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How Cement Mixers Work

In addition to the mixing energy applied to the fresh concrete (i.e. shearing during mixing), the shear history after mixing is also important. This applies especially to binder rich concretes like the different types of high performance concrete (HPC). With this in mind, the shear rate is analyzed inside a drum of a concrete tank truck. The objective is to better understand the effect of transport of fresh concrete, from the ready mix plant to the building site. The analysis reveals the effect of different drum charge volume and drum rotational speed. Also, the effect of yield stress and plastic viscosity is investigated. The work shows that the shear rate decreases in an exponential manner with increasing drum charge volume. It is also shown that for a given drum speed, the shear rate decreases both with increasing plastic viscosity and yield stress.

Since civilizations first started to build, the human race has sought materials that bind stones into solid formed mass. After the discovery of Portland cement in 1824 (year of patent), concrete has become the most commonly used structural material in modern civilizations. The quality of the concrete structure is of course dependent on the quality of each constituent used in the concrete mix. However, this is not the only controlling factor. The quality also depends very much on the rheological properties of the fresh concrete during placement into the formwork [1]. That is, the concrete must be able to properly flow into all corners of the mold or formwork to fill it completely, with or without external consolidation depending on workability class. Tragic events may sometimes be traced back to concrete of unsuitable consistency resulting in, for example, coldjoint and honeycombing. Therefore, one of the primary criteria for a good concrete structure is that the fresh concrete exhibits satisfactory rheological properties during casting [1]. The use of simulation of flow to analyze such behavior is something that has been increasing in popularity for the last decade [2], [3], [4], [5], [6], [7], [8], [9]. In 2014, a RILEM state-of-the art report (TC 222-SCF) was made specifically on this subject [10]. Here, such method is used to analyze the shear rate inside a concrete truck mixer for a wide range of cases. Previously in [11], such simulation was reported for the case of yield stress 50 Pa and plastic viscosity 50 Pa ⋅s, in which the aim was to verify a special truck mixer simulator.

In addition to the energy applied during mixing (i.e. shearing during mixing) [12], [13], [14], the shear history after mixing is also important [15], [16], [17]. This applies especially for binder rich concretes like the (rich) high performance concrete (HPC). This is due to the influence that the binder exerts on the concrete as a whole in terms of thixotropic- and structural breakdown behavior (these two terms are well explained in [18]). The rheological state of the binder depends heavily on the shear rate and especially on its history [15], [16], [17]. That is, in a highly agitated system (high shear rate), the cement particles will disperse, making the overall fresh concrete more flowable. While in a slowly agitated system, the cement particles will coagulate and thus thicken the overall fresh concrete.

The rheological properties of the fresh concrete depends on the proportions of each constituent as well as on their quality. However, as is apparent from the above paragraph, conditions like the shear rate during transport can play a major role on final workability. That is, a concrete batch with seemingly target rheological behavior at the ready mix plant can become unsuitable at the building site due to thixotropic thickening, caused by insufficient agitation during transport (i.e. low shear rate). The decrease in the slump during transport in truck mixer can be up to 90 mm, which corresponds to a deviation of one and a half consistency class [11]. Such could lead to the refusal of acceptance, or in the case of acceptance, make successful casting in awkward sections or through congested reinforcement difficult, resulting for example in honeycombing [1], [11].

In this work, the shear rate is analyzed inside the drum of a concrete fuel tank truck. This is done to better understand the potential effect of transport, from the ready mix plant to the building site, in terms of the concrete final rheological state. From Section 1.2, a higher shear rate will imply increased dispersion of the cement particles and thus more flowable concrete during the casting phase. Likewise, a lower shear rate will imply insufficient agitation, increased thixotropic rebuild and thus stiffer concrete during casting.

Because the shear rate within the drum is highly non-uniform and time dependent, meaning , a two step integration is most necessary to generate quantifiable values for analysis and comparison, which is shown later. The final outcome is given by  and is simply referred to as “shear rate”. Here, this shear rate is analyzed as a function of drum rotational speed f = 0.03, 0.07, 0.11, 015, 019 and 0.23 rps (revolutions per second) and drum charge volume V = 2.6 m3, 5.4 m3 and 8.2 m3. In addition to this, the effect of yield stress τ0 = 0, 150 and 300 Pa and plastic viscosity μ = 25, 75 and 125 Pa ⋅s, is analyzed.

The simulation software used in this work is the OpenFOAM. It is licensed under the GNU General Public License (GNU GPL) and available at http://openfoam.org, without charge or annual fee of any kind. The benefits of using a GNU GPL licensed code rather than a closed commercial code, is that the user has always a full access to the source code, without any restriction, either to understand, correct, modify or enhance the software. Here, this is a highly desirable feature since a custom made solver is used for the current analysis. The software OpenFOAM is written in C++. As such, an object-oriented programming approach is used in the creation of data types (fields) that closely mimics those of mathematical field theory [19]. For the code parallelization and communication between processors, the domain decomposition method is used with the Message Passing Interface, or MPI [20]. In OpenFOAM, the collocated mesh system (in Cartesian coordinates) is applied in conjunction with the finite volume method (FVM).

The mesh in Fig. 1 is generated with a native OpenFOAM mesh utility called blockMesh. To investigate the mesh dependency of the numerical result, two different mesh densities (or mesh resolutions) are used, namely 58,888 and 372,568 cells, which are shown in the left and right illustrations of Fig. 1, respectively. For the former case, 88% of the cells are hexahedra, while it is 99% for the latter case. In either case, the remaining cells consist of prisms, tetrahedra and polyhedra. In the end of the mesh generation, its quality is checked with another native OpenFOAM utility, named checkMesh.

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