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The effect of mixing sequence and a mixing time on the properties of SCC were studied by Takada et al. using a gravity mixer (G) and a forced paddle mixer (F). It was reported that for the same water-to-powder ratio by volume (V_w/V_p), longer mixing times for 7.5 and 3.5 minutes in gravity mixtures required lower SP dosages and resulted in higher slump flow values and low V-funnel flow times as compared to shorter mixing times of 5.5 and 2.5 minutes. However, in the case of forced pan mixers, in order to obtain the same degree of deformability, mixtures that mixed for 5 minutes required higher dosages of SP than the corresponding mixtures with the same V_w/V_p that mixed for 3.5 minutes.

 

While such results might not be representative, interpretation of the results suggests that it is not the mixing time, but rather the sheer energy and sheer rate that count. It is recognized that with a given mixture, completely different flow curves are obtained by varying these two parameters.

 

The effect of delayed addition of SP on the slump flow and V-funnel time was studied by Domone and Jin.[i]  In their study the delay in the time of addition of SP varied from zero to 6 minutes in increments of 1 minute. Fig. 8 shows the mixing sequence in which the powder, sand and 80% of the water were mixed for 2 minutes and then the SP and 20% of the water were incorporated.

 

 

Three types of superplasticizers were used: naphthalene-based SP, melamine-based SP and polycarboxylic ether-based SP. It was observed that delayed addition increased the fluidity of the SP as measured by both the flow spread and V-funnel flow time. The optimum addition time ‘window’ was found to be 2 to 4 minutes for the naphthalene- and melamine-based admixtures, but 0 and 0.5 minutes for polycarboxylic-based admixture.

 

Deshpande and Olek prepared 27 mixtures of RSSCC and found that the time in which the superplasticizer was added affects the rheological properties. The results of this study indicate that early addition of superplasticizer enhances dispersion of cement and increased the flowability of the RSSCC mixture, especially for mixtures mixed in mortar mixer. Addition of silica fume also leads to reduction of the mixing time for mortar mixers. It was also observed that for the same w:p ratio and mixing sequence, the total mixing time in a mortar mixture was shorter that that in the pan mixer.

 

Chopin and his co-workers[ii] studied the effects of mixing time on robustness. The parameters varied in the study included the quantity of powder, use of limestone filler, and various types and contents of silica fume and SP. The authors concluded that although the SCC mixtures generally require longer mixing times than conventional mixtures, their mixing time can be reduced by increasing the fine particle content, (with a constant w:c ratio), increasing the total water amount, and replacing part of the cement by silica fume.