Laboratoire Matériaux et Durabilité des Constructions

This is a review of the theoretical and applied progress made based on the Constructal law of design and evolution in nature, with emphasis on the last decade. The Constructal law is the law of physics that accounts for the natural tendency of all flow systems (animate and inanimate) to change into configurations that offer progressively greater flow access over time. The progress made with the Constructal law covers the broadest range of science, from heat and fluid flow and geophysics, to animal design, technology evolution, and social organization (economics, government). This review presents the state of this fast growing field, and draws attention to newly opened directions for original research. The Constructal law places the concepts of life, design, and evolution in physics.

In damp environments, indoor building materials are among the main proliferation substrates for microorganisms. Photocatalytic coatings, including nanoparticles of TiO2, could be a way to prevent microbial proliferation or, at least, to significantly reduce the amount of microorganisms that grow on indoor building materials. Previous works involving TiO2 have already shown the inactivation of bacteria by the photocatalysis process. This paper studies the inactivation of Escherichia coli bacteria by photocatalysis involving TiO2 nanoparticles alone or in transparent coatings (varnishes) and investigates different parameters that significantly influence the antibacterial activity. The antibacterial activity of TiO2 was evaluated through two types of experiments under UV irradiation: (I) in slurry with physiological water (stirred suspension); and (II) in a drop deposited on a glass plate. The results confirmed the difference in antibacterial activity between simple drop-deposited inoculum and inoculum spread under a plastic film, which increased the probability of contact between TiO2 and bacteria (forced contact). In addition, the major effect of the nature of the suspension on the photocatalytic disinfection ability was highlighted. Experiments were also carried out at the surface of transparent coatings formulated using nanoparticles of TiO2. The results showed significant antibacterial activities after 2 h and 4 h and suggested that improving the formulation would increase efficiency.

Kaolin samples of the same mass were treated at 700 °C for the same duration of 30 min by varying the rate of calcination (1, 2.5, 5, 10, 15 and 20 °C/min) in order to obtain metakaolins which were used to produce geopolymers. Depending on the nature of each type of material, kaolin, metakaolins and geopolymers were characterized using thermal analysis, chemical analysis, XRD, FTIR, particle size distribution, specific surface area, bulk density, setting time and compressive strength. FTIR and XRD analyses showed that metakaolins except at 1 °C/min contained residual kaolinite whose quantity increased with the rate of calcination of kaolin and which influenced the characteristics of geopolymers. Thus as the rate of calcination of kaolin increased, the setting time increased (226 min (rate of 1 °C/min)–773 min (rate of 20 °C/min)) while the compressive strength reduced (49.4 MPa (rate of 1 °C/min)–20.8 MPa (rate of 20 °C/min)). From the obtained results the production of geopolymers having high compressive strength along with low setting time requires that the calcination of kaolin be carried out at a low rate.

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 Seff is introduced in the model. Seff 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.

Bacterial respiration of nitrate is a natural process of nitrate reduction, which has been industrialized to treat anthropic nitrate pollution. This process, also known as “microbial denitrification”, is widely documented from the fundamental and engineering points of view for the enhancement of the removal of nitrate in wastewater. For this purpose, experiments are generally conducted with heterotrophic microbial metabolism, neutral pH and moderate nitrate concentrations (<50 mM). The present review focuses on a different approach as it aims to understand the effects of hydrogenotrophy, alkaline pH and high nitrate concentration on microbial denitrification. Hydrogen has a high energy content but its low solubility, 0.74 mM (1 atm, 30 °C), in aqueous medium limits its bioavailability, putting it at a kinetic disadvantage compared to more soluble organic compounds. For most bacteria, the optimal pH varies between 7.5 and 9.5. Outside this range, denitrification is slowed down and nitrite (NO2−) accumulates. Some alkaliphilic bacteria are able to express denitrifying activity at pH levels close to 12 thanks to specific adaptation and resistance mechanisms detailed in this manuscript, and some bacterial populations support nitrate concentrations in the range of several hundred mM to 1 M. A high concentration of nitrate generally leads to an accumulation of nitrite. Nitrite accumulation can inhibit bacterial activity and may be a cause of cell death.

The building industry is progressively trying to use self-compacting concrete (SCC) in order to improve many aspects of construction, principally reinforced concrete. However, the problem of its durability still exists, particularly in terms of physicochemical properties which are essential to avoid corrosion of rebars. The purpose of this project was to qualify the ‘potential’ durability of self-compacting concrete and reference vibrated concrete (VC) with similar compressive strength according to French recommendations. To do this, general indicators of durability (water porosity, chloride diffusion, oxygen permeability) and additional properties necessary for a better understanding (mercury porosity, water absorption by capillarity, carbonation and ammonium nitrate leaching) were examined. Various mixes of SCC and VC were therefore made with the same raw materials in identical proportions (except for the high-performance concrete). The tests conducted on the concretes studied revealed that the durability of both concretes could be regarded as equivalent. So, at the same level of compressive strength, self-compacting concrete can be considered to be as durable as vibrated concrete.

