Self- healing of cracks in concrete long-term exposed to different types of water : results after 1 year exposure

SUMMARY The aim of the project was to find out if cracks in concrete exposed to water can heal so that chloride ingress in the crack is effectively obstructed. If this is the case it might be possible to accept wider cracks in concrete than is allowed today. 18 concrete specimens with induced cracks, 0.2 and 0.4 mm wide, were exposed to sea water, brackish water and tap water for 1 year. For most specimens the crack width has been locked mechanically. For four specimens the crack had the possibility to relax. Two types of water exposure have been used: * Permanent immersion * Cyclic immersion and drying in lab air Photographs were taken of the crack before and after exposure. Photos are shown in APPENDIX 1 and 2. The photographs indicate that some healing has occurred, particularly for specimens permanently immersed in sea water. The effect of relaxation seems to be marginal. After terminated exposure the chloride content in the crack walls on different depths was determined. The chloride content diminishes with the crack depth which indicates that the precipitation of minerals in the crack has the ability to somewhat obstruct chloride ingress. There was no big difference between cracks 0.2 mm and 0.4 mm wide or between locked and relaxed cracks. SEM-EDS analyses of precipitations in the cracks showed that these mainly consisted of calcium hydroxide crystals of various size, and sometimes calcium carbonate crystals (calcite). In sea water needle-like sulphur containing crystals (e.g. ettringite) were frequent. For concrete that was exposed to cycles of sea water and drying magnesium hydroxide crystals (brucit) were found. This report presents the results of part 1 of the project. In part 2 specimens will be exposed in sea water for about somewhat more than 2 years (28 months)