Role of Climatic Cycles on the Duality of Chemical and Physical Modes of Sulfate Attack in Concrete

The possibility of the synergic occurrence of the chemical and physical modes of sulfate attack at the evaporating front is one of the un-resolved issues concerning the case of the partially immersed concretes. Some of the past studies put the accent on chemical sulfate attack as the dominant mechanism, while others identified physical sulfate attack responsible for the degradation of the evaporating front. To fill out this knowledge gap and distinguish the dominant mechanism at work, the presented study focused on the role of climatic condition. Accordingly, visual characteristics, phase composition, and ultrasonic pulse velocity of the immersed and upper-dry regions of the partially immersed concretes in Na2SO4 solution were analyzed under different climatic cycles. The designed cycles offered low and high evaporation rates, allowing the thenardite/mirabilite transitions in the Na2SO4.nH2O system. It was found that regardless of the evaporation rate, the mechanisms of physical sulfate attack were favored over the chemical ones. Furthermore, the differences in the evaporation rates of the environments determined the location of the physical sulfate attack. Under high evaporation rate, such aggression was realized beneath the mortar layer of the concrete, leading to its internal microcracking and spalling.