Mechanical properties of concrete with light burnt MgO-based expansive additive under different restrained conditions

Compressive strength, split tensile strength, ultimate tensile strain and axial tensile strength of concrete with 0%, 5%, 8% and 12% light burnt MgO-based expansive agent were tested. By means of scanning electron microscope and optical microscope, the microstructure of concretes was examined. The results indicate that restraint may improve mechanical properties of concrete to some extent, and three-dimensionally restrained concrete shows best mechanical properties when concrete contains the same amount of MgO. Compressive strength of three-dimensionally restrained specimens with 5%, 8% and 12% MgO increases by 16.9%, 22.0% and 16.0% respectively at 360 d, and splitting tensile strength increases by 32.93%, 31.03% and 33.33% respectively at 180 d compared with unrestrained specimens. Ultimate tensile strain of three-dimensionally restrained specimens with 5% and 8% MgO increases by 9.5% and 3.1% respectively, and tensile strength increases by 5.7% and 6.9% respectively compared with unrestrained specimens. Under the free-curing condition, microstructure of expansive cement paste and concrete is loose, with cracks in the paste and matrix-aggregate interface. Restraint could improve the microstructure of expansive concrete with light burnt MgO, especially by densifying and intensifying the matrix-aggregate interface. As a result, the mechanical properties of concrete are improved.