Abstract
Cracking of concrete structures during the hardening phase often seriously compromises not only structure integrity but also durability and long-term service life. Especially for large massive structures, for example, concrete submerged tunnel, the reliable crack risk evaluation at the hardening phase is critical to the successful design. Mineral additives such as silica fume (SF), blast furnace slag (BFS), and fly ash (FA) have been used extensively in production of high-performance concrete in the last decades. The mineral additives such as FA and BFS not only reduce the hydration heat during the hardening phase but also have significant influence on the development of mechanic and viscoelastic properties at an early age. The main objective of the research is to propose a design methodology to select the appropriate composition of concrete for construction of the submerged tunnel. The influence of mineral additives such as FA and BFS on the risk of cracking during the hardening phase was investigated for the massive concrete structure. Five types of concrete mixes denoted as SV40, 40% BFS, 60% BFS, 40% FA, and 60% FA concrete are considered in the current study, and the measurement to reduce the initial temperature is also considered for 60% FA concrete. First, the well-documented material models are verified by calibration of restraint stress development in the TSTM test by using the finite element method (FEM), and then the 3D thermal-structural analysis is performed to assess the cracking risk for the submerged tunnel during the hardening phase. Based on analysis results, the 60% FA concrete has both the lowest maximum temperature and the lowest stress/strength ratio, and the cracking-free design based on the current study ensures the successful construction of the submerged tunnel.
Highlights
In the past, prediction of the early-age cracking was almost exclusively based on temperature criteria. e temperature development in the young concrete was calculated, and cracking was predicted from the maximal temperature difference in the massive concrete structure
In North America, the second Midtown Tunnel built under the Elizabeth River from 2013 to 2016 is the first deepwater concrete immersed-tube tunnel and only the second all-concrete immersed tunnel in the U.S e all-concrete tunnel design allows for a strong, durable structure with substantial economic savings compared to a more conventional design using a steel tube encased in concrete, and it is extensively used across Europe. e all-concrete tunnel design is selected for the submerged tunnel built in Oslo
Similar temperature and stress contour distribution. e maximum temperature appears at the corner between the inner wall and the top slab, while the critical locations, regarding the risk of through cracking determined as the ratio between maximum tensile stress and tensile strength, are in the center of the inner wall and approximately 0.6–1.2 m above the foundation slab
Summary
Prediction of the early-age cracking was almost exclusively based on temperature criteria. e temperature development in the young concrete was calculated, and cracking was predicted from the maximal temperature difference in the massive concrete structure. E main drawback of the temperature-based crack risk estimation is that the other important factors in stress calculation are not considered: restraint conditions, material properties, and shrinkage. E cracking risk at the hardening phase is the main concern in the design of the concrete submerged tunnel built in Oslo. Simulation of the hardening structure in general has to take into account temperature development due to hydration, development of material properties, and restraint conditions of the particular structure [10, 11]. A design methodology is proposed based on both comprehensive tests and advanced numerical simulations. (i) Suggest candidate concrete with different compositions (ii) Establish material models through a comprehensive test program (iii) Calibrate material models against temperaturestress testing machine (TSTM) tests (iv) Perform advanced thermal-structural numerical simulation (v) Recommend the concrete composition with lowest cracking risk. Candidate Concrete e concrete proposed in the current design includes one typical construction concrete (SV40) and four other concrete types with different percentages of mineral additives (FA or BFS), and the composition of concrete is presented in Table 1. e materials tests and the mechanical properties do not include the steel reinforcement (bars, etc.)
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