Characterization of Delayed Ettringite Formation in Maryland Bridges

Abstract

The research investigated the significance of Delayed Ettringite Formation (DEF) presences in the Maryland Bridge Inventory. The objective of the research included investigating possible presence of DEF, correlations between the presence of DEF in cast-in-place concrete and moist map cracking, and correlations between presence of DEF in cast-in-place concrete and air entrainment agents (AEA). The research required coring a target population, so that scanning electron microscope (SEM) with energy dispersive analysis x-ray (EDAX) could be utilized to identify DEF in the concrete. Combination of visual identification, SEM and elemental identification with EDAX are used to verify the presence of DEF in the concrete samples.

The research was conducted in two phases with the first suggesting a possible link between moist map cracking and DEF. The research identified numerous ettringite morphologies in bridge concrete. Characterization of the ettringite morphologies suggested that lamellar ettringite could be linked to DEF-related damage. From this work, a second phase was developed to establish a link between degree of moist map cracking and DEF. Furthermore, other research showed that ettringite grew well in a solution of AEA. The second phase research developed a population to test both hypothesis and attempted to show positive or negative correlations between these two theories and DEF presence.

Phase 2 research shows no correlation between concrete mixes with AEA and DEF. Therefore, the conclusion is that AEA does not have any measurable adverse effects regarding DEF. Whereas, the research shows a strong correlation between widespread moist map cracking and significant DEF quantities, but that a DEF presence is not necessarily indicative of DEF-related damages. Once a threshold is exceeded, DEF-related damages are observed and are found throughout the affected concrete. These results are independent of alkali-silica reaction (ASR), since ASR was not found in any of the Phase 2 samples selected.

Overall, both phases of research shows that Maryland bridge concrete contains ettringite, but that not all ettringite formations appear to lead to damage of the concrete. Moist map cracking, a suggested DEF-related damage, appears to be correlated with lamellar ettringite formations and high quantities of DEF with in the concrete.