Concrete workability

The commonly used ratios of admixture to cement for the purpose of shrinkage compensation are 9-11 (admixture) to 91-89 (cement). At these ratios the properties of CSA concrete are similar to Portland cement concretes of similar mix proportions. At admixture dosages exceeding 11% however, concrete workability and strength decrease, while expansion and air entrainment increase. When expansion is umestrained and exceeds 0.3%, strength is reduced [74]. 

Dhir, R.K. et al. (1988) Contribution of PFA to concrete workability and strength development. Cement and Concrete Research 18,221—239. 

The second, which also reduces the level of activity of the concrete, Is by the Introduction of boron. In the form of non-soluble boron frits, to the concrete mix. Prior to the adoption of these Ideas, testing would of course be required to confirm that they gave the anticipated enhanced shielding properties, to establish procedures for producing the concrete and to check that neither caused any unacceptable effects on concrete workability, heat of hydration, creep, shrinkage, strength and durability. 

ASTM C 494 (Specification for Chemical Admixtures for Concrete) classifies several types of concrete admixtures. Of particulcir use for steel in concrete are the high range water reducers that are classified as type F or G admixtures. These chemicals significantly improve concrete workability at low w/c ratios, which is needed to produce low permeability concrete. 

Slump test (ASTM Cl 43, Standard Test Method for Slump of Hydraulic Cement Concrete). This test is the oldest (it first appeared as an ASTM standard in 1922) and most widely used test of concrete workability. Because it is a static test, it is not a good indicator of the workability of FRC. However, according toACI Committee 544 (2), once it has been established that a particular FRC mixture has satisfactory handling and placing characteristics at a given slump, the slump test may be used as a quality control test to monitor the FRC consistency from batch to batch . 

P. Balaguru, and V. Ramakrishnan, Properties of fiber reinforced concrete workability, behavior under long-term loading, and air-void characteristics . Ad Mate/ V. 85,1988,189-196. 

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).

This has already been dealt with in previous parts of this section where it can be seen that a reduction in the water-cement ratio by use of admixtures is beneficial in reducing the permeability of the concrete. Even when cement contents are reduced, whilst maintaining the workability and strength characteristics of corresponding mixes containing water-reducing admixtures, there is no deleterious effect on the permeability. 

Direct addition of water-reducing admixture This increases the workability of the concrete. The effect of all types of water-reducing admixture under these conditions is invariably to increase the shrinkage and creep of the concrete. Some typical values are shown in Table 1.31. The considerable increases in both shrinkage and creep in the presence of admixtures containing calcium chloride and triethanolamine are clearly illustrated. 

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