The gravity-induced flow was measured with cement paste having water-cement ratio of 0.4 and that incorporating superplasticizer. The final spread diameter of mini-slump flow is 15.5 cm and 35.5 cm and the final spread length of channel flow is 21.0 cm and 49.5 cm. The margin of ±1.0 cm is an error of mini-slump flow test and ±2.5 cm is of channel flow test. The channel flow test has higher sensitivity as mix proportions or admixtures rather than the mini-slump.
When the rheological experiment is conducted, the best sensor seems to be the concentric cylinders due to its high reproducibility and no shear induced migration. The parallel plate sensors having a large diameter also bring stable results. Especially, the slippage between the sensors and samples could be prevented by attaching sandpaper on the plates. The rheological properties of a cement paste are well fitted into the Herschel-Bulkey model in the up step while the measured values of down step follows the power-law model. When cement powder have been stored in a sealed condition (lowering humidity of the powder), the rheological test shows high reproducibility.
Incorporating HRWAR also provides good reproducibility due to the homogeneous cement dispersion in the suspension. The sample mixed with a high shear mixing shows lower reproducibility than hand- mixed samples. That is because the paste becomes stiff and highly thixotropic under the influence of high shear. From the gravity-induced flow simulation, the effective range of shear rate was found. The flow curve in the range of 0.1 to 50 s-1 is critical to simulate mini-slump flow or channel flow. This range was reflected to the design of rheological test protocol. With the parallel plate attached sandpaper, the flow curve that was measured with 10 steps for each 5 seconds in the range of 0.1 to 50 s-1 precisely simulates the gravity-induced flow. For example, the sample of 40% water-cement ratio and that incorporating 0.2% HRWRA have the yield stress (the result of curve fitting to the Herschel- Bulkey model) of 55.36 Pa and 8.14 Pa, respectively. The measured yield stresses finally provide accurate simulation within the error of 1.5 cm spread in the neat paste. But the more conditions need to set to measure the rheological properties in non-Newtonian fluid having thixotropy under various mix designs.
References
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Appendix: erroneous measurement on a cement suspension
Even though a rheometer is successfully calibrated with a standard oil, erroneous measurement on a cement suspension can be obtained in some cases. The following two problems need caution for the rheological testing:
(1) Placing difficulty: It is not easy to load thick paste sample with an exact amount for accurate measurement. This problem is severe with the coaxial cylinder sensors. The cement paste does not flow in the outer cylinder. The use of parallel plates sensor allows us to handle comparatively thicker paste samples. In this study, the water-to-cement ratio of 35% sample was possibly measured with the parallel plates of 35 mm diameter. Figure A shows that the shear stress had been persisted for a period of time.
Figure A. w/c 0.35, shear rate 100
(2) Shear-induced migration: Cement grains and sometimes whole volume including them are susceptible to be migrated due to the density difference between cement grains and water. It is not good with parallel plates for high rate measurement. The shear-induced migration causes the results of the lower yield stress than the real value. The use of coaxial cylinder reportedly prevents the migration problem. (See the Figure B)
(3) Slippage: The slippage phenomenon was observed by human at a lower rate of shear that causes the lower yield stress. To prevent slippage, the sandpaper was attached on the plates.
Whether or not the sandpaper is attached to the plates have a lot of different with the yield stress values (Banfill et al., 2003).
Figure B. Shear-induced migration