# Investigations of Mixing Technique on the Rheological Properties of Self-Compacting Concrete
## 1 Introduction
The difficulties linked to homogeneously mixing SCC can be attributed to the peculiar mixture characteristics of \"three high and one low\" (high powder dosage, high admixture dosage, high superplasticizer dosage, and low water-powder ratio), as well as to the rheological characteristics of high fluidity. Compared with common concrete, SCC requires a longer mixing time to achieve homogeneity due to its high paste volume and low water-cement ratio. The time period required to achieve a uniform dispersion of a granular substance into a mix is associated with the particle size of the substance. Repartition of coarse aggregates is much faster than that of sand and fine cement particles [16]. The power consumption curve is often used as the index of concrete homogeneity. When the power curve attains a relatively stable state, the concrete reaches homogeneous state. The larger the water-powder ratio, the shorter the power consumption curve. The time period to reach homogeneity is also influenced by the content of fine powders and the amount of superplasticizer (SP). [PERSON] et al. [17] found that the cement paste wrapped aggregate method can significantly improve the fluidity of SCC. [PERSON] [18] and [PERSON] et al. [19] stated that the SP subsequently added method can reduce the mixing time but is not favorable for the dispersion of SP. [PERSON] et al. [20] and [PERSON] et al. [21] reported that the SP subsequently added method can reduce the adsorption of SP molecules on the surface of cement particles, thereby improving the concrete workability. Such reduction is considered to be associated with the type of SP, as claimed by [PERSON] et al. [22]. Naphthalene water reducer, relying on its electrostatic potential to resist congregation, has a great capability in flow ability. Polycarboxylate SP shows steric hindrance, readily leading to a dispersion between fine particles. This advantage has been confirmed previously by workers like [PERSON] [23] and [PERSON] et al. [24]. According to the studies by [PERSON] et al. [25], the water-powder ratio has an impact on the dispersion phenomenon as observed in the SP subsequently added method, and the slump-flow properties of the mixtures become increasingly sensitive with a higher water-powder ratio. [PERSON] [18] observed a higher fluidity, caused by a longer mixing time (3-7 min), for ultra-high-performance concrete (UHPC). Nevertheless, over-mixing may wear off fine aggregates, thus lowering down the workability of the mixtures, a finding reported by [PERSON] et al. [26]. Investigations from [PERSON] et al. [27] and [PERSON] et al. [28] indicated that a properly higher mixing speed could result in a shorter mixing time and meanwhile a larger mixing efficiency. On the other hand, adverse effects on the workability and rheological properties of concrete due to high-speed mixing have also been found. The main reason can be related to the fact that high-speed mixing may wear off aggregates and subsequently reduce the dispersion degree for both cement particles and SP [29; 30]. Traditionally, infrastructures were built up by emphasizing design over construction. As such, the quality problems caused by poor workability of concrete are nowadays common. Self-compacting concrete (SCC), whose rheological properties are highly influential on the performances of cast-in-situ concrete structures, remains limited in practical applications. The combination type SCC, which possesses remarkable rheological properties and high robustness, has been utilized on a large scale in the CRTS III slab ballastless track of China's high-speed railway. Plenty of publications have been reported on the influencing factors on rheological properties of concrete. [PERSON] [31] evaluated the change of yield stress and viscosity as a function of the mix proportion of concrete. Incorporations of SP, which are crucial for self-compaction, to causing loss of yield stress have been intensively reported [31; 32]. Admixtures can be included to increase the viscosity and further to keep the stability of SCC [33].