In Fig. 9, the relative viscosity is shown for all the tests performed. The relative viscosity is defined as the ratio of the measured viscosity for a given test to the viscosity measured for the respective control mix C10 or C20. The addition of the first dosage of HRWRA to the control mix C10 resulted in a decrease in viscosity as measured by both the truck and the ICAR rheometer. Further additions of HRWRA to the same mix, however, produced different results from the two devices. The ICAR rheometer indicated that further additions of HRWRA resulted in further reductions in viscosity while truck measurements indicated that the viscosity began to increase. The use of a viscosity-modifying admixture (VMA), a product intended to increase viscosity, for mix C14 resulted in an increase in viscosity as recorded by the ICAR rheometer but a decrease in viscosity as recorded by the truck.
In the second series of mixtures, the addition of the first dosage of HRWRA resulted in a decrease in viscosity as recorded by both the truck and ICAR rheometer. Like the first series, the magnitude of the decrease in relative viscosity was greater for the ICAR rheometer than for the truck measurement. For the final two mixtures, the setting of the concrete began to dominate the rheology. The addition of a second dosage of HRWRA for mix C22 resulted in an increase in viscosity as measured by the ICAR rheometer, while the truck rheometer recorded a value of viscosity that was essentially the same as mix C21. Finally, the addition of water for mix C23 resulted in a decrease in viscosity as recorded by both the truck and the ICAR rheometer.
In addition to the flow curve measurements indicated above, a second flow curve was measured with the ICAR rheometer over a lower range of rotation speeds (from 3.1 rad/s to 0.31 rad/s). The concrete sample was remixed by hand between each flow curve measurement; therefore, the second flow curve was measured approximately 3 min to 4 min after the first flow curve. In the first measurements (C10 to C14), the second series of flow curve points simply extended the original flow curve closer to the origin. However, as the day progressed and the concrete began to set, torque readings at low rotation speeds suggested a negative slope of the curve at low shear rates, as illustrated in Fig. 10. Since the plastic viscosity is defined as the slope, it would imply that this procedure resulted in the calculation of a negative viscosity, which is not physically possible. The final mixture exhibited not only a negative lope, but an upward shift of the entire curve. This same negative slope was evident in the truck measurements shown for mixture C12 in Fig. 4. When the rotation speed is sufficiently slow, the microstructure of the concrete is able to reform−that is, the cement particles are able to agglomerate during the shearing process resulting in an increase in torque. This phenomenon of a negative slope of the shear stress-shear rate curve was also observed for cement paste rheological tests where the value of the plastic viscosity was strongly dependent on the shear rate applied and the degree of hydration of the cement paste. Further explanations are presented in [17].
The correlations between the ICAR rheometer and truck measurements for the viscosity values are shown in Fig. 11. No apparent relationship exists for plastic viscosity. This disappointing result could be attributed to the lack of precision of the pressure gauge (± 0.34 MPa or 50 psi accuracy), especially at pressures less than 3.45 MPa (500 psi) where the lack of graduation below this lower value prevented measurements. Another possible reason for the discrepancy is the shear-thinning nature of concrete, resulting in a plastic viscosity that is shear rate dependent [17]. It is easy to state that the ICAR rheometer and the truck do not shear the concrete at the same shear rate, leading to different plastic viscosities. This does not explain the different trends in the relative viscosities. Nevertheless, the results show that the truck is able to sense differences between the mixes as the plastic viscosity value varies between 1500 kPa·s and 4000 kPa·s.