Analysis And Prediction Of Temperature Field In Large Volume Concrete Test Blocks

In recent years,with the rapid development of China’s economy,the proportion of construction of large and super-large buildings has been growing,and large volume concrete structures have gradually occupied an increasingly high percentage in the actual engineering applications.Due to the large size and poor thermal conductivity of such concrete structures,it is difficult to dissipate the heat of hydration of cement generated inside the concrete during the construction process.The heat accumulation inside the structure is likely to produce excessive temperature gradients,which leads to a significant increase in temperature stresses and thus temperature cracks,which ultimately poses a threat to the safety of the entire project structure.The study of temperature field for mass concrete as an important issue in temperature control,is directly related to the quality of structural construction.In order to better study this problem and to provide reference and basis for the actual construction and maintenance.This paper firstly starts from the hydration temperature rise law of mass concrete,based on the cubic concrete block(1m × 1m × 1m)cast on site,and studies the characteristics of temperature change at different locations inside the test block and the role of related factors on the temperature rise by analyzing the temperature curves at its different measurement points.Secondly,in order to be able to calculate the temperature rise value at the core location inside the concrete more easily and accurately,and to meet the demand of temperature prediction in actual construction.Based on the experimental results,this paper improves and optimizes this calculation method by introducing the concept of cement hydration degree from the calculation theory of traditional one-dimensional difference method.Finally,the test blocks were modeled with the finite element software COMSOL Multiphysics,and the temperature calculation results at different locations of the test blocks were analyzed to compare and verify the temperature variation patterns obtained from the tests.In addition,the advantages and disadvantages of in situ monitoring,one-dimensional difference method and numerical simulation and their applicability are discussed from the perspective of temperature prediction,respectively.Among them,the main findings of the paper are as follows:(1)Comparing the trends of temperature rise curves at different measurement points,it can be found that the deformation trends of temperature curves at different points are generally similar,and they are all characterized by "rising-declining-steady" phase changes.Among them,the contact degree between the upper measurement points of the test block and the external environment is closer than other measurement points,and is vulnerable to the influence of the external environment.While the heat accumulation in the central measurement point and not easy to dissipate,the value of its temperature curve is significantly higher than other measurement points.And because the internal core position is farther from the surface of the test block and less affected by external interference factors,the temperature curve changes more steadily and smoothly.(2)For two concrete blocks with different reinforcement ratios,the analysis of their temperature profiles reveals that there are different exothermic mechanisms in the concrete at each stage of temperature change.Among them,the exothermic reaction of cement hydration is mainly concentrated in the rising section,and the temperature difference between the lower and inner surfaces of this stage increases significantly,which makes it easy to have temperature cracks.The cement hydration reaction in the descending section gradually becomes weaker,and the temperature gradient formed between the external environment and the surface temperature affects the heat transfer inside the concrete.Entering the smooth section,the cement hydration reaction is basically finished,the heat of its polymerization has been dissipated,and the ambient temperature becomes the main influencing factor.In addition,comparing the temperature trends and temperature rise values of the two test blocks,it can be found that the reinforcement has no significant effect on the heat transfer inside the test block,so it is not recommended to increase the reinforcement rate to reduce the temperature of the concrete core area in actual construction.(3)By comparing the characteristics of the temperature curve changes of each measurement point inside the test block,it can be found that there is a small transition interval between the descending and ascending sections of the curve,which can be used as the basis for the stage division of the temperature rise curve.By converting the temperature rise curve into an absolute temperature rise rate curve,this transition interval becomes a horizontal line segment with values close to zero.Further set the temperature rise rate threshold to reject the spurious data,the K-Means clustering algorithm can be applied to extract the time data of each point in the segment,and the average time in the segment can be used as the time boundary between the falling segment and the smooth segment.The time when the maximum temperature rise occurs is also used as the time boundary between the rising section and the descending section,so as to establish a method that can quantify the time boundary of different stages and provide a basis for the construction and maintenance of concrete.(4)Based on the temperature measurement data of test blocks,this paper introduces the cement hydration degree to improve the analysis of the one-dimensional difference method commonly used in concrete temperature prediction,which effectively improves the calculation accuracy of the one-dimensional difference method.The study combines the concept of equivalent time with the calculation method of cement hydration degree,thus introducing the dynamic process of thermal parameters such as heat of hydration,specific heat capacity and thermal conductivity with hydration degree into the calculation equation of the one-dimensional difference method.The results show that the revised one-dimensional differential method is more accurate,and the calculated maximum temperature value and the time of maximum temperature value occurrence are basically similar to the real temperature curve,which can effectively guide the construction and maintenance of concrete on site.(5)Based on COMSOL Multiphysics finite element analysis software,the concrete test blocks were modeled and analyzed in this paper.Comparing the temperature clouds and temperature rise curves at different locations at different times,it can be found that the simulation can well reflect the real temperature change law and its trend inside the test block,and its calculated value has a high degree of agreement with the actual measured value.This shows the reliability and accuracy of the simulation for predicting the temperature of concrete.(6)In order to meet the needs of concrete construction and maintenance for temperature prediction,this paper compares the advantages and disadvantages of field temperature monitoring,one-dimensional difference method and finite element simulation analysis and their applicability.It is considered that although on-site temperature monitoring is good for monitoring the actual temperature and enables early warning judgments,it is difficult to predict the trend of the temperature rise curve.The one-dimensional differential method is more suitable for concrete members with shapes similar to large flat plates but not for complex members,and is influenced by the thickness of the delamination.Finite element simulation analysis is powerful and can be applied to the temperature field calculation of various complex environments and various irregular bulk concrete members,but it has many calculation parameters,complex calculation process,high professional knowledge requirement and long calculation time.