In accordance with a fresh accident by severe expansion cracks of structural elements,based on systematic detection and analyses such as X-ray diffraction,differential thermal analysis,scanning electron microscory,chemical analysis,petrographic analysis, electronic probe analysis,and atomic absorption spectroscopy analysis, it is pointed out that the dominant reasons lie in the hydration reaction of magnesia in concrete aggregates, resulting in a volume expansion in structure members.A wholly new corresponding strengthening method is applied to the cracked elements and ted out to be effective.