Study On Drying Shrinkage Of Cement Mortars

Drying shrinkage is a kind of the common strains occurring in concrete, and isone of the major causes of the cracking of concrete. The shrinkage component inconcrete is the cement paste. With non-uniform restraint, drying shrinkage caninduced tensile and cause cracking if the induced tensile stresses exceed the tensilestrength of the concrete, which leads to adversely affecting durability of concrete.Therefore it is important to study the question about the drying shrinkage andshrinkage-compensated properties of cement paste and concrete. In the thesis, two drying shrinkage methods of cement mortars were studied. Inorder to develop a new test method for drying shrinkage, the relationship betweendrying shrinkage of cement mortars and that of concrete was studied. The effect ofwater-binder(w/b) ratio, mineral admixtures, relative humidity, curing time and curingtemperature on drying shrinkage of cement mortars was investigated. The mechanismof influence of mineral admixtures and curing temperature on drying shrinkage ofcement mortars was studied by SEM, determination of non-evaporable water contentand pore structure. Under drying condition of 20℃ and 50%RH, the correlation of mortar shrinkagewith concrete shrinkage was highly significant at all ages. Considering of kinds ofcement and procedure of test, the test method with constant w/b ratio was feasible. Drying shrinkage of cement mortars was related to the w/b ratio. The higher w/bratio was, the larger the drying shrinkage of cement mortars was. The mineraladmixtures such as fly ash (FA) and ground granulated blast slag (GGBS) had a largeinfluence on drying shrinkage of cement mortars. Drying shrinkage of cement mortarswas reduced with the increase of the mass fraction of the FA replacement of thecement. As drying shrinkage of cement mortars with GGBS, there was an optimumadding amount. Drying shrinkage of cement mortars was reduced with the increase ofthe mass fraction of GGBS, and when it was added to exceed the optimum addingamount, drying shrinkage of cement mortars was increased with the increase of the III李顺凯:水泥砂浆的干缩研究 2004 年 4 月mass fraction of GGBS, inferior to drying shrinkage of cement mortars withoutmineral admixtures. The influence of mineral admixtures on drying shrinkage ofcement mortars is depended on lower activity and the microsized particle effect ofmineral admixture in mortar. When mineral admixtures replace the cement, thehydration rate of cement mortars reduce, the mineral admixtures play a role ofmicrosized particle in hydrated cement paste, and affect the pore structure of mortarsand change degree of the hydrate. Coarsening pore and reducing degree of the hydratedecrease drying shrinkage of cement mortars. Curing condition had an important influence on the drying shrinkage of cementmortars. As curing time was prolonged, drying shrinkage of cement mortars withmineral admixtures would be reduced, except for the mortar with 60% the massfraction of FA. When relative humidity was lower, the drying shrinkage of cementmortars would be reduced with the increase of the mass fraction of mineral admixturereplacement of cement. When relative humidity was higher, the influence of the massfraction of mineral admixture on drying shrinkage of cement mortars was un-notable,but drying shrinkage of cement mortars with humidity increase would be reduced. Ascuring temperature before drying was raised from 20 ℃ to 60 ℃, drying shrinkage ofPortland cement mortars without mineral admixtures was remarkably decreased, anddrying shrinkage of cement mortars with mineral admixtures was increased with theincrease of the mass fraction of the mineral admixtures. The influence of curingtemperature on drying shrinkage of Portland cement mortar without mineraladmixture is depended on the microstructure of...