Experimental Observing Damage Evolution in Cement Pastes Exposed to External Sulfate Attack by in situ X-ray Computed Tomography

The paper presents experimental investigation results of crack pattern change in cement pastes caused by external sulfate attack(ESA).To visualize the formation and development of cracks in cement pastes under ESA,an X-ray computed tomography(X-ray CT)was used,i e,the tomography system of Zeiss Xradia 510 versa.The results indicate that X-CT can monitor the development process and distribution characteristics of the internal cracks of cement pastes under ESA with attack time.In addition,the C3A content in the cement significantly affects the damage mode of cement paste specimens during sulfate erosion.The damage of ordinary Portland cement(OPC)pastes subjected to sulfate attack with high C3A content are severe,while the damage of sulfate resistant Portland cement(SRPC)pastes is much smaller than that of OPC pastes.Furthermore,a quadratic function describes the correlation between the crack volume fraction and development depth for two cement pastes immermed in sulfate solution.

A time−space porosity computational model for concrete under sulfate attack

The deterioration of the microscopic pore structure of concrete under external sulfate attack(ESA)is a primary cause of degradation.Nevertheless,little effort has been invested in exploring the temporal and spatial development of the porosity of concrete under ESA.This study proposes a mechanical–chemical model to simulate the spatiotemporal distribution of the porosity.A relationship between the corrosion damage and amount of ettringite is proposed based on the theory of volume expansion.In addition,the expansion strain at the macro-scale is obtained using a stress analysis model of composite concentric sphere elements and the micromechanical mean-field approach.Finally,considering the influence of corrosion damage and cement hydration on the diffusion of sulfate ions,the expansion deformation and porosity space−time distribution are obtained using the finite difference method.The results demonstrate that the expansion strains calculated using the suggested model agree well with previously reported experimental results.Moreover,the tricalcium aluminate concentration,initial elastic modulus of cement paste,corrosion damage,and continuous hydration of cement significantly affect concrete under ESA.The proposed model can forecast and assess the porosity of concrete covers and provide a credible approach for determining the residual life of concrete structures under ESA.