Christophe Lanos

One-Dimensional-Time Study of the Dehydration of Plasterboards Under Standard Fire Condition (ISO 834): Thermo-Chemical Analysis

Gypsum materials are often used as a technical solution for passive fire protection in many civil engineering applications. In this study, the dehydration of plasterboard with large thickness, 40 mm, is studied using a one-dimensional (along the thickness) and time methodology analysis. Such materials are subjected on one side to a standard fire curve (ISO 834). The imposed heat transfer directly to the exposed face of the board induces combined actions due to the gypsum dehydration. The thermal barrier efficiency evolves significantly versus time. The local measurement of the temperature field through the sample and the local water content combined with the analysis of the local chemical structure helped us to model and analyze the dehydration processes.

Behaviour of Fire-Proofing Materials Containing Gypsum, Modifications Induced by Incorporation of Inert Filler

The fire-proofing materials used to make fire breaks are usually calcium sulphate dihydrate or calcium silicate panels. During fire exposure, the gypsum panels are characterized by an isothermal stage on the unexposed side due to the latent heat consumption. This latent heat is induced by two endothermic phase changes during transformation of gypsum into hemihydrate and anhydrite. These phase changes cause thermal shrinkage that provokes panel cracking and the increased temperature on the unexposed side. To improve the thermomechanical properties of gypsum panels, a specific filler is added to the plaster before hydration. Thermomechanical analyses show that this filler reduces the shrinkage in gypsum during heating. Fire tests show that this decrease in thermal shrinkage reduces crack appearance. Unlike gypsum plaster without filler, the heat flow is reduced even after gypsum dehydration and the temperature of the unexposed side after the isothermal stage is stabilized. The inert filler decreases the latent heat and the thermal conductivity of the materials which also modifies the thermal behaviour of panels.

Hysteretic Behaviour and Moisture Buffering of Hemp Concrete

Moisture transfer in hygroscopic building materials affects the indoor air quality by exchanging moisture and buffering the ambient relative humidity variations. The paper deals with experimental and numerical study on hysteretic sorption behaviour of the hemp concrete sorption process. Experimental intermediate scanning curves of hemp concrete are measured and used to compare two hysteresis models, Huang’s model and Carmeliet’s model. An original method is achieved to fit the numerical results on the experimental ones leading to the identification of the main desorption curve. The most relevant model, Huang’s model, is implemented in a heat and moisture transfer model based on Künzel formalism. The transient hydric response of hemp concrete submitted to cyclic hydric loadings is investigated and compared to experimental results issued from the literature. These investigations show the relevance to consider the hysteresis phenomenon into the model. Then, the influence of initial conditions is discussed. The results point out that transient response of hemp concrete strongly depends on the initial hydric state (initial moisture content as well as initial relative humidity).

Some Closed-form Solutions to Simple Beam Problems Using Nonlocal (Gradient) Damage Theory

In this article, a family of damage models which leads to the analytical solvability of the nonlocal evolution problem of a homogeneous bar in tension is defined. Explicit gradient damage models and implicit gradient damage models are investigated in a simple structural framework. The natural boundary conditions are derived from a variational principle, and are obtained at the boundary of the damage zone. It is shown that these damage models are the only ones leading to a linear differential equation of the strain variable. Some closed-form solutions are then available, providing a useful framework for the verification of computational models. Furthermore, these continuum damage mechanics models are well suited for the tension behavior of quasi-brittle materials, such as rock or concrete materials. It is theoretically shown that the damage zone evolves with the load level. This dependence of the localization zone to the loading parameter, is a basic feature, which is generally well accepted, from an experimental point of view. The strain profiles are also theoretically obtained, and corroborate well with what is usually numerically found for such nonlocal models. An imperfection analysis shows that the softening evolution problem is well posed in presence of strength imperfections. However, explicit gradient damage models lead to physically questionable results, in presence of imperfections. It is then recommended to use implicit gradient damage models when modeling realistic structures. It is hoped that the present article could serve as a benchmark to numerical gradient-damage finite element codes.

Creep Failure in Concrete as a Bifurcation Phenomenon

Softening and time-dependence of fracture are two complex and coupled phenomena that have to be taken into account in order to simulate realistic concrete behaviour. Understanding the interaction between these two phenomena is important to design reliable civil engineering structures subjected to high level-loading for a long time. The aim of this paper is to develop a simple time-dependent softening model applied to concrete. Presentation is restricted to compression behaviour. A constitutive viscodamage model describes concrete phenomena like relaxation, creep and rate-dependent loading using a unified framework. The model could be viewed as a generalisation of a time-independent damage model and is based on strong thermodynamical arguments. The determination of the material parameters linked to the proposed constitutive equation results from constant strain rate experiments. Using these parameters values, creep and relaxation numerical tests give satisfactory qualitative responses. Phenomena as creep failure under high-sustained load are explained quite simply within stability theory. Creep failure appears as the manifestation of a bifurcation phenomenon. Conversely, for low-sustained load, the motion asymptotically converges towards an equilibrium configuration. Consequently, this model is able to predict creep failure for various stress levels. Implementation of this rheological model in a structural code is envisaged in the future: a non-local approach would be probably necessary in order to simulate objective structural behaviour.

