Here is another excerpt from a BFI investigation where concrete core samples were extracted and examined. As a result, Akalai-Silica Reaction (ASR) was found, contributing to defective concrete. The following excerpt is from the Concrete Petrographic Report:
"All of the concrete represented by the cores was susceptible to alkali-silica reaction (ASR) and presented evidence of ASR in the form of pop outs. All of the concrete represented by the cores contained opaline shales and cherts, which are potentially alkali-silica reactive per ASTM C294. Opaline shales are notoriously reactive and were commonly observed at the base of the pop outs. Opaline shale was observed in all of the concrete cores. Examination of the concrete in transmitted light indicated the presence of radial fracturing around opaline shales and the presence of microfractures filled with ASR gel. In some cores particles of chert also presented evidence of ASR.
Figure 1: Plane-polarized transmitted light photomicrograph showing typical opaline shale ASR texture: red arrows indicate internal microfractures; yellow arrows indicate concentric external microfractui e; purple arrow indicates paste microfracture.