| With the rapid development of construction industry in our country,a large number of large urban public buildings,civil buildings and super high-rise industrial buildings have emerged.The large volume concrete structure has been widely used.During the construction of mass concrete,due to the existence of different degrees of thermal expansion and cold contraction inside and outside the structure,resulting in great tensile stress outside the concrete and compressive stress inside the concrete.In the actual construction,measures such as embedded cooling water pipes and scientific maintenance are often taken to control the tensile stress caused by temperature changes within the allowable range.If the control is improper,Cracks occur on the surface of concrete when tensile stress exceeds tensile strength.Secondly,large-scale concrete projects tend to have many cross operations and large pouring volume.Especially when large and complex structures are involved,the construction conditions are relatively complex and the construction technical requirements are high,which leads to difficult construction quality control and easy cracks in concrete.Based on this background,this paper takes the concrete large-scale concrete raft as the research object,relies on the engineering practice,adopts the methods of theoretical calculation,numerical simulation and field monitoring to conduct a systematic study on the mechanism of crack generation,uses the real-time monitoring data to guide the construction,and achieves the purpose of avoiding harmful cracks.Finally,combined with the engineering practice,it puts forward the crack prevention and control measures.The main research contents and methods of this paper are summarized as follows:(1)According to domestic and foreign literature,systematically combs the progress of crack mechanism and control methods of mass concrete,summarizes the temperature field and temperature stress theory,mainly covering concrete conduction heat and boundary value conditions,concrete constraint theory and temperature stress theory under three-way stress state,etc.From the theoretical level,it lays a theoretical foundation for the subsequent finite element calculation of thermal parameters of hydration,calculation of temperature field by difference method,temperature and strain monitoring.(2)Finite element software was used to establish the finite element model of the actual engineering raft foundation,and the changes and distribution rules of the temperature field and stress field were analyzed.Through the field monitoring of the raft foundation,the measured data and the numerical simulation results were compared and analyzed,and it was concluded that the numerical simulation could agree with the field measured results to a large extent.At the same time,the temperature and strain in the construction process of real-time monitoring and dynamic warning,combined with the monitoring data for dynamic adjustment of the curing stage,to achieve digital construction of mass concrete,so as to prevent the formation of harmful cracks.(3)The crack control measures of embedded cooling water pipes are often used in raft foundation.The finite element simulation is used to calculate the temperature and stress field changes of concrete under two working conditions with or without cooling water pipes.The temperature is continuously monitored during the construction process,and the simulated temperature is compared with the measured value to confirm the rationality of the numerical simulation results.At the same time,the anti-cracking safety factor was calculated under the two working conditions to evaluate the cracking risk.(4)The crack generation mechanism of mass concrete is a complex problem with multi-factor coupling.Based on the summary of raw materials,structural design,construction,maintenance and other factors that affect concrete cracks,this paper analyzes the degree of influence of different factors on cracks,analyzes the crack grade of mass concrete and various influencing factors,and establishes an evaluation system for crack grade of mass concrete.After determining the weight of influencing factors through AHP,fuzzy comprehensive rating method is used to evaluate the crack grade of mass concrete.The evaluation results can provide reference for other mass concrete construction.In this paper,aiming at the crack problem of large volume concrete raft foundation,fine modeling and finite element simulation method are adopted,combined with the field temperature and strain monitoring,so as to obtain the development and change process of the internal temperature and strain of concrete under the three-way stress state,and then analyze the hidden danger of concrete cracking and the internal mechanism of cracks.Finally,this paper discusses the main factors of early cracking intensification and crack control measures,and introduces the effective measures of crack prevention and control from the aspects of raw materials,design,construction and maintenance according to the engineering practice,so as to provide reference for avoiding the cracking of mass concrete raft foundation. |
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