FACTORS AFFECTING DEF AND ASR IN THE CONCRETE DAM AT VRANÉ NAD VLTAVOU

Abstract

Two degradation mechanisms, delayed ettringite formation (DEF) as well as the alkali-silica reaction (ASR), were investigated in the concrete dam at Vrané nad Vltavou, CZE with an aim to: (1) find those aggregates causing the degradation of the concrete; (2) define those parts of the concrete most affected by the degradation; and (3) correlate the water attack with the extent of degradation. The following methods were used: measurement of residual expansion of the concrete (following the CSN 72 1179 standard), optical microscopy, petrographic image analysis, and scanning electron microscopy combined with energy dispersive spectrometer (SEM/EDS). Eight different sampling points were selected according to the cracks observed on the concrete's surface. DEF was found to be the most important degradation mechanism in the concrete samples; ASR and calcite-rich rims are regarded as minor. Influence of freezing/thawing degradation is also possible. The highest residual expansion was measured on one concrete sample periodically washed by water (this sample taken from an area one meter above the water level). The same sample shows the highest content of ettringite and microcracks, as well as minor contents of calcite-rich rims and alkali-silica gels. All samples that were taken below the water level as well as samples without any contact with water, showed lower contents of ettringite and microcracks, and very low (or no) contents of alkali-silica gels. The highest degree of degradation by DEF, freezing/thawing and ASR is explained by the following factors: (1) position of sampling points one meter above the water level; and (2) coarse, as well as fine aggregates that are mainly composed of acid volcanic rocks (alkaline rhyolite, tuff and tuffite).