SEM OBSERVATIONS OF THE MICROSTRUCTURE OF FROST DETERIORATED AND SELF-HEALED CONCRETES

The microcracking and self healing mechanisms of concrete exposed to r apid freezing and thawing in water and subsequently kept in water have been investigated by Scanning Electron Microscopy (SEM). Non air entr ained concretes of water/binder ratio 0.40 with 0 and 5 % silica fume were studied. Damage was measured as loss in resonance frequency and c ompressive strength. After frost exposure, concrete beams were stored three months in water. During this time resonance frequency largely re covered, whereas compressive strength showed smaller recovery. On Seco ndary Electron Images (SEI) of fractured surfaces hydration products m ainly of the C-S-H type were seen traversing cracks at several locatio ns after self healing, but not directly after freeze/thaw. Back Scatte red Electron Images (BSEI) showed that the cracks due to freeze/thaw t esting were of 1 - 10 mu m width. The cracks traversed the paste and f ollowed the interfaces of most larger aggregate particles. On BSEI sel f healing was seen on 300 - 1000 X magnification as partly closing of several cracks smaller than 5 mu m. This was most clearly seen by swit ching between SEI and BSEI modes. In BSEI-mode the re-hydration produc ts appeared less dense and the cracks appeared wider than in the SEI-m ode.