Alain Ehrlacher

Analyses and models of the autogenous shrinkage of hardening cement paste: I. Modelling at macroscopic scale

Abstract After having studied phenomena linked to hydration and self-desiccation, one notes that capillary depression is the main origin of the autogenous shrinkage of hardening cement paste. Based on this mechanism, modelling at macroscopic scale is undertaken for a commonly used cement paste (CPA 55) with a W C ratio = 0.42 . It consists in introducing a macroscopic stress due to the capillary depression and characterizing the viscoelastic aging behavior of the material. The result is in satisfactory agreement with measurements.

RC two-way slabs strengthened with CFRP strips: experimental study and a limit analysis approach

This paper deals with strengthening of reinforced concrete (RC) two-way slabs with carbon fibre reinforced plastic (CFRP) strips bonded to the tensile face. The first part deals with an experimental study. The fibre reinforced plastics (FRP) strengthened slab test presents a failure mode with debonding of the external FRP strips from the slab. The second part deals with a limit analysis modelling. The strengthened slab is designed as a three-layered plate. A simplified laminated plate model is used to describe the behaviour of three-layered plate supported in four sides, which is subjected to a load in the centre. The upper bound theorem of limit analysis is used to approximate the ultimate load capacity and identify the different collapse mechanisms. Experimental results are compared with theoretical predictions.

Numerical and experimental analysis of chemical dehydration, heat and mass transfers in a concrete hollow cylinder submitted to high temperatures

Abstract This paper presents a thermo-hydro-chemical model for concrete at high temperatures. Non-linear phenomena, heat and mass transfers, evolution of the phases constituting the porous medium are taken into account in a full three phases coupled analysis. The proposed model does not take into account mechanical aspects, i.e. the solid skeleton is considered as rigid. An experimental set-up and a numerical simulation are also presented. A hollow cylinder has been heated up to 523.15 K (250 °C) on the internal side and submitted to gas pressure/temperature measurements. A numerical simulation of the cylinder has been performed, showing a good correlation with the experimental observations.

Interlaminar stress analysis with a new Multiparticle modelization of Multilayered Materials (M4)

Abstract We propose a new model for calculating the interlaminar stresses in a multilayered medium. It is well known that the classical analysis methods for multilayered materials do not provide sufficient information about the interlaminar stresses that cause damage. The model we propose describes the multilayered medium as a surface with as many particles in each point of the surface as the number of layers in the medium. For this reason, it is referred to as a Multiparticle Model of Multilayered Materials (M4 in the following text). It permits the direct calculation of the stresses as interactions between particles at the same point of the surface. In a certain sense, the M4 model belongs to the same family as the well known shear-lag model (Garett and Bailey, J. Mater. Sci., 12, 1977;1 Macquire et al., Composites Polymers, 5, 19922) and Paganos global-local model (Pagano and Soni, Int.J. Solids Structures, 19, 207–228, 19833). It is more general than the shear-lag approach and simpler than the Pagano global-local model. In this paper we present the equations of the model and establish an analytical solution for a cylindrical bending multilayered plate problem. Through this example, we show that we may obtain good predictions of displacements and shear stresses and that the M4 model is an appropriate tool for the study of interlaminar stresses.

Analyses and models of the autogenous shrinkage of hardening cement paste II. Modelling at scale of hydrating grains

Abstract Our previous paper (Hua/1995/) has studied the autogenous shrinkage at macroscopic scale. One can also model the autogenous shrinkage at the scale of the hydrating grains without going into the colloidal details of the hydrates. At this scale, hydrates (including the immobilised water held by micropores) are considered as homogeneous isotropic viscoelastic and locally non-ageing. For the ensemble of the material, only the percentages of heterogeneities (hydrates, residual anhydrous cement, capillary water, etc.) vary during the process of hydration. The modelling is based on the mechanism of capillary depression for the same cement paste as that studied in the previous paper with a modelling at macroscopic scale (CPA 55 with W C = 0.42 ). The result agrees well with measurement.

