Master Dissertation Research On Crack Control Technology For Mass Concrete

At initial stage of pouring completion of mass concrete, due to the sudden temperature rise of concrete resulting from the cement hydration heat, the temperature difference inside and outside of concrete excesses its limit. In addition, moisture dissipation on surface layer of the concrete results in shrinkage deformation, and then generates hazardous penetrating crack. Surface cracks caused by drying shrinkage and autogenous shrinkage of concrete though dose not affect the safety performance of the structure, some of them exceed the requirements of Specifications will reduce the durability, waterproofing and integrity of the structure which bring about steel corrosion and other adverse consequences. Therefore, whether they are unsightly and unharmful surface cracks or the harmful cracks which endanger the structural safety must be strictly controlled in construction.Taking the nuclear island basemat of a nuclear power plant as an example, mainly carrying out the following aspects research and discussion:(1) Describing the causes and development mechanisms of mass concrete cracks;(2) Combining finite difference method with MATLAB software bases on the heat conduction principle, carrying out predictive calculation for the nuclear island basemat mass concrete temperature field, achieving the visualization of early concrete internal temperature filed accurately, receiving the temperature curve chart and temperature rise during the period of construction program;(3) Analyzing the temperature development law in the condition of different placement temperature and layer thickness, to determine the main restricted factors of temperature field development;(4) Using the model of a long wall on the foundation to calculate the temperature stress of basemat under the external constraints on the basis of temperature field, checking the temperature stress extremum according to crack resistance standard, obtaining the optimal placement temperature of concrete in advance. These prophase efforts provide a theoretical basis and guidance for formulating construction programs and curing arrangement scientifically;(5) Discussing the economic rationality of concrete placement temperature as well as the supplementary role of other construction measures;(6) For the reason and characteristics of cracks, drawing up effective response measures is the key to control the formation and development of cracks. This paper elaborates the actual crack control scheme, mainly involves structure design and site construction, summarizes and analyzes a series of effective technical measures, at the same time proposes a variety of new technology and improved construction methods to control temperature stress and temperature cracks. The specific methods and ideas include the selection of raw materials, formulating mixing proportion scientifically, improving the quality of concrete placement, and strictly controlling the temperature rise and the temperature peak, delaying the cooling rate, inhibiting concrete shrinkage, increasing concrete tensile strength value, releasing or discharging constraints suffered by nuclear island basemat concrete and strengthening the monitoring of temperature and temperature stress.etc;(7) The article compares the predicted results with measured situation, tests the reliability of the prediction calculation, summarizes the basic context with age evolution of mass concrete temperature field and temperature stress. And thus proposes some suggestions and advice about crack control of mass concrete.