Water reducing admixtures concrete

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. 

The way in which the four types of water-reducing admixtures discussed so far affect the strength gain characteristics of concrete containing them is shown in Fig. 1.2. The four concrete mixes have been designed to have approximately the same 28-day compressive strength, i.e. the admixture-containing mixes would contain approximately 10% less cement than the control mixes. 

Fig. 1.2 Compressive strength development of concrete containing various types of water-reducing admixture.

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... 

The initial plastic state of the fresh concrete subsequent to the mixing process, where properties such as the air content, density and workability are normally measured by relevant standard tests, and utilized as a means of control of production. The magnitude of these properties is affected by the addition of water-reducing admixtures, either intentionally or as a side effect, which could result not only in a change in the characteristics in the plastic state, but could also be reflected in changed properties in the hardened state. 

The subsequent hardened state during the life of the structure where the concrete material must fulfill its structural or aesthetic role without deterioration. It is important that the durability of the concrete should not be adversely affected by the presence of a water-reducing admixture. 

The presence of a water-reducing admixture can alter the air content of concrete, either as a deliberate measure (the air-entraining water-reducing admixtures) or as a side effect of the material in lowering the surface tension of the aqueous phase. 

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. 

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. 

Fig. 1.35 Concretes containing water-reducing admixtures tend to have more structure than a plain concrete at a given workability (after Hewlett).

The major physical attributes of concrete as a construction material are a high compressive strength and stiffness, an ability to protect and restrain steel and, most important of all, to retain these properties over a considerable period of time. The effects that water-reducing admixtures have on these properties can be considered from the point of view of design parameters, i.e. those properties of concrete at a relatively early age (usually 28 days) which are used for structural calculations, and longer-term aspects or durability. 

The overall conclusion from the available data is that when a concrete mix is designed incorporating a water-reducing admixture of the normal or retarding type, then the properties of the resultant concrete at 28 days will conform to the normal relationships used for concrete not containing an admixture at the same water-cement ratio. 

The durability of concrete is the ability of the material to maintain its structural integrity, protective capacity, and aesthetic qualities over a prolonged period of time. It is important that the benefits conferred to concrete in the plastic and early hardened state by water-reducing admixtures are not negated by any adverse effect on the long term durability. 

Direct measurement of the effect of aggressive reagents on concrete durability appears to be confined to sea water and sulfate attack, where in both areas it is recognized that the lower the water-cement ratio, the greater will be the resistance to attack and the use of a water-reducing admixture will be obviously helpful. This is confirmed by work carried out in Holland [95] and Japan [84] and a general conclusion is that a reduction in the water- cement ratio from 0.5 to 0.40, would allow a reduction in thickness of cover of the reinforcement by about 50%. 

Table 1.21 Sulfate resistance of concrete mixes containing water-reducing agents (no mix design changes other than addition of water-reducing admixture)...

Table 1.25 Freeze-thaw resistance of air-trained concrete containing water-reducing admixtures...

It can be concluded from the assessment of the data in this section that inclusion into a concrete mix of a water-reducing admixture of the lignosulfonate, hydroxycarboxylic acid and air-entraining type should not lead to any deterioration in the durability of that concrete to freeze-thaw cycling. Indeed there are strong indications that, when used either as a means of reducing the water-cement ratio or, alternatively, of reducing the cement content, more durable concrete may result. 

In order to study the effect that water-reducing admixtures may have on the role that concrete plays in protecting steel reinforcement, it is necessary to consider the following aspects. 

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