Publikationer vid avdelningen för Betongbyggnad

Senast publicerade artiklar från avdelningen för Betongbyggnad

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. In this paper, 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. 

On failure probability in thin irregular shotcrete shells

Tunnels through hard jointed rock are commonly reinforced with a combination of fibre reinforced shotcrete (sprayed concrete), FRS, and rock bolts. The design of such reinforcement is a complex task. First, the interaction between rock bolts, FRS and rock should be considered. Secondly, a natural variation in important parameters such as thickness of the shotcrete, fracture energy, and bond strength between shotcrete and rock exists. In this paper, a numerical framework for non-linear analyses of FRS suitable for Monte Carlo simulations is presented. As a case study, a 2D FE-model of a bolted shotcrete lining subjected to load from a pushing block was used to perform a sensitivity analysis for the variation in thickness. Results indicate that an irregular shotcrete thickness highly affects the failure load but has a smaller impact on ductility.

Safety levels in concrete slabs-on-grade

Concrete pavements and industrial concrete floors are two examples on slabs-on-grade. None of them is considered as a load-carrying structure and is therefore not designed according to codes for structural concrete. These codes are based on probabilistic concepts and prescribed values of probability of failure. Concrete pavements and industrial concrete floors are designed differently but has in common that neither safety level nor probability of failure, severe cracking or other malfunction are included in the design. The safety levels in concrete pavements and industrial concrete floors designed according to Swedish practice are discussed in this Paper. Proposals for improvements and further research are given.

Numerical simulations of a concrete bridge deck loaded to shear failure

This paper presents numerical simulations of the shear failure of a bridge slab previously tested in full scale on an existing bridge. Using the non-linear finite element method, a model of the bridge is assembled with the purpose to simulate the test procedure and realistically capture the failure load and behaviour. This in order to conclude what type of shear failure that occurred. Furthermore, the shear capacity of the bridge is calculated according to current design codes. A parametric study is conducted on the FE model with the aim to study the influence of key variables on the outcome of the analyses. From the studied parameters, it is observed that a combined reduction of the tensile strength and fracture energy, together with a low fixed crack coefficient has the largest influence. It is also observed that the location of the failure and the ultimate load is dependent on how the loading was applied to the model, i.e. via load control or deformation control. In the final FE analysis, the model fails at a load which slightly exceeds the experimental ultimate load. The mode of failure obtained in all the analyses are the result of a large shear crack propagating from the edges of the loading plate, through the slab to the slab/girder-intersection. This indicates that the type of failure that occurred in the full scale test was primarily due to a one-way shear mechanism with a secondary punching effect. The design values calculated with current codes results in very conservative values when compared to the obtained failure load from the experiment.

Influence of varying ambient conditions on time-dependent deformations inconcrete using multi-field modelling

Time-dependent deformations, such as creep and shrinkage, are important when dealing with durability aspects of concrete. In the current study, a multi-field analysis method is described, verified and used in a numerical study to investigate the influence of short and long term variations in temperature and relative humidity. It is found that especially the creep behaviour is significantly influenced by the seasonal variations in climate conditions and also to a lesser extent the daily variations.

Thermally-induced cracking of a concrete arch dam using COMSOL Multiphysics

The design and maintenance of concrete dams in cold regions is a challenging task, in large due to the temperature difference between summer and winter. In order to enhance the knowledge of this, theme A of the 14th International Benchmark Workshop on Numerical Analysis of Dams is dedicated to the prediction of the extent of cracking in a concrete arch dam due to temperature variations. The current study proposes a solution to this using the finite element software COMSOL Multiphysics. A global model is set up to analyze the transient temperature variations as well as the displacements given the assumption of a linear material behavior. To predict the extent of cracking, a rate-dependent isotropic damage model is implemented as an extension of the built-in functionality of COMSOL Multiphysics. Furthermore, a submodel is created to allow for a higher mesh resolution in the non-linear analysis. The results indicate that the considered arch dam suffers a large risk of cracking due to temperature variations, especially on the downstream side. Most cracks propagate during the winter, although some cracks appear already when static loads are applied.

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