Concrete at high temperatures: hygro-thermo-mechanical degradation of concrete

The main aim of the presented work is the development of a reliable and coherent solution approach to investigate thermo-hygro-mechanical behaviour of concrete, especially under severe heating conditions. The work focuses on the development and extension of an existing analytical, numerical and constitutive model developed at the University of Glasgow. This is then used as a predictive modelling tool to investigate the response of concrete structures subject to combined thermo-mechanical loads. The thesis focuses initially on the coupled heat and mass transport model. A novel alternative formulation for sorption isotherms, that is applicable to both normal and high strength concrete, is developed. Furthermore, the effect of the slip flow effect is included and several state equations, including relative permeability and saturation vapour pressure, are adopted. Additionally, the effect of polypropylene fibres as a spalling prevention technique is modelled via a modification of the intrinsic permeability formulation. The transport model is further coupled with a damage-based mechanical model for concrete. The fully coupled thermo-hygro-mechanical model is presented through validation and verification problems and case studies. The model is implemented in a finite element formulation and behaves in a robust manner. The predictions of moisture state for the benchmark problems of drying and heating compare well with experimental results. Classical behaviour associated with heated concrete, such as moisture clog, gas pressure build-up, etc. are all captured by the presented model. The thesis concludes by considering the analysis of prestressed concrete pressure vessel

Fully coupled, hygro-thermo-mechanical sensitivity analysis of a pre-stressed concrete pressure vessel

Following a recent world wide resurgence in the desire to build and operate nuclear power stations as a response to rising energy demands and global plans to reduce carbon emissions, and in the light of recent events such as those at the Fukushima Dai-ichi nuclear power plant in Japan, which have raised questions of safety, this work has investigated the long term behaviour of concrete nuclear power plant structures.<p></p> A case example of a typical pre-stressed concrete pressure vessel (PCPV), generically similar to several presently in operation in the UK was considered and investigations were made with regard to the extended operation of existing plants beyond their originally planned for operational life spans, and with regard to the construction of new build plants.<p></p> Extensive analyses have been carried out using a fully coupled hygro-thermo-mechanical (HTM) model for concrete. Analyses were initially conducted to determine the current state of a typical PCPV after 33+ years of operation. Parametric and sensitivity studies were then carried out to determine the influence of certain, less well characterised concrete material properties (porosity, moisture content, permeability and thermal conductivity). Further studies investigated the effects of changes to operational conditions including planned and unplanned thermal events.<p></p> As well as demonstrating the capabilities and usefulness of the HTM model in the analysis of such problems, it has been shown that an understanding of the long-term behaviour of these safety–critical structures in response to variations in material properties and loading conditions is extremely important and that further detailed analysis should be conducted in order to provide a rational assessment for life extension.<p></p> It was shown that changes to the operating procedures led to only minor changes in the behaviour of the structure over its life time, but that unplanned thermal excursions, like those seen at the Fukushima Dai-ichi plant could have more significant effects on the concrete structures.<p></p&gt

Mechanical and Durability Properties of Concrete Under Low Temperatures

Frost attack can heavily damage concrete materials and concrete structures. Therefore, it is important to predict the service life of concrete structures considering the effect of the deterioration process. A theoretical model based on Mori-Tanaka micromechanics method was developed to estimate the deterioration of elastic properties of concrete under low temperatures. The model took into account a continuous pore size distribution for cement paste, freezing temperatures with different pore sizes, and damage development in cement paste. The model was able to characterize the stiffening effect due to the formation of solid ice and the weakening effect of the damage development due to excessive ice formation. The accuracy of model predictions was validated with published experimental data. Moreover, the effective diffusivities of concrete under low temperatures were predicted based on a multiple-scale generalized self-consistent (GSC) model. The results show that the diffusivities of concrete under low temperatures vary based on the amount of ice formation and the extent of damage production. An experimental study was conducted to determine the coupling parameter for the effect of thermal gradient on moisture transfer. The theoretical models for the elastic and transport properties and the coupling parameter were implemented in a finite element code. The estimations of temperature distributions by the finite element code were compared with the experimental data, and a good agreement was obtained. In order to mitigate the frost damage, nano-silica was incorporated in concrete mix design to optimize the pore structure of cement paste and thus to improve the mechanical properties of concrete. A comprehensive experimental study was conducted to evaluate the effect of nano-silica on mechanical and durability properties of the concrete, particularly on its low temperature resistance. The results indicated that nano-silica can be used as an effective additive to improve the resistance of concrete to the frost attack

