## 4 Conclusions  
This study developed a collaborative sensing approach that combines the pre-embedded sensor method and the impact-echo technique to enhance the accuracy of grout filling quality assessment for precast concrete sleeve connections. The key findings and conclusions drawn from the research are as follows:  
1. The pre-embedded sensor method, based on the principle of vibration energy attenuation, can provide continuous monitoring of the grout filling process. However, its accuracy is limited at low filling degrees due to the exposure of the sensor to air;
2. The impact-echo technique, utilizing the principle of impact elastic wave propagation, demonstrates a high degree of accuracy in evaluating the grout filling degree across different filling levels. It can reliably detect the internal density and impedance changes within the sleeve connections;
3. The collaborative sensing approach establishes a functional relationship between the vibration energy values obtained from the pre-embedded sensor method and the grout filling degree, thereby leveraging the strengths of both testing methods. This approach can provide a comprehensive and accurate assessment of the grout filling quality, even in field conditions where direct access to the reinforcement ends is not feasible;
4. The field demonstration has validated the effectiveness of the collaborative sensing approach, with the calculated grout filling degree values closely matching the set values. This indicates the practical applicability of the proposed method in real-world precast concrete construction projects;
5. Practical guidelines have been provided for the implementation of the collaborative sensing approach, including the pre-installation of embedded sensors, the grout filling and testing procedure, the collaborative data analysis, and the quality assurance and decision-making process. These guidelines can assist construction teams in the effective implementation of this approach in their projects.  
The collaborative sensing approach developed in this study offers a reliable and comprehensive solution for assessing the grout filling quality in precast concrete sleeve connections, contributing to the overall structural integrity and performance of precast concrete structures. By integrating the strengths of both the pre-embedded sensor method and the impact-echo technique, this approach can overcome the limitations of each individual method and provide a robust and accurate evaluation of the grout filling quality.  
Future research directions may include the investigation of advanced data analysis techniques, the exploration of wireless sensor integration, and the development of automated monitoring and decision-making frameworks to further streamline the implementation of the collaborative sensing approach in the construction industry.  
Conceptualization, Y.Y. and B.J.; methodology, S.W.; software, Q.X.; validation, Y.Y. and B.J.; formal analysis, S.W.; investigation, B.J. and S.W.; resources, Q.X.; data curation, Q.X.; writing--original draft preparation, B.J.; writing--review and editing, B.J. and Y.Y.; project administration, S.W.; funding acquisition, Q.X. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Science and Technology Research Project of Chongqing Education Commission (Grant numbers: KJQN202205802, KJQN202305802).  
Not applicable.  
The raw data supporting the conclusions of this paper will be made available by the authors by request.  
The authors declare no conflicts of interest.