Building materials can be exposed to microorganisms (mainly bacteria, fungi and algae) in almost every aqueous medium or damp environment, water being the indispensable condition for life development. The activity of microorganisms can be responsible for mineralogical, chemical and microstructural damage to the material (biodeterioration). Deleterious effects can also concern the aesthetics of a building (proliferation of colored biological stains on façades and roofs) or the quality of indoor air (presence of microorganisms in damp buildings). However, microorganisms can also have positive effects (healing of materials) and their action is explored through the development of bio-based protective systems intended for building materials. In all cases, understanding interactions between building materials and microorganisms is an indispensable step toward the development of more sustainable, better quality, safer structures in many environments. This paper presents two examples where the action of microorganisms has—or is likely to have—strong impact on the durability and safety of concrete structures. The first example concerns the biodeterioration of concrete in agricultural and agro-food environments. The second example is that of the abiotic and biotic reactivity of nitrates in repository of intermediate-level long-lived nuclear wastes. The paper presents the approaches used to explore and understand the phenomenology of bio-geo-chemical interactions in these complex environments. These studies notably comprise the development of test methods and experimental pilots to enable these explorations to be carried out. Current shortcomings in the scientific literature and in the standardization environment are also highlighted.

Physicochemical characteristics of Hibiscus cannabinus (kenaf) fibers from Burkina Faso were studied using X-ray diffraction (XRD), infrared spectroscopy, thermal gravimetric analysis (TGA), chemical analysis and video microscopy. Kenaf fibers (3 cm long) were used to reinforce earth blocks, and the mechanical properties of reinforced blocks, with fiber contents ranging from 0.2 to 0.8 wt%, were investigated. The fibers were mainly composed of cellulose type I (70.4 wt%), hemicelluloses (18.9 wt%) and lignin (3 wt%) and were characterized by high tensile strength (1 ± 0.25 GPa) and Young’s modulus (136 ± 25 GPa), linked to their high cellulose content. The incorporation of short fibers of kenaf reduced the propagation of cracks in the blocks, through the good adherence of fibers to the clay matrix, and therefore improved their mechanical properties. Fiber incorporation was particularly beneficial for the bending strength of earth blocks because it reinforces these blocks after the failure of soil matrix observed for unreinforced blocks. Blocks reinforced with such fibers had a ductile tensile behavior that made them better building materials for masonry structures than unreinforced blocks.

The aim of this work is to valorize volcanic scoria by using them as starting material for geopolymers production. Nevertheless, volcanic scoria possesses low reactivity. Various amounts of metakaolin (5%, 10%, 15%, 20% and 25%) were added into two volcanic scoria (ZD and ZG) in order to improve their reactivity. Two alkaline solutions were used to activate the aluminosilicate materials. The starting materials were characterized by particle size distribution, specific surface area, chemical and mineralogical composition. The geopolymers were characterized by the setting time, XRD, FTIR, SEM and compressive strength. The results indicated that volcanic scoria have low specific surface area (2.3 m2/g for ZD, 15.7 m2/g for ZG), high average particle size (d50 = 13.08 μm and 10.68 μm for ZD and for ZG respectively) and low glass phase contents. Metakaolin have a smaller average particle size (d50 = 9.95 μm) and high specific surface (20.5 m2/g). The compressive strength of geopolymers increased in the ranges of 23–68 MPa and 39–64 MPa for geopolymers from ZD–MK and ZG–MK respectively. This study shows that despite the low reactivity of volcanic scoria it can still be used to synthesize geopolymers with good physical and mechanical properties.

To evaluate the degree of corrosion, reinforcements of 14-year-old and 17-year-old concrete members were completely exposed. These 3 m long beams were stored in three-point flexion in an aggressive environment consisting in sequences of drying and wetting by a salt fog (35 g/l of NaCl). The total chloride content was also measured at the level of all reinforcements. The chloride content appears to be significantly higher than the threshold that is generally used to evaluate corrosion initiation. A comparative analysis carried out on these experimental results shows that the steel–concrete interface quality (physical adhesion between steel and concrete) is greatly determinant to predict the initiation of corrosion in reinforced concrete members.

