Workability superplasticizer dosage

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. 

When high dosages of superplasticizer are used to effect low W/C ratio and high workability. 

Many concrete producers use a combination of superplasticizer and a conventional water-reducing, set-modifying or air-entraining admixture to achieve the desired performance. The superplasticizer provides the major portion of the required water reduction and the conventional admixture is added to achieve one or more of the following objectives (1) further water reduction (2) admixture economy (3) the desired air content (4) increased workability and (5) extension of set and workability. Coimnercial conventional admixture formulations used for this purpose are usually based on sodium lignosulfonates, hydroxycarboxylic acids or processed carbohydrates. Such combinations, besides reducing the dosage of the... 

Previous work on superplasticized Portland cement concrete containing fly ash or blast furnace slag has shown that such mixes require 10% less admixture than reference Portland cement concrete to attain the same workability. Therefore, a given dosage may produce higher water reduction. The reason for the reduced admixture requirement has not been determined. It is probably due to the lowering (dilution) of the C3A content... 

High dosages of a superplasticizer permit the replacement of a large amount of the cement by fly ash, while maintaining workability and strength. This is typical high-volume fly ash concrete. 

High performance concrete (HPC) may be defined as a concrete that, with particular care in the selection and proportioning of its constituents, shows a clear improvement in one or more of the properties with respect to ordinary concretes [1]. In practice, the term high-performance concrete is applied to concretes with a low water/binder ratio (0.3-0.35 or even lower), elevated cement content (400-550 kg/m ) and the addition of 5-15% silica fume with respect to the total mass of binder. Sometimes fly ash or GGBS are also added. In order to achieve a suitable workability with mixes typical of HPC, a high dosage of superplasticizer is required. 

Chemical admixtures in small amounts are added to concrete to enhance the physical, mechanical, chemical, and durability characteristics of concrete. Superplasticizers are admixtures that have the ability to increase the workability of concrete (for easy placement), but also produce high strength concretes. The relative rates of hydration of cement containing superplasticizers at different temperatures are conveniently followed by conduction calorimetry. In Fig. 11 both the rates of hydration and the cumulative amounts of heat developed in cement pastes hydrated at temperatures of20,40, and 55" C are plotted as a function of time. In the figure, SMF refers to the superplasticizer based on sulfonated melamine formaldehyde. The addition of the superplasticizer retards the hydration of cement. The retardation increases with the dosage of the superplasticizer. Also, the retardation effect becomes less significant as the temperature of hydration is increased. 

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