For tunnels in hard rock, fibre-reinforced shotcrete (sprayed concrete) is an essential part of the rock support system. Due to the complexity of estimating the load on the shotcrete, design philosophies and guidelines vary between countries. This also applies to fibre type, where some countries only allow fibres of a certain material. With this background, the suitability of using steel, synthetic and basalt fibres in shotcrete linings was studied in a recently finalized project. The aim was to evaluate the structural performance through experimental testing, investigate the environmental impact of the fibres and summarize the knowledge regarding the long-term performance of the fibres. The project has shown that all three fibres are suitable to use in tunnels from a structural perspective, but some questions regarding long-term performance still exist. From an environmental perspective, the choice of fibre type has, compared to shotcrete, a negligible effect on greenhouse gas emissions.

Structural integrity for energy dissipation of spillways is essential to maintain a high dam safety level. In some Swedish low-head facilities (< 15 m), damages appear also where flow velocities are expected to be too low to cause cavitation. To better understand the reason for damages a study including desktop survey, hydraulic scale model tests and numerical modelling with CFD has been initiated at Vattenfall R&D. Most common damages found in an inventory were erosion of bedrock in stilling basins, often in or adjacent to weak zones (clay filled cracks or crushed rock). Sometimes the damages progressed under concrete structures. A flow velocity above 15 m/s atthe end of the crest or at the intersection with the downstream water level seems to be a limit for when damages occur in the studied spillways. Scale model tests (1:17.5) of a case with damages has been performed to study the energy dissipation and potential causes for initiation of the damages. A CFD-model (OpenFOAM) of the same case, in model scale, has been set up to make a comparison. Measurements of flow velocities give that the tested scenario could cause flow velocities above 15 m/s in prototype scale. The pressures given by the CFD-model where in line with the ones from the model tests. From the model test the pressures, near the transition from the crest to the stilling basin, were low but not sub atmospheric. In the CFD-model the indicated risk for cavitation was high and in the same area as the real damages. Excluded from both model tests and CFD-model is the fact that cracks where visible in the real structure. Therefore, it cannot be ruled out that the initiation of damages also was stagnation pressure in the cracks. The study will continue with further tests with more pressure gauges and simulations. and the validation of the numerical model and conclusions presented are therefore preliminary.

Concrete dams in cold regions are designed to resist static loads from the ice sheet forming on the reservoir. Current guidelines suggest design ice loads of 50-250 kN/m based solely on geographical location, overlooking site-specific conditions. For many dams, this constitutes a considerable part of the total horizontal load and could theoretically cause problems with stability, especially for lower dams. However, the knowledge about the actual magnitude of ice loads remains limited, and no stability failure initiated by ice loads has been reported. This paper presents results from ice load measurements using a 1 X 3 m2 load panel installed at two dams in Sweden. The results from ice load measurements are presented alongside external factors such as water level, ambient temperatures, precipitation, and ice thickness, offering a more comprehensive understanding of the variables affecting ice loads on concrete dams. 

Fibre-reinforced shotcrete (sprayed concrete) is an essential part of the rock support for tunnels in hard rock. In Sweden, steel fibres have exclusively been used to reinforce shotcrete in traffic tunnels since the 1980s. Today, several alternative fibres exist on the market, e.g. basalt and synthetic fibres. These alternative fibres could potentially offer improvements such as reduced environmental impact or a reduced need for maintenance and repair. Therefore, this report focuses on investigating the performance of shotcrete reinforced with fibres made of steel, basalt and synthetic. A large experimental campaign, including cast and sprayed beams and panels, was performed to investigate the structural capacity and sprayability of shotcrete reinforced with various dosages of each fibre type. Numerical simulations using the finite element method were performed to investigate the impact of different test methods, i.e. comparing beams and panels. A literature study was performed to investigate how the design philosophies of rock support affect the required dosages of fibres and the potential problems regarding the durability of each fibre type.