In this paper we develop flow architectures for “vascularizing” smart materials that have self-healing capabilities. The flow architectures are configured as two trees matched canopy to canopy. A single stream flows through both trees and bathes every subvolume (crack site) of the material. Several types of tree-tree configurations are optimized. Trees that have only one level of branching and bathe a rectangular domain have optimal external shapes that are nearly square. They also have optimal ratios of channel sizes before and after branching. Trees optimized on square domains perform nearly as well as trees on freely morphing rectangular domains. The minimized global flow resistance decreases slowly as the number of subvolumes increases. It is more beneficial to bathe the entire volume with a single (optimized) one-stream architecture than to bathe it with several streams that serve small clusters of volume elements. These conclusions are reinforced by an analytical optimization of the same class of architectures in the limit of a large number of assembled subvolumes. We also show that the freedom to morph the design and to increase its performance can be enhanced by using tree-tree architectures with more than one level of branching.

This paper describes the conceptual design and performance of balanced two-stream counterflow heat exchangers, in which each stream flows as a tree network through its allotted space. The two trees in counterflow are like two palms pressed against each other. The paper develops the relationships between effectiveness and number of heat transfer units for several tree-counterflow configurations: (i) constructal dichotomous trees covering uniformly a rectangular area, (ii) trees on a disk-shaped area, and (iii) trees on a square-shaped area. In configurations (ii) and (iii) each stream flows between the center and the periphery of the area. Configurations (i) and (ii) are trees with minimal resistance to fluid flow. Configuration (iii) is designed by minimizing the length of each duct in the network. The paper reports the formula for the number of heat transfer units in each configuration. Unlike in counterflows formed by two parallel streams, in which the longitudinal temperature gradient is constant, in the counterflow formed by two trees the longitudinal temperature gradient is steeper as one approaches the periphery of the tree canopy. The application of dendritic heat exchangers to devices with maximal transport density is discussed, e.g., electronics cooling, fuel cell architectures, etc.

The migration of neutrally buoyant finite sized particles in a Newtonian square channel flow is investigated in the limit of very low solid volumetric concentration, within a wide range of channel Reynolds numbers Re = [0.07-120]. In situ microscope measurements of particle distributions, taken far from the channel inlet (at a distance several thousand times the channel height), revealed that particles are preferentially located near the channel walls at Re > 10 and near the channel center at Re < 1. Whereas the cross-streamline particle motion is governed by inertia-induced lift forces at high inertia, it seems to be controlled by shear-induced particle interactions at low (but finite) Reynolds numbers, despite the low solid volume fraction (<1%). The transition between both regimes is observed in the range Re = [1-10]. In order to exclude the effect of multi-body interactions, the trajectories of single freely moving particles are calculated thanks to numerical simulations based on the force coupling method. With the deployed numerical tool, the complete particle trajectories are accessible within a reasonable computational time only in the inertial regime (Re > 10). In this regime, we show that (i) the particle undergoes cross-streamline migration followed by a cross-lateral migration (parallel to the wall) in agreement with previous observations, and (ii) the stable equilibrium positions are located at the midline of the channel faces while the diagonal equilibrium positions are unstable. At low flow inertia, the first instants of the numerical simulations (carried at Re = O(1)) reveal that the cross-streamline migration of a single particle is oriented towards the channel wall, suggesting that the particle preferential positions around the channel center, observed in the experiments, are rather due to multi-body interactions.

Earthen plastering mortars are becoming recognized as highly eco-efficient. The assessment of their technical properties needs to be standardized, but only a German standard exists for the moment. An extended experimental campaign was developed in order to assess multiple properties of a ready-mixed earth plastering mortar and also to increase scientific knowledge of the influence of test procedures on those properties. The experimental campaign showed that some aspects related to the equipment, type of samples, and sample preparation can be very important; although others seemed to have less influence on the results and the classification of mortars. It also showed that some complementary tests can easily be performed and considered together with the standardized ones, whereas others may need to be improved. The plaster satisfied the requirements of the existing German standard but, most importantly, it seemed adequate for application as rehabilitation plaster on historic and modern masonry buildings. Apart from their aesthetic aspect, the contribution of earthen plasters to eco-efficiency and, particularly, to hygrometric indoor comfort should be highlighted.

Natural porous structures are heterogeneous with multiple scales that are distributed nonuniformly. Few large pores (fissures, channels, and cracks) are accompanied by numerous finer channels. Can this type of flow architecture be attributed to a principle of maximization of global flow access? Features similar to those of multiscale porous structures are exhibited by tree-shaped flow structures. Trees have been deduced from the maximization of flow access between a point and a volume, a point and an area, and a point and a curve (e.g., circle). In this paper we invoke the same principle and consider fundamentally the question of how to bathe with minimal flow resistance a microchannel structure that globally behaves as a porous medium. We develop completely multiscale configurations that guide the flow from one side of the porous structure to the other (line to line and plane to plane) and show analytically the advantages of tree structures over the usual stacks of parallel microchannels. The “porous medium” that has tree-shaped labyrinths is heterogeneous, with multiple scales that are distributed nonuniformly. These features justify comparisons with the design of natural porous structures.

