A concrete wastewater tank, less than three years old, had sustained severe cracks around entire tank with efflorescence products oozing from some of the cracks.
Section of wastewater tank showing severe cracks. (photo)
Cores were taken and evaluated by:
Scanning Electron Microscopy
Photo from Petrographic Analysis showing micro crack, micro porous paste, dense patches of paste and entrained air. (photo)
Petrographic analysis revealed internal cracks, micro porous paste with dense paste patches, indicative of re-tempered concrete, and was air-entrained.
Scanning electron microscopy, SEM revealed copious amounts of ettringite in various stage of formation.
Cement specified for Concrete formulation was Type I/II
Most often when specifications calls for Type I/II; the preferred cement is Type II.
Type II has lower C3A content (8%); Type I can have as much as 13% C3A content in the cement.
Information on the source of cement was not available. It was also known that Type II cement was not readily available in the vicinity of the plant. Highly probable that the cement used was a Type I with high C3A content which reacted with the high sulfate ions in the waste water to form ettringite. A deleterious hydration product which grows in voids, fill them up and burst out of the voids creating cracks and micro cracks.
Crystal growth of ettringite in a void – Seen via Scanning Electron Microscopy (SEM) (photo)
Advanced growth of ettringite, filling void and breaking out of it – Seen via Scanning Electron Microscopy (SEM)