The test campaign revealed that all fibre types can achieve commonly used requirements regarding structural capacity. However, steel and basalt fibres seem to be more competitive if high residual strength is required at low deformations, while synthetic fibres are more competitive if large deformations are allowed. Thus, the design philosophy affects the potential of using certain fibre types. The results from the numerical simulations highlight that tests on panels could reduce the required dosage of fibres. This results from the longer fracture planes in the panels, which reduce the impact of local variations in the amount and orientation of fibres. For steel and basalt fibres, the shotcrete mix is important regarding durability. For steel, problems with corrosion are limited to locations of cracks since steel is initially protected from corrosion due to the alkaline environment in the shotcrete. Basalt fibres corrode in an alkaline environment and are therefore protected by a coating. Synthetic fibres seem to be chemically resistant to the environment in the shotcrete and tunnel. In conclusion, this report has identified that fibres made of basalt and synthetic can potentially be used in Swedish tunnels but also highlights some important topics for future studies.

Fiberarmerad sprutbetong är en viktig del av bergförstärkningen för tunnlar i hårt berg. I Sverige har stålfiber uteslutande använts som armering i sprutbetong sedan 1980-talet. Idag finns flera alternativ tillgängliga på marknaden, t.ex. basalt och syntetiska fiber. Möjligtvis kan dessa alternative fiber leda till en minskad klimatpåverkan eller ett minskat behov av reparation och underhåll av sprutbetongen. Därför fokuserar denna rapport på att utvärdera prestandan för sprutbetong armerad med stål, basalt och syntetiska fiber. För att utvärdera bärförmåga och sprutbarheten genomförde en stor experimentell studie som innehöll gjutna och sprutade balkar samt plattor armerade med olika fiber typer och doseringar. Numeriska simuleringar med finita element metoden genomfördes för att undersöka hur provmetoden, balkar och plattor, påverkade resultaten. En litteraturstudie genomfördes även för att undersöka hur design filosofin av bergförstärkningar påverkar rekommenderade doseringar av fiber samt potentiella problem med nedbrytning för de olika fibertyperna.

Resultaten från experimenten visade att alla fibertyperna can uppnå de vanligt använda kraven på bärförmåga. Däremot är fiber av stål och basalt vanligen mer konkurrenskraftiga när en hög residualbärförmåga krävs vid små deformationer medans syntetiska fiber är mer konkurrenskraftiga när stora deformationer tillåts. Strategin och metoden för att dimensionera sprutbetong påverkar därmed vilken fibertyp som kan vara lämplig. Resultat från numeriska simuleringar visar att fiberdoseringen kan minska om prover genomförs på plattor istället för balkar. Detta beror på att plattan har längre brottytor vilket minskar lokala effekter av fibermängd och orientating i sprickplanen. Sprutbetongens sammansättning är viktig för nedbrytningen av stål och basaltfiber. Stålfiber är skyddade mot korrosion av den alkaliska miljön och problematiken med korrosion är begränsad till sprucken sprutbetong. Basaltfiber korroderar dock i en alkalisk miljö och är skyddade mot detta genom en hinna runt fiberna. Syntetiska fiber tycks vara kemiskt stabila i en betong- och tunnelmiljö. Sammanfattningsvis har denna rapport visat att basalt och syntetiska fiber möjligen kan användas i svenska tunnlar men samtidigt lyfts några viktiga frågor för fortsatt forskning fram.

I Sverige har fiberarmerad sprutbetong sedan början av 80-talet varit den dominerande bergförstärkningsmetoden. Under alla dessa år har stålfiber nästan uteslutande använts i all sprutbetong i tunnelmiljö. Nu finns dock flera alternativa fiber tillgängliga på marknaden och med det ständigt pågående arbetet att minska klimatutsläppen från byggsektorn är frågan om alternativa fiber av syntet och basalt kan användas och reducera klimatpåverkan från tunnelbyggandet.

