Microcracking and Chloride Ion Diffusion of Concrete Under Sustained Uniaxial Compression

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Title: Microcracking and Chloride Ion Diffusion of Concrete Under Sustained Uniaxial Compression

Author(s): C. C. Lim, N. Gowripalan, and V. Sirivivatnanon

Publication: Special Publication

Volume: 221

Issue:

Appears on pages(s): 893-910

Keywords: chloride diffusion; microcracking; sustained compressive stress

Date: 5/1/2004

Abstract:
The rate of chloride ingress in concrete not only depends on the intrinsic proper-ties of concrete but also on the magnitude of applied stresses and the nature of micro-crack propagation under these stresses. Limited information is available on the influence of these factors on the chloride ion penetration into concrete. The significance of applied stresses and the corresponding microcracking behaviour on the transport properties of concrete could provide useful information on the service life prediction of the concrete structure. To date, studies on the chloride ion transport into concrete are primarily based on concrete specimens that are not subjected to any stresses, particularly under sustained uniaxial compression. In the present study, the characteristics of microcracking and chloride diffusion into Grade 20 and 40 concretes are being investigated jointly by UNSW and CSIRO. The concrete specimens were loaded uniaxially in compression and sustained for a maximum duration of 18 months. Chloride ion penetration and micro-crack evaluation of these specimens were monitored periodically. This paper presents some early results on the apparent chloride diffusion coefficient obtained from Grade 20 and 40 concrete specimens that have been subjected simultaneously to sustained compressive stresses and 3% NaCl solution immersion for 90 days. Three levels of sustained compressive stresses at 20%, 35% and 50% of the ultimate strength were investigated. In addition, microcrack evaluation of the companion specimens (subjected to the same stress levels for 90 days) was also carried out. Microscopy technique was used to deter-mine the bond crack length in the concrete after the 90-day sustained period. At 35% sustained stress level, microcracks appear to be stable. However, the apparent chloride diffusion coefficient (Da) was found to decrease when compared with the unloaded control specimen. At 50% sustained stress level, a further reduction in D. was observed even though microcracks appear to have propagated.