Microcracking and chloride permeability of concrete under uniaxial compression

Abstract

In the previous studies on microcracks and rapid chloride permeability tests, microcracks were quantified in terms of total crack length. This was carried out by examining concrete slices after compression tests. No attempts have been made to characterise the microcracks during the compression test prior to the chloride permeability test. In the present study, concrete cylinders were loaded under uniaxial compression between 30% and 95% of the ultimate strength. A non-destructive method of microcrack evaluation was used to study the progressive microcracking in concrete cylinders during compression tests. After the compression test, a rapid chloride permeability test (RCPT) (ASTM C1202) was carried out on a specimen cut from the same cylinder. The total crack length was also determined from the same specimen to compare with the observed microcracking behaviour, assessed by the non-destructive testing. The characteristics of the microcracks in terms of the specific crack area are different when a concrete is under a load and when it is completely unloaded. The chloride permeability of a concrete (after it was unloaded) appears to be influenced by the occurrence of a certain stress level known as the critical stress. When the critical stress is exceeded in a concrete specimen, a comparatively large chloride permeability was measured. Where the critical stress in a specimen is not exceeded, the increase in the permeability is marginal in spite of the large increase in microcracks.