Water cements

In these early reactions the reactivities of the individual phases are important in determining the overall reaction rate. However, as the cement particles become more densely coated with reaction products, diffusion of water and ions in solution becomes increasingly impeded. The reactions then become diffusion-controUed at some time depending on various factors such as temperature and water—cement ratio. After about 1 or 2 days, ie, at ca 40% of complete reaction, the remaining unhydrated cement phases react more nearly uniformly. 

Cathodic protection can be used to protect steel in concrete (see Chapter 19). There is no fear of damage by H2 evolution due to porosity of the mortar. Local corrosion attack can be observed under extreme conditions due to porosity (water/ cement ratio = 1) and polarization (f/jq = -0.98 V) with portland cement but not with blast furnace cement, corresponding to field IV in Fig. 2-2 [53]. However, such conditions do not occur in practice. 

Factors that affect cell formation are the type of cement, the water/cement ratio and the aeration of the concrete [6]. Figure 12-1 shows schematically the cell action and the variation of the pipe/soil potential where there is contact with a steel-concrete structure. The cell current density is determined by the large area of the cathode [see Fig. 2-6 and Eq. (2-44)]. In industrial installations the area of steel surface in concrete is usually greater than lO m ... 

The specific electrical resistance of concrete can be measured by the method described in Section 3.5. Its value depends on the water/cement value, the type of cement (blast furnace, portland cement), the cement content, additives (flue ash), additional materials (polymers), the moisture content, salt content (chloride), the temperature and the age of the concrete. Comparisons are only meaningful for the... 

A diagnosis of possible damage should be made before beginning repairs with other construction measures [48,49]. There should be a checklist [48] of the important corrosion parameters and the types of corrosion effects to be expected. Of special importance are investigations of the quality of the concrete (strength, type of cement, water/cement ratio, cement content), the depth of carbonization, concentration profile of chloride ions, moisture distribution, and the situation regarding cracks and displacements. The extent of corrosion attack is determined visually. Later the likelihood of corrosion can be assessed using the above data. 

It functions as a water-reducing plasticizer, producing a flooring composition with good workability at low water/cement ratios. 

Figure 9.3b Conductivity/time curves (a) of cements, showing maxima. Prepared from ZnO ignited at 600 C with A-.-A dry eugenol 0...0 eugenol + 1% water O-.-O eugenol + 1% chloracetic acid ... eugenol+1% acetic acid ... eugenol+1% acetic acid+1% water. Cement powder/liquid ratio = 2-5 g cm (Crisp, Ambersley Wilson, 1980).

Fig. 3.4.8 One-dimensional SPI drying profiles drying are indicated by the symbols ( ) and of concrete moist-cured for 28 days and of a (O), respectively. The measurement para-0.6 water-cement ratio [9]. The specimen was meters were field of view (FOV) 150 mm, sealed except for one face and exposed to a acquisition points 64, tp = (55 - 300 ps, 8 val-drying regime at 38 °C and 40% relative humi- ues), a = 6°, TR = 100 ms, acquisition time dity for 28 days. The spatial moisture content 3.5 min per encoding time, after 28 days of moist curing and 28 days of...

The following is a list of recommendations that builders can use to utilize the foundation as a mechanical barrier to radon entry. Foundation walls and floor slabs are often constructed of poured concrete. Plastic shrinkage, and therefore cracking, is a natural function of the drying process of concrete. Many factors, such as the water/cement/aggregate ratio, humidity, and temperature, influence the amount of cracking that occurs in a poured concrete foundation. Cracking may be minimized by... 

When portable barricades are needed, they can be moved into position rapidly. To provide them with added strength and stability, they are typically anchored to buttress boxes that are located on either side of the road. These buttress boxes, which may or may not be permanent, are usually filled with sand, water, cement, gravel, or concrete to make them heavy and aid in stabilizing the portable barrier. In addition, these buttresses can help dissipate any impact energy from vehicles crashing into the barrier itself. 

The water-reducing admixtures are the group of products which possess as their primary function the ability to produce concrete of a given workability, as measured by slump or compacting factor, at a lower water-cement ratio than that of a control concrete containing no admixture. 

By the addition of the admixture with a reduction in the water-cement ratio, a concrete having the same workability as the control concrete can be obtained, with unconfined compressive strengths at all ages which exceed those of the control. 

In order to understand more fully the effect that water-reducing admixtures have on the plastic properties of fresh concrete, and to gain an insight into the mechanism of action of this category of materials, it is useful to study the effect on the water-cement system. The topic can be considered from the... 

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. 

The fluidity of the cement paste can be measured in rheological terms by the torque transmitted to a stationary bob inside a revolving outer cylinder placed in a water-cement system as shown in Fig. 1.10. The shear stress measured at the stationary bob is plotted against the rate of applied shear when, for pastes of varying water-cement ratios, the results shown in Fig. 1.11 are obtained for readings taken of the shear stress as the shearing rate is increased (the up curve). 

Fig. 1.11 Shear-stress-shear-rate relationships for cement pastes at various water- cement ratios.

Material Viscosity reduction index (P m g ) (water cement ratio = 0.30)... 

The amount of calcium lignosulfonate adsorbed on to hydrating cement is almost independent of initial water-cement ratio within the range 0.4 to 1.5 [34]. 

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