Monitoring internal sulphate reactions by X-ray tomography
Résumé
For certain cement chemical compositions and under specific temperature and humidity conditions, concrete deterioration can be attributed to internal sulphate reactions. This pathology mainly affects massive structures (e.g. bridge piers, dams, nuclear power plants) or precast concrete structures that undergo specific heat treatments. Without an external sulphate source, sulphate reaction is characterized by delayed ettringite formation leading to swelling of the cementitious matrix and cracking. Nevertheless, the mechanisms and the impact of this pathology on the microscopic and structural scales are still today not thoroughly understood. A performance based approach (accelerated treatment) was recently proposed by IFSTTAR (test method n 66) in order to determine the sensitivity of a cementitious mixture to the delayed etttringite formation. It consists in longitudinal swelling measurements on cylindrical specimens submitted to drying and wet cycles followed by a water curing at 20 C. In this work, the internal sulphate reaction in a concrete specimen that has followed an accelerated treatment is monitored by means of X-ray tomography. X-ray tomography is a non-destructive test that provides information on the 3D mesostructural evolutions. Using a post-treatment method implemented in Matlab, the evolution of several parameters (porosity, cracking) is quantified. The interfacial zone between aggregate and paste matrix is also studied. Supplementary observations performed by means of optical and scanning electron microscopy confirm that the delayed ettringite is principally formed in the large pores and in the interfacial transition zone of the cementitious matrix. Finally, using a 3D numerical reconstruction method aggregates can be seperated from the cementitious matrix to create a finite element mesh of the specimen at the mesoscale level that can be used for numerical simulations.