Superplasticizers workability loss

In order to study the effect of SO3 on slump loss in superplasticized cement, Khalil and Ward l adopted the conduction calorimetric technique. The heat effects were used to explain the workability loss. It was concluded that for cements cured at 25°C, the optimum SO3 content was higher in the presence of the superplasticizer. 

By the addition of a superplasticizer the water content of concrete can be reduced by 20-30% without loss of workability. 

Skarp et al. [14], however, pointed out that poor stability, that is bleeding or segregation, and loss of workability are two main concerns when working with SCC. A concrete mixture is said to be workable if it can be maintained in fluid form until the casting moment. The term workability time is defined as the time the concrete mixture remains workable. They attribute the instability to deficiencies in mix design and the loss of workability to incompatability between the cement and the superplasticizer. 

Higher than normal workability of concrete containing a superplasticizer is maintained for about 30-60 minutes, and then there is a rapid decrease in slump, termed slump loss. To control or extend the workability, the superplasticizer should be added at the point of discharge of concrete. Some admixtures are added to superplasticizers to control slump loss. The acrylate-based superplasticizers are claimed to possess good slump retention qualities. 

The use of superplasticizers, e.g., pol5maphthalene formaldehyde (PNS) and pol mielamine formaldehyde (PMS), appear to improve flowability of CAC concrete loss in workability appears to be too rapid. Normal range water reducers, e.g., hydrocarboxylic acids, function in a manner similar to their role in portland cement concrete. 

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