Petrographic identification of alkali–silica reactive aggregates in concrete from 20th century bridges

Cracks and white coatings have been observed in a number of concrete bridges constructed during the 20th century. The observed damage phenomena have been attributed to the alkali–silica reaction (ASR) after following detailed macroscopic examination. Several analytical approaches – optical microscopy of thin sections, petrographic image analysis and SEM/EDS study – were employed in order to: (1) quantify the extent of the damage (volume percentage of alkali–silica gel); (2) determine the causes of the damage. The presence of alkali–silica gels is controlled by the petrography of aggregates where monomineral quartz aggregates, metagreywacke, pellite and metasedimentary rock fragments contribute significantly to ASR development. Other phases (feldspars) and fragments of magmatic native rocks (granite, diorite, and volcanic rocks) have not been shown to have any influence on ASR. The volume of alkali–silica gels in the studied concrete samples (ranging from 0.1 to 0.9 vol.%) increases with increasing age of the bridge.