the velocity profile of the air within the crack approaches zero in all directions, leading to the heat convection within the crack being approximately negligible. Since the heat conductivity of air is very low, the air within the crack acts as a thermal insulator to some extent. [PERSON]'s group [4] worked with reduced-scale segment joints (1:3). They measured the surface temperature of their specimens with a thermal infrared imager immediately after turning off the combustor, and they found that the surface temperature of the concrete was highest near the joints. This implies that heat had transferred into the joint gap, and the nearby concrete was heated on two sides. Both the concrete cracks in the literature [5] and the joint gaps in reference [4] belong to narrow crevices, however, the influences of them on heat transfer within the crevices seem to be different. That motivated the first part of this study in which fire tests on full-scale segment joints, which were preformed to investigate the influence of the joint gap on the temperature distribution in inter-segment joints. After that, the fire-damaged full-scale segment joints were strengthened in the second part of this study, and then the strengthening effect was examined by loading these joints till fail. Steel plates or fiber-reinforced plastic (FRP) are the two materials most commonly applied for strengthening subway tunnels. A group led by [PERSON] [6] tested full-scale shield tunnel lining rings strengthened with 20 mm-thick steel plate rings. They found that such full-ring steel reinforcement significantly improves the bearing capacity and overall stiffness of tunnel structures. Further, such strengthened linings fail due to local bond failures between the steel plates and the segments. [PERSON] [7] describes how tunnels of the Taipei Rapid Transit System are strengthened with an inner steel lining. Such strengthening methods satisfy the mechanical requirements for a tunnel, but they use a lot of steel, and can therefore be uneconomic. Furthermore, the weight of the steel can cause additional settling during construction and in future operations. [PERSON] [8] describes a method for reinforcing existing tunnel structures using thin steel panels, it involves elaborate design and construction work. A group led by [PERSON] [9] investigated using arch supports made with steel I-beams, and they found that the strength and stiffness of steel supports depends largely on the confinement to which the support is subjected. Thin steel plates and I-beams can suffer from local buckling at the waist of a tunnel lining, which prevents the full exploitation of the steel's strength. [PERSON] and his colleagues [10] investigated numerically the reinforcement of the tunnel linings with FRP. Other researchers [11; 12; 13] have pursued retrofitting a tunnel using FRP grids and corrugates. Evidently, FRP strengthening parts only enhance the integrity of the tunnel ceiling, they have almost no reinforcing effect on the waist of a tunnel lining subjected to negative bending moments. Concrete- filled steel tubes (CFSTs) have also been proposed to support tunnel structures. [PERSON] [14] and [PERSON] [15] have described using CFSTs in a mine tunnel. In a CFST the most prominent natural characteristics of steel and concrete are used to advantage. The concrete is confined by the steel tube, and the concrete core supports the steel tube against the local buckling. The result is improved strength, ductility, stiffness, and energy absorption capacity for the combination [16]. That suggests investigating the feasibility of using CFSTs to strengthen fire-damaged segment joints in tunnels. In this way, the second part of this study focused on the effectiveness of CFSTs for restoring segment joints damaged by a thermal shock. As compared with the traditional steel plate rings [6], the steel consumption and weight of the CFSTs employed in this paper are only about 40% and 65%, respectively.