Search

Highlights • Rheological properties have a good relation with the absorption of SAP. • The thixotropy shows the effect of SAP absorption behavior compared with plastic viscosity and maximum shear stress. • Large SAP's particles led to a high absorption rate and a long absorption time. Abstract Superabsorbent polymer (SAP) is an important internal curing agent for concrete with low or ultra-low water-to-binder ratio (w/b). When SAP is added, it will absorb water from fresh concrete fast. Thus, water absorption of SAP will affect the rheological properties of cement-based materials, especially at low w/b. Moreover, SAP with different particle size has a different absorption rate which will also affect the rheological properties. In this paper, two SAPs with different particle size are used to investigate its effects on the maximum shear stress, yield stress, plastic viscosity and thixotropy of cement-based materials within 80 min. Basic w/b of 0.18 was specified as reference values. Rheological results show that the absorption of SAP changed the evolution of yield stress, plastic viscosity and thixotropy of fresh mortar with time. Large SAP's particles led to a high absorption rate and a long absorption time which increased the yield stress and plastic viscosity. The above effects of SAP are closely related to the availability of water near SAP in cement mortar which causes to be a nonlinear relationship between the amounts of SAP and extra water required. In addition, the amount of extra water should be adjusted according to particle size of SAP and the addition level of SAP. [ABSTRACT FROM AUTHOR]

Epoxy adhesives for concrete cracks repair may necessitate the addition of reactive diluent to decrease their viscosity. However, research on the effect of different reactive diluents on the various properties of epoxy adhesives remains insufficient. In this paper, three reactive diluents, including monofunctional alkyl C12–C14 glycidyl ether (AGE), butyl glycidyl ether (BGE) and difunctional 1,4-butanediol diglycidyl ether (BDDE) are utilized to comparatively study the different effects on the viscosity, exothermic temperature, mechanical properties, and bond strength with cement mortar of the adhesives. The results show that the addition of BDDE increases the maximum curing temperature of the adhesives, whereas AGE and BGE both causes a decrease. The epoxy adhesive with AGE or BGE shows greater ultimate tensile strain, but a more obvious reduction in Young's modulus than that with BDDE at the same concentration. BDDE improves the bond strength of the adhesive-mortar interface whereas AGE and BGE both decrease it. Moreover, the bond strength shows a positive correlation with the hydroxyl content on the surface of cured epoxy. Comparing with AGE and BGE, BDDE increases the hydroxyl content, resulting in the strongest bond strength. [ABSTRACT FROM AUTHOR]

Air entrainment in cement-based materials is believed to be affected by low air pressure (AP) which is the atmospheric characteristic in high plateaus, and thus the rheology may be affected. This paper attempts to study rheology and air entrainment of fresh Portland cement mortar in simulated low APs. The results indicate that the rheological responses with the varying AP gradients are relevant to the air entrainments in fresh mortar mixtures. The volume changes of air voids in fresh mortars are linearly related to the varying APs, which is owing to not only the AP difference between the inside and outside air voids but also the additional pressure on the liquid films of air voids stemming from the surface tension changes. Moreover, the Bingham capillary number is calculated to account for the air void deformability. Then rheological parameters are correlated to the content and deformability of air voids in fresh mortars. [ABSTRACT FROM AUTHOR]

Improving the melt strength of poly(lactic acid) (PLA) is of continuing strategic research interest since a low melt strength results in poor processability of foaming and blowing film. PLA was modified in various ways including crosslinking, chain extension, grafting, blending, plasticizing and nucleation to improve its melt strength. This work focuses on the effect of melt strength after various modifications on foaming behaviour and cell structures of PLA foam. Cell density was increased and cell structure became more uniform with increasing melt strength, in particular when the melt strain was also increased after modification. With increasing melt strength the phenomenon of open and ruptured cells was decreased. The foaming window was widened with increasing melt strength, in particular after crosslinking, chain extension and blending with elastomer. Molecular weight, thermal properties and viscosity of PLA after various modifications were also studied and used to explain the foaming behaviour and foam structures. Copyright © 2012 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

Pumping changes the rheological properties, workability and construction performance of fresh concrete. However, there were limited knowledges on influences of discharge rates and pumping distances on the fresh properties of concrete. In this paper, effects of various discharge rates and pipeline lengths on rheological properties of self-consolidating concrete (SCC) after pumping were studied. A total of 12 SCC mixtures with the slump flow of 650–745 mm were pumped in pipelines with measured lengths of 342 m, 545 m, and 1044 m by constant discharge rates ranging between 5.1 and 11.4 L/s. Rheological properties were measured before and after pumping. Physical conditions of SCC during pumping, including pressure, shear, and temperature were estimated theoretically and experimentally. Moreover, the total organic carbon in the pore solution of SCC mixtures was measured to evaluate the change of superplasticizer adsorptions after pumping. Test results indicated that, due to pumping, the yield stress increased; the initial tangential viscosity decreased and the shear-thickening phenomenon was eliminated. The changes of yield stresses and viscosity were encouraged by discharge rates. Besides, lower initial slump flow of SCC exhibited a larger slump flow loss after pumping due to a higher shear rate experienced during pumping. The yield stress and viscosity after pumping increased with the increase of pipeline length at similar discharge rates, but decreased with the pipeline length under similar pumping pressures. The adsorption of superplasticizer increased after pumped at relative low discharge rates (5.2–5.7 L/s), but decreased at discharge rates of 7.7–10.8 L/s. [ABSTRACT FROM AUTHOR]

Air bubble stability in cementitious materials in low air pressure (AP) environments is influenced by the Ostwald ripening process, which causes coarsening behavior of air bubbles. This research reports the enhancement of air bubble stability in low AP via the adoption of rheology and surface tension methods. Besides, the evolution of air bubbles during the cement early hydration process is proposed. It was found that incorporating viscosity-modifying agents (VMAs) and shrinkage-reducing admixtures (SRAs) resulted in improving air bubble stability by 37.7 % and 23.4 %, upon undergoing AP change from 100 to 60 kPa. Calculation of air bubbles via ripening equations under low AP revealed that VMA induced a notable decrease in the effective diffusion flux within fresh cement mortar, leading to a reduction of 9.8 % in air bubble ripening within 120 min. Conversely, SRAs were primarily aimed at reducing the initial dimensions of air bubbles. [ABSTRACT FROM AUTHOR]

The influence of pumping on the rheological properties of self-consolidating concrete (SCC) is widely recognized, but there is still limited understanding of its behavior within the pipeline. In this study, we propose a novel approach using equilibrium distances to investigate the evolution of rheological properties along the pipeline. Full-scale horizontal pumping circuits spanning 328 m were established to assess the axial development of SCC's rheological properties. Thirteen mixtures, with water-to-cement (w/c) ratios ranging from 0.22 to 0.27, were pumped at discharge rates varying from 5.77 to 12.69 L/s. Our findings show that for pumping setups and material studied, both the w/c ratio of SCC and the discharge rate significantly affected the influence of rheological equilibrium distances on pressure loss gradient changes. In all tested samples, viscosity values required the entire length of the pipeline to reach post-pumping levels, while yield stress values rapidly achieved their fully developed state upon entering the pipeline. Importantly, the predicted pressure loss displayed a strong correlation with experimental measurements, with an accuracy of 89.42% across all tested mixtures. Our predictive model showed an average improvement of 4.6% in accuracy compared to the linear approach, with the most significant enhancement observed in samples with higher w/c ratios. This research sheds new light on the behavior of SCC during pumping and provides valuable insights for optimizing the pumping process. • The equilibrium distance of rheology is introduced to describe SCC flow behavior along the pipeline. • Horizontal pumping circuits spanning 328 m is used. • Factors affecting the equilibrium distance of rheology are discussed. • The prediction accuracy of the model is validated. [ABSTRACT FROM AUTHOR]

• Changes in rheological properties of high-strength concrete due to pumping were systemically investigated. • Fresh concrete was sampled in dismantled pipes for the rheological test. • Shear-thickening in high-strength concrete was eliminated after pumping. • Shearing energy during agitation and pumping was estimated and correlated to the changes in viscosity. Pumping of high-strength concrete (HSC) in high-rise or long-span structures is of paramount importance in constructions. However, pumping technology in practice is highly dependent on practical field experience, and full-scale testing that is carried out for complex projects involving new materials or long pumping distances. In this paper, a long pumping circuit was built to study changes in fresh properties of HSC due to pumping. In total, 10 HSC mixtures with strength grades of C60 to C100 were pumped at constant flow rates in pipelines measuring 348, 600 and 924 m in length. Rheological properties, workability characteristics, and concrete temperature were measured on concrete specimens sampled before and after pumping. Rheological properties of concrete sampled in dismantled pipes shortly after the end of pumping were also determined. Test results indicated that the yield stress increased, the initial tangential viscosity decreased and the shear-thickening phenomenon was eliminated due to pumping. The increase in yield stress mostly occurred in the first half of the pipeline, and the tangential viscosity decreased nearly linearly along the pipe length. The levels of drop in initial tangential viscosity were found to positively correspond to the calculated shearing energy, which is estimated given the shear rate, apparent viscosity and elapsed time during agitation and pumping. [ABSTRACT FROM AUTHOR]

Many alkali-activated materials (AAMs) exhibit poor workability in mixing owing to great viscosity of the paste and fast setting. Chemical admixtures commonly used in Portland cement-based materials, such as rheology control additives and superplasticizers, show lower efficiency in AAM systems. Understanding the rheological features of various AAMs and their control can lead to a valuable guidance for their application in large-scale, especially in the field of pumping and placement, grouting and 3D printing, etc. This paper aims to deliver a comprehensive review of the effects of composition factors including activators, precursors, admixtures, additions, aggregates and fibers on the rheology of AAMs, as well as models for describing their rheological behavior. The current progress has shown that the large knowledge gap needs substantial research from scientific mechanisms to robust formulations of paste, mortar and concrete to meet both workability and mechanical requirements. [ABSTRACT FROM AUTHOR]