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Numerical models for degradation of concrete in hydraulic structures due to long-term contact with water
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.ORCID iD: 0000-0002-4015-3373
2018 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The durability of concrete is of major concern in all types of concrete structures where the combined effect of exposure conditions and the type and quality of the concrete material usually determines the rate of degradation. Furthermore, there are synergy effects between different deterioration mechanisms, which means that the combined rate of degradation is higher than the sum of the individual rates of each mechanism. Therefore, to accurately predict the residual service life of existing structures or when designing new structures, it is essential to consider all these aspects. This means that various chemical and physical processes, as well as how these interact, must be taken into account in models aiming to be used for service life predictions.

This thesis presents the first part of a research project with the aim to investigate common deterioration mechanisms of concrete in hydraulic structures, and to improve the knowledge how these and other related phenomena can be described using mathematical models. The objective is also to study how different mechanisms interact and to find suitable approaches to account for these interactions in the models. To this end, a literature survey on commonly detected damage in hydraulic structures is presented. In addition, it also addresses in what types of and where in hydraulic structures the various damage types are usually observed. The mathematical models presented in this part of the project are focused on long-term water absorption in air-entrained concrete as well as on freezing of partially saturated air-entrained concrete. Both models are based on a multiphase description of concrete and poromechanics to describe the coupled hygro-thermo-mechanical behaviour. The thesis also presents some of the basic concepts of multiphase modelling of porous media, including discretization of the models using the finite element method (FEM). Furthermore, it covers the simplifications that are usually introduced in the general macroscopic balance equations for mass, energy and linear momentum when modelling cement-based materials.

To verify the developed models and to show their capabilities, simulation results are compared with experimental data, in situ measurements and other simulations from the literature. The results indicate that both models perform well and can be used to predict long-term moisture conditions in hydraulic structures as well as freezing-induced strains in partially saturated air-entrained concrete, respectively. Even though no interactions with other deterioration mechanisms are included in the models, the development and use of these have given insights to which parameters that are important to consider in such extensions. Furthermore, based on the insights gained, the complexity of describing the full interactions between several mechanisms in mathematical models is also discussed. It is concluded that models aiming to be used for service life predictions of hydraulic structures in day-to-day engineering work need to be simplified. However, the type of advanced models presented in this thesis can serve as a basis to study which aspects and parameters that are essential to consider in simplified prediction models.

Abstract [sv]

Beständigheten hos betong är av avgörande betydelse i alla typer av betongkonstruktioner där den kombinerade effekten av exponeringsförhållanden samt typ och kvalitet på betongmaterialet vanligtvis avgör nedbrytningshastigheten. Dessutom finns synergieffekter mellan olika nedbrytningsmekanismer som innebär att den kombinerade nedbrytningshastigheten är större än summan av de enskilda nedbrytningshastigheterna. För att noggrant kunna prediktera den återstående livislängden hos befintliga konstruktioner eller vid design av nya konstruktioner är det därför viktigt att ta hänsyn till samtliga av dessa aspekter. Detta innebär att olika kemiska och fysikaliska processer, samt hur dessa interagerar med varandra, måste tas i beaktande i modeller som avses användas för livslängdsbedömningar.

Den här licentiatuppsatsen presenterar den första delen av ett forskningsprojekt där målet är att studera vanligt förekommande nedbrytningsmekanismer i vattenbyggnadskonstruktioner och att öka kunskapen om hur dessa och andra relaterade fenomen kan beskrivas med matematiska modeller. Målet är också att studera hur olika nedbrytningsmekanismer samverkar och att hitta lämpliga tillvägagångssätt att ta hänsyn till dessa interaktioner i modellerna. För detta ändamål presenteras en litteraturstudie avseende vanligt förekommande skador i vattenbyggnadskonstruktioner. Dessutom behandlar denna i vilka typer av vattenbyggnadskonstruktioner och var i dessa som de olika typerna av skador vanligtvis observeras. De matematiska modeller som presenteras i denna del av projektet är inriktade på långtidsabsorption av vatten i lufttillsatt betong samt på frysning i delvis vattenmättad lufttillsatt betong. Båda modellerna är baserade på en multifasbeskrivning av betong samt poromekanik för att beskriva det kopplade hydro-termo-mekaniska beteendet. Uppsatsen presenterar också några av de grundläggande koncepten gällande multifasmodellering av porösa material, inklusive diskretisering av modellerna genom användning av finita elementmetoden (FEM). Dessutom beskrivs de förenklingar som vanligtvis införs i de generella makroskopiska balansekvationerna för massa, energi och rörelsemängd då cementbaserade material modelleras.

Simuleringsresultat från de utvecklade modellerna jämförs med försöksdata, fältmätningar samt andra simuleringsresultat från litteraturen för att verifiera modellerna samt visa hur de beter sig. Resultaten visar att båda modellerna ger tillfredställande resultat och kan användas för att uppskatta de långsiktiga fuktförhållandena i vattenbyggnadskonstruktioner samt frysinducerade töjningar i delvis vattenmättad lufttillsatt betong. Även om inga interaktioner mellan andra nedbrytningsmekanismer inkluderades i modellerna, så har utvecklingen samt användandet av dessa gett insikter gällande vilka parametrar som är viktiga att beakta i sådana vidareutvecklingar. Baserat på dessa insikter diskuteras också komplexiteten i att beskriva interaktionen mellan flertalet mekanismer i matematiska modeller. Det konstateras också att modeller som avses användas i dagligt ingenjörsarbete för livstidsbedömningar av vattenbyggnadskonstruktioner behöver förenklas. Däremot kan den typ av avancerade modeller som presenteras i denna uppsats användas som en grund för att studera vilka aspekter och parametrar som är viktiga att beakta i förenklade modeller.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. , p. xii, 90
Series
TRITA-ABE-DLT ; 185
Keywords [en]
degradation, deterioration mechanisms, hydraulic structures, air-entrained concrete, multiphase model, long-term moisture conditions, pore size distribution, freezing, partially saturated, finite element method
National Category
Civil Engineering
Research subject
Civil and Architectural Engineering
Identifiers
URN: urn:nbn:se:kth:diva-225125ISBN: 978-91-7729-734-5 (print)OAI: oai:DiVA.org:kth-225125DiVA, id: diva2:1194245
Presentation
2018-05-08, B1, Brinellvägen 23, Stockholm, 12:30 (English)
Opponent
Supervisors
Note

