Drying Shrinkage Of Hardened Cement Paste Based On Full-Field Deformation

With the increasing use of concrete,the durability of concrete has become increasingly important.The key factor affecting the durability of concrete is the shrinkage cracking of concrete.In the shrinkage of concrete,the drying shrinkage has become an important and most complex research object due to its high total proportion,long existence time,and many uncontrollable factors.The traditional method of measuring shrinkage deformation ignores the shrinkage gradient inside the concrete and cannot reflect the true deformation information inside the sample.The shrinkage deformation of concrete mainly comes from cement paste.In order to reveal the shrinkage deformation mechanism of hardened cement paste,researchers have successively proposed some models and theories of shrinkage,but these models and theories are still very controversial.Focusing on the controversial issues of the shrinkage mechanism of hardened cement paste,this thesis uses Digital Volume Correlation(DVC)and high-resolution X-CT imaging to calculate the full-field deformation of hardened cement paste in the range of 100%-50%RH,systematically studied the deformation process of the first and second shrinkage of the hardened cement paste.Based on the two sets of CT image data before and after drying and the DVC method,the full-field deformation data of the hardened cement paste is obtained.Through systematic dry shrinkage comparison experiments,the spatial distribution trends of local drying shrinkage(LDS)with time is obtained,and the following conclusions are obtained in conjunction with research results of literatures:(1)The initial shrinkage test can confirm that the shrinkage process of the hardened cement paste corresponds to two shrinkage mechanisms.Combined with experimental data in literature,the shrinkage deformation of the hardened cement paste in the higher humidity range is mainly due to the capillary theory dominated by capillary stress,and the shrinkage deformation in the lower humidity range is mainly due to the irreversible shrinkage caused by the reorganization of the C-S-H gel microstructure.(2)The LDS results of the second shrinkage corroborate the irreversible shrinkage due to changes in the microstructure of the C-S-H gel during the first shrinkage.(3)Based on the dry shrinkage mechanism,the whole process of dry shrinkage deformation of the hardened cement paste is completely described.