Browsing by Author "Puttonen, Jari, Prof., Aalto University, Department of Civil and Structural Engineering, Finland"

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  • Long-term ageing of concrete structures in Finnish rock caverns as application facilities for low- and intermediate-level nuclear waste
    (2015) Kari, Olli-Pekka
     School of Engineering | Doctoral dissertation (article-based)
    In the Finnish disposal concept concrete-made engineered barriers ensuring the safety of low- and intermediate-level waste repositories in rock caverns are designed to remain serviceable for hundreds of years. With regard to the ageing of the concrete, the lifetime of the disposal facilities will be divided into the periods before and after the closing of the caverns, when the concrete will be exposed to two different sets of environmental conditions. The research pointed out that the ageing of concrete in the disposal conditions is a complex physiochemical phenomenon that cannot be predicted by diffusion models based only on Fick's second law for periods considerably exceeding the period covered experimentally. A thermodynamic model, which considers ion-ion and ion-cement hydrate interactions as well as the properties of the gas phase in multi-ionic transport in concrete, was introduced in the study. A statistical methodology was presented for evaluating the effect of uncertainties on the results. A range of experimental data extracted from sulphate-resistant Portland cement concretes exposed to natural carbonation or the penetration of aggressive substances under submersion for 13 years in a controlled environment was used for evaluating the thermodynamic model. The theory of the model was shown to give a plausible basis for estimating the long-term ageing phenomena of concretes, particularly in practical applications where the design periods of structures are hundreds of years. The thermodynamic simulation method also makes it possible to observe the latent factors involved in the deterioration of the concrete. The method considered is general and can be applied to different concrete mixes by defining their case-specific initial values. Generally, the sulphate-resistant type of concrete with a water-to-cement ratio of 0.43 performed satisfactorily during the simulated period of 500 years, but the initiation of the corrosion of the steel reinforcement cannot be excluded. The leaching of calcium-bearing components from the concrete affects the strength of the concrete. However, the main detrimental reactions of the concrete take place at a depth of less than 50 millimetres from the exposed surface, which is typically a relatively small part of the whole structural thickness and, especially, of the total set of multiple technical barriers. Statistical analyses of the factors and the consideration of uncertainties in the simulation results can be used for improving the reliability of the thermodynamic model. The role of the aggregates in the deterioration of the concrete should also be evaluated in the future and more sophisticated models for the alteration of calcium silicate hydrate are necessary if the timescale is significantly extended beyond 500 years.
  • Managing the delivery and commissioning of nearly zero-energy building projects
    (2015) Kantola, Mikko
     School of Engineering | Doctoral dissertation (article-based)
    EU has set ambitious goals to reduce CO2 emissions in the future. As a part of these goals, according to the two directives (EPBD and RES), all new buildings have to be nZEB buildings after the year 2020. Renewable on-site energy and high energy efficiency are distinctive measures for the nZEB definition. In the northern climate zone, the technical features to achieve those two goals are thick insulation, air-tight envelope, state-of-the-art building services, and renewable energy management systems such as solar PV-panels and ground source heating. The dissertation investigates the risks and challenges related to the fast paced transition period to the nZEB directives. In addition, solutions and counter-measures to the discovered challenges and risks are suggested. Four types of research methods were used in the study. Literature review was the source of inductive reasoning yielding many of the results. Background study on renewable energy prices was also conducted using price data gathering via Internet and phone. Finally, the major part of the findings yielded from four interviews and two expert brainstorming workshops. In the context of the study, ground source heating, wood chip heating, and biogas energy had lower life-cycle costs than a traditional district heating and grid electricity combination. The price of renewable energy and nZEB technology is a major issue, but the life-cycle cost of the nZEB buildings to be built in the 2020s and 2030s will most probably be determined through quality factors. In those projects the main concern of the experts was the management of quality. At the moment the competence level of designers, constructors, and owners was considered insufficient to cope with the transition. Training, lobbying, and official designer competence requirement are offered as part of the solution. Modern project delivery systems enhancing collaboration, such as CM At-Risk and Integrated Project Delivery, and procurement options, such as Competitive Dialogue and Competitive Negotiated Procedure, were found to be highly compatible with nZEB technology. Compared to the traditional project delivery and contracting efforts, more experts are attached to the project from its early stages and throughout the project with the modern procedures. Commissioning quality assurance was found as a useful asset to nZEB project management. Additionally, a commissioning process with specific nZEB alterations was developed in the study.
  • Orientation of short steel fibres in concrete: measuring and modelling
    (2014) Eik, Marika
     School of Engineering | Doctoral dissertation (article-based)
    This research focuses on a cementitious composite formed by mixing of concrete matrix with short steel fibres, SFRC. This composite is quite extensively employed in the construction industry, for example in floors resting on soil and even in some load-bearing structures, such as floor-slabs. The complexity of SFRC is the presence of the anisotropic behaviour occurring due to different alignments of short fibres. The examinations performed comprise two research branches: measuring of fibre orientations from the hardened concrete matrix and modelling of composite properties considering the orientation distribution of fibres. Two methods of measuring fibre orientations are developed: slicing method and X-ray micro-tomography. Parts extracted from the full-size floor-slabs are used as samples. The slicing with photometry approach is improved by DC-conductivity testing joined with image analysis. X-ray micro-tomography is performed on sufficiently large samples and the orientation of fibres is specified by the analysis of 3D voxel images of scanned fibres. The received measuring results proved that both DC-conductivity testing combined with photometry and X-ray micro-tomography have high accuracy and they can be applied in defining fibre orientations from real concrete samples reliably. The material model developed for one meso-volume element of SFRC is based on an orthotropic hyperelastic material model where the second-order terms of the strain energy function are employed resulting in orthotropic St. Venant-Kirchhoff model. The orthotropic meso-symmetry of the composite is modelled by the structural tensors based on the eigenvectors of the second-order alignment tensor, which represent the dominating alignment of fibres. The material model developed for SFRC consists of an isotropic part presenting the concrete and the orthotropic part including the influence of short steel fibres. The orientation distribution function of fibres is utilized to evaluate the orthotropic effect in the defined material symmetry directions. The advantage of the material model developed is that it uses the full orientation information of fibres and employs tensor quantities, which are independent of any reference frame. Finally, the implementation of the model is demonstrated by examples based on fibre orientations measured from test samples.