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The effect of infilling of air voids by ettringite on resistance of concretes to freezing and thawing was studied. Nine concrete mixtures, made with five cements with or without Class C fly ash, were exposed to freezing-thawing cycles following 110 to 222 days of moist curing. Prior to freezing-thawing, the specimens were examined by a low-vacuum scanning electron microscope (SEM) for the degree of infilling. It was found that the extent of the infilling depends on the length of moist curing as well as the wet/dry treatment. The infilling implies that these air voids are water-accessible. The function of the air-void system to protect concrete from freezing and thawing has been compromised due to the presence of water in some air voids. The infilling seems also to increase effective spacing factor. These might cause concrete to be more vulnerable to freezing-thawing damage.

This project originated because of premature deterioration of concrete pavements in Wisconsin. The deterioration took the form of a “V” at ,the joints of the pavements. A number of hypotheses had been put forward by various investigators of the damaged concrete. These included filling of the air voids by ettringite, which was thought to reduce the ability of the air void system to protect the concrete against frost damage. The purpose of the work reported here was to recreate the damage mechanism in the laboratory and investigate the sequence of events leading to the deterioration of the concrete. Three cements produced from the same raw materials were used in the project. Two were commercial Type I and Type II cements; the third was made by intergrinding the Type I cement with additional gypsum to increase the amount of available sulfate in the concrete. Concrete prisms 3 x 3 x ll-l/ 4 inches (75 x 75 x 285 mm) were subjected to the conditions specified by ASTM C 666 Procedure A, except that 3% NaCl solutions either with or without added gypsum (to simulate road salt) were used instead of water. The freeze/thaw cycles were interrupted over the weekends, when the specimens were allowed to dry out in laboratory air. The specimens were tested to destruction in most cases. Companion specimens were examined petrographically during the course of the test period in order to establish a sequence of ettringite deposition and damage. Damage was measured by mass loss, length change, and relative dynamic modulus. The findings show that the ettringite deposited in the air voids did not cause cracking, nor did it contribute to the propagation of existing cracks. Rather, it appears to have been opportunistic: cracks due to frost damage created space for ettringite crystals to grow.

The effects of chemical transformation processes on the frost and frost-deicing salt resistance of concrete are much less significant than the physical effects, but they are nevertheless significant. Our investigations showed that monosulfate (AFm phase) is particularly instable and will transform to ettringite (AFt phase) under frost and also under frost-deicing salt attack. This delayed formation of ettringite, which is supported by thermodynamic conditions at low temperatures, may reduce considerably the frost and frost deicing salt resistance of concretes without air-entrainment.

During the past few years delayed ettringite formation (DEF) has probably received more attention, and been involved in more controversy, than any other concrete deterioration mechanism. Even its name has been subject to dispute. Our extensive experience on the investigation of many occurrences of DEF is presented here as a series of questions, with some answers. Where answers have become available, they have explained phenomena that have greatly bothered us and other investigators. Where answers are not available, the questions will provide directions for needed research.

The formation of ettringite in hardened concrete is not only a problem of heat treatment. Ettringite also occurs in no heat-treated concrete, which is exposed only to normal climatic conditions. In some cases the mechanism of damage in concrete pavements correlates with this ettringite formation in the hardened concrete. Structural changes by ettringite formation were caused above all by varying moisture conditions and, as a result, by transportation of moisture and substances within the concrete structure, which also lowers the pH value of the pore solution. The primary ettringite from the paste is microcrystallin at normal pH of 13.5 to 14 in the pore liquid. Thus ettringite may dissolve in the pore liquid and recrystalize at a lower pH in larger spaces, where the capillary transportation is interrupted. This recrystallized ettringite in the air voids was stable up to 60°C. But the mechanism of this ettringite formation is supported and accelerated by higher temperatures (e.g. 60°C) because of the intensive drying. Microstuctural defects like microcracks may be created by alternating temperatures and later on filled and may be widened by ettringite crystals. In concrete pavements no indications were found for recrystallized ettringite itself to be the primary cause of crack formation. The expansion of concrete is reduced by introducing artificial air voids, because there is more available space for accumulation of ettringite. But the combined action of freezing and thawing and de-icing salt after filling the artificially entrained air voids with ettringite crystals may causedamages.