This work critically reviews stable isotope fractionation of essential (B, Mg, K, Ca, Fe, Ni, Cu, Zn, Mo), beneficial (Si), and non-essential (Cd, Tl) metals and metalloids in plants. The review (i) provides basic principles and methodologies for non-traditional isotope analyses, (ii) compiles isotope fractionation for uptake and translocation for each element and connects them to physiological processes, and (iii) interlinks knowledge from different elements to identify common and contrasting drivers of isotope fractionation. Different biological and physico-chemical processes drive isotope fractionation in plants. During uptake, Ca and Mg fractionate through root apoplast adsorption, Si through diffusion during membrane passage, Fe and Cu through reduction prior to membrane transport in strategy I plants, and Zn, Cu, and Cd through membrane transport. During translocation and utilization, isotopes fractionate through precipitation into insoluble forms, such as phytoliths (Si) or oxalate (Ca), structural binding to cell walls (Ca), and membrane transport and binding to soluble organic ligands (Zn, Cd). These processes can lead to similar (Cu, Fe) and opposing (Ca vs. Mg, Zn vs. Cd) isotope fractionation patterns of chemically similar elements in plants. Isotope fractionation in plants is influenced by biotic factors, such as phenological stages and plant genetics, as well as abiotic factors. Different nutrient supply induced shifts in isotope fractionation patterns for Mg, Cu, and Zn, suggesting that isotope process tracing can be used as a tool to detect and quantify different uptake pathways in response to abiotic stresses. However, the interpretation of isotope fractionation in plants is challenging because many isotope fractionation factors associated with specific processes are unknown and experiments are often exploratory. To overcome these limitations, fundamental geochemical research should expand the database of isotope fractionation factors and disentangle kinetic and equilibrium fractionation. In addition, plant growth studies should further shift toward hypothesis-driven experiments, for example, by integrating contrasting nutrient supplies, using established model plants, genetic approaches, and by combining isotope analyses with complementary speciation techniques. To fully exploit the potential of isotope process tracing in plants, the interdisciplinary expertise of plant and isotope geochemical scientists is required.

Abstract The aim of RILEM TC 247-DTA ‘Durability Testing of Alkali-Activated Materials’ is to identify and validate methodologies for testing the durability of alkali-activated concretes. To underpin the durability testing work of this committee, five alkali-activated concrete mixes were developed based on blast furnace slag, fly ash, and flash-calcined metakaolin. The concretes were designed with different intended performance levels, aiming to assess the capability of test methods to discriminate between concretes on this basis. A total of fifteen laboratories worldwide participated in this round robin test programme, where all concretes were produced with the same mix designs, from single-source aluminosilicate precursors and locally available aggregates. This paper reports the mix designs tested, and the compressive strength results obtained, including critical insight into reasons for the observed variability in strength within and between laboratories.

The aim of this paper is to compare hemp shiv and sunflower pith properties when they are used as plant aggregates incorporated into a pozzolanic matrix. Scanning electron microscope observation of these aggregates showed distinct microstructures, which could be responsible for the differences in mechanical and thermal behaviour observed between composites including these two types of plant aggregates. The long-term behaviour of composites revealed the mineralization of plant aggregates, in particular hemp shiv, by calcium compounds. L’objectif de cet article est de comparer les propriétés de la chènevotte du chanvre et de la moelle de tournesol dans le cadre de leur valorisation en tant que granulats végétaux associés à une matrice pouzzolanique. L’observation par microscopie électronique à balayage de ces granulats a permis de mettre en évidence leurs microstructures distinctes. Ceci expliquerait les écarts de comportement mécanique et thermique des composites incorporant ces granulats végétaux. Concernant le comportement a long terme des composites, les résultats mettent en évidence la minéralisation des particules et en particulier de la chènevotte par des composés de nature calcique.

This paper presents the physical, chemical and mechanical characteristics of sewage sludge ash (SSA), in order to compare its properties with the standard requirements for coal fly ash (FA). It also provides an evaluation of its environmental behaviour when used as a mineral admixture in cement-based materials. Results show that, compared with FA, SSA has a high amount of calcium and phosphorus but a relatively low fraction of silica. SSA is composed of irregular grains having a high specific surface area and thus leading to a high water demand. The high water absorption of SSA requires an increase of the water–binder ratio of mortars, or the use of a water-reducing admixture. The strength activity index (25% replacement of cement) reaches more than 90% after 28 days, showing a long-term positive effect, which might be related to the pozzolanic activity. The amount of leached elements of SSA mortars is slightly higher than for the reference mortar without residue, but it remains within the same order of magnitude. The comparison of the characteristics of SSA with the FA specifications given by American and European standards shows that SSA does not satisfy the requirements to be directly considered as a mineral admixture. Nevertheless, these preliminary results show that SSA could be classified between filler and pozzolanic admixtures. Thus SSA may not necessarily be excluded from use in cement-based materials.

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