Hydro power facilities in Sweden are in general exposed to large temperature variations betweensummer and winter, high moisture levels and high hydrostatic pressure on water retaining structures.In concrete structures with insufficient properties these factors co-operate and can result indegradation by e.g., frost action, erosion, leaching and stresses that can aggravate leakage of waterand further degradation or cracking of the concrete material. In concrete dams and hydro power plants different kind of joints are installed. Cold joints anddilatation joints are two common types. Dilatation joints must be designed and constructed sothey are both water retaining and flexible enough to follow thermal movements of the concretewithout rupturing. A joint that is not well constructed with a good performance can jeopardize thedam safety for a hydro power facility with uncontrolled leakage, ice-formation, degradation orproblematic erosion on the down-stream side. A laboratory study of concrete from an dilatation joint has been performed in this project. Thestudied materials are taken from a larger concrete slab of a spillway sill of a concrete dam. Thescope of the project has been to study the characteristics of the concrete around the water stop anddilatation joint. The goal was to learn more about the mechanisms behind the degradation andleaking of expansion joints.

Shotcrete (sprayed concrete) differs from ordinary cast concrete through the application technique and the addition of set accelerators that promote immediate stiffening. The bond strength development between shotcrete and rock is an important property that depends on the texture of the rock, the type of accelerator, and application technique. This investigation focuses on the development of the microstructure in the interfacial transition zone (ITZ) and the strength of the bond at the shotcrete-hard rock boundary. The results show that the bond strength is related to the hydration process—that is, the strength gain of the shotcrete—and remains low before the acceleration period of the cement hydration. With a scanning electron microscope (SEM), it is possible to observe changes over time for the early development of the interfacial zone, both before and after proper cement hydration. Results from tests with wet-sprayed concrete on granite rock are presented. The test method—using both bond strength and the SEM to investigate the development of the microstructure at the ITZ—is interesting, but has to be more broadly examined. Different mixtures, accelerators, and rock types have to be used.

Shotcrete (sprayed concrete) differs from ordinary cast concrete through the application technique and the addition of set accelerators that promote immediate stiffening. The bond strength development between shotcrete and rock is an important property that depends on the texture of the rock, the type of accelerator, and application technique. This investigation focuses on the development of the microstructure in the interfacial transition zone (ITZ) and the strength of the bond at the shotcrete-hard rock boundary. The results show that the bond strength is related to the hydration process—that is, the strength gain of the shotcrete—and remains low before the acceleration period of the cement hydration. With a scanning electron microscope (SEM), it is possible to observe changes over time for the early development of the interfacial zone, both before and after proper cement hydration. Results from tests with wet-sprayed concrete on granite rock are presented. The test method—using both bond strength and the SEM to investigate the development of the microstructure at the ITZ—is interesting, but has to be more broadly examined. Different mixtures, accelerators, and rock types have to be used.

In Scandinavian traffic tunnels soft drains covered with shotcrete are often installed to lead away un-wanted water, giving little resistance to shotcrete shrinkage, which may cause severe cracking. Mapping of shrinkage cracks was done in situ, followed by analyses focused on stresses due to drying shrinkage and various time of waiting between turns of spraying, with or without water curing. The effect of dilatation joints has also been investigated. A recently developed laboratory test set-up with shotcrete on instrumented granite slabs represent shrinkage of shotcrete on soft drains. The test results indicate that addition of glass fibres could reduce the cracking problem.

This study investigates shrinkage of accelerated shotcrete (sprayed concrete), especially in the case of shotcrete sprayed on drains, a part of a tunnel lining not continuously bonded to the rock. One of the goals is to find methods of avoiding shotcrete shrinkage cracks in such drain structures. If cracks yet develop the crack distribution is of great importance, i.e. several fine cracks instead of one wide. By using both steel and glass fibres this may be achieved. A newly developed test set-up for shrinking, end-restrained shotcrete slabs is also presented and evaluated. The performed tests show that the addition of very fine glass fibres could be a solution to the cracking problem. The newly developed test equipment using concrete interacting with an instrumented granite slab represents a realistic way of testing restrained shrinkage. The on-going research focuses on the optimization of the glass fibre addition and the understanding of the interaction between shrinkage and creep of shotcrete.

This study investigates early age bond strength of shotcrete (sprayed concrete), in the case of shotcrete sprayed on hard rock. Shotcrete differs from ordinary, cast concrete through the application technique and the addition of set accelerators which give immediate stiffening. The bond between shotcrete and rock is one of the most important properties. During the very first time after spraying the physical properties and the bond to the rock depend on the set accelerator and the micro structure that is formed. In this work a laboratory test method for measuring early bond strength for very young or early age shotcrete is presented. The newly developed method was tested and evaluated and proved that it can be used for bond strength testing already from a couple of hours after shotcreting.