Surface Strain Experienced by Mortar in Wetting-Drying Cycles and Deicer Salt Application

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Title: Surface Strain Experienced by Mortar in Wetting-Drying Cycles and Deicer Salt Application

Author(s): Peter P. Hudec and Martin Ondrasik

Publication: Special Publication

Volume: 170

Issue:

Appears on pages(s): 853-878

Keywords: Deicing; drying; expansion; mortars (materials); osmotic forces; sands; shales; water-cement ratio; welting.

Date: 7/1/1997

Abstract:
The scaling of concrete and mortar involves sub-parallel de-laminations of material from the surface. To produce this phenomenon, differential stresses parallel to the surface and resulting in differential strain must be active. This research measured the differential strain developed along the surface of specially shaped mortar bars upon their wetting, drying, and osmosis due to application of deicer salts. Mortar bars were made at a w-c ratio of 0.4 and 0.6 with shaley sand and high quality dolomite as aggregate. The sand is known to cause surface scaling. The bars were cast in a ‘half circle’ shape. The normally cured samples were dried, and all but the outer surface of the ‘half circle’ were sealed.. This allowed the ingress of water and solutions from one direction only, such as would occur in an ‘infinite’ concrete surface. Steel pins were secured to the ends of the half circle to facilitate measurement of the strain. The strain of the specimens was measured during the following states: 1. dry samples, 2. saturating in water, 3. drying, 4. saturated, placed in saline solutions, 5. then placed in pure water. The results show that as the water entered or left the surface, stresses developed which were sufficient to deform the ends of the half circle up to 0.6% of the diameter distance. Largest deformations took place upon wetting, followed by those on drying, and the least deformation resulted from osmotic forces. When the samples had equilibrated, i.e., became either fully saturated, dried, or the pore fluid composition equaled that of the saturating medium, the strain was relaxed. Water-cement ratio influenced the time of maximum strain development and aggregate and cement type determined the magnitude of the strain.