A Um novo método para a determinação da retração de argamassas nas primeiras 24 horas de hidratação (new method for determining mortar shrinkage in the first 24 hours of hydration)

The shrinkage of cementitious materials is a complex phenomenon of volume reduction that can result in cracking, affecting the durability of these materials. The Brazilian standard ABNT NBR 15261 addresses long-term shrinkage without emphasis on the initial phase (< 24h) of hydration, which contains the plastic state of the cementitious material. Therefore, a method was proposed to measure the shrinkage in the first 24h of mortar hydration, on a laboratory scale, from prismatic samples, using molds from the Brazilian standard ABNT NBR 15261 (285x25x25mm) and rigid elements that transmit horizontal displacements of the sample to dial gauges. Cement mortar specimens were studied with the water/dry materials ratio fixed at 0.168 and varying the binder/aggregates ratio linearly: 33% (1:3); 25% (1:4); and 17% (1:6). Different exposures to the environment (sealed, open and wind) were applied. The analysis indicated that the proposed method was sensitive enough to indicate statistically significant relationships between the maximum initial shrinkages and the variation in the concentration of the binders, as well as indicating variations in the behavior of the initial shrinkage. The increase in cement concentration caused an increase in the average initial shrinkage of 76% in the samples exposed to air, 54% in those exposed to 5 m/s wind, and 22% in the sealed samples.

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