Self compacting concrete forging solid "muscles" for deep and medium channels
2024-06-18
On June 16th, the super project located in the core area of the Guangdong Hong Kong Macao Greater Bay Area - the Shenzhen Zhongshan Passage passed the handover acceptance. The Shenzhong Channel is a cross sea cluster project that integrates bridges, islands, tunnels, and underwater interconnectivity, and currently has the highest comprehensive construction difficulty in the world. It has a total length of about 24 kilometers. The immersed section of the underwater tunnel is 5035 meters long and is constructed using a new type of steel-concrete composite structure, consisting of 32 pipe sections and a final joint. As one of the important participants in the construction of the submarine tunnel project of the Shenzhong Channel, China Communications Fourth Navigation Engineering Research Institute Co., Ltd. (hereinafter referred to as CCCC Fourth Navigation Research Institute), a state-owned enterprise, is responsible for the research and development of steel shell immersed tube concrete. "Faced with difficulties and challenges, China Communications Fourth Aviation Research Institute has been exploring for four years since 2015, conducting experiments such as scaled models, floating pouring, full-scale models, and simulated pouring. Finally, in 2019, a self compacting concrete" super formula "for the steel shell sinking tube pouring process was formulated, which effectively ensured the smooth pouring of the steel shell concrete sinking tube in the deep and middle channels, laying a solid foundation for the timely opening of the deep and middle channels." Deng Chunlin, Chief Engineer of the Building Materials Institute of China Communications Fourth Aviation Research Institute, introduced. The reporter learned that the Shenzhong Tunnel is the first underwater tunnel in China to be constructed using a steel-concrete composite structure. The steel shell immersed tube adopts self compacting concrete, which can uniformly fill the steel shell compartment without vibration, forming a sandwich shaped steel-concrete composite structure. Compared with traditional reinforced concrete immersed tube structures, this type of structure has advantages such as strong load-bearing capacity, high seismic adaptability, and good waterproof performance. The reporter found through research that there are no reference cases for large steel shell sunken pipes both domestically and internationally. The immersed tube of the Shenzhong Tunnel adopts a sandwich structure with inner and outer double-layer steel shells and poured concrete in the middle. The construction team uses steel shells to build "tendons" for the immersed tube, and fills the inside with concrete as the "muscles" of the immersed tube. Each section of the steel shell concrete standard immersed tube in the deep middle channel is divided into 2255 compartments, each of which is not connected. In order to enhance the overall integrity of the structure, we need to pour a type of high fluidity concrete, namely self compacting concrete, into each compartment According to Yu Fang, Deputy Chief Engineer of the Building Materials Institute of China Communications Fourth Aviation Research Institute, "The impression of concrete has always been that it can be used to build various projects by mixing some cement, sand, and stones with water. However, it is not easy to mix concrete into a high fluidity fluid. Self compacting concrete is relatively delicate.". Its self leveling performance and stability are greatly affected by factors such as raw materials, discharge time, pumping distance, and environmental temperature. Therefore, members of the team from the Building Materials Institute of China Communications Fourth Aviation Research Institute conducted a large number of indoor tests on self compacting concrete in the early stage of the project. The success of indoor experiments is far from enough. To truly test the performance, it is necessary to conduct full-scale model pouring tests of self compacting concrete. As early as January 2016, the research team of China Communications Fourth Navigation Research Institute began a full-scale sea model pouring test of self compacting concrete sunken pipes for deep and medium channels at the Jiangmen Xinhui Prefabrication Yard. With the close cooperation of team members, the concrete was successfully poured. Dynamic control of concrete performance and quality is different from the traditional method of pouring concrete sunken pipes. Deep and medium channel steel shell sunken pipes use ultra long concrete ground pump equipment for concrete pouring, which means that the concrete is pressed into each compartment through 200 meter long pipes. During this process, the performance of concrete before entering the pipe and after being pumped out of the pipe must meet the performance requirements of self leveling. However, the building materials team found during the testing process that the traditional control variable method is no longer applicable, and there is an urgent need to propose a new analysis method in order to establish a more accurate performance prediction model. Therefore, during the construction process of E1-E4 immersed pipes, Yu Fang led a team to collect performance indicators of hundreds of batches of raw materials such as cement, fly ash, slag powder, and more than 2000 sets of performance test data before and after pumping self compacting concrete. After more than half a year, the team overturned more than 20 plans and ultimately found a suitable analysis method to quantify the correlation between key technical indicators of various raw materials and concrete performance, achieving dynamic control of the quality of concrete raw materials and concrete workability. They also established a predictive model for the performance of self compacting concrete before and after pumping, proposed a set of technical indicators for the robustness control of self compacting concrete, and formed the Key Technical Guidelines for the Preparation and Construction of Self compacting Concrete for Steel Shell Immersed Pipes in Deep and Medium Channels, guiding the smooth pouring of steel shell immersed pipes. "This set of guidelines can predict the changes in the workability of concrete after pumping based on the indicators of raw materials and the performance of concrete after discharge, achieving dynamic correlation between raw material performance and concrete mix ratio, and thus achieving dynamic control of concrete performance quality." Introduction by Deng Chunlin. Since the construction of the Shenzhong Corridor, the building materials team of China Communications Fourth Aviation Research Institute has been involved and has conducted a large amount of research in design, construction, quality control, and other aspects. The research results have been comprehensively applied in engineering design and construction, providing important technical support for engineering construction Song Shenyou, Director and Chief Engineer of the Shenzhong Channel Management Center, said. (Lai Xin She)
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