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The influence of air entrainment on rheological behavior of fresh cementitious materials has attracted a lot of attention in the past years, but the results are still in controversy. In this research, cement mortars were prepared with equivalent air contents using different air-entraining admixtures (AEAs) to measure their fresh-state performance and hardened air-void system at variable temperature conditions, which aimed to clarify and unveil the coupled effects of different surfactants and temperature for the first time. The adsorption amount of AEA, zeta potential of cement, along with hydration kinetics of mixtures were evaluated to characterize the interaction of air bubbles with cement grains. It was found that for mixture without superplasticizer (SP), the impact of air entrainment on rheological parameters was dependent on the interaction intensity of AEA molecules with cement. For SP-plasticized mixture, air entrainment tended to cause a shear-thinning behavior with decreased plastic viscosity, while its influence on yield stress was in accordance with the situation without SP. The hardened samples with anionic surfactant contained more small voids and lacked big voids, whereas the samples with cationic surfactant showed opposite outcomes. With temperature decreasing, both the average chord length and the spacing factor of air voids increased owing to the reduced yield stress of mixture. The results will provide in-depth illustrations for understanding how air bubbles change the rheological behavior of mixture, and more importantly it will explain the seemingly inconsistent findings reported by previous literatures. • How air bubbles affected rheological behavior of mixtures with various states was clarified. • The coupled influence of surfactant type and temperature on air-void system were explored. • The crucial role of surfactant interaction with cement for fresh-state performance was revealed. • The reasons why temperature decreasing at fresh-state degraded air-void structure were analyzed. [ABSTRACT FROM AUTHOR]

The thixotropic behavior of fresh cement-based materials has been found to be derived from the reversible breakage and agglomeration of flocculated cement grains. The existence of thixotropy can lead to significant variations in shear stress and apparent viscosity under the same shear rate, which brings great challenges for measuring rheological performance. This paper highlights the difference between strong and weak particles flocculation for the first time, and then reveals the intrinsic interactions of various thixotropic and rheological parameters. The thixotropic behavior and dynamic yield stress were evaluated for cement paste samples with different resting time and temperature conditions. It was found that the longer resting time, the higher temperature, and the larger solid volume fraction increased the difficulties for both the structural breakdown and the subsequent structural re-buildup processes, indicating the lower reversibility of thixotropic behavior; These factors also led to higher dynamic yield stress, more amount of strongly-bound flocs, but less weakly bound flocs. After removal of shear force, the microstructure reconstruction ability of fresh cement paste was highly correlated to the recovery of dispersed weak flocs, but with little regard to strong flocs. The dynamic yield stress value could be dependent on the breakage degree of flocculation, while the prolonged duration of last several steps in the descending portion of the rheometric procedure enhanced the dynamic yield stress due to structural re-buildup. The results will provide new in-depth insights into the roles of strong and weak flocs in the reversible thixotropic behavior of fresh cement-based materials. • The roles of strong and weak flocs in thixotropy of cement paste were separately clarified. • The influence of resting time and temperature on thixotropy of cement paste were explored. • The higher binding strength of flocs resulted in the less reversibility of thixotropic behavior. • The higher dynamic yield stress was measured with the prolonged duration of last shear steps. • The intrinsic relations among various thixotropic and rheological parameters were established. [ABSTRACT FROM AUTHOR]

This paper aims to provide insights into the coupled influence of typical anti-freezing agents (AFAs) and cold environment on rheological performance of cement mortar with/without different superplasticizer (SP) and air-entraining agent (AEA) components. Fresh mixtures were prepared with chemical admixtures singly or in groups (AFA; AFA/SP; AFA/SP/AEA) for the continuous measurement of their time-dependent rheological behavior at variable temperatures (24 °C; 0 °C; −5 °C; −10 °C). Hydration kinetics, pore solution and air-void system were systematically evaluated to provide evidences supporting for the variations in rheological parameters. It was found that the yield stress of mixture decreased and the plastic viscosity increased with the temperature falling from 24 °C to 0 °C, while both the yield stress and the plastic viscosity increased rapidly with the temperature falling from 0 °C to −10 °C. Temperature could affect the rheological parameters development by changing the interaction of SP with ions. For the SP-plasticized mixtures under low temperature, the dosage of calcium nitrite (CN) as AFA should be carefully restricted. For the air-entrained mixtures, the incorporation of CN enhanced the shear stress in the high shear rate region, while the incorporation of ethylene glycol (EG) showed a converse behavior. The results will provide theoretical supports for the manufacture and the early-age properties control of flowable concrete during winter construction. [ABSTRACT FROM AUTHOR]

Highlights • Coupling Influence of vibration parameters and rheology on concrete segregation was presented. • Segregation led to severer degradation on transport than mechanical performance of concrete. • Chloride ion permeability of the top concrete exponentially increased with the degradation of ITZ. Abstract Vibrating consolidation process is widely applied to field construction of cement concrete. Improper vibration can easily lead to the instability of fresh concrete and then induce performance degradation of hardened concrete. In this study, two poker vibrators with different vibrating duration were adopted for consolidating fresh concrete mixture with variable rheological performance. The variation of apparent density along casting direction was employed to evaluate the segregation state. Rebound hammer, chloride ion penetration and cement-aggregate interfacial transition zone (ITZ) microhardness were performed on selected samples at different casting depths of hardened concrete prisms. Results show that either higher amplitude or longer duration of poker vibration causes the severer segregation of fresh concrete and the plastic viscosity plays a main role in controlling vibration-induced segregation. Both mechanical performance and chloride permeability variation along casting direction tends to linearly increase with the segregation level. The severe segregation presents a worse harmful impact on the long-term durability than on the mechanical performance. Chloride ion permeability of the top concrete exponentially increased with the thicker ITZ width or the lower ITZ bonding for all vibrated concretes. [ABSTRACT FROM AUTHOR]

