Sem observations of the microstructure of frost deteriorated and self-healed concretes

• Published in: Cement and concrete research 1995-12, Vol.25 (8), p.1781-1790
• Main Authors: Jacobsen, Stefan, Marchand, Jacques, Hornain, Hugues
• Format: Article
• Language: English
• Subjects: Applied sciences; Buildings. Public works; Concretes. Mortars. Grouts; Exact sciences and technology; Freezing; Frost effects; Hydration; Materials; Microcracks; Microstructure; Natural frequencies; Properties and performance of fresh concrete; Scanning electron microscopy; Silica; Thawing; Water
• ISSN: 0008-8846; 1873-3948
• DOI: 10.1016/0008-8846(95)00174-3

The microcracking and self healing mechanisms of concrete exposed to rapid freezing and thawing in water and subsequently kept in water have been investigated by Scanning Electron Microscopy (SEM). Non air entrained concretes of waterbinder ratio 0.40 with 0 and 5 % silica fume were studied. Damage was measured as loss in resonance frequency and compressive strength. After frost exposure, concrete beams were stored three months in water. During this time resonance frequency largely recovered, whereas compressive strength showed smaller recovery. On Secondary Electron Images (SEI) of fractured surfaces hydration products mainly of the C-S-H type were seen traversing cracks at several locations after self healing, but not directly after freeze/thaw. Back Scattered Electron Images (BSEI) showed that the cracks due to freeze/thaw testing were of 1–10 μm width. The cracks traversed the paste and followed the interfaces of most larger aggregate particles. On BSEI self healing was seen on 300–1000 X magnification as partly closing of several cracks smaller than 5 μm. This was most clearly seen by switching between SEI and BSEI modes. In BSEI-mode the re-hydration products appeared less dense and the cracks appeared wider than in the SEI-mode.