Ye Qian, Ph.D.

Abstract: Nanoclay, a thermal treated, purified attapulgite clay, has been used to increase thixotropy. However, the effect of nanoclay in presence of water reducing agent, such as PCE, on rheology, and the compatibility between nanoclay and PCE have not been well studied. The dynamic yield stress, thixotropic index, characteristic time, and microstructure of fresh cement pastes with combination of nanoclay and PCE addition are measured. It is found that nanoclay has a good compatibility with… Show more

Abstract: Nanoclay, a thermal treated, purified attapulgite clay, has been used to increase thixotropy. However, the effect of nanoclay in presence of water reducing agent, such as PCE, on rheology, and the compatibility between nanoclay and PCE have not been well studied. The dynamic yield stress, thixotropic index, characteristic time, and microstructure of fresh cement pastes with combination of nanoclay and PCE addition are measured. It is found that nanoclay has a good compatibility with PCE. Nanoclay increases the dynamic yield stress and enhances the thixotropy of fresh cement pastes with and without PCE addition. The nanoclay addition agglomerates the microstructure at high PCE addition and increases the thixotropic index from bottom value to a high value. This study gives insight of achieving low dynamic yield stress yet high static yield stress and thixotropy mixtures. Show less

Abstract: Polycarboxylate ether surperplasticizer (PCE) has been commonly used and studied to enhance flowability of cementitious materials. However, the effect of PCE addition on thixotropy and flocculation state has not gained enough attention. In this study, Focused Beam Reflectance Measurement (FBRM)-rheometer is coupled. The coupled setup could simultaneously measure both flocculation states in terms of chord length distribution of cement particles/agglomerates and rheological properties… Show more

Abstract: Polycarboxylate ether surperplasticizer (PCE) has been commonly used and studied to enhance flowability of cementitious materials. However, the effect of PCE addition on thixotropy and flocculation state has not gained enough attention. In this study, Focused Beam Reflectance Measurement (FBRM)-rheometer is coupled. The coupled setup could simultaneously measure both flocculation states in terms of chord length distribution of cement particles/agglomerates and rheological properties including dynamic yield stress and thixotropy. The effect of various PCE additions is explored, firstly by measuring the adsorption of PCE surfactants and the concentration of PCE surfactants in cement pore solution. It is found that the effect of PCE addition on dynamic yield stress, thixotropy and chord length distribution of paste particles/agglomerates are all bi-fold. The turning point is the Critical Micelle Concentration in cement pore solution. Results of more agglomerated flocculation states at low PCE addition are measured for the first time and further discussed. Show less

Abstract: The dynamic and static yield stress of fresh cement mortar were measured in a rotational rheometer with a vane geometry using shear rate and shear stress-controlled protocols, respectively. Through a shear rate-controlled steady-state protocol, the equilibrium flow curve is measured and fitted with the Bingham model to obtain dynamic yield stress. A negative slope in the equilibrium flow curve, shear banding and stick-slip phenomena are observed and discussed. Through a… Show more

Abstract: The dynamic and static yield stress of fresh cement mortar were measured in a rotational rheometer with a vane geometry using shear rate and shear stress-controlled protocols, respectively. Through a shear rate-controlled steady-state protocol, the equilibrium flow curve is measured and fitted with the Bingham model to obtain dynamic yield stress. A negative slope in the equilibrium flow curve, shear banding and stick-slip phenomena are observed and discussed. Through a stress-controlled creep-recovery protocol, viscosity bifurcation behavior is captured and static yield stress is marked as the creep stress when the bifurcation occurs. Finally, the discrepancy between dynamic and static yield stress is tied to thixotropy. Show less

Shear-induced particle migration is widely recognized to be a challenge in characterizing the rheological properties of fresh cement-based mortars. In this study, we aim to quantify shear-induced particle migration by characterizing the stress decay process during constant shear flow with the aid of a modified thixotropy/migration model. It is found that a conventionally used single exponential model is not sufficient to fit the stress decay and describe the destructuration and sand migration… Show more

Shear-induced particle migration is widely recognized to be a challenge in characterizing the rheological properties of fresh cement-based mortars. In this study, we aim to quantify shear-induced particle migration by characterizing the stress decay process during constant shear flow with the aid of a modified thixotropy/migration model. It is found that a conventionally used single exponential model is not sufficient to fit the stress decay and describe the destructuration and sand migration of mortar under shear. Instead, a two exponential model is needed to capture the interaction of sand particles and the suspending cement paste phase. Model parameters are used to quantify the effect of sand volume fraction, clay addition, and applied shear rate on the kinetics and intensity of colloidal deflocculation and sand migration. Results provide evidence that the colloidal and granular contributions to the overall stress decay of mortars can be represented by each of the two exponentials. Show less

In this study, a creep recovery shear rheological protocol was applied to fresh cement pastes. A iscosity 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… Show more

In this study, a creep recovery shear rheological protocol was applied to fresh cement pastes. A iscosity 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. Show less

Freeze-thaw cycle is one of the major damage factors of concrete patch repair. Not only the material itself but also the adhesive interface is damaged under freeze-thaw cycles (FTC). Air-entraining agent has long been used to increase the freeze-thaw resistance of concrete materials. However, the effect of air-entraining agent on the adhesive interface has not been explored. The degradation mechanism and failure mode of concrete repair system under FTC has not been studied, either.

In… Show more

Freeze-thaw cycle is one of the major damage factors of concrete patch repair. Not only the material itself but also the adhesive interface is damaged under freeze-thaw cycles (FTC). Air-entraining agent has long been used to increase the freeze-thaw resistance of concrete materials. However, the effect of air-entraining agent on the adhesive interface has not been explored. The degradation mechanism and failure mode of concrete repair system under FTC has not been studied, either.

In this study, three kinds of substrate concrete were casted and repaired by two kinds of ordinary Portland cement mortars and one kind of polymer-modified cement mortar (PCM), respectively. With up to 150 FTC, splitting tensile strength and failure modes of composite specimens were experimented. Results showed that air-entraining agent in the repairing mortar greatly influenced adhesive tensile strength under FTC. The water cement ratio and air-entraining agent of substrate concrete insignificantly affected the adhesive interface, but affects failure mode. The adhesive tensile strength of PCM-repaired composite specimens decreased faster than that of ordinary Portland cement mortar-repaired composite specimens although PCM itself showed stronger freeze-thaw resistance than ordinary mortar. Show less