Deterioration of Aggregates--The Underlying Cause

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Title: Deterioration of Aggregates--The Underlying Cause

Author(s): Peter P. Hudec

Publication: Special Publication

Volume: 100

Issue:

Appears on pages(s): 1325-1342

Keywords: adsorption; aggregates; capillarity; concrete durability; deiciers; deterioration; expansion; freeze-thaw durability; porosity; General

Date: 4/1/1987

Abstract:
Freezing and thawing cycles in northern latitudes have resulted in the breakdown of some aggregates and concrete. Deicing salts have accelerated the problem. However, freezing of water cannot be the principal cause of deterioration, since in the fine-grained aggregates and cement paste the pores are too small to allow freezing. Yet it is these materials that deteriorate the most. Deicing salts likewise lower the freezing point and the number of freeze-thaw cycles, yet cause increased breakdown. The same materials susceptible to freeze-thaw breakdown also deteriorate significantly under repeated wetting-drying cycles. Laboratory experiments show these materials to expand on wetting and contract on drying. NaCl solution causes significantly greater expansion. Ice formation in the pores, therefore, is not the primary cause of breakdown. The answer may be found in the nature of the water in the small pores--water affected by the capillary and surface forces of the pore material. The pore water has lower vapor pressure, which prevents it from freezing, but which results in osmotic pressure differential, causing expansion. Deicing salt cations are preferentially adsorbed and concentrated on pore surfaces, further increasing the osmotic potential, expansion, and breakdown. Methods that determine the pore size distribution, surface sorption characteristics of the pore walls, and volume changes on wetting are suggested as more definitive measures of aggregate (and concrete) durability, compared to some of the currently accepted tests. This paper presents an overview of processes causing the aggregate breakdown, based on the theoretical and laboratory-derived evidence accumulated in the author's lab over the last 15 years.