Sulfate Attack - Cases
Elbe River bridge piers in Magdesburg, East Germany
The pier sinking operation in a closed caisson opened up a spring. The spring water contained 2040 mg/liter SO4. Over 4 years, the expansion of the concrete lifted the piers by 8 cm and caused extensive cracking. Due to the sulfate attack, the piers had to be demolished and rebuilt. A situation like this could have been avoided by performing a thorough survey of environmental conditions and by providing suitable protection against sulfate attack.
The U.S. Bureau of Reclamation performed a study in regard to sulfate attack. In some cases, the soluble sulfate content of soil was as high as 4.55 percent, and sulfate concentration of was up to 9900 mg/liter. There were many cases of serious deterioration reported in structures ranging from 5 to 30 years old. It was found that sulfate-resisting cements containing 1 to 3 percent C3A performed better than cement containing 0 percent C3A.
The concrete structure was exposed to sulfate for 20 years. The sulfate content of the groundwater was 10,000 mg/liter and it was found that large amounts of gypsum had formed in the hydrated portland cement paste. As a result of the sulfate attack, the concrete was rendered relatively porous or weak and consequently reduced to a noncohesive mass.
A long-time investigation was performed on concrete performance in sulfate soils located in Sacramento. The soil contained approximately 10 percent sodium sulfate and concrete specimens made with three different types of portland cement and three different cement contents were used. The deterioration of the concrete specimens was evaluated and the results showed that the permeability of the concrete (cement content) had more influence on the sulfate resistance than the composition of cement.
This study shows that soil, groundwater, seawater, and industrial waters are not the only sources of sulfate. Deterioration was reported between the cantilevered precast concrete girders and the cast-in-place concrete bleacher girders. The drypack grout was not compacted properly during construction and the deteriorated material showed considerable amounts of ettringite and gypsum formed as a result of sulfate attack. It was determined that because of inadequate drainage, rainwater had accumulated near the mortar. Sulfates were present in the rainwater due to air pollution and they caused the deterioration. This situation could have been avoided if precautions were made for drainage of the rainwater or if the concrete were not as permeable.
A detailed investigation on durability of concrete from a 60-year-old pier was conducted right before the structure was demolished. The non-air-entrained concrete mixture consisted of 350 kg/m3 cement and a w/cm ratio of 0.53. Concrete cores were taken from the deck and there was no evidence of sulfate attack or any other form of chemical attack. The cores also demonstrated excellent strength impermeability after 60 years of exposure to seawater. The sulfate attack associated with the formation of ettringite was confined to the surface of the concrete up to a depth of 5 mm. Some microcracks were observed 10-35 mm deep, but they were attributed to thermal stresses during construction. This case study supports Mehta’s belief that permeability rather than cement composition is the most important parameter for the chemical resistance of concrete exposed to sulfates.