Effects of stress on concrete expansion due to delayed ettringite formation

Delayed ettringite formation (DEF) is a sulfate attack affecting civil engineering structures. This chemical reaction takes place within the concrete matrix of structures and causes damage in concrete and tension in reinforcements. For managers and owners, the ability to predict and reassess the mechanical behaviour of such structures is a major challenge. The influence of both reinforcements and prestress on DEF expansion were studied in the present work. Several tests were performed in laboratory: expansion under both uni and tri-axial restraint due to reinforcements and expansion under prestress (stress level of 14.5 MPa). Uniaxial restraint led to decreased strain in the restrained direction. For prestressed concrete, the loaded direction exhibited creep strain. In both cases, expansions were not impacted in transversal free directions. Therefore, DEF expansion under uniaxial stress is anisotropic. Cracks were observed parallel to the restrained direction. The final volumetric expansion was lower than in stress-free conditions (decrease of 27% for uni-axially restrained condition). For triaxially restrained tests and prestressed specimens, the volumetric decreases were 56% and 34% respectively. Data provided by these results will be used for numerical reassessment of DEF damaged structures.