Analysis of early damage characteristics and late mechanical behavior of cement specimens

Damage fracture occurs easily in an early cement stage when there is a small disturbance due to the early weak strength. The early damage characteristics of cement are complex,and the damage degree has a great influence on its late mechanical behavior. Based on the theoretical analysis of early cement damage characteristics,the tests of the three-day early uniaxial damage compression in different stress rate and twenty-eight-day late bearing mechanics were conducted on 5 groups of 10 cement specimens of 100 mm×100 mm×100 mm by rigid testing machine,the damage mechanism and the basic characteristics of early stage cement were analyzed,the damage coefficient concept of early stage cement was introduced,the solution equation of early stage cement damage quantity was established under the condition of total stress-strain curve,the early damage threshold was determined,the influence of early different damages (damage quantity and stress rate) on the late cement strength,deformation and their relationships of elastic modulus,Poisson’s ratio were studied,and the late bearing instability modes of early stage damage cement were proposed.The results showed that the damage curves of early stage cement were divided into three stages:compaction damage,damage accumulation and damage burst. The full typical stress-strain curves existed in the uniaxial compression failure process,the complete damage model equation of cement specimen could be divided into two sections,which characterized well the damage characteristics of the early stage cement under different stress ratios. When stress ratio increased from 75% to 100% ,the axial strain,the transverse strain,Poisson’ s ratio and damage amount of the early stage cement specimens increased,and the transverse strain increased more than the axial strain,while the elastic modulus showed attenuation trend. The damage threshold of the early stage cement was 0. 48 (80% stress ratio),when the early stage cement damage exceeded the threshold,the internal crack of the cement specimen expanded and penetrated to the final instability,and the late strength of cement specimen converted to damp from the previous increasing strength.With the damage quantity of early stage cement specimen increasing,the uniaxial compressive strength and the elastic modulus of the late cement specimen firstly increased and then decreased,and the axial strain,the transverse strain and Poisson’s ratio increased wholly. The late instability modes of early cement damage specimens was divided into accumulation penetrated-crack instability and initiation divergence-crack one,the instability of high damage cement specimen was caused by unilateral failure,while the one of low damage cement specimen was caused by imbalance among blocks.