Tien-Tung Ngo

Rheology of ordinary and low-impact environmental concretes

This study deals with the rheological properties of concrete mixtures incorporating various types of mineral additions as a partial replacement of cement in order to produce a low-impact environmental concrete. The control mixture contained only Portland cement as the binder, while the remaining mixtures incorporated binary cementitious blends of slag, limestone filler, and fly ash with different rates of replacement. After mixing, the plastic viscosity and yield stress of the concretes were evaluated at different slump values using a rheometer apparatus. The results showed that the type of mineral additions and the rate of substitution affect the rheological parameters of low-impact environmental concrete. Indeed, increasing the degree of substitution leads to an increase in the plastic viscosity of the concrete made with different types of additions used in this study.

Design of portable rheometer with new vane geometry to estimate concrete rheological parameters

AbstractThis paper presents the development of a portable vane rheometer to estimate concrete plastic viscosity and yield stress. The apparatus can be used not only in laboratory but also on construction site. In this study, new blade geometry was proposed to minimize the effect of segregation of concrete during testing, and also to expand the wide range of concrete workability with a slump of approximately from 7 cm to fluid concrete, and concrete with high plastic viscosity such as concrete with mineral additions. The used blade (U shaped and reversed) allows reducing the vibration of the apparatus, and obtaining more stable measurements. The obtained results permit validating the rheometer test procedure and confirmed that the results are reliable, with a low coefficient of variation of 9% for repetitive test and of 5.8% for reproductive tests.

Influence of recycled sand and gravel on the rheological and mechanical characteristic of concrete

Abstract The objective of this paper is to investigate the influence of type and dosage of recycled sand (RS) and gravel on the fresh and mechanical properties of concrete. Experimental program was conducted on concretes made with different ratio of substitutions (15, 30, 70, and 100%) of natural sand and gravel with RS and gravel. At the fresh state, slump, air content, and density were measured at the exit of the mixer, and then at 30, 60, and 90 min after mixing. Tests were also performed for compressive strength at the age of 1, 7, and 28 days, whereas elastic modulus measurements were done at 28 days. The results indicated that maintaining the workability of recycled aggregate concrete depends on sand or gravel substitution and their rates. Up to 30 min, slump values were decreased, but after that, no substantial change in slump values was observed. Air content increased and density decreased, with increasing recycled aggregate content (sand or gravel). Mechanical properties, such as compressive strength and modulus of elasticity, were lower than those of reference concrete.

Relationships between concrete composition and boundary layer composition to optimise concrete pumpability

Concrete pumpability is determined by the friction at the interface between the concrete and the wall of the pumping pipes (which are generally made of steel); called the concrete–steel interface. This friction is related directly to the thickness and composition of the boundary layer (BL) that occurs during the movement of fresh concrete in a pipe. These BL characteristics are rigorously linked with concrete composition parameters. To highlight this complicated relationship, an apparatus called a ‘tribometer’ was used in two experimental programs. This apparatus allows measurement of the steel–concrete interface friction and deduction of interface parameters (also called the pumping parameters), namely viscous constant and interface yield stress. The first program focuses on the effect of the concrete composition on pumping parameters, and the second program focuses on the relationships between the concrete composition and the BL composition. The results of the first program show that increases in cement paste volume, water/cement ratio and super-plasticiser dosage enhance concrete pumpability. Increased content of fine sand in concrete induces negative effects on the interface frictions and on the pumpability. The results of the second program show that the BL is formed by water, cement and fine sand particles with a diameter lower than 0.25 mm. It also shows that the proportions of water and cement volume in BL and micro-concrete (concrete with the largest grain diameter lower than 0.25 mm) are almost the same. The relative enrichment produced by fine sand in the BL decreases with increasing proportion of fine sand volume in the concrete. La pompabilité d’un béton est déterminé par le frottement à l’interface entre le béton et la paroi des tuyaux de pompage généralement en acier (appelé l’interface béton-acier). Ce frottement est relié directement à l’épaisseur et la composition de la couche limite (note BL) formée au cours d’écoulement du béton frais dans un tuyau de pompage. Ces caractéristiques de la BL sont rigoureusement liées avec les paramètres de composition du béton. Afin de mettre en évidence cette relation complexe, les auteurs ont utilisé dans ses deux programmes expérimentaux son appareil, qui a été récemment développé, appelé ‘tribomètre’. Cet appareil permet de mesurer les frottements à l’interface béton - acier et d’en déduire les paramètres d’interface (appelé aussi les paramètres de pompage), à savoir la constante visqueuse et le seuil d’interface. Le premier programme se focalise sur l’influence des paramètres de composition du béton sur les paramètres de pompage et le deuxième programme se focalise sur la relation entre la composition du béton et la composition de la couