Investigation of low compressive strengths of concrete in paving, precast and structural concrete
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2000-08-01
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Abstract:This research examines the causes for a high incidence of catastrophically low compressive strengths, primarily on structural concrete, during the 1997 construction season. The source for the low strengths was poor aggregate-paste bond associated with air void clusters and poorly formed cement paste in the interfacial region adjacent to the aggregate. An interaction between the "synthetic" air entraining admixtures, used as substitutes for vinsol resin, and low alkali cements was directly tied to the problem with high summertime temperatures also contributing to the problem. The "synthetics" appear to be more hydrophobic, form thinner-walled air bubbles and develop rapid draining bubble flocculations more readily than vinsol resin, all of which can lead to significant reductions in strength. As an interim measure, the Department specified the sole use of vinsol resin air entraining agents along with water reducers, as needed, in April, 1998 and these measures have minimized the incidence of low strengths. Petrographic analysis of cores taken from low strength concretes failed to reveal the causes of the low strength problem but SEM microscopy clearly showed the failure mechanism. Laboratory testing of concrete mixes with various air entraining admixtures demonstrated an interaction was taking place with one cement and petrographic and chemical analysis of the cements used in the testing implicated alkali sulfates as a potential source of the interaction. Testing of the "synthetic" air entraining admixtures showed they have substantially different properties compared to vinsol resins. Mixtures of the "synthetics" and vinsol resin with 50% or more vinsol resin behaved similarly to vinsol alone.
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