Multi-field Coupling Analysis Of Early-Age Concrete And Its Application Considering Changing Of Behavior

The tensile strength of concrete is low in the early age after pouring, and mainlyeffected by its own heat generation of hydration, shrinkage deformation and otherfactors, surface temperature cracks are easy to appear in concrete structures, and thecracks will have a significant impact on the safety and quality of construction of thestructures. Based on ABAQUS and its secondary development platform, a subroutinefor calculation thermal、 mosture and stress field has been developed, and has beenapplied to practical engineering. In this paper, the main research works are as follows:(1) Deeply research on the secondary development of ABAQUS software has beencarried out, and the relevant subroutine used in thermal field and structure fieldsimulation of the early age concrete are emphatic introduced. Meanwhile, theestablishment of ABAQUS secondary development platform and subroutine callingprocess is described in detail.(2) Based on ABAQUS secondary development platform, and using FORTRANlanguage, the thermal field subroutine UMATHT which takes the degree of hydrationtheory and the change of thermal parameters into account and FILM which simulatesthe third type of boundary conditions are developed. And based on the maturity theory,a viscoelastic structure field calculation subroutine which is based on double powercreep function (Double Power Law, DPL) is composed. The validity of the developedsubroutines is demonstrated through various numerical examples.(3) Different caculation models of early concrete thermal field, including thetraditional caculation models, the calculation model of considering the hydrationreaction rate affected by hydration degree and the model of considering hydrationdegree and thermal parameters changing, are used to study the differences of differentthermal field calculation models. It is found that the numerical calculation results ofthese three thermal field calculation models are not the same. In this paper it is alsofounded that it is necessary to consider the specific heat and the changing process ofthermal conductivity varied with the degree of hydration in simulation of early concretethermal field, or it would result in a lower structure temperature and a smallertemperature difference between inside and outside, and thus the design may be unsafe.(4) According to the similarity beteween humidity and thermal field theory, and with consideration to the effect of time and temperature on humidity diffusioncoefficient, the simulation of humidity field of early age concrete has been achieved byusing the subroutine of thermal field. The result shows that: diffusion process ofhumidity field is very slow, a greater humidity gradients will only appear in the shallowparts of the surface even the time has been360h after pouring.(5) Using multi-field sequential coupling method, the subroutines developed inthis paper have been applied to practical engineering, and a numerical simulationresearch of early stress and cracking risk of a concrete long wall has been carried out.The specific heat, thermal conductivity, creep, drying shrinkage, elastic modulus andtensile strength have been considered synthetically in the calculation. The result showsthat: shrinkage deformation mainly has great effect on the surface stress; cracks canappear at surface of the structures, and internal has a greater likelihood of cracking,which means enough attention should be paid to this in practical engineering.