Slump retention

One of the significant limitations of the use of first- and second-generation superplasticizers in ready-mixed flowing concrete is the rapid decrease in the initially achieved high workability and this constitutes one of the chief constraints to their wider acceptance. Therefore, a number of major producers of admixtures have sponsored active research to improve the workability retention characteristics of their superplasticizers. Some recent developments [48] have shown promise, among these are materials based on acrylate polymers (AP). The AP-based materials are reported to be more effective than SNF- or SMF-based surplasticizers in terms of water reduction, slump increase and slump retention. Figure 7.19 shows the remarkable improvement in the retention of workability produced by the AP type superplasticizer compared to an SNF type. 

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 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... 

Features Good water retention props. does not interfere with color development improves sag and slump resist. 

Polychloroprene is commercially produced by emulsion polymerization and exists as a one- and two-component system. The one-component polychloroprene sets to touch in 4-6 hours and cures slowly (3-7 daysj.t The two-component system is a solvent-free, non-shrinking, slump resistant sealant which sets quickly, but cures slowly Polychloroprene sealants have poor color retention (usually ate pigmented with black pigments), and high shrinkage. 

As it can be seen, vapor depletion phenomena control FP retention before the failure of the core support plate, since temperatures of the RCS atmosphere are too high, what inhibits vapor nucleation and condensation onto Inert aerosols. On the contrary, core slump at the same time that FP vapors revolatilize promotes their conversion into the condensed phase therefore, aerosol generation and transport mechanisms determine the final escape of radioactivity to the auxiliary building atmosphere before vessel breach. 

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