Localization in the Vibration of an Axially Loaded Two-Span Weakened Column

This paper is devoted to the dynamic analysis of two connected beam columns with a variation of the bending connection and minor perturbations of the length of each span. The point of reduced bending stiffness represented by a rotational spring may result from a crack. This rotational spring can also be associated with a semi-rigid connection in the field of steel or composite structures, for instance. The dynamics of this axially loaded two-span weakened column appears to exhibit strong localization for small values of stiffness of the rotational spring. The vibration mode shapes indicate a strong confinement of the vibration level to a fraction of the column. A quantitative criterion of localization is established and is correlated to well-known phenomena such as curve veering effect or close eigenvalues. Such a result is quite encouraging as localization is strongly associated with the flexibility values of the rotational spring. When considering the open crack analogy, localization only appears for severely damaged columns. It can then be understood as an indicator of the damage level of the global structure.

Vers une réelle rhéométrie adaptée aux bétons frais

ABSTRACT In the first section of this paper, the rheological behaviour of concretes obtained with different rheometers is investigated from a procedure allowing the plot of the flow curve of sheared material. Secondly, the rheological properties of industrial concretes are studied with a high torque vane rheometer. The flow properties of concrete are evaluated and compared, as well as the reproductibility of the making process of concrete.

Thermal and mechanical properties of gypsum–cement foam concrete: effects of surfactant

The mineral foams studied were mainly composed of a reactive, partially anhydrous calcium sulphate (90%) and Portland cement (10%), mixed with a low water-to-binder ratio (W/B = .35) leading to a highly concentrated suspension. The foaming method used allowed a wide foam density range (250 < ρ < 820 kg/m3) to be obtained after hardening and drying, with the same mineral suspension composition. To ensure that foam formed and remained stable until the material had set, six surfactants were chosen. The influence of the surfactants on paste properties in the fresh state was evaluated and the thermal and mechanical performances of gypsum–cement products (foamed and unfoamed) were analysed. The thermal conductivity of gypsum–cement foams evolved linearly with density but compressive strength values were widely dispersed and depended on the porous structure, which was strongly dependent on surfactant type and content. The performance levels of gypsum–cement foams were compared with those of lightweight foam concrete from litterature. The most efficient surfactants and the optimum content range leading to the best thermomechanical compromise were identified. They ensured a minimal compressive strength of 2 MPa, and thermal conductivity lower than .2 W/(m.K).

Study of tribological behaviour of fresh mortar against a rigid plane wall

The interaction between a fresh cement-based material and a solid wall represents a considerable consumption of energy during an extrusion proces. In this article, the tribological behaviour of extrudible mortar and rigid wall friction is studied using a tribometer especially developed in the laboratory. Dynamic friction coefficient is highlighted. For fixed wall roughness, the friction velocity effect is analysed. Then, for fixed friction velocity, the roughness effect is analysed. Finally, the material structure evolution near the wall and the material erosion are analysed. L,interaction de matériaux à base cimentaire frais et une paroi solide représente une consommation énergétique importante dans les procédés d,extrusion. Dans cet article, le comportement tribologique d,un mortier extrudable et d,une paroi rigide est étudié au moyen d,un tribomètre développé et mis au point au laboratoire. Dans les travaux, le coefficient de frottement dynamique est tout d,abord mis en évidence. Pour une paroi de rugosité fixée, l,effet de la vitesse de frottement est ensuite analysé. Dans une partie suivante, à vitesse de frottement fixée, l,effet de la rugosité de la paroi est analysé. Enfin, l,évolution de la structure du matériau au voisinage de la paroi est présentée et discutée ainsi que l,érosion.

Transient hygrothermal modelling of coated hemp-concrete walls

The use of hemp concrete, a sustainable material, in constructions is developing significantly. The hygrothermal properties of this hygroscopic porous material promise a good potential in terms of increasing occupants’ comfort. In this paper, numerical simulation is used to assess hygrothermal behaviour of a hemp-concrete wall submitted to transient boundary conditions. First, benchmarking the model against a well-documented experimental test case from literature proves the ability of the model to simulate heat and mass transfer in porous building material. Then, experimental results from a full-scale hemp-concrete wall exposed to transient climatic conditions are compared to numerical simulation, and a sensitivity analysis is performed. Good agreement between numerical results and experimental data is obtained for both the temperature and the relative humidity field in hemp concrete. Finally, the influence of coating layer on the hygric behaviour of the wall is investigated numerically. The ability of coating to affect moisture regulation is pointed out and the coating thickness is discussed.