Alkali-Silica Reaction—Possible Cause of Map Cracking in Gunite

In this case study, a number of gunite swimming pools exhibited extensive map or alligator cracking that caused rough and uneven surfaces. The nature of the cracking pointed to alkali-silica reaction (ASR) as the cause of the cracking. Four test methods, the uranyl acetate method, petrographic examination, chemical analysis by electron microprobe, and X-ray diffraction, were used to investigate the presence of ASR within the gunite. Results showed (1) some evidence of ASR gel within the gunite using uranyl acetate; (2) the potential existence of alkali silica gel throughout the cement and within the fractures of the sand grains based on petrographic examination; (3) elevated levels of total alkali, indicating a potential for ASR to occur based on chemical analysis; and (4) tiny inclusions of thermonatrite (commonly found in ASR gel) based on X-ray diffraction. Results from each of the four test methods suggest, albeit inconclusively, the existence of ASR in the gunite. Additionally, the results of all of the four test methods did not contradict each other. Accordingly, it can be stated with some certainty that the distress in the gunite was due to ASR.