对火灾破坏后的混凝土结构的安全评价，传统判断方法是通过混凝土强度损失率来判断的。通过采用超声回弹综合法测强法，测定高温煅烧试件前后强度的变化，得到混凝土强度损失率与损坏程度无单纯的线形关系，传统方法的使用具有一定的局限性。通过在二维平面构建的“声速-回弹值”曲线的分形维数值，得出了该分形维数值与高温破坏时间的关系。得出不能单纯采用强度损失率作为判断破坏时间长短依据的原因和采用分形方法的优点。

For the safety evaluation of the fire hazard destroyed concrete, the traditional method uses the strength loss ratio to distinguish the fired concrete strength loss. The research work used the ultrasonic-rebound synthesis method to testing the strength change before and after high temperatures destroys, and it gained that there was no simplex linear relationship between the damage grade and the strength loss ratio, and that the traditional method showed a definite limination. The research work through investigating the “ultrasonic velocity-rebound value” curves fractal dimensions in the two dimensions plane, gained the corelation between the fractal dimension change and the high temperatures destroy time. It was concluded that using the strength loss ratio alone was able to distinguish the high temperature destroy time, and that using the fractal methods was more advantageous.