• Multi-scale solid particles optimization for Eco-SCC was developed by DEM simulation. • DEM models gave insights into interactions among aggregates with different gradation. • The flow behavior and continuous motion traces of fresh Eco-SCC could be presented. • Optimized Eco-SCC showed lower environmental loads than normal SCC mixtures. The development of mix design method for low-binder ecologically-friendly self-compacting concrete (Eco-SCC) has attracted interest in recent years. Most studies investigated this topic by evaluating mixtures with various ingredient proportions experimentally and then establish empirical correlations. This research attempts to take the first step towards a novel numerical discrete element method (DEM) simulation to optimize multi-scale particles size distribution of Eco-SCC at a constant mixing proportion. The two-phase DEM model was generated as an assembly of hard core with soft shell based on the thickness of mortar layer covering gravels and rheological behavior of suspending mortar, to characterize fresh properties of Eco-SCC. The rolling resistance standard contact model was adopted for simulating fresh mortar matrix lacking in excess paste. It was found that the blended cementitious powders closer to Fuller ideal curve and the aggregates gradation with higher fractal dimension were beneficial to the flowability of mixture. However, potential negative impacts on stability or passing ability of Eco-SCC might also be induced by an overmuch incorporation of slag or a higher portion of large-size gravel. The numerical results not only showed good agreements with experimental verifications (deviations lower than 7% for flow spread prediction), but also provided insights into the interactions among particles along with their continuous motion traces. Thereby, DEM modelling can be used as an efficient tool to reveal the influential mechanism of aggregates on fresh Eco-SCC and attain a general improvement in rheological performance via multi-scale particles optimization, with the measured slump flow increasing from 510 mm to 730 mm (by 43.1% at most) in this study. The eventually optimized Eco-SCC mix can be obtained with slump flow at 670 mm, V-funnel time at 4.1 s, J-ring spread at 635 mm, VSI = 0, and the cost intensity of 7.09 CNY·m−3·MPa. [ABSTRACT FROM AUTHOR]

• Stabilization effect of VMA on entrained air bubbles in fresh concrete was clarified. • Improvement of VMA on air voids system of hardened vibrated concrete was validated. • The relation of air-void system parameters to rheological properties was presented. The influence of viscosity-modifying agent (VMA) on air-void system of self-compacting concrete has attracted considerable attention in recent years. But for conventional concrete with a sharp loss of air content due to vibration operation, the role of VMA on air-void system remains unclear. This paper aims to validate and clarify the positive effects of VMA on air bubbles stability and air voids homogeneity in vibrated concrete. Three typical VMAs with variable dosages were incorporated to prepare twenty air-entrained fresh mixtures. Rheological properties of mortar matrix portion of fresh concrete, surface tension of aqueous solution and time-dependent air content were evaluated for every mix. The air-void system in hardened concrete was inspected at three casting depths of prism specimen. Results showed that the surface tension of VMA solution played a key role in the stabilization effect on air bubbles, which strongly varied with VMA molecules. A higher plastic viscosity induced by the higher VMA dosage was favorable for maintaining the homogeneous distribution of entrained small air bubbles and reducing the formation of large voids in vibrated concrete. Therefore, a proper incorporation of VMA can improve the freeze-thaw resistance of vibrated concrete with lower spacing factor and smaller chord length. [ABSTRACT FROM AUTHOR]

This paper investigated the spatial distribution of steel fibers and air bubbles in UHPC cylinder specimen by using X-ray CT system. Four levels of superplasticizer dosage were adopted to obtain cement mortar mixtures with different rheological properties and then steel fibers were added by volume of 1%, 2% and 3% for every cement mortar mixture respectively. Cylindrical specimens with diameter of 100 mm and height of 200 mm were prepared under the same condition for every mixture and the content of steel fibers and air bubbles at different casting depth was determined by X-ray CT measurement. On the other hand, the cylinder specimen was cut into four pieces for evaluating the splitting tensile strength variation with the casting depth. The results reveal that steel fibers and air bubbles are uniformly distributed in UHPC specimen when the fresh mixture is prepared by adding a lower dosage of superplasticizer. This homogeneity is also proved by the splitting tensile strength for samples at different depths. When the yield stress of fresh mortar was reduced by the higher dosage of superplasticizer, the steel fiber sedimentation and air bubble rise occurred and the splitting tensile strength was significantly decreased at the top portion of cylinder specimen. The influence of steel fiber content on splitting tensile strength can be regressed into a good quadratic equation. Therefore, the balance between homogeneity and flowability should be fully considered for manufacturing UHPC elements with a high depth. [ABSTRACT FROM AUTHOR]