Environmental and technical evaluation of cement reduction and test methods for fibre reinforced shotcrete in tunnels

University essay from KTH/Betongbyggnad

Author: Elin Brodd; Lina Östlund; [2022]

Keywords: ;

Abstract: The dominating support method for hard rock tunnels today is use of fibre reinforcedshotcrete in combination with rock bolts. The fibre reinforced shotcretesecures smaller blocks, while rock bolts are used to support larger blocks in therock. Application of shotcrete is done by spraying against the rock surface usingcompressed air. The use of accelerators result in fast strength developmentand adhesive properties, which are two characteristics of great importance whenconstructing tunnels.This thesis aims at increasing the understanding of climate impact from fibre reinforcedshotcrete in tunnel construction. The focus is on reducing the climateimpact with two methods: reducing the share of cement in the shotcrete mixturethrough substitution with addition materials and using better test methods for fibres.Cement is one of the most important ingredients in concrete, however alsothe largest contributor to CO2 emissions. Reducing the cement amount is thereforea way of reducing the emissions of concrete. In addition, when testing the performanceof fibres, different methods can lead to a spread in the results, causing anoveruse of fibres in the shotcrete.First, the thesis investigated the use of alternative binder materials, especiallyGround Granulated Blast Furnace Slag (GGBS), as a substitute for cement. Experimentaltesting was performed in a laboratory to evaluate the compressive strengthfor shotcrete with different amounts of GGBS. Testing was performed after one andseven days in order to evaluate the early strength. Second, the thesis investigatedthe use of fibre reinforcement and the possibilities of reducing the fibre dosagewhen changing fibre type and test method. Numerical modelling was performedfor two test methods, beam and panel testing, based on experimental data. Thethesis evaluated the environmental performance in terms of Global Warming Potentialfor both fibres and binder.The results show that substituting cement with GGBS has the largest potential tolower the CO2 emissions from fibre reinforced shotcrete. In addition, the fibredosage can be lowered by changing fibre type, but also test method. Also thislowers the emissions, however the main emissions origins from the binder part.

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