| 1. |
- Hedlund, Hans
(författare)
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Hardening concrete : measurments and evaluation of non-elastic deformation and associated restraint stresses
- 2000
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- If a structural member of hardening concrete is restrained to some degree against expansion and contraction during the heating phase of the hydration process and the subsequent contraction phase, stresses will be induced. The issue of primary interest is whether or not these induced stresses will lead to cracking. Volume changes in hardening concrete are not only ruled by the thermal movements. High Performance Concrete (HPC) is usually based on high binder content mixed with small amounts of water. The low water-to-binder ratio leads to significant autogenous deformations, which has to be included in stress analyses. For water-to-cement ratios below about 0.40, self-desiccation results in a significant drop in the pore humidity even at early ages. In contrast to the moisture flux, self-desiccation is reflected all over the structure, and at restraint conditions this may be a significant contribution to the risk of early age cracking. Models for descpition of autogenous shrinkage and different evaluation techniques how to split between moisture and thermal deformations are presented in the thesis. Laboratory tests and modelling in early age of the stresses caused by restraint volume changes have been performed and discussed for high performance and normal strenght concrete. Evaluated parameters are presented for a great number of concrete mixes. It has been shown that all material data for stress calculations can be taken from material related tests on each individual property without any correction when comparison is done with measured stresses in the relaxation test-frame.
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| 2. |
- Sayahi, Faez
(författare)
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Plastic Shrinkage Cracking In Concrete : Mitigation and Modelling
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Early-age (up to 24 hours after casting) cracking may become problematic in any concrete structure. It can have a negative influence on the aesthetics of the concrete structure, as well as decreasing the durability and serviceability by facilitating the ingress of harmful materials into the concrete bulk. Moreover, these cracks may expand gradually during the member’s service-life due to long-term shrinkage and/or loading. Early-age cracking is caused by two driving forces: 1) plastic shrinkage cracking which is a physical phenomenon and occurs due to rapid and excessive loss of moisture, mainly in form of evaporation, 2) chemical reactions between cement and water which causes autogenous shrinkage. In this PhD project only the former is investigated.Rapid evaporation from the surface of fresh concrete causes negative pressure, known as capillary pressure, in the pore system. This pressure pulls the solid particles together and decreases the inter-particle distances, causing the whole concrete element to shrink. If this contraction is hindered in any way, the induced tensile stresses may exceed the low tensile strength of the concrete and cracking starts. The phenomenon, occurring shortly after casting while the concrete is still in the plastic stage, is mainly observed in elements with high surface to volume ratio such as slabs and pavements.Many parameters may affect the probability of plastic shrinkage cracking. Among others, effect of water/cement ratio (w/c), fines, admixtures, geometry of the element, ambient conditions (i.e. temperature, relative humidity, wind velocity and solar radiation), etc. has been investigated previously. In the presented research, in addition to studying the influence of various parameters, i.e. w/c, cement type, coarse aggregate content, superplasticizer dosage, admixtures, and steel fibres, effort is made to reach a better and more comprehensive understanding about the cracking governing mechanism. Evaporation, capillary pressure evolution and hydration rate are particularly investigated in order to identify their relationship.This project started with extensive literature study which is summarized in Paper I. Then, the main objective was set upon which series of experiments were defined. The utilized methods, material, investigated parameters, and results are presented in Papers II-IV. A model was, then, proposed in Paper V, to estimate the cracking severity of the plastic concrete.It has been observed that evaporation is the driving force behind the plastic shrinkage crackingin concrete. However, a correlation between evaporation, rate of capillary pressure development and the duration of dormant period governs the severity of the phenomenon. Among other things, the results show that rapid capillary pressure development in the pore network accompanied by slower hydration significantly increases the cracking risk.
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| 3. |
- Stelmarczyk, Marcin
(författare)
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Applied Modeling of Moisture Phenomena in Concrete
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis contains new and improved calculation models, based on applied research merging numerical modeling and material research, in the areas of hydration and chemical binding of water, sorption of moisture and transport of moisture in concrete. The proposed model for hydration and chemical binding of water is based on general kinetics modeling and calculates degree of hydration directly, without using of equivalent time of maturity. It handles dependencies on temperature as well as availability of binder and water. It models the dormant phase in the beginning of hydration. It allows also for linear variability with temperature of how much water is bounded per amount of binder, which models cross-over effects in development of chemical binding of water, measured at different temperatures. In the area of moisture sorption a family of models is proposed, building on a domain-based approach combined with explicit modeling of sorption site concentration for various desorption and absorption conditions by a matrix of adaptable values. The proposed modeling idea offers a spline-like possibility of choosing the amount of adaptation parameters and the precision of the model. Two formulations of the sorption model are presented – for both isothermal and non-isothermal conditions. A selection of methods to adapt the parameters to measured data is proposed for various situations, also covering both isothermal and non-isothermal conditions. Their properties are to some degree mathematically shown/proved and their performance is discussed.In the area of moisture transport, a family of models is proposed, conceptually similar to the proposed sorption models. It also builds on a domain-based approach combined with explicit modeling of concentration of contributions to the overall transport, for various desorption and absorption conditions by a matrix of adaptable values. The proposed modeling idea offers a spline-like possibility of choosing the amount of adaptation parameters and the precision of the model. Three formulations of the transport model are presented – one for isothermal conditions, one generalized for non-isothermal conditions and one simplified for non-isothermal conditions where capillary suction is assumed to dominate transport phenomena. A selection of methods to adapt the parameters to measured data is proposed for various situations, covering all three proposed model versions. Their properties are to some degree mathematically shown/proved and their performance is discussed.
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