Reference: Martys, N.S. and Ferraris, C.F., First
North American Conference on the Design and Use of Self-Consolidating Concrete.
Proceedings. Chicago, IL, November 12-13, 2002, pp. 27-30, 2003.
(PDF Version of Original Paper)
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Nicos Martys, Chiara F. Ferraris
National Institute of Standard and Technology, Gaithersburg MD 20899
In principle, self-consolidating concrete (SCC) should allow for the easy movement of concrete around flow obstructions under its own weight without the use of external vibration. The flow of concrete around barriers will depend on a variety of factors, including the concrete or mortar viscosity, the yield stress and the size distribution and shape of the coarse aggregate. Modeling the flow of complex fluids like concrete presents a great research challenge because of the necessity of accounting for the polydisperse motion of the aggregate, while, simultaneously solving the Navier-Stokes equations for the liquid phase in which they are immersed. Numerous schemes [1] have been developed using many standard computational approaches for solving the Navier-Stokes equations with moving rigid bodies but, in general, they are rather complicated and demand very large computational resources. Recently some novel approaches [2], based on cellular automata methods (lattice Boltzmann and dissipative particle dynamics (DPD)) have shown great promise for modeling a variety of complex flow problems. DPD has been shown to be particularly well suited for modeling complex systems like suspensions. This paper will give some highlights of the method and some examples of applications to SCC technology.
Fresh concrete is a complex fluid that necessitates the understanding of phenomena on many length scales. On one scale, one can think of concrete as a suspension composed of a fluid phase (mortar) and a solid phase (coarse aggregate). Similarly, for mortar the fluid phase is the cement paste and the sand (the aggregates) is the solid phase [3, 4]. Also, in cement paste, the cement could be considered the particle and the water is the matrix. When modeling suspensions using DPD at any scale, the fluid phase is represented by particles that correspond to mesoscopic regions of fluid. Interactions between the particles are incorporated so that their motion is consistent with the equations of hydrodynamics (Navier-Stokes equation and continuity equation). To model the motion of the solid bodies, a subset of the DPD fluid particles are "frozen" [1] together to present individual aggregates. Once the forces on each rigid body are determined, the particles move according to the Euler equations [5]. Details of modeling suspensions with the DPD method are available in Ref. [6, 7, 8].
Of course, this model needs to be validated before it can be used to predict concrete flow. At first, we chose some simple cases where results were clearly known from the literature or available data. The following cases were selected, each of which revealed excellent agreement between the DPD prediction and the experimental results:
Although the validation of computational approaches can be a challenge because of the dearth of known analytical solutions for complex flow problems, it is believed that, based on these four comparisons, DPD produces reasonable results and can be used as a predictive tool when used carefully. Further, an advantage of DPD is that it naturally accommodates flow in many complex geometries like flow around rebars or various rheometers. Some examples of concrete flowing in complex geometries will be now shown.
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