Publikationer vid avdelningen för Betongbyggnad

Senast publicerade artiklar från avdelningen för Betongbyggnad

Influence of air voids in multiphase modelling for service life prediction of partially saturated concrete

The purpose of this study is to show the influence and significance of including water filling of air pores when studying moisture conditions in concrete structures cast with air-entrained concrete and in contact with free water. Especially if the aim is to assess the risk for frost damages in different regions of the structure, based on a critical degree of saturation, in order to ultimately perform a service life prediction. A hygro-thermo-mechanical multiphase model that includes the effect of water filling in air pores, recently presented by the authors, is briefly described and applied in two numerical examples. The results show moisture distributions that would not be possible to capture without the air pore filling included in the model. More importantly, the general shape of these distributions complies well with measured distributions in real concrete structures as well as with distributions obtained in laboratory measurements.

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.

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.

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