The Rheological Properties of Concrete

The application of plasticizers leads to the complete dispersion of cement grains in water, without conglomerates formation. It is assumed that the molecules of superplasticizer are adsorbed on the surface of cement grains which is neutralizing the 

12 assumed ratio of water in the free space, not belonging to the snrface layer. 

Several methods were applied in the studies of rheological properties of paste those based on the acoustic wave velocity, as well as on the measurements of resistance against the force in of penetrator (similar to Vicat apparatus for setting time measurements), also based on the pressure transfer in the paste and different rotation methods with spindle and finally viscometers with co-axial cylinders [5]. 

The majority of measurements have been done in dynamical conditions, with the use of co-axial cylinders. However, this method does not give the possibility of precise measurement of the yield stress threshold value. This will be discussed later in this chapter. 

This studies have shown that the rheological parameters are affected by the type of measuring device, composition of the paste and rheological history of the sample. The pastes can be attributed to the Newtonian or non-Newtonian types of liquid between the later the plastic (Bingham), pseudo-plastic or exhibiting the dilatancy phenomena can be differentiated. Moreover, the pastes reveal basically the thixotropy or anti-thixotropy properties. 

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The Rheological Properties of Concrete Table 5.1 Interparticle forces. [21]... 

The rheological properties of concrete mixture, which control the workability, are affected by Cj A content in clinker. The content of gypsum and its rate of dissolution is also very important this problem is discussed in Sect. 4.1.2. 

It is known that some of the properties of fresh concrete can be considered in terms of the rheological properties of the cement paste contained in the concrete. Thus a high water-cement ratio concrete will contain a paste content which is more fluid than that of a low water-cement ratio concrete. 

There are some paste properties which influence the behaviour of fresh concrete. The workability of concrete mixture is the function of the rheological properties of paste which depend on the fine fraction content in cement. The yield stress value, being the measure of concrete mixture consistency at final stage of mixing, is increasing with the rise of cement specific surface area (Fig. 5.12). The initial setting time is significantly shorter in the case of this cement property. 

In a superplasticized cement a suitable suUbnated naphthalene-based superplasticizer is interground with the binder (Rosetti et al, 1992 Bouzoubaa et al, 1998). Under these conditions the rheological properties of the produced concrete are more uniform than in cases in which the superplasticizer is added to the concrete mix in the course of mixing. The added superplasticizer also acts as a grinding aid. 

A comprehensive overview of the properties of superabsorbent polymers, specific water absorption and desorption behavior of superabsorbent pol5uners in fresh and hardening concrete, the effects of the superabsorbent pol5uners addition on the rheological properties of fresh concrete, the changes of cement paste microstructure and mechanical properties of concrete have been reviewed (42). 

The workability of a concrete mix is by no means dependent only on the physical properties of the cement paste it contains, but an understanding of it requires one of those properties. For some specialist uses in which cement is used without an aggregate, the latter are directly relevant. The most important properties are concerned with rheology, and this section deals primarily with these properties in Portland cement pastes, free from admixtures, prior to setting. From the chemical standpoint, this period comprises that of initial reaction and induction period. From the practical standpoint, it includes those of mixing, placing and compaction. 

In another study using crumb rubber (size 0/0.7 mm and quantity 15% and 18%), it was also found that the characteristic and rheological properties of the bitumen improved. However, the storage stability requirement of the rubberised modified bitumen (RmB) was not satisfied. When wet and dry processes were compared on extracted RmB from an asphalt concrete mixture (AC 8 mm), it was found that the effect of crumb rubber was more profound when the wet process was used (Lukac and Valant 2011). 

Another type of rheometer with parallel plates was designed and tested by Kuder et al. (2007) to evaluate the rheological behaviour of the fibre-reinforced cement matrices that are too stiff for testing with ordinary rheometers as shown in Figure 12.3. It appeared from the tests of fibre-reinforced mortars that at lower fibre amounts the properties of the mix were decided by the matrix and the fibres only reduced its yield stress and viscosity. At higher fibre contents, the mechanical interlocking and entangling of fibres determined the mix behaviour. Similar influence of the fibres may be expected for concrete mixes also. 

The particle size and SSA of a cement powder have a direct influence on hydration kinetics. This is illustrated in Figure 10.1, as the particle size of alite (main phase of cement) decreases, and consequently SSA increases, a significant acceleration of the hydration is observed. In addition, these parameters have a direct impact on the rheological and mechanical properties of concrete and on the interaction between the cement and chemical admixtures such as superplasticisers. Both parameters are routinely measured during cement production for quality control purposes. 

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