Concrete workability loss

Workability Loss Most WRA also have a retardation effect. Therefore, they reduce the workability loss, which is usually described in terms of slump (measure of concrete consistency) loss. For normal WRA, a distinction should be made between concrete mixes with a specified waterrcement ratio and a specified slump. At a given waterxement ratio the slump of concrete with WRA increases considerably. Although the rate of slump loss increases upon normal WRA use, the higher slump value at the beginning causes later slump values to be still higher than those of the concrete without WRA. On the other hand, for the same initial slump, the workability loss is more rapid in normal WRA incorporated concretes than that of control concretes. A similar but more pronounced effect of loss of workability is observed in high range WRA incorporated mixes. 

The relationship of the concentration of the admixture to the water reduction produced has been found to be linear (Fig. 7.6). Two conclusions were reached from the study which included several brands of cement [15] (1) it is the addition rate of the admixture which is important, and (2) the chemical nature of the admixture does not play a part in water reduction, but is an important factor in determining the retention (or loss) of slump in concrete with age. For example, the extended workability of the more... 

The workability of any fresh concrete decreases with time after mixing, but this effect, which is called slump loss, is more marked if a water reducer is used. The slump nevertheless remains higher than if the latter was absent. Slump loss is caused by the slow commencement of the hydration reactions, and its increased magnitude in concrete containing water reducers is probably due to the gradual absorption of the admixture by the hydration products. Delay in adding the admixture until a few minutes after mixing minimizes it. 

The rate of slump loss of concrete containing a normal water-reducing admixture is generally similar to or greater than that of the equivalent plain concrete mix depending on whether the admixture is being used to increase workability or reduce water. 

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

The effect of air entrainment is to increase yield, improve workability but reduce strength. These three factors are taken into account in mix design such that the strength loss is minimised. A method for doing this is outlined i n the DoE publication, Design of normal concrete mixes. 

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. 

Adding fibres to concrete reduces the workability and the loss of workability is proportional to the volume concentration of the fibres. To compensate for this, WRA or HRWRA may be used [29]. Impact resistance, flexural fatigue resistance, and resistance to surface deterioration is increased by fibre reinforcement. 

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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