دانلود رایگان مقاله استفاده از پروتکل بازیابی خزش برای اندازه گیری تنش های استاتیک خمیر سیمان تازه

Abstract

In this study, a creep recovery shear rheological protocol was applied to fresh cement pastes. A viscosity bifurcation behavior was observed through applying a range of creep stresses. When applied stress is sufficiently low viscosity increases and the material yields, exhibiting viscoelastic solid-like behavior. Beyond a critical stress viscosity decreases and the material flows, exhibiting viscoelastic liquid-like behavior. Through examining this bifurcation behavior we found that the transition of viscosity occurs at very low strains. The strains at which this transition occurred were compared with critical strains measured through low amplitude oscillatory shear. Results provided support that the solid-liquid transition occurs beyond the critical stress measured through creep, thereby tying it to static yield stress. The protocol was implemented to probe pastes modified with attapulgite clays, a highly thixotropic system, and was found to be effective in characterizing static yield stress and thixotropic rebuilding.

5. Conclusions

The creep recovery test is applied to probe the viscoelastic behavior of cement paste and a viscosity bifurcation is observed. Within a critical stress the strain approaches equilibrium, indicating no flow. Beyond a critical stress the strain increases dramatically and no equilibrium value is reached, indicating flow is initiated and continues. The critical stress corresponding to this viscosity bifurcation is considered to mark the solid-liquid transition of the material and so is considered to be a measure of static yield stress. Measures of critical strain by LAOS are found to support this. The methods are applied to characterize the thixotropic rebuilding of cement pastes modified with nanoclays up to 0.5% by mass of cement. The creep protocol is found to effectively capture the static yield stress of highly thixotropic systems. Further, the increase in static yield stress over resting time is found to be higher for cement paste with 0.5% nanoclay over neat paste. This helps to elucidate how nanoclays can reduce formwork pressure over time. Nanoclays are also found to decrease critical strain measured by LAOS, which provides evidence that they are increasing the interconnectivity of the fresh-state microstructure.