Fundamental Factors in the Drying Shrinkage of Concrete Block


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Title: Fundamental Factors in the Drying Shrinkage of Concrete Block

Author(s): George L. Kalousek

Publication: Journal Proceedings

Volume: 51

Issue: 11

Appears on pages(s): 233-248

Keywords: no keywords

Date: 11/1/1954

Shrinkage of concrete block dried to stable condition successively at 85, 70, 50, and 25 percent relative humidity was not always related to moisture loss. In one series of tests, block underwent slight expansion for a time while losing moisture at 70 percent relative humidity. Sand and gravel block dried at 25 percent relative humidity showed no expansion during rehumidification at 50 and 70 percent relative humidity although up to 17 percent of the total moisture content was readsorbed. Measurements of apparent surface areas by water vapor and nitrogen adsorption suggested a clue as to the mechanism by which water caused volume changes in concrete block. The amounts of water vapor adsorbed were approximately twice those required to cover the true internal surface as determined by nitrogen adsorption. The amount of this excess water, designated "interlayer" water, was about 0.7 times as large for autoclaved units as for normally cured units. Since the autoclaved block showed a shrinkage about 0.6 times as large as that manifested by the normally cured ones it appeared that the amount of shrinkage may be proportional to the amount of the "interlayer" water. The volume changes with adsorption and desorption of water, at least at moderate to low humidities, could be caused by the change in size of the unit cell of the structure of the cementitious phase. The manner in which the capillary phenomenon may contribute to volume change is considered. The capillary-tension force is probably operative only at high humidities.