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In a number of isolated cases in the U.K., expansion and cracking has occurred in precast concrete elements subject to early heat treatment followed by wet or moist exposure, and in some in-situ concretes of large section and high cement content, again subject to wet or moist exposure. The present paper discusses some of the work relating to the cracking of these field concretes and to the laboratory expansion testing of concretes and mortars subject to high early temperatures which has been carried out by the British Cement Association. It is shown that expansion is caused by expansion of the cement paste fraction. It is shown that concretes and mortars made using a number of Portland cements can be induced to show abnormal expansion when subject to wet curing after a severe early cure and that the parameters influencing expansion are probably total sulfate as SO, magnesium oxide and alkali contents of the cement and cement fineness. Given our present understanding, it does not seem possible to eliminate the expansion by a suitable choice of portland cement composition. However, it is shown that limiting the concrete temperature at all times to below 70 C in precast concrete and limiting the cement content in in-situ concrete of large section are probably secure ways of avoiding late expansions associated with ‘delayed ettringite formation’ in concretes subject to prolonged wet or moist exposure.

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.

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 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.

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.