How does the thermal expansion of rebars not compromise the structure of encasing concrete?

Steel has a coefficient of about 10.8 and concrete about 12. As mentioned before these are pretty close. But, they are different.

In this case it is most likely that the concrete will be applying force to the steel rather than the other way around (with thermal stresses). When a material is constrained we can take the thermal change convert into amount of linear expansion and plug that into the modulus of elasticity to find out the equalizing force is to the expansion.

Basically, change in temperature creates stress within the system. If it becomes too much you see fracturing.