## 3 Conclusions  
Experimental results showed that when the surface of the CFRTs was heated to approximately 26 \({}^{\circ}\)C, each measure point's temperature on the surface was almost similar when the structure was in good condition. Once the void phenomenon appeared, temperature differences would occur in the void area.  
For a single model, the temperature of the maximum void height was the highest, the temperature increase rate was the fastest, and the cooling rate was the slowest. The temperature of other measure points decreased with the decrease of the void height.  
For the model with different void forms, the higher the void, the higher was the temperature. The average temperature difference of the measure points in the same position was above 1 \({}^{\circ}\)C. Because of the void's existence, the heat absorbed by the upper surface transferred slowly into void area and then accumulated.  
At the same time, there was a sudden temperature change on both sides of the interface. The temperature difference of the heating section was above 1.8 \({}^{\circ}\)C, and the temperature difference of the cooling section was above 1.2 \({}^{\circ}\)C, which were easy to identify. The more serious the void was, the wider was the gap between the void and the solid area. In addition, although the degree of temperature variation decreased after changing the heating direction, the temperature distribution could still be identified.  
Numerical simulation showed that, due to the input of heat flux, the temperature difference of the concrete-filled steel tube structure between the void and the solid parts became greater, and the sudden temperature change of the interface also increased. The trend of this phenomenon was consistent with the experimental results. Moreover, the highest temperature point was also located in the largest void height. The temperature distribution of the whole model was uniform in the solid region and the temperature in the void region was higher than that in the solid region. There was an obvious and abrupt temperature change at the void interface.  
In summary, the arch rib's temperature was almost the same when the structure was good condition. However, when voids existed, the temperature difference of the surface was above 1 \({}^{\circ}\)C, and the temperature difference at the edge of the voids was even more serious. Therefore, the authors could evaluate the structure's void. It is evident that the method used and based on a distributed temperature measurement is effective and feasible. This method has the advantages of low cost, the simple arrangement of sensors, and no need for a high-altitude operation. However, this method is suitable for the case of large voids in concrete-filled steel tubes, but not for the identification of voids.  
In the future, researchers using this method would have to consider the natural environment's influence and other factors, as well as the optimal placement of sensors to recognize the void interface of concrete-filled steel tubes and obtain a better qualitative and quantitative identification. Furthermore, the question of whether this method is suitable for concrete-filled steel tubular structures with surface coatings would also need further discussion.  
Methodology, S.P.; Software, J.M.; Resources, D.L.; Writing-Original Draft Preparation, Y.Z.  
This research was funded by National Natural Science Foundation of China grant number 51678110 and 51108058 and Research Funds of Key Laboratory of Heating and Air Conditioning, The Education Department of Henan Province grant number 2017 HAC103.  
The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.