Diagnosing delayed ettringite formation in concrete structures

Abstract

There has been a number of cases involving deteriorated concrete structures in North America where there has been considerable controversy surrounding the respective contributions of alkali–silica reaction (ASR) and delayed ettringite formation (DEF) to the observed damage. The problem arises because the macroscopic symptoms of distress are not unequivocal and microscopical examinations of field samples often reveal evidence of both processes making it difficult to separate the individual contributions. This paper presents the results of an investigation of a number of concrete columns carrying a raised expressway in North America; prior studies had implicated both DEF and ASR as possible causes of deterioration. Although the columns were not deliberately heat-cured, it is estimated that the peak internal temperature would have exceeded 70 °C and perhaps even 80 °C, in some cases. The forensic investigation included scanning electron microscopy with energy-dispersive X-ray analysis and expansion testing of cores extracted from the structure. Small-diameter cores stored in limewater expanded significantly (0.3 to 1.3%) and on the basis of supplementary tests on laboratory-produced concrete specimens it was concluded that expansion under such conditions is caused by DEF as the conditions of the test will not sustain ASR. In at least one column, DEF was diagnosed as the sole contributory cause of damage with no evidence of any contribution from ASR or any other deterioration process. In other cases, both ASR and DEF were observed to have contributed to the apparent damage. Of the columns examined, only concrete containing fly ash appeared to be undamaged. The results of this study confirm that, under certain conditions, the process of DEF (acting in isolation of other processes) can result in significant deterioration of cast-in-place reinforced concrete structures.

Introduction

In a previous paper [1] presented the results of a forensic evaluation conducted on core samples retreived from precast, prestressed concrete bridge girders. These girders were exhibiting extensive cracking, which was fortunately observed before the girders were placed in service, and were then subjected to examination by numerous investigators. It was generally agreed by all who examined samples from these girders that the concrete was undergoing some form of internal chemical attack, however, there was considerable debate as to whether the principal cause of deterioration was alkali–silica reaction (ASR) or delayed ettringite formation (DEF). Scanning electron microscopy (SEM) revealed evidence of reacted siliceous aggregate particles, alkali–silica gel and abundant quantities of ettringite filling the cracks and voids in the concrete. However, whereas there was firm evidence of expansion caused by ASR in the form of reacting aggregate particles with cracks emanating into the surrounding paste, there was little evidence of significant expansion of the paste due to DEF and it was concluded that ASR was the primary cause of distress and was a precursor to the subsequent precipitation of ettringite in the resulting cracks [1]. This type of ettringite formation is frequently referred to as secondary ettringite formation and is the result of the continuous dissolution of ettringite in fine cracks and voids and its reprecipitation to form larger crystals in bigger empty spaces [2], a thermodynamically-driven spontaneous process known as Ostwald ripening. There is no evidence to suggest that this process leads to damage.

In most of the reported “cases” of DEF some other mechanism of deterioration, usually ASR, has also been present and it is difficult to determine the actual contribution made by DEF to the damage or, indeed, whether DEF played a significant role at all in the deterioration of the concrete [1], [3]. Based on the number of confirmed cases of DEF damage in the field, it would appear that the risk of this process causing premature deterioration in real concrete structures is low compared to other more widespread deterioration processes. However, recently Sahu and Thaulow [4] diagnosed DEF as the sole cause of deterioration of precast concrete railroad ties (railway sleepers) in Sweden. This paper presents another such case, i.e. where DEF (in the absence of ASR) has been isolated as the sole contributory cause of premature deterioration in cast-in-place reinforced concrete bridge columns in North America.