Gilles Escadeillas

Characterization of biological stains on external concrete walls and influence of concrete as underlying material

Investigations were carried out on concrete walls stained by biological growth. Pieces of this material were removed down and observed using optical microscopy, low-vacuum scanning electron microscopy (LVSEM) and normal SEM. The results show that biological stains are due to two different kinds of microscopic algae, Chlorophyceae and Cyanophyceae, whose presence depends on the amount of moisture on the concrete wall. Accelerated laboratory tests of biological growth on mortar samples that were performed show that algal developments increase with the porosity of the underlying material. Thus, it seems that the use of dense, high-performance mortars can slow down or even inhibit microorganism growth.

Microstructural features of concrete in relation to initial temperature—SEM and ESEM characterization

Both high initial and subsequent curing temperatures of concrete are known to induce decreases in strengths at 28 days of age. This paper deals with observation of the concrete microstructure to point out some differences that would be responsible for these changes in strength. For this purpose, conventional electron microscopy (SEM) and environmental electron microscopy (ESEM) techniques were used to study the microstructural features of normal strength concrete prepared at two initial temperatures (20 °C and 50°C) and cured at either 20°C or 35°C. Using either SEM or ESEM, three major results can be observed on fracture surfaces. When initial temperatures are increased, the bulk paste seems amorphous, with a local poorly textured morphology. Moreover, air voids and capillaries can be seen close to the paste-aggregate interface. High initial temperatures also promote crystallization. Particularly, intermingling of little ettringite rods and large crystals of calcium hydroxide are observed on aggregate imprints, which results in an open microstructure of the paste. This latter observation explains the decrease in concrete strength as a result of a weakening of the interfacial transition zone. There are also outstanding differences between SEM and ESEM observations, which indicate artifacts of drying when using SEM.

Study of the degree of hydration of concrete by means of image analysis and chemically bound water

Abstract This article deals with the effect of both initial temperature and subsequent high curing temperature on the degree of hydration of normal strength concrete at the age of 28 days. Variations in compressive strength were observed and presented previously. Here, their causes are investigated through a difference in the hydration state in relation with the initial mixing and curing conditions. We present two ways of measuring the degree of hydration and compare the results, first by means of an original technique of image analysis on flat polished sections observed by scanning electron microscope, and second by classical measurement of the chemically bound water. Two kinds of specimens are studied. Cylinders of 11 cm × 22 cm were prepared with two different temperature formulae of constituents: 20°C (water)—20°C (aggregate)—20°C (cement); and 20°C (water)—70°C (aggregates)—70°C (cement), so as to obtain an initial temperature of the mixes between 20°C and 50°C, respectively. Specimens were sealed and cured under either controlled laboratory conditions or simulated conditions of hot weather. In each case, all the specimens were stored in water up to 28 days.

Use of plant aggregates in building ecomaterials

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.

Evaluation of natural pozzolan for use as supplementary cementitious material

This paper presents preliminary study results on the use of local natural pozzolan (NP) as a mineral addition. The natural pozzolan was obtained from a source in north-western Algeria and was composed of zeolite and plagioclase, which are mineralogical materials containing large quantities of SiO2 and Al2O3. A number of cements were prepared in which portland cement (OPC) was replaced by NP in the range of 0–25%. The parameters investigated included pozzolanic activity, setting times, heat of hydration, compressive strength, and the crystalline hydration products (by XRD).The pozzolanic activity index was evaluated on the basis of physical and mechanical parameters (standards ASTM C618 and ASTM C311) and through a chemical analysis (European standard NF EN 196-5). The performance of natural pozzolan cement exposed to aggressive solutions (sulphuric and nitric acids) and their resistance to chloride permeability were also analysed. Cet article présente des résultats d’études préliminaires sur l’utilisation d’une pouzzolane naturelle locale (NP) comme addition minérale. Cette Pouzzolane naturelle provient de la region nord-Ouest d’Algérie. Elle est composée de zeolite et plagioclase qui sont des matériaux minéralogiques contenant de grandes quantités de SiO2 et Al2O3. Des mortiers ont été préparés dans lesquels le ciment Portland (OPC) est remplacé par la pouzzolane naturelle dans des proportions variants de 0 - 25%. Les paramètres recherchés sont l’activité pouzzolanique, le temps de prise, la chaleur d’hydratation, les proprieties mécaniques. L’indice de l’activité pouzzolanique a été évalué sur la base des paramètres physiques et mécaniques (normes ASTM C618 et ASTM C311) et par une analyse chimique (norme européenne NF EN 196-5). La performance de ces mortiers exposés à des solutions agressives (acides sulfuriques et nitriques) et leur résistance à la perméabilité des chlorures furent aussi analysées.

Drops in concrete strength in summer related to the aggregate temperature

Drops in compressive strengths may occur for hot weather concreting. They are observed on specimens for manufacturing control tested in laboratory at 28 days. On the other hand, 7-day compressive strength seems not to be affected. An experimental program was carried out. It constituted a quantitative approach to evaluate the effect of the aggregate temperature on the performances of plain concrete. So, this paper presents the results of tests for compressive and splitting tensile strengths conducted on normal strength concrete specimens. Cylinders (11 cm × 22 cm) were prepared with different aggregate temperatures ranging from 20–70 °C and cured under either controlled laboratory conditions (20 °C) or simulated conditions of hot weather (35 °C). Results show that both the 28-day compressive strength and the splitting tensile strength of concrete are reduced with the increase in aggregate temperature by as much as 15% and 17% respectively. The aggregate temperature rise also implies an increase in the water demand of the concrete mixes which, in turn, cannot fully explain the drops in strength.

