Shotcrete (sprayed concrete)
Investigation of non-linear drying shrinkage for end-restrained shotcrete of varying thickness
Tunnels in hard, jointed rock are commonly reinforced with shotcrete (sprayed concrete) applied directly on the irregular rock surface. The thickness for such linings can be as small as 50 mm, which result in a fast drying. The resulting shrinkage of the restrained lining is a well-known phenomenon, which leads to cracking. The installation of drainage systems also results in an end-restrained shotcrete lining that is more prone to shrinkage cracking. The drying process is a complex problem that depends on multiple factors such as cement content, porosity and ambient air conditions (i.e. temperature, relative humidity and wind speed). Two numerical models capable of capturing the structural effects of drying shrinkage were compared in this study. It was found that inclusion of non-linear drying shrinkage is important for accurately describing crack initiation in an end-restrained shotcrete slab. The best fit to the experimental data was obtained when the rate of drying was described as a non-linear decreasing function.
Numerical simulations of restrained shrinkage cracking in glass fibre reinforced shotcrete slabs
Modern tunnels in hard rock are usually constructed by drill and blast with the rock reinforced by shotcrete (sprayed concrete) in combination with rock bolts. The irregular rock surface and the projection method of shotcrete leads to a tunnel lining of varying thickness with unevenly distributed stresses that affect the risk of cracking during shrinkage of the young and hardening material. Depending on water conditions, shotcrete is either sprayed directly onto the rock surface or over a drainage system, creating a fully restrained or an end-restrained structural system. A method for non-linear numerical simulations has been demonstrated, for the study of differences in stress build up and cracking behaviour of restrained shotcrete slabs subjected to shrinkage. Special focus was given to the effects of the irregular shape and varying thickness of the shotcrete. The effects of glass fibre reinforcement and bond were implemented in the study by changing the fracture energy in bending and in the interaction between shotcrete and the substrate. The study verifies that an end-restrained shotcrete slab is prone to shrinkage induced cracking, and shows the importance of a continuous bond to avoid wide shrinkage cracks when shotcrete is sprayed directly onto the rock.
Laboratory testing of early age bond strength of shotcrete on hard rock
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.
Contact: Anders Ansell
Structural dynamic and stress wave models for analysis of shotcrete on rock exposed to blasting
During blasting in tunnels and mines, the interaction between shotcrete (sprayed concrete) and rock is influenced by propagating stress waves. Shotcrete support in hard rock tunnels is studied through numerical analysis using three different modelling approaches. The stress response in the shotcrete closest to the rock when exposed to P-waves striking perpendicularly to the shotcrete–rock interface is simulated. The first model tested is a structural dynamic model that consists of masses and spring elements. The second is a model built up with finite element beam elements interconnected with springs. The third is a one-dimensional elastic stress wave model. The models give comparable results, although the definition of the dynamic loads is different. The analysis results can be used to estimate whether the shotcrete will fail or not for a prescribed distance to detonating explosives inside the rock.
Investigation of shrinkage cracking in shotcrete on tunnel drains
The investigation combines in situ observations, measurements, testing and theoretical modelling. The in situ work was done to map and evaluate the shrinkage related cracking of shotcrete on short and long sections of soft, plastic drains. The occurrence of variation in shotcrete thickness and crack widths were of particular interest. The theoretical analysis focuses on the stresses that can occur due to uneven drying shrinkage in the two-layered shotcrete. The models used include variation in shotcrete thickness and in time of waiting between turns of spraying, with or without watering of the shotcrete. Watering will delay the shrinkage but has no effect on the strength development. Long times of waiting without watering before spraying a second layer will increase the tensile stresses in the shotcrete. It is recommended that the further work is directed towards establishing guidelines for the design of future drain constructions with shotcrete. Different methods for repair and strengthening of cracked shotcreted drains must be developed, tested and evaluated.
Contact: Anders Ansell