Abstract
In this study, cement minerals, aluminates, and alkali-free accelerators incorporated with steel fiber were used to scrutinize the influence of accelerating agents on the long-term performance of tunneling shotcrete. Performance tests were identified based on the core compressive strength of mix shotcrete specimens with different types of accelerating agents throughout timeframes of 1, 3, 6, and 12 months. Here, 37 kg of steel fiber was incorporated into the cement mineral and aluminate mixes, and 40 kg of steel fiber was incorporated in an alkali-free mix for the shotcrete mix design. The KSF 2784 and ASTM 214 standards were followed for specimen fabrication and core cutting. For all specimens, shotcrete test panels of 250 × 600 × 500 mm were manufactured for core compressive strength tests conducted using 100, 75 and 55 mm diameter cylindrical molds and a length-to-diameter ratio of 2. The 1-month compressive strength of all test variables satisfied the Korea Expressway Co. standard of 21 MPa. The core compressive strength of the shotcrete specimens showed a tendency to increase with age, but a strength reduction occurred in 6 months and increased again at 12 months. Moreover, the impact of the diameter changes in the shotcrete core specimens was analyzed based on the mixing. For 12 months, a large increase in the core compressive strength occurred, particularly in the alkali-free specimens. The comparison also focused on the relative strength compared with a cast concrete mold and shotcrete core specimens. It is necessary to use alkali-free accelerators considering the long-term performance of tunnels and worker safety.
Highlights
Shotcrete is a sprayed concrete with a blend of cementitious material, coarse aggregate, and fine aggregate incorporated with other admixtures through a hose by compressed air [1]
The main purpose of this study was to consider the cement mineral, aluminate, and alkali-free accelerators incorporated with a steel fiber for analyzing the performance of shotcrete in a tunnel project site based on the Korean Expressway Corporation (2003)
To evaluate the performance of the shotcrete, we focused on the core compressive strength at 1, 3, 6, and 12 months of all mixes and, here, considered the three samples of each specimens
Summary
Shotcrete is a sprayed concrete with a blend of cementitious material, coarse aggregate, and fine aggregate incorporated with other admixtures through a hose by compressed air [1]. The accelerator agents mainly used in highway tunnels have a high alkali content, which lowers the long-term strength, and this alkaline component is eluted together with the groundwater over a long period of time. The construction performance of shotcrete depends on the use of alkali-free, aluminate-type accelerators [7]. This is because the accelerators enhance the primary buildup thickness and decrease the amounts of rebound and dust. Based on the purpose of the present study, shotcrete accelerators are categorized as aluminate, silicate, alkali-free, and cement-mineral accelerators. Cement mineral accelerators (C12A7) were used to estimate the performance of the shotcrete of the 28 days compressive and flexural strength with enhanced durability [10,11]. Three types of accelerators were used to determine the mechanical long-term performance of the tunnel shotcrete, and accelerators incorporated with steel fibers were applied for a better shotcrete performance
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