Stress analysis in adhesive joints with a multiparticle model of multilayered materials (M4)

Abstract We propose a new model to calculate the stresses in adhesive joints. It is well known that the classical models only deal with adhesive joints with constant layer thickness. The model we propose is up to calculate the stresses in adhesive joints with any geometry and with anisotropic adherends. If we consider an adhesive joint as a multilayered medium, our model describes it as a surface with as many particles in each point of the surface as the number of layers in the medium (three layers for a simple lap joint and six layers for a double lap joint). For this reason, it is named multiparticle modelisation of multilayered materials (M4 in the following of the text). The M4 models are developed at ENPC Chabot B. These de l’ecole nationale des fonts et chaussees, 1997; Ehrlacher A, Caron JF, Chabot A, Doucot E, Naciri T. Modelisation multiphasique des plaques composites en flexion, 1er Congres de Mecanique, 13–16 Avril 1993, E.N.I.M Rabat; Naciri T, Ehrlacher A, Chabot A. Compos. Sci. Technol. 1998; 58:337 for the calculation of interlaminar stresses in composite materials. The model we propose is an application of a M4 model to adhesive joints. It deals with every geometry which we can find in the literature. It only suffices to know the layer thickness in each point on the overlap to calculate the stresses in the joint. In this paper, we firstly present the classical steps to build the M4. This construction is based on the Hellinger–Reissner (Reissner E. JMath Phys, 1950; 29: 90–5) formulation. Secondly, through some joint shapes, we show that we can have a good prediction of stresses in the adhesive layer and that the M4 is a pertinent tool to study adhesive joints.

RC beams strengthened with composite material: a limit analysis approach and experimental study

This paper deals with the flexural strengthening of reinforced concrete beams by means of thin carbon fibre reinforced plastic (CFRP) plates. A simplified laminated plate model is used to describe the behaviour of three-layered plate in cylindrical bending which were subjected to third-point line loads. The upper bound theorem of limit analysis is used to approximate the ultimate load capacity for multi-layered plates and identify different collapse mechanisms. A reinforced concrete beam strengthened by CFRP is designed as a three-layered plate. Experimental results are obtained and a comparison with theoretical predictions made.

Modelling the kinetics of transverse cracking in composite laminates

Abstract A micromechanical model of transverse cracking in composite laminates under monotonie loading is proposed. To compute stresses, we simplify the problem and use a uniaxial model, often called a shear-lag analysis, which allows an analytical determination of the stress fields. In the model the laminate is discretised into cells including potential cracks. The initial strengths of these cells are randomly distributed according to a law which is experimentally identified from a series of tests on unidirectional samples taking into account the size effects. Then, by using a single series of tests, we can simulate the transverse cracking of 90 ° plies in any kind of laminate. The predictions of the simulations are validated on [02,902]s carbon/epoxy composite specimens. We have observed and counted cracks during experimental tension tests and compared them with simulations. We observe good agreement between predictions of the model and experimental results.

Probabilistic analysis of failure in adhesive bonded joints

We propose herein a strength probability law to estimate the shear strength of an adhesive joint. A simplified model enables generating an analytical solution of the shear stress present in the adhesive. The strength probability law takes into account the scale effects experimentally established for the adhesive joint shear strength and leads to studying the influence of both adhesive thickness and overlap length on joint strength.

Modelling of fatigue microcracking kinetics in crossply composites and experimental validation

Abstract We describe a micromechanical model of transverse cracking in [ α ,90 p ] sym composite laminates under fatigue loading. Several features are recalled which have been the subject of another article. These concern static aspects (such as stress fields, strength distributions, etc.) and have permitted us to predict the number of cracks created during the first cycle of loading. This paper presents the principles of the fatigue model and also proposes several experimental aspects concerning the identification of parameters and validation of the model. A micromechanical hypothesis is suggested, which is based on a physical interpretation of damage to predict the reduction in residual strength of 90° plies during fatigue loading. The model is incremental and allows us to describe in a simple but non-linear way the increase of damage in the 90° plies of a [ α ,90 p ] s laminate. We then include this fatigue aspect in our more general tool which describes a static description of the transverse cracking kinetics of a [ α ,90 p ] sym composite. The predictions of the simulations are validated on [0 2 ,90 2 ] s carbon/epoxy specimens. We have observed and counted cracks during experimental tension–tension tests and compared the results with simulations. We observe good agreement between predictions of the model and experimental results.