Abstract: The concrete was modified with blast furnace nickel-iron slag and steel fiber, and then the mechanical properties and thermal damage law of the modified concrete after thermal cycling were studied through strength test and ultrasonic speed test. The strength test shows that thermal cycling has an accumulative effect on the strength degradation of the modified concrete. An appropriate amount of blast furnace nickel-iron slag and steel fiber improve the heat resistance of the concrete, but too much steel fiber will reduce the heat resistance of the concrete. The optimal amount of fiber is 0.5% (volume fraction) to 1.0%. The ultrasonic speed test shows that the ultrasonic speed in the concrete decreases continuously with the increase of the number of thermal cycles, similar to change law of compressive and tensile strength. According to the results of ultrasonic speed test and strength test, the relationship between ultrasonic speed and strength is established, and a generalized damage factor expression is established. Compared with the damage factors of different parameters, the results shows that the sensitivity of each parameter to thermal cycling is: wave velocity>tensile strength>compressive strength; the improvement effect on thermal cycling damage is: steel fiber>blast furnace nickel-iron slag; low water-cement ratio>high water-cement ratio.

Key words: blast furnace nickel-iron slag, steel fiber, thermal cycling, strength, ultrasonic velocity, damage law