Cj Pearce

Coupled Heat and Moisture Transport in Concrete at Elevated Temperatures—Effects of Capillary Pressure and Adsorbed Water

ABSTRACT The importance of capillary pressure and adsorbed water in the behavior of heat and moisture transport in concrete exposed to high temperatures is explored by incorporating their behavior explicitly into a computational model. The inclusion of these two phenomena is realized with a formulation of a modified model, which represents an extension to the significant work of Tenchev et al. Comparative studies were carried out, using a benchmark problem, and it was determined that while the Tenchev formulation underestimated the capacity for fluid transport in the concrete, resulting in an overprediction of pore pressures (which may affect the prediction of mechanical damage and spalling), the inclusion of capillary pressure had little effect on the results. More important was the accurate representation of the free water flux, which has a significant effect on the prediction of vapor content and subsequently pore pressure. It was furthermore found that, while the adsorbed water flux may be minimal when concrete is exposed to high temperature, its presence has a significant effect on the fluid transport behavior and the prediction of pore pressures.

Viscoplastic Hoffman consistency model for concrete

Abstract A novel viscoplastic model for concrete and its performance under dynamic loading is presented. The model is based on the so-called viscoplastic consistency model, recently proposed by Wang. The model makes use of the Hoffman yield function in its isotropic form and is augmented to include seperate hardening/softening behaviour in tension and compression. An implicit backward Euler integration scheme is adopted for the integration of the rate equations. The performance of the model is illustrated on a number of numerical examples at the material point level and on the structural level with the cylinder split test.

Relationship between brittleness and moisture loss of concrete exposed to high temperatures

The effect of moisture loss at high temperatures on the brittleness of concrete was investigated by conducting three-point bending tests on preheated notched beams. The relationships of moisture loss represented by mass loss with heating temperature and exposure time could be established. Higher heating temperature always led to higher mass loss and lower brittleness. Longer exposure time led to higher mass loss and lower brittleness, but this effect was more significant at the early exposure stage and became insignificant thereafter. When concrete is exposed to high temperatures, the brittleness is reduced. The evaporation of gel water was more closely related to the brittleness.

Three-dimensional brittle fracture: configurational-force-driven crack propagation

SUMMARY This paper presents a computational framework for quasi-static brittle fracture in three-dimensional solids. The paper sets out the theoretical basis for determining the initiation and direction of propagating cracks based on the concept of configurational mechanics, consistent with Griffiths theory. Resolution of the propagating crack by the FEM is achieved by restricting cracks to element faces and adapting the mesh to align it with the predicted crack direction. A local mesh improvement procedure is developed to maximise mesh quality in order to improve both accuracy and solution robustness and to remove the influence of the initial mesh on the direction of propagating cracks. An arc-length control technique is derived to enable the dissipative load path to be traced. A hierarchical hp-refinement strategy is implemented in order to improve both the approximation of displacements and crack geometry. The performance of this modelling approach is demonstrated on two numerical examples that qualitatively illustrate its ability to predict complex crack paths. All problems are three-dimensional, including a torsion problem that results in the accurate prediction of a doubly-curved crack. Copyright

ASSESSMENT OF TOUGHNESS OF CONCRETE SUBJECT TO ELEVATED TEMPERATURES FROM A COMPLETE LOAD-DISPLACEMENT CURVE--PART 2: EXPERIMENTAL INVESTIGATIONS

Part I of this paper summarized and derived six energy-based and deformation-based toughness indexes for assessing the concrete toughness, based on a two-portion complete load-displacement relationship comprising parameters such as load, characteristic displacements, and geometric coefficients by considering the effect of self-weight of the beam specimen under three-point bending. In this paper, the effects of heating temperature, exposure time, and curing age on the fracture and toughness of concrete were experimentally investigated by conducting three-point bending tests on a total of 55 notched plain concrete beams that had been preheated at temperatures between 100 deg C and 600 deg C over varied exposure times up to 168 hours and for four ages from 7 days to 90 days. The relationships of these parameters with heating scenarios and curing age were established. Thus, six toughness indexes could be used to quantitatively assess the toughness of concrete in varied heating scenarios and curing ages. A higher heating temperature always led to a larger toughness. A longer exposure time also led to a larger toughness, but such effect was more significant at the early exposure stage under 12 hours and became weaker thereafter. A longer curing age only led to a lower toughness in the first 28 days and did not significantly affect concrete toughness after 90 days.