Modelling of heat and moisture transfer in concrete at high temperature

Moisture diffusion and related fluid pressures play a key role in cracking and spalling of concrete subject to high temperatures. This paper describes recent developments of a mode for moisture and heat transfer in porous materials, to be combined with an existing and well tested meso-mechanical model for concrete. Liquid and gas flows are formulated separately, yet later they can be combined in terms of s single variable, Pv. The material pore distribution curve is taken as the basis for developing a new physically-based desorption isotherm alternative to the traditional Bazant & Thonguthai’s model. A simple academic example for temperatures between 27 and 800ºC is presented to show the behaviour of the model

Lattice Modeling of Early-Age Behavior of Structural Concrete.

The susceptibility of structural concrete to early-age cracking depends on material composition, methods of processing, structural boundary conditions, and a variety of environmental factors. Computational modeling offers a means for identifying primary factors and strategies for reducing cracking potential. Herein, lattice models are shown to be adept at simulating the thermal-hygral-mechanical phenomena that influence early-age cracking. In particular, this paper presents a lattice-based approach that utilizes a model of cementitious materials hydration to control the development of concrete properties, including stiffness, strength, and creep resistance. The approach is validated and used to simulate early-age cracking in concrete bridge decks. Structural configuration plays a key role in determining the magnitude and distribution of stresses caused by volume instabilities of the concrete material. Under restrained conditions, both thermal and hygral effects are found to be primary contributors to cracking potential

Three-Dimensional Network Model for Coupling~of~Fracture and Mass Transport in Quasi-Brittle Geomaterials

Dual three-dimensional networks of structural and transport elements were combined to model the effect of fracture on mass transport in quasi-brittle geomaterials. Element connectivity of the structural network, representing elasticity and fracture, was defined by the Delaunay tessellation of a random set of points. The connectivity of transport elements within the transport network was defined by the Voronoi tessellation of the same set of points. A new discretisation strategy for domain boundaries was developed to apply boundary conditions for the coupled analyses. The properties of transport elements were chosen to evolve with the crack opening values of neighbouring structural elements. Through benchmark comparisons involving non-stationary transport and fracture, the proposed dual network approach was shown to be objective with respect to element size and orientation

Long-term deformations of fastening systems under sustained loads

Fastening elements are used to connect structural members with each other and with appliances. In general post-installed fastening systems are characterized as either mechanical anchors or so called bonded or chemical anchors. The working principle of the former is friction or mechanical interlock while the performance of the latter is based on the adhesive properties of polymer mortars – mostly epoxy or vinyl-ester based. Both mechanical as well as chemical anchors undergo an approval process during which their performance is certified. These tests are performed according to strict guidelines. In recent years two prominent disasters created significant concerns regarding the performance of bonded anchors under sustained loads. Thus, current approval guidelines regarding the qualification of bonded anchors systems are being challenged and the introduction of penalty factors for sustained loads is being discussed. Interestingly, all long-term deformations are being attributed to the adhesive only, neglecting contributions from concrete as well as damage and system effects. In this contribution we attempt to quantify the effect of (a) concrete creep, and (b) stress redistribution and damage mechanisms based on state of the art numerical simulations, calibrated by material tests and validated by system tests. Specifically, an epoxy based and a vinyl-ester based system are investigated and compared to the performance of headed-studs. The numerical framework is able to model the coupled problem of heat- and moisture transport, hydration, ageing of material properties, shrinkage and creep in a rate-type, pointwise form. For the constitutive model the Lattice Discrete Particle Model (LDPM) is used