EVOLUTION OF NORTH AMERICAN STANDARDS FOR SULFATE RESISTANCE: AN HISTORICAL PERSPECTIVE AND RECENT DEVELOPMENTS

摘要

Current standards for sulfate resistance in North America include restrictions on water-to-cementitious-materials ratio, minimum strength, prescriptive limits on cement composition, and expansion limits when cementing material combinations are used in mortar bar tests. Since the 1920's, even before the chemical reactions were understood, it was found that low w/cm and low unit water contents of concrete mixtures were essential to obtaining resistance to the actions of sulfate soils. Shortly afterwards, the negative role of cement C_3A was realized, and the first sulfate-resistant cements were formulated in the 1930's. The first standard performance test for evaluating cements was ASTM C 452, but it was found not to be suitable for evaluation of blended cements and supplementary cementing materials. This led to the development of the ASTM C 1012 mortar bar expansion test, where bars are immersed in 5% sodium sulfate solution after reaching 20MPa strength. There is no performance standard for testing concrete, due to the extended time it would take, so limits on w/cm and strength have been maintained. In the ACI 318 Building Code, the ASTM C 1012 test can be used to qualify cementitious binders, but for concrete there are also maximum w/c and minimum strength limits for each of the different severities of exposure. While these measures were thought to provide protection against sulfate deterioration, since the 1990's, a number of sulfate-related problems have been identified which may not be adequately addressed in current standards. These include delayed ettringite formation, sulfate salt crystallization, and thaumasite sulfate attack. This contribution will address the various forms of sulfate-related distress and discuss the development of sulfate standards in North America, including the need for new guidance and standards.