Workable concrete

When the superplasticizer is used to produce highly workable concrete, more air will be entrained than a control concrete. This is particularly true of SNF and polyacrylate types (where air-detraining agents are sometimes intentionally added into the formulations) and less so with the SMF type. 

With a direct addition of superplasticizer to obtain highly workable concrete, initial and final setting times are invariably increased in the order SMF < SNF < polyacrylates. At normal dosages this increase rarely exceeds two hours for materials that are not intentionally formulated to retard. 

In general, superplasticizers will not have an adverse effect on strength, or strength development of concrete when added to produce highly workable concrete without reducing the water content. In fact several studies have indicated that an increase in strength may occur, and in some cases this can... 

If the addition of the air-entraining agent is maintained at a constant level, a more workable mix will entrain more air than a less workable one. However, for very workable concrete of slump greater than 180 mm, the air will be more rapidly lost before placing. 

These volumes are selected from empirical relationships to produce concrete with a degree of workability suitable for usual reinforced construction. For less workable concrete such as required for concrete pavement construction they may be increased about 10 percent. For more workable concrete see Section 5.3.6.1. 

Concrete, Mortar, and Plaster. Citric acid and citrate salts are used as admixtures in concrete, mortar, and plaster formulations to retard setting times and reduce the amount of water requited to make a workable mixture (172—180). The citrate ion slows the hydration of Portland cement and acts as a dispersant, reducing the viscosity of the system (181). At levels below 0.1%, citrates accelerate the setting rate while at 0.2—0.4% the set rate is retarded. High early strength and improved frost resistance have been reported when adding citrate to concrete, mortar, and plaster. 

Substrates protected from different environmental conditions basically include the metals (steel, zinc, aluminum and copper), inorganic materials (plaster, concrete and asbestos), and organic materials (wood, wall-board, wallpaper and plastics). Metals may be surface coated to improve their workability in mechanical processing. 

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. 

If the admixture is added directly to a concrete as part of the gauging water with no other changes to the mix proportions, a concrete possessing similar strength development characteristics is obtained, yet having a greater workability than the control concrete. 

A concrete with similar workability and strength development characteristics can be obtained at lower cement contents than a control concrete without adversely affecting the durability or engineering properties of the concrete. 

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 later plastic state when the concrete may be transported, handled and placed and where changes in properties such as workability and the ability of the mix to resist segregation and bleeding may affect these operations. 

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. 

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

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