Study On The Dry Shrink Agedeformation Of Hardened Cement Paste Under Humidity Gradient Dased On DVC

Cement-based materials are widely used in construction currently.Its durability is the focus of attention in the industry,especially drying shrinkage and the cracking caused by it.Although the drying shrinkage of cement-based materials has been systematically studied in literatures,most of the researches focus on the phenomenon and law of drying shrinkage.Due to the limitation of research methods,the mechanism of drying shrinkage has always been controversial,especially the humidity range and degree of several mechanisms have not reached a consensus yet.This thesis takes the drying shrinkage of hardened cement paste(HCP)as the object and uses the strain field calculated by Digital Volume Correlation(DVC)and the relative humidity(RH)field of HCP to explore the relationship between drying shrinkage strain and RH,which is expected to further understand the drying shrinkage mechanism.First,the HCP was dried in a pure one-dimensional drying condition under 50%RH,and a humidity sensor was used to monitor the change of its RH with time.Combined parameters were obtained by the humidity prediction model,and then the RH field at any position in HCP with drying time was obtained.Then,with the help of high resolution X-Ray Computed Tomography(CT),the 3D image data of HCP was used as the input data of DVC algorithm to calculated the full-field deformation of HCP under50%RH environment,what is the previous static shrinkage measurement can not be observed.Last,the relation between drying shrinkage strain and RH of HCP is obtained by matching the RH and strain at the same positions.Combining the HCP humidity field and strain field,the main scientific findings of this thesis are as follows:(1)In the mapping between shrinkage strain and RH,it is found that the change of shrinkage with humidity is not always linear.In the range of 80-100%RH,due to the decrease of capillary driving effect and the increase of elastic modulus,the rate of change of strain with humidity gradually decreases.Below 80%RH,the structural reorganization of C-S-H is intensified,leading to shrinkage deformation and increasing the rate of change of strain with humidity.When the humidity is reduced to 70-74%RH,the strain no longer change with the decrease of humidity,which may be caused by the disappearance of capillary force and the intrinsic structure of gel pores.(2)It is discovered that there are differences in the strain when the HCP reaches the same humidity through different drying systems during the drying shrinkage process,which is a phenomenon that cannot be seen in the traditional balanced drying method.This thesis proposes drying shrinkage historical effect to analyzes and shows that drying shrinkage is not only a state function of RH,but also related to the drying process.