Moreover, permeability is found to be correlated to effective porosity. This is a sound basis for investigating the impact of the interfacial transition zone on water and gas permeability. Effects of partial saturation are demonstrated different for water and gas transport. Possible mechanisms underlying these permeability characteristics are discussed in this paper.

Although an earlier developed numerical methodology for permeability estimation of cement pastes can provide satisfactory results in comparison with experiments, it would be of engineering interest to find a simpler way to perform the same task. This method referred to by "the shorter approach" is presented in this paper. In the approach, water permeability is only correlated to the water-filled porosity of the specimen. A mathematical model is proposed to approximately calculate the water permeability using the water-filled porosity as the only input parameter. The confidence limit is found to yield an appropriate level of reliability. To better understand the proposed mathematical relationship, pore throat size and connectivity of the capillary pores are separately shown as a function of the water-filled porosity. Their respective and successive impacts on permeability is illustrated in this way. (C) 2016 Elsevier Ltd. All rights reserved.

Permeability of virtual cement seems to exceed experimental data by several orders of magnitude. The difference may not be that dramatic, since samples are in practise not always fully saturated as generally assumed. This has enormous effects on permeability. Therefore, a numerical study is conducted on water and gas permeability of partly saturated cement paste based on simulated microstructures. The results show that water permeability declines with the decreasing saturation degree. A similar but inverse relationship exists between gas permeability and saturation degree. The changes of pore size distribution and pore connectivity will be discussed in this paper. The results in terms of relative water and gas permeability have been validated against lattice Boltzmann simulations and experimental data, respectively. A satisfactory agreement is found.

Realistic three-scale simulation by DEM of particulate materials like concrete is nowadays possible by fast developments in computer technology. This would be an attractive alternative for systematic physical experimenting that is more time-consuming, labour-intensive and thus expensive. The paper discusses the simulation on meso-level of the aggregate structure and on micro-level of the fresh binder material that is enclosed by aggregate grains. In the latter case, it will be followed by hydration simulation to obtain the matured material. This can be expanded to involve particle packing of the C–S–H on nano-level, completing the three-scale approach. For assessment of mechanical properties, packing density is a crucial issue that can be straightforwardly studied. A subject of major engineering relevance is porosity because of durability issues. This requires techniques for delineating the capillary pore network structure to be able assessing its topological and geometric properties. By combining such features with hydraulic properties in a so called tube network model, basically transport properties of concrete can be estimated that are underlying long-term strength issues. Influences of technological parameters on packing characteristics are of interest for optimising packing, strength and durability of cementitious composites. The paper will offer the main lines of a complete methodology involving a modern three-scale analogue DEM approach, hydration simulation, and pore delineation and measuring. Intension is not to discuss separate “building blocks” of the methodology, because they have been presented, discussed and validated earlier in publications. Additionally, typical capabilities of the methodology will be indicated. To do so, results will be presented for illustrative purposes on the shape issue involved in aggregate packing, on the positive effects of cement blending by fine-grained rice husk ash on pore characteristics and resulting permeability, and on permeability estimation in the practical case of pores containing capillary water.

Path length measuring is a relevant engineering problem. Leonardo Da Vinci designed for the military appropriate equipment, the podometer, to do so. Modern equipment such as step meters and map meters are quite similar to Da Vinci's design, despite geometrical statistical - stereological - methods based on theorems of Cauchy and Buffon that were potentially available for a long period of time for doing a better job. The theorems have moreover been applied earlier for the engineering purposes indicated in this paper. Even Saltikov's reintroduction for quantitative image analysis purposes in 1945 was ignored. Gradually, the last half of a century, stereological methods became more popular in concrete technology. Nevertheless, the stereology-based global averaging operation required to make the step from materials technology to engineering properties is inherent to making errors, as the literature demonstrates.The methodological framework has been described in earlier papers by this author as to path length measurements in sections of cracked concrete and on X-ray images of steel fibre reinforced concrete (SFRC). By laying side by side in this paper the direct engineering approach (from Da Vinci to map meter) and the science (stereology)-based approaches, the profit in economy and reliability can nevertheless be stressed. Particularly the element of geometric averaging based on simple mathematical-statistical notions is highlighted, because the literature reflects still major violations to these scientific principles. Three-dimensional information in the indicated fields of materials engineering is also readily obtained, provided proper sampling is guaranteed.