DETECTION OF DEF SYMPTOM USING MICROSCOPIC ANALYSIS ON CONCRETE WITH COMBINED ASR AND DEF POTENTIALS

Abstract

Delayed ettringite formation（DEF）and alkali-silica reaction（ASR）need to be carefully distinguished due to their similar macroscopic appearance. In this study, the cement paste matrix of concrete specimens with DEF and ASR potentials was analyzed by using EPMA（Electron Probe Micro Analyzer）. Mapping analysis of CaO, Al₂O₃ and SO₃ was carried out and the results of the analysis were plotted on a ternary diagram, showing that in specimens with DEF potential, the C-S-H composition data groups were distributed extending in the direction of the ettringite composition. When these data groups were plotted on the BSE（backscattered electron）image, the portion of the data groups overlapped the lighter C-S-H microstructure. Lighter C-S-H is a microstructure formed during high-temperature curing and appears lighter in BSE images, and is thought to sorb Al₂O₃ and SO₃ and contribute to DEF expansion. As the specimen lost DEF potential, the lighter C-S-H composition was distributed in the direction of the monosulphate composition. For specimens with combined DEF and ASR potential, ASR expansion occurred first and DEF potential was identified even after the expansion had ceased. It is not easy to distinguish between ASR and DEF, and careful diagnosis using a combination of several methods including petrographic analysis, residual expansion testing of concrete cores and analysis of DEF potential by using microscopic analysis, is recommended to identify DEF potential.