Digital Microscopy Applied to Damage Rating Index for Alkali-Silica Reaction in Concrete

Abstract

The Damage Rating Index (DRI) has been in use since the early 1990s to quantify petrographic features of the alkali-silica reaction (ASR) in concrete. To perform a DRI, ASR petrographic features are counted on a cut and polished concrete surface using a stereomicroscope. For this research program, in addition to the traditional stereomicroscope approach, digital images were collected from the same surfaces using a flatbed scanner and using an automated digital microscope equipped with a motorized stage. In both cases, a single image encompassing the entire polished surface was recorded. An ImageJ macro was used to consecutively extract and display adjacent 1 × 1 cm square regions on a monitor at a magnification of 16×, along with an interface to manually record the observed features. DRI measurements were made using all three approaches on concrete prisms made with a reactive coarse aggregate after ASR expansion testing. DRI results were equivalent across the different DRI measurement methods. Four different concrete binder systems were tested: a Portland cement, an alkali-activated slag, an alkali-activated fly ash, and an alkali-activated metakaolin. In addition to expansion measurements, the leaching of alkalis was monitored, as well as the chemical composition of ASR products. Extensive alkali leaching was observed for concrete made with alkali-activated binders, but alkali concentrations within the concrete remained well above the recognized lower threshold required for expansion in prism testing. Potassium was the dominant alkali ion in ASR products formed within the portland cement concrete, and sodium was the dominant alkali ion in ASR products formed within the alkali-activated concrete.