Papers by Keyword: Hydration Heat

Abstract: This paper deals with the stress analysis and prediction of crack formation caused by the development of hydration heat and shrinkage in the expansion block of the Nové Heřminovy dam. During the design of the concrete gravity dam, extensive analysis were carried out in order to predict the occurrence of cracks induced by hydration heat development and shrinkage within the first 10 years after the start of construction. The aim of the analysis was to predict the location, depth, and width of cracks under given conditions and to provide recommendations for minimizing crack formation in the structure. In the first stage, a parametric study on a sectional model was carried out, which led to the optimization of technological procedures and the concreting schedule. Subsequently, a global model of a typical gravity block of the dam was created to simulate the boundary conditions of the structure as accurately as possible during construction and subsequent operation, and to obtain results regarding stress distribution and areas with a risk of cracking. The analysis was performed using nonlinear numerical calculations in ATENA software, while also incorporating transport analysis of temperature and environmental humidity.

Abstract: The article deals with the analysis of the development of hydration heat in massive concrete structures and focuses mainly on risk factors which could lead to the appearance of early age cracking in the structure shortly after concreting. As part of the analysis, a parametric study was carried out, which was focused not only on the size of cracks on the surface of the structure, but also on the evaluation of the temperature gradient, where the ratio of the temperature difference and the distance between two points was investigated. During the study, different variants of the type and length of surface covering and different boundary conditions in terms of ambient temperature during individual seasons were considered. The numerical analysis was performed using nonlinear numerical calculations in ATENA software. At the conclusion, an evaluation of the effect of different types of covering of the concrete structure on the reduction of the number and size of surface cracks caused by hydration heat is carried out.

Abstract: The aim of this paper is to present results of an experimental analysis focused on the mechanical waves passing through fine-grained materials, prepared from the CEM I 52.5 R Portland cement and the water coefficient w/c = 0.40, during the setting and hardening phase. In the experiment differences in the wave propagation in cement pastes and mortars were investigated. After mixing, both materials were poured into a cylindrical mould with the diameter of 75.0 ± 0.3 mm. These test specimens were then used to observe, in particular, the development of the amplitude of mechanical waves using a measuring setup which was composed for this purpose from the equipment of the Brno University of Technology. The results show that the time of „critical changes" in the internal structure of the material can be determined. These changes are related to the quality of the bonds of the particles in the internal structure which are reflected in the propagation of the mechanical waves within the material. It is also expected that the experimental analysis will help extend the knowledge of mechanical waves propagation in cement-based composite materials during setting.

Abstract: The paper describes problems of water-tight concrete constructions, their design, manufacture and testing of quality in accordance with regulations and directives used in the Czech Republic and abroad. Current knowledge of water-tight constructions and water-tight concrete imply that main focus is on problems of use of water-tight concrete for construction of so-called white bath and preventing crack formation in concrete. Laboratory work is focused mainly on design of concrete based on individual regulations. Subsequently, influence of various types of cement and additions partly replacing cement on properties of water-tight concrete is examined. Research of heat development during hydration at early stages of maturing is and inseparable part of the experimental work, as well as development of compressive strength, water-tightness and frost resistance of concrete

Abstract: The effects of triethanolamine on the compressive strength and setting time of fly ash cementitious materials, slag cementitious materials, and limestone cementitious materials were investigated. The results show that the dosage of 0.04% of triethanolamine can significantly improve the 3d, 7d and 28d compressive strength of fly ash cementitious materials. It possesses less impact on the setting time. The dosage of 0.04% of triethanolamine can significantly improve the 3d and 7d compressive strength of slag cementitious materials while the effect of late strength is not obvious. It extends the initial setting time and the final setting time is not changed, which has retarding effect. The results of hydration heat and XRD show that triethanolamine extends the induction period of cement hydration, and gypsum is exhausted in advance during the hydration process. Triethanolamine promotes the transformation of AFt to AFm and there is no effect on the type of hydration products.

Abstract: The paper deals with the influence of level of grinding of the recycled concrete. Level of grinding mostly influence the hydration heat evolution of cement paste. Measurement of hydration heat evolution is one of the ways to detect reactive properties of recycled concrete powder modified by high speed mill device. Basic characteristics that most influence the hydraulic properties of recycled concrete powder are grin size diagram and chemical composition of the original concrete. Non-hydrated inner part of cement grains could further react if it is exposed to water. In this experiment the recycled concrete from recycled railroad sleepers and structural layers of the D2 highway was used. Recycled concrete powder from highway was divided into 4 more variants which differ by grinding process during production. To detect difference in hydration process four mixtures of cement paste with different types of recycled concrete powder were made and further subjected to calorimetric measurement. The results were after compared with the hydration heat rate of reference cement paste. According to results the type of milling process had minor influence on hydraulic properties of recycled concrete powder compared with influence of origin of recycled concrete.

Abstract: The design of concrete structures exposed to environmental attack requires serious attention for concrete durability. Early age cracking due to autogenous deformations should be avoided.In this work the study of the structural effects of hydration heat and rheological behaviour of a massive concrete casting is presented. The object of the study is a skyscraper foundation slab. Aim of the work is the numerical simulation of what occurs to the structure during the hardening, in order to avoid unforeseen autogenous cracking and therefore the choice of a tailor-made concrete mixture able to fulfil the performance criteria.Non-linear finite element coupled thermal and mechanical analyses have been performed taking into account: hydration heat generation and dispersion, dimension and sequence of the casting, evolution of concrete mechanical properties in time during the hardening reaction, creep and differential shrinkage.

Abstract: In order to get the various strength grade of the cement and know the characterics of the hydration heat of multi-component cementitious binders, in this paper, we get the component-cement with low content clinker and high content admixture. The cement properties, the hydration heat properties and the self-construction properties of composite cementitious binders with different rations of the mixture are tested.The results showed that the 56^th compressive strength can get 58.4Mpa which is 4.4MPa higher than the traditional cement.The hydration heat of cement with high dosage admixture has a greatly reduction. The heat amount and the hydration release speed are both lower than the tradtional . The value of self-constriction is obviously decreased.

Abstract: Many factors play important role during concreting the base slabs, industrial floors and concrete pavements. Often, there is a tendency to design the structure so that the crack does not arise or arise only with the limited crack width. The parametric study of selected climatic factors which have crucial affect on the temperature or cracking development of the structure at an early stage is elaborated in the paper.The results were obtained using thermodynamic and stress numerical model TD&SM, which was developed in the last four years at the Department of Concrete Structures and Bridges. The outputs from program TD&SM were tested and compared with experimental measurements on real structures, and showed very good agreement. It is possible to calculate the temperature development at any time and place of base slab, industrial floor or concrete pavement at the early stage using this model . It enables afterwards to investigate the risk of crack creation.

Abstract: The paper deals with the topical theme of concrete for watertight tunnel lining, currently solved in the Czech Republic and abroad. The focus is on high water-tightness of concrete for production of these specific structures and other requirements from the point of durability of concrete for watertight tunnel lining. Experimental work followed previous research and was focused on testing of selected properties of concrete for watertight tunnel lining, either with only cement as a binder (reference mixtures) or with partial replacement of cement with fly ash from Detmarovice power plant. Manufactured specimens of fresh and hardened concrete were used for testing of development of compressive strength, depth of penetration with pressure water and development in hydration heat in time.