This study analyzed the effect of accelerating agents, such as aluminate, cement mineral, and alkali-free accelerators, on the long-term performance of steel-fiber-reinforced shotcrete. The shotcrete performance was studied based on the type and amount of steel fiber added. Performance tests were performed to identify the accelerator providing better long-term performance to the steel-fiber-reinforced shotcrete. Changes in strength and flexural performance over time were investigated. The compressive strength and flexural strength tests on 1-, 3-, 6-, 12-, and 24-month-old test specimens were performed, wherein 37 kg of steel fiber was added to the cement mineral and aluminate mixes, and 40 kg of steel fiber was added to the alkali-free mix. The 1-month compressive strength result of all the test variables satisfied the Korea Expressway Corporation standard. The compressive strength of the cast concrete and shotcrete specimens increased with age, demonstrating a strength reduction, particularly in the 24-month-old shotcrete specimens. Thus, the shotcrete performance may deteriorate in the long-term. In the 24-month-old specimen, substantial flexural strength reduction was observed, particularly in the aluminate and alkali-free specimens. The relative strength of the specimens was compared with that of the cast concrete mold specimens. The results suggest the use of alkali-free accelerators, considering the long-term performance of tunnels and safety of workers. Moreover, increasing the steel fiber performance rather than the amount of low-performance steel fiber must be considered.

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.

Since it is impossible to reconstruct the top level that has collapsed, a formwork is constructed to squeegee mortar or spray mortar, and repair kits are being used in Korea to chip away the damaged concrete of the bridge deck structure. In Korea, a technique called hydro-demolition replaces water blasting and water jetting by using high-pressure water to remove not only asphalt but also old and broken concrete. Additionally, dry materials including cement, aggregate, and additives are carried via the inside of a hose to the field using compressed air, where they meet water and are ejected at a high rate of speed. This technique is known as dry mix shotcrete. Using the devised automatic hydraulic dismantling technology and high-performance dry-mix shotcrete, field application results are discussed in this study.