Test Results and Discussion

Various self-consolidating concretes were produced with desired flowing ability and air content. HRWR produced the self-consolidation effect, whereas AEA provided the desired air contents. The initial and successive dosages of HRWR worked very well to fulfil the performance criteria for flowing ability with respect to slump and slump flow. Also, consistent slump and slump flow were maintained during all test stages since the air-void stability could be affected by the flowing ability of the concretes. The ability of AEA to reduce the surface tension is generally decreased at lower flowing ability (Khayat and Assaad 2002). Thus, bigger and less stable air-voids could be produced. Also, more air-voids can be entrapped at lower flowing ability resulting in higher total air content. These drawbacks were eliminated in the present study, as the flowing ability of the concretes was almost the same at all test stages.

The measured slump and slump flow have been presented in Table 5 and Table 6, respectively. It can be seen from Table 5 that the slump was always consistent for each concrete. On the whole, the slump has varied from 270 to 280 mm, with an average of 275 mm and with a standard deviation of only about 2%. Moreover, Table 6 shows that the slump flow varied in the range of 670 to 720 mm. For each concrete, the slump flow was also consistent throughout the testing period. The average slump flow for all concretes was about 697 mm with a standard deviation of 11%. The slump of SCC usually varies from 250 to 280 mm (Ferraris et al. 2000). Again, the slump flow of SCC generally ranges between 600 and 800 mm (Khayat 2000, Xie et al. 2002). These criteria were maintained in the present study by split use of the saturation dosages of HRWR. For this, higher saturation dosages of HRWR were required in the presence of 15 and 20% RHA, as can be seen from Table 4. This is due to an increase in flocculation forces resulting from greater amount of fine particles and higher specific surface area. Therefore, the demand for HRWR was increased to improve the dispersion of the binding materials.

Table 5: Variation of Slump with Time for Different Self-consolidating Concretes

Concrete

Series

Concrete

Designation

Slump (mm)

T = 15 min.

T = 30 min.

T = 45 min.

T = 60 min.

C3 5/0/4

275

275

275

270

S1

C3 5/15/4

280

275

275

275

C3 5/20/4

275

275

275

275

T = 15 min.

T = 30 min.

T = 60 min.

T = 90 min.

C3 5/0/8

275

275

275

275

S2

C3 5/15/8

280

275

275

275

C3 5/20/8

275

275

275

270

Table 6:

Variation of Slump Flow with Time for Different Self-consolidating Concretes

Concrete

Series

Concrete

Designation

Slump Flow (mm)

T = 15 min.

T = 30 min.

T = 45 min.

T = 60 min.

C3 5/0/4

700

700

700

680

S1

C3 5/15/4

720

705

700

700

C3 5/20/4

690

695

700

695

T = 15 min.

T = 30 min.

T = 60 min.

T = 90 min.

C3 5/0/8

670

680

700

695

S2

C3 5/15/8

720

700

710

695

C3 5/20/8

695

690

695

690

The results for air content of various SCC have been presented in Figures 1 and 2. It can be seen from Figure 1 that the air content varied from 3.5 to 4.3% for concretes under series 1. Conversely, the air content ranged from 7.5 to 8.6% for concretes under series 2, as can be seen from Figure 2. Hence, the actual air contents deviated from the design air contents (4 and 8%) within the range of ±0.6%. This is below the acceptable tolerance for air content measurement of ±1.5% (ACI Committee 201, 2001). In a few cases, the air content observed during second stage of testing at 30 minutes from concrete batching was slightly higher than the initial air content. This is possibly due to enhanced dispersion of the entrained air under more mixing action. The overall test results indicate that the air-void stability in all fresh self-consolidating concretes was good. The maximum loss of air content over the period of 60 and 90 minutes was less than 1.0% for all concretes. This is below the generally occurring air loss of 1 to 2% due to transportation of concrete (Kosmatka et al. 2002). Also, there was no significant difference in loss of air content between series 1 and 2 due to post-mixing and agitation. It suggests that the concrete placement can be delayed up to 60 to 90 minutes from the time of batching, while maintaining the desirable air content. This time length is adequate for the transport of concrete from ready-mixed plant to the construction site.

30 45

2

Conclusions

1. Air-void stability in various fresh self-consolidating concretes was not affected by post-mixing and agitation, as the air content at various test stages did not differ significantly.

2. Air-void stability in various fresh self-consolidating concretes was good over the period of 60 to 90 minutes, as the maximum loss of air content remained below 1%.

3. Rice husk ash increased the demand for air-entraining admixture for a given air content but it did not affect the overall air-void stability in fresh self-consolidating concretes.

4. The flowing ability of various self-consolidating concretes had no effect on air-void stability, as the slump and slump flow of the concretes were kept consistent in all test stages.

5. The split addition of the saturation dosage of HRWR maintained consistent flowing ability at all test stages but it did not affect the air-void stability in fresh self-consolidating concretes.

Acknowledgements

The authors are thankful to Mr. Cameron Monroe, Manager of Technical Services of Degussa Master Builders, Ltd. for supplying chemical admixtures and to Mr. Michael Rich for obtaining the rice husk ash required for the experimental investigation. The authors are also grateful to Lafarge North America Inc. for the supply of the cement needed for the entire project.