Nanoindentation Study of Resin Impregnated Sandstone and Early-Age Cement Paste Specimens

Nanoindentation testing requires well prepared samples with a good surface finish. Achieving a good surface finish is difficult for heterogeneous materials, particularly those with weak and fragile structures/phases, which are easily damaged or lost during preparation. The loss of weak structures can be drastically reduced by impregnating the sample with a resin before cutting and polishing. This technique is commonly used in SEM microscopy but has not been used for nanoindentation-testing before. This paper reports an investigation to extract micro-mechanical properties of different phases in resin impregnated sandstone and 1-day hydrated cement samples. The results appeared to show that it is feasible to use resin impregnated specimens for nanoindentation study of both materials.

A micromechanics-enhanced finite element formulation for modelling heterogeneous materials

Abstract In the analysis of composite materials with heterogeneous microstructures, full resolution of the heterogeneities using classical numerical approaches can be computationally prohibitive. This paper presents a micromechanics-enhanced finite element formulation that accurately captures the mechanical behaviour of heterogeneous materials in a computationally efficient manner. The strategy exploits analytical solutions derived by Eshelby for ellipsoidal inclusions in order to determine the mechanical perturbation fields as a result of the underlying heterogeneities. Approximation functions for these perturbation fields are then incorporated into a finite element formulation to augment those of the macroscopic fields. A significant feature of this approach is that the finite element mesh does not explicitly resolve the heterogeneities and that no additional degrees of freedom are introduced. In this paper, Hybrid-Trefftz stress finite elements are utilised and performance of the proposed formulation is demonstrated with numerical examples. The method is restricted here to elastic particulate composites with ellipsoidal inclusions but it has been designed to be extensible to a wider class of materials comprising arbitrary shaped inclusions.

HIGHER-ORDER DISCONTINUOUS MODELING OF FRACTURING IN CONCRETE USING THE NUMERICAL MANIFOLD METHOD

This paper presents an overview of the numerical manifold method (NMM) as a unifying framework for modeling fracturing phenomena in quasi-brittle materials, and particularly in concrete. NMM is attractive due to its potential to provide an efficient framework for modeling the entire transition between continuum to discontinuum, from a continuum point of view, without remeshing. To achieve this, the following characteristics are employed: (1) discontinuities are introduced in a discrete manner, without remeshing; (2) the approximation is improved locally, for any arbitrary level, without remeshing. Furthermore, essential boundary conditions are enforced using projection matrices and higher-order boundary issues are identified.

Aspects of Permeability in Modelling of Concrete Exposed to High Temperatures

The development of heat and moisture transport in concrete is critical to the development of pore pressures, which are thought to be a primary driver of damage and thermal spalling in concrete exposed to elevated temperatures. In the light of uncertainty and variation in the value of certain material properties and constitutive or parametric descriptions found in the literature, various sets of numerical experiments were conducted to investigate the significance of the intrinsic permeability, the evolution of permeability related to temperature and the relative permeability of the fluid phases as functions of saturation in predicting and analysing the behaviour of concrete drying under normal, low temperature, isothermal conditions and under exposure to very high temperature conditions as might be encountered during a fire. A fully coupled hygro-thermo-mechanical finite element model for concrete was employed with the permeability values and parametric functions altered in the model as required. Results of mass loss and the development of gas pressures with time were considered in relation to the potential for the occurrence of damage and thermal spalling, which is thought to be variously related to these processes. The analyses showed that permeability, and its variation with temperature, are very important in controlling the predicted behaviour at both low and high temperatures. Most significant of all were the relationships chosen to define the relative permeabilities. These were shown to strongly control the results of analyses of both low and high temperature problems and to potentially imply apparently different permeability values for the same concrete.

Efficient numerical analysis of bone remodelling.

This paper presents a formulation for the three-dimensional numerical simulation of mechanically regulated bone adaptation. Attention is focussed on a phenomenologically-based approach to bone remodelling that can be used as a computationally efficient tool to provide insight into the overall response of bone to mechanical loading. A discretisation approach is developed based on a hybrid finite element formulation where displacement, stress and density fields are approximated independently. The paper also discusses a solution algorithm tailored for shared memory multi-core computers. The performance of the model is demonstrated by two numerical examples.