QC 20180403

Available from: 2018-04-03 Created: 2018-03-29 Last updated: 2018-04-03Bibliographically approved
List of papers
1. A hygro-thermo-mechanical multiphase model for long-term water absorption into air-entrained concrete
Open this publication in new window or tab >>A hygro-thermo-mechanical multiphase model for long-term water absorption into air-entrained concrete
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Many concrete structures located in cold climates and in contact with free water are cast with air-entrained concrete. The presence of air pores significantly affects the absorption of water in the concrete, and it might take decades before these are fully saturated. This generally improves the long-term performance of such structures and in particular their frost resistance. To study the long-term moisture conditions in air-entrained concrete, a hygro-thermo-mechanical multiphase model is presented, where the rate of water filling of air pores is described as a separate diffusion process. The driving potential is the concentration of dissolved air, obtained using an averaging procedure with the air pore size distribution as the weighting function. The model is derived using the Thermodynamically Constrained Averaging Theory (TCAT) as a starting point. Two examples are presented to demonstrate the capabilities and performance of the proposed model. These show that the model is capable of describing the complete absorption process of water in air-entrained concrete, and yield results that comply with laboratory and in situ measurements.

Keywords
Air-entrained concrete, Multiphase model, Long-term absorption, Diffusion, Pore size distribution
National Category
Infrastructure Engineering
Research subject
Civil and Architectural Engineering
Identifiers
urn:nbn:se:kth:diva-225114 (URN)
Note

QC 20180403

Available from: 2018-03-29 Created: 2018-03-29 Last updated: 2018-04-03Bibliographically approved
2. Freezing of partially saturated air-entrained concrete: A multiphase description of the hygro-thermo-mechanical behaviour
Open this publication in new window or tab >>Freezing of partially saturated air-entrained concrete: A multiphase description of the hygro-thermo-mechanical behaviour
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Even though air-entrained concrete is usually used for concrete structures located in cold climates that are exposed to wet environments, frost damages are frequently detected during inspections. However, it is often hard to assess the extent and severity of these damages and, thus, there is a need for better tools and aids that can complement already established assessment methods. Several studies have successfully shown that models based on poromechanics and a multiphase approach can be used to describe the freezing behaviour of air-entrained concrete. However, these models are often limited to the scale of the air pore system and, hence, hard to use in applications involving real structures. This study proposes a hygro-thermo-mechanical multiphase model which describes the freezing behaviour of partially saturated air-entrained concrete on the structural scale. The model is implemented in a general FE-code and two numerical examples are presented to validate and show the capabilities of the model. The first concerns a series of experimental tests of air-entrained cement pastes, whereas the second aims to show the capability of the model to account for an initial non-uniform distribution of moisture. The results show that the model can reproduce the freezing behaviour observed in the experimental tests on a structural scale as well as being capable of describing freezing induced deformations caused by non-uniform moisture distributions.

Keywords
freezing, partially saturated, air-entrained concrete, structural scale, finite element modelling
National Category
Infrastructure Engineering
Research subject
Civil and Architectural Engineering
Identifiers
urn:nbn:se:kth:diva-225115 (URN)
Note

QC 20180403

Available from: 2018-03-29 Created: 2018-03-29 Last updated: 2018-04-03Bibliographically approved
3. Influence of air voids in multiphase modelling for service life prediction of partially saturated concrete
Open this publication in new window or tab >>Influence of air voids in multiphase modelling for service life prediction of partially saturated concrete
2018 (English)In: Computational Modelling of Concrete Structures / [ed] Günther Meschke, Bernhard Pichler, Jan G. Rots, London, UK: CRC Press, 2018, p. 317-326Conference paper, Published paper (Refereed)
Abstract [en]

The purpose of this study is to show the influence and significance of including water filling of air pores when studying moisture conditions in concrete structures cast with air-entrained concrete and in contact with free water. Especially if the aim is to assess the risk for frost damages in different regions of the structure, based on a critical degree of saturation, in order to ultimately perform a service life prediction. A hygro-thermo-mechanical multiphase model that includes the effect of water filling in air pores, recently presented by the authors, is briefly described and applied in two numerical examples. The results show moisture distributions that would not be possible to capture without the air pore filling included in the model. More importantly, the general shape of these distributions complies well with measured distributions in real concrete structures as well as with distributions obtained in laboratory measurements.

Place, publisher, year, edition, pages
London, UK: CRC Press, 2018
National Category
Infrastructure Engineering
Research subject
Civil and Architectural Engineering
Identifiers
urn:nbn:se:kth:diva-225113 (URN)2-s2.0-85061340478 (Scopus ID)978-1-138-74117-1 (ISBN)978-1-315-18296-4 (ISBN)
Conference
Euro-C 2018 Computational Modelling of Concrete and Concrete Structures
Note

QC 20180403

Available from: 2018-03-29 Created: 2018-03-29 Last updated: 2019-04-26Bibliographically approved

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