Comparison of Effects of Chloride Exposure on Damage Gradients and Loss of Mechanical Properties due to Freezing and Thawing (05-0739)
Ufuk Dilek, MACTEC Engineering and Consulting, Inc.
Michael L. Leming, North Carolina State University, Raleigh

This article discusses a comparison of changes in dynamic elastic Young's modulus with depth of concrete slabs in service, after exposure to freezing and thawing with and without the application of deicer salts. Frost damage and associated frost durability is currently assessed in a laboratory environment using two different test methods for two different damage mechanisms; rapid freezing and thawing resistance (ASTM C 666) and scaling resistance of concrete surfaces exposed to deicing chemicals (ASTM C 672). ASTM C 666 involves determination of dynamic modulus loss on beam specimens after frost exposure while ASTM C 672 visually evaluates the extent of surface scaling. As part of an engineering assessment of concrete slab-on-grade sections showing surface distress under frost exposure, concrete samples were evaluated to identify causes of surface distress and the mechanisms contributing to the distress. Concrete core samples were evaluated for water-soluble chloride gradients to identify areas exposed to deicer salts. In addition, concrete disks approximately 25 mm (1 in.) thick were analyzed throughout the depth of cores, starting with the immediate exposed surface. Dynamic elastic Young's modulus of the concrete disks was determined non-destructively using the resonant frequency of the disks. A comparison of changes in dynamic elastic Young's modulus with depth for two different exposures was made, between a visually distressed area that had been subjected to deicing chemicals and an area that had not been subjected to de-icing chemicals but showed apparent frost damage. The results indicated that damage due to freezing of water in the absence of deicing chemical application caused a reduction in dynamic elastic Young's modulus with a well defined gradient throughout the concrete slab, possibly reflecting the effects of temperature gradients and progression of water intrusion into the slab. On the other hand, the loss in dynamic elastic Young's modulus was limited to the first 25 mm (1 in.) of near surface concrete in an adjacent slab that had a chloride gradient with higher concentrations near the surface; a gradient in the mechanical properties of the member was not observed in this location. The primary findings of this study provide important insights into frost damage. Damage exhibited in both degradation of elastic modulus and scaling was occurring at the same time and loss in elastic modulus can occur without any visual indications of distress. The information related to damage gradients presented in this paper were possible primarily through determination of the dynamic elastic Young's modulus of relatively thin sections of concrete. The use of this technique is very promising for both forensic and research applications since it provides a quantitative measure of an important, fundamental mechanical property, and the changes in this property with depth, under exposure to damage mechanisms.