CORRELATION BETWEEN MAP CRACKING AND DELAYED ETTRINGITE FORMATION IN FIELD SPECIMENS

摘要

The study investigated the possible presence of Delayed Ettringite Formation (DEF) in the Maryland Bridge Inventory. Maryland is a state in the eastern part of the United States. The objective of the study included investigating the possible presence of DEF in cast-in-place concrete. Two nondestructive test (NDT) methods, potassium autoradiography and impact-echo, were utilized in the study. In addition, cores were taken for analysis with a scanning electron microscope and energy dispersive analysis x-ray, which identify DEF by crystal morphology and elemental composition. The focus of this study was on bridges exhibiting wet map cracking on the surface, which is regarded as a surrogate for DEF. The PONTIS bridge management database provided a means for selecting a sampling population. In order to identify potentially damaged bridges, the PONTIS database was queried for concrete elements with descriptions containing the phrase "map cracks". A representative number of bridges from this set were selected from each district in proportion to the total number of bridges in that district. The study found DEF throughout the sampled Maryland bridges, indicating that DEF is not simply a regional problem that affects only a few states like Texas. Of the twenty-nine cores taken, twenty-six exhibited DEF and two bridges without visible map cracking also contained DEF. Alkali-silica reaction (ASR) gel, which is commonly said to be associated with map cracking, was observed in only a limited number of bridges with map cracking. Therefore, map cracking is not necessarily a definitive indicator of ASR. The study also showed cases of coexistence of DEF and ASR in six of the ten bridges cored. In the bridges that were studied, DEF was found in cast in place bridge elements. This therefore refutes the theory that DEF happens only as a result of high temperature curing. Correlations between concrete surface conditions and four observed DEF morphologies are suggested.