From clinker to concrete—an overall modelling

By overall modelling we mean a relationship which links concrete strength to some major practical concerns such as cement and water contents, cement fineness and lapse of time, the constants being characteristics of the clinker only. The experimental basis covers concretes and standard mortars prepared with 52 laboratory OPCs, made from 14 industrial clinkers ground to different grades of fineness. The model allows a synthesis between the engineer’s and the chemist’s approaches, taking into account both compacity and chemical reactivity. Though empirical, it seems to be in line with current scientific knowledge.ResumeL’expression ‘modèle global’ désigne une relation entre, d’une part, la résistance en compression à différentes échéances et, d’autre part, les paramètres importants de la formulation du béton (dosages en eau et en ciment, finesse du ciment), les constantes qui interviennent étant caracté-ristiques du clinker et de lui seul. Pour établir cette relation, des bétons et des mortiers ont été fabriqués à partir de 52 CPA de laboratoires issus de 14 clinkers industriels. Le modèle offre une synthèse entre les points de vue de l’ingénieur et du chimiste puisqu’il tient compte aussi bien de la compacité granulaire que de la réactivité chimique. Bien qu’il soit empirique, il s’accorde convenablement avec les connaissances scientifiques actuelles sur le béton.

Influence of various parameters on heat-induced internal sulphate attack

The work presented in this article studies the effects of some parameters that can be considered, a priori, as important in the appearance of delayed ettringite formation (DEF). The parameters studied are the nature of the binder, the water/cement (W/C) ratio, the preliminary microcracking (wetting–drying cycles) and the curing conditions. The results presented show that the chemical composition of cement is one of the main parameters controlling the appearance of DEF. The other parameters studied, (W/C ratio, wetting–drying cycles and temperature of curing water) principally modify the kinetics of the expansion: it seems that, the more favourable the conditions are to ionic and hydrous transfers, the sooner the expansion starts. Finally, the renewal of the curing water strongly affects the start of DEF. In one case of the study, the renewal of the curing water only accelerated the start of the expansion. In the second, it led to late starting of DEF (four years) whereas the same mixture cured continuously in water that was not renewed still had not expanded after more than five years. L’étude présentée dans cet article a pour objectif d’observer les effets de certains paramètres que l’on peut qualifier a priori d’importants vis-à-vis de l’apparition de la DEF. Les paramètres étudiés sont la nature du liant, le rapport E/C des mélanges, le préendommagement des éprouvettes (cycles d’humidification-séchage) et, enfin, les conditions de conservation des échantillons. Les résultats présentés montrent que la composition chimique du ciment est l’un des principaux paramètres contrôlant l’apparition de la DEF. Les autres paramètres étudiés (rapport E/C, cycles d’humidification-séchage et la température de l’eau de conservation des échantillons), modifient principalement la cinétique de l’expansion: il semble que plus les conditions sont favorables pour améliorer les transferts ioniques et hydriques, plus l’apparition des expansions est accélérée. Enfin, le renouvellement de l’eau de conservation affecte fortement le début de l’apparition de la DEF. Dans un cas d’étude, il n’a fait qu’accélérer le début de l’expansion. Dans un second cas, le renouvellement de l’eau de conservation a permis le déclenchement de la DEF au bout de 4 ans, alors que le même mélange conservé en permanence dans l’eau qui n’a pas été renouvelé n’avait pas encore montré de gonflement après plus de 5 ans.

Effect of wetting-drying cycles on mortar samples affected by DEF

This article, which deals with the effect of wetting–drying (W-D) cycles on materials affected by delayed ettringite formation (DEF), has two main objectives. The first is to observe whether W-D cycles pre-damage samples, by using the replica technique. Scanning electron microscopy (SEM) observations of replicas show that W-D cycles lead to microcracks in the samples. The second objective of the study is to observe whether the cycles are necessary for development of DEF or if they only accelerate DEF. A study of the expansion of the samples shows that cycles accelerate the appearance of expansion but they are not a prerequisite for triggering DEF. Cet article, qui porte sur l’effet des cycles d’humidification-séchage des matériaux atteints de DEF, a deux objectifs principaux. Le premier objectif est d’observer par l’utilisation de la technique de la réplique, si les cycles d’humidification-séchage pré-endommageaient les échantillons. Les résultats obtenus par le suivi des répliques ont permis de conclure que les cycles pré-endommagent les éprouvettes. Le second objectif de cette étude, est d’observer si les cycles sont nécessaires au développement de la DEF ou s’ils ne font qu’accélérer la DEF. Le suivi des expansions a montré que les cycles permettent d’accélérer l’apparition des expansions, mais qu’ils ne sont pas un préalable nécessaire pour le déclenchement de la DEF.

Ammonium Nitrate Attack on Cementitious Materials

Of the factors that can quickly degrade concrete, ammonium salts are among the most aggressive (Lea 1965; Biczok 1972). Ammonium nitrate, a very commonly used fertilizer, is at the origin of severe degradation of concrete structures, especially in factories manufacturing these fertilizers or in storage silos.