Water cement ratio

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 ... 

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. 

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 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. 

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]. 

When a normal, accelerating, or retarding water-reducing admixture is utilized to increase the workability of a concrete mix by direct addition, it would be reasonable to assume that the extent of the effect would be markedly affected by changes in mix design parameters such as cement content, aggregate size, shape and grading, and the water-cement ratio. A study of many hundreds of results, however, indicates that this is not the case and Fig. 

This independence of efficiency in relation to mix design parameters is only true with regard to workability increases where a concurrent change in water-cement ratio is made, a number of variables must be considered and this will be discussed later. 

Fig. 1.28 The relationship between the slump and the water-cement ratio for mixes with and without a water-reducing admixture (Howard).

The most widely used application of water-reducing admixtures is to allow reductions in the water-cement ratio whilst maintaining the initial workability in comparison to a similar concrete containing no admixture. This, in turn, allows the attainment of a required strength at lower cement content to effect economies in mix design. 

The amount of water reduction possible is also a function of the way in which an admixture is added to the concrete if a period between mixing with water is allowed prior to the addition of the admixture, greater adsorption of the admixture on to the initial hydrates is obtained and a higher workability or alternatively a greater reduction in water-cement ratio is obtained, as can be seen from Table 1.14 [73]. 

Fig. 1.33 Reductions in water-cement ratio as a function of aggregate-cement ratio for lignosulfonate and hydroxycarboxylic-acid-based water-reducing agents.

Designed slump (BS 1881) (mm) Reduction in water- -cement ratio (%)... 

Water-reducing admixture type Addition ievei Water-cement ratio... 

Method of addition of retarder (0.225% calcium lignosulfonate by wt cement) Water-cement ratio Slump Water reduction (mm) (%)... 

In the case of lignosulfonate water-reducing agents, the effectiveness in reducing the water-cement ratio diminishes with an increase in either the the C3A or alkali content. In a comparative experiment with three... 

Products based on hydroxycarboxylic acid salts are more effective than lignosulfonates in reducing the water-cement ratio as illustrated in Table 1.15 [75],... 

---