The core of dam safety is to safely store and discharge water and thereby prevent uncontrolled release of water. Excess water, e.g. in a flooding situation, is commonly discharged via different types of spillways (gated, weirs, surface, bottom outlets etc.). The turbine discharge is typically not included in the capacity as flooding scenarios could cause a grid blackout rendering the turbine incapable of operating. This paper describes a project that has tested island operation of a hydropower unit equipped with a developed dump-load device. The dump-load device allows the unit operating even during a black-out without risk for failure of the unit and thus assure the discharge capacity via the waterways connected to the unit. Stable operation of the dump-load verified in the tests together with high reliability opens for the turbine discharge to be included inthe total discharge capacity of a facility. A cost-benefit analysis of the suggested solution and a brief description of the past and planned development of the device is given. In addition to the extra discharge capacity, the new system can also be used to reduce the risk for surge during start and stop sequences. Implementation of the system in two different facilities, with different purpose, is planned and details on the coming installations and the technical solutions around them is given. A cost comparison between conventional spillway gates and dump-load indicates a factor of ten less for dump-load for a given increase in discharge capacity. The safety analyses shows that the turbine discharge should be included in the total discharge capacity of a facility if a dump load device is present.

Dam safety measures on the Vattenfall hydropower facility Lilla Edet, in Sweden are, ongoing by replacing parts of the old spillway dam from the 1920s to increase discharge capacity. A climate enhanced concrete without needs for any post-cooling to avoid thermal cracking has been developed for the project. A series of tests from initial laboratory tests via development of a production mix and a full-scale mock-up preceded the construction works on site. Increase of the maximum aggregate sizes from today’s commonly used 27 mm up to 45 mm was introduced to reduce the cement content. Basic tests to verify fresh properties, air void content, strength development, shrinkage etc. was done for a series of different supplementary cementitious materials before selecting fly ash from combustion of pulverized hard coal (PFA) as cement replacement. Examination of the durability aspects like freeze-thaw and carbonation resistance by accelerated test showed no or limited impact on the properties. Development and optimization of the production mix, with use of locally available materials, was done in cooperation with the concrete supplier. The mock-up tests included two large size beams, one with reference concrete and one with the developed concept concrete. Representative reinforcement (dimensions and spacing) was installed, a full-scale concrete pump used and workers from the project executed the casting. The outcome was successful without blockage during pumping, no honeycombing and happy workers. By using installed gauges and optic fibers, early age behavior in terms of temperature and strain development was measured to be able to estimate the risk for thermal cracking. The concept concrete showed to give a substantially reduced riskfor early age thermal cracks in comparison to the reference concrete. With support from the investigations, a decision to deviate from accepted levels of cement replacement in standards was taken. This was possible since the concrete concept proved to fulfill requirements on expected service-life and robustness during construction without cost increase. By using fly ash from combustion of pulverized hard coal as cement replacement, a reduction of the CO2-emissions by almost 30% has been possible and the need for post-cooling has been eliminated completely.

Large volumes of steel fibre reinforced shotcrete (sprayed concrete) and steel bolts are commonly used to support tunnels in hard rock. This generates a high CO2-footprint which must be reduced in order to decelerate the continuously increasing average temperature worldwide caused by the emissions of greenhouse gases. Thus, alternative design methods and the possibility to use other materials than steel are currently investigated. Work is ongoing on testing the load-bearing capacity of shotcrete reinforced with fibres of steel, basalt and synthetic materials. This also includes a comparison between tests using Round Determinate Panels (RDP) and four-point bending of beams. Moreover, the practical use of RDP testing as a quality control methodology is also investigated in situ. Here, the goal is to identify several shotcrete mixes suitable for use in tunnelling so that the right material and fibre volume can be used in the right place.

Large volumes of steel fibre reinforced shotcrete (sprayed concrete) and steel bolts are commonly used to support tunnels in hard rock. This generates a high CO2-footprint which must be reduced in order to decelerate the continuously increasing average temperature worldwide caused by the emissions of greenhouse gases. Thus, alternative design methods and the possibility to use other materials than steel are currently investigated. Work is ongoing on testing the load-bearing capacity of shotcrete reinforced with fibres of steel, basalt and synthetic materials. This also includes a comparison between tests using Round Determinate Panels (RDP) and four-point bending of beams. Moreover, the practical use of RDP testing as a quality control methodology is also investigated in situ. Here, the goal is to identify several shotcrete mixes suitable for use in tunnelling so that the right material and fibre volume can be used in the right place.