Daugherty and Hou Present at 33rd ICMA Conference
Ann Daugherty, Senior Petrographer & Manager, and Hugh Hou, Petrographer, presented a paper titled "Petrographic Study of Concrete: Two Case Studies Involving Internal and External Sulfate Attacks" at the 33rd International Conference on Cement Microscopy, hosted by the International Cement Microscopy Association (ICMA) on Mon., Apr. 18 at 11:15-11:45 a.m. The conference was held at the Omni Hotel & Resort in San Francisco, CA Apr. 17-20.
The presentation illustrated the value of concrete petrography through two case studies. The first case involves a municipal concrete building in Michigan that exhibits substantial foundation expansion and cracking. Petrographic examination indicates that the distress is attributed to thaumasite sulfate attack (TSA), based on the presence of cracks, microcracks and ring gaps around aggregates filled with abundant thaumasite secondary deposits, and observed paste alteration to thaumasite. Petrographic examination also reveals the sulfate is internally sourced, from the dolomite coarse aggregate in the concrete. To the best of our knowledge, TSA due to an internal sulfate source from aggregate has not been reported so far. Implications of the findings from this case study to the current research regarding TSA and to cement and concrete practices will also be discussed.
The second case involves concrete walls of a sewer tunnel about 100 years old. The exposed sides of the inner walls have experienced significant deterioration and spalls. Petrographic examination revealed that concrete deterioration is due to sulfate attack associated with accumulation of hydrogen sulfide gas within the structure. Distinct physical, compositional and textural variations in paste along the exposed surface are observed. Depth of deterioration has been determined to be shallow from the existing surface, although the near-surface damage is severe.
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