Analysis Of Temperature,Humidity And Coupled Stress Field Of Prestressed Concrete Cylinder Pipe At Early Age

Prestressed Concrete Cylinder Pipe(PCCP)is widely used in urban water supply projects and long-interval water transmission trunk projects because of its good impermeability and earthquake resistance,long economic life and convenient construction.In the production process of PCCP,in order to speed up the hardening of concrete and speed up the production process,high temperature steam curing is usually adopted.Due to the hydration and heat release of early age concrete,the temperature difference between inside and outside PCCP structure is large,resulting in temperature stress.At the same time,the early age concrete will cause obvious self-shrinking and drying shrinkage due to the change of humidity.If PCCP is subject to external constraints,it will produce a large dry shrinkage stress.When the coupling stress of temperature and humidity exceeds the tensile strength of concrete under the current age,PCCP will produce cracks.Therefore,it is necessary to analyze the temperature field,humidity field and stress field of PCCP under early age to avoid the occurrence of concrete cracks caused by improper high temperature steaming system and harsh external construction environment.The main research contents and results of this paper are as follows:(1)The calculation model of thermal parameters and adiabatic temperature rise of concrete based on hydration degree is introduced.The temperature field subroutine is developed and verified.On this basis,the early temperature field distribution of PCCP under actual steam curing system and ambient temperature was analyzed,and the variation law of thermal parameters of concrete with hydration degree was obtained.(2)According to the similarity between the theoretical solution models of humidity field and temperature field,the early-age humidity field analysis of PCCP was carried out with reference to the temperature field analysis method,and the humidity diffusion coefficient related to the temperature of concrete and the water self-dissipation function related to the current hydration degree of concrete were considered.The results show that after the end of steaming,the relative humidity of the socket end and the inner and outer walls of PCCP decreases with the increase of curing age,while the relative humidity inside PCCP almost stays at 1.(3)The tensile and compressive strength and elastic modulus of concrete based on equivalent age are introduced,and the control equations of concrete temperature stress and shrinkage stress are introduced.In this paper,the temperature,humidity field and coupled stress field of PCCP at early age were analyzed by sequential coupling method.Firstly,the temperature field is analyzed,and then the humidity field and humidity stress are analyzed.Finally,the coupling stress field of PCCP under the combined action of temperature and humidity is obtained by superimposing the dry shrinkage strain and temperature strain.The results show that in the stage of steam curing and cooling,the stress of concrete increases sharply.After removing the formwork,the shrinkage deformation has a great influence on the stress of concrete on the surface of PCCP.(4)The temperature rise and cooling rate of steam curing system and the influence of different external environment on the temperature,humidity and coupling stress field of PCCP were analyzed,and the cracking risk of early PCCP was evaluated.The results show that the first principal stress of concrete at all monitoring points does not exceed the tensile strength of concrete at the current age under different heating or cooling rates during the72 h period before steam curing.Therefore,under the existing steam curing system,the heating or cooling rate can be set to 10 ° C /h and 20 ° C /h.During summer construction,PCCP is not easy to crack,but during winter construction,when the maximum ambient temperature is 5 ° C and the minimum daily ambient temperature is 0 ° C,after the end of evaporation,the cracking risk coefficient η of each monitoring point at the socket end reaches 0.91(greater than the specified 0.7),and the socket end is easy to crack.Therefore,in this environment construction,should adopt the corresponding construction measures.