Abstract
The present thesis deals with numerical modelling of form filling with Self-Compacting Concrete (SCC). SCC differs from conventional concrete by its increased fluidity, which enables it to fill out the form work without any vibration. The benefits of casting with SCC as compared to conventional concrete may be a decreased construction time and a better working environment if the SCC is managed properly. However, also obstacles may arise from casting with SCC such as issues related to robustness, form work pressure, static segregation and flow induced aggregate migration, thus numerical modelling of form filling with SCC includes a lot of topics. In this thesis it is chosen to focus on the following three topics by the usage of Finite Difference Method (FDM) / Finite Volume Method (FVM) based Computational Flyid Dynamics (CFD) models developed in both FLOW-3D and MATLAB.
The first investigation focussed on the complications involved with modelling a yield stress fluid with a bi-viscosity material model, which is a typical material model used when capturing the non-Newtonian flow behaviour of SCC. The study was carried out by comparing the numerical result and the yield stress based analytical solution of the LCPC-box test. The comparison showed that a relatively good agreement was obtained for both the FLOW-3D and MATLAB model. In addition, the study identified that the agreement improved when the initial viscosity was increased, thus it was impossible for the applied numerical models to be in full agreement with the analytical solution. Based on the investigation it was also found that the LCPC-box test is a highly recommended test to carry out in order to get a better understanding of the numerical settings' implication for a given CFD solver.
Following this, two numerical approaches were developed to investigate their capability of predicting gravity induced aggregate migration in SCC castings. The two FDM/FVM based CFD models differentiated from each other by their aggregate representation, which was a discrete approach (one way momentum coupling) for one of them and a scalar approach for the other. It was found that it was less complicated to implement criteria for the model with the scalar aggregate representation. Subsequently, experimental results from an SCC-like model fluid casting and a real SCC casting were compared with numerical results from the model with the scalar aggregate representation and showed a good agreement. In the case of the SCC though, it was found out that a coupling back from the aggregates to the rheological parameters of the SCC was needed. The study showed also an obstacle for the scalar approach which was the need of a parameter dictating the viscosity of the surrounding fluid in which the aggregate settled. The parameter did not seem to change when changing the casting velocity, but only a future study will show how it changes with different mix compositions of the SCC and thereby finally judge the potential of numerically predicting gravity induced aggregate migration with this scalar approach.
Finally, a single objective genetic algorithm was coupled to the numerical model with the scalar aggregate representation in order to investigate its applicability. Two studies were carried out with the objective to obtain a homogeneous aggregate distribution in a beam SCC casting. The primary difference between the two studies was the implementation of constraints that enabled more realistic and usable casting scenarios to be found. In both studies non-trivial casting scenarios were obtained, which indicated that the coupling between a numerical model capable of predicting gravity induced aggregate migration and an optimization algorithm can be a useful tool. An obstacle for the numerical model used in this study is the calculation time. In the case of evaluating a large vertical casting it was found that the simulation most likely would be too time consuming to finish the optimization study in a reasonable time, but that an algorithm which splits the pressure and velocity calculation may give the necessary calculation speed up.
The first investigation focussed on the complications involved with modelling a yield stress fluid with a bi-viscosity material model, which is a typical material model used when capturing the non-Newtonian flow behaviour of SCC. The study was carried out by comparing the numerical result and the yield stress based analytical solution of the LCPC-box test. The comparison showed that a relatively good agreement was obtained for both the FLOW-3D and MATLAB model. In addition, the study identified that the agreement improved when the initial viscosity was increased, thus it was impossible for the applied numerical models to be in full agreement with the analytical solution. Based on the investigation it was also found that the LCPC-box test is a highly recommended test to carry out in order to get a better understanding of the numerical settings' implication for a given CFD solver.
Following this, two numerical approaches were developed to investigate their capability of predicting gravity induced aggregate migration in SCC castings. The two FDM/FVM based CFD models differentiated from each other by their aggregate representation, which was a discrete approach (one way momentum coupling) for one of them and a scalar approach for the other. It was found that it was less complicated to implement criteria for the model with the scalar aggregate representation. Subsequently, experimental results from an SCC-like model fluid casting and a real SCC casting were compared with numerical results from the model with the scalar aggregate representation and showed a good agreement. In the case of the SCC though, it was found out that a coupling back from the aggregates to the rheological parameters of the SCC was needed. The study showed also an obstacle for the scalar approach which was the need of a parameter dictating the viscosity of the surrounding fluid in which the aggregate settled. The parameter did not seem to change when changing the casting velocity, but only a future study will show how it changes with different mix compositions of the SCC and thereby finally judge the potential of numerically predicting gravity induced aggregate migration with this scalar approach.
Finally, a single objective genetic algorithm was coupled to the numerical model with the scalar aggregate representation in order to investigate its applicability. Two studies were carried out with the objective to obtain a homogeneous aggregate distribution in a beam SCC casting. The primary difference between the two studies was the implementation of constraints that enabled more realistic and usable casting scenarios to be found. In both studies non-trivial casting scenarios were obtained, which indicated that the coupling between a numerical model capable of predicting gravity induced aggregate migration and an optimization algorithm can be a useful tool. An obstacle for the numerical model used in this study is the calculation time. In the case of evaluating a large vertical casting it was found that the simulation most likely would be too time consuming to finish the optimization study in a reasonable time, but that an algorithm which splits the pressure and velocity calculation may give the necessary calculation speed up.
| Original language | Danish |
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| Place of Publication | Kgs. Lyngby |
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| Publisher | Technical University of Denmark |
| Number of pages | 198 |
| Publication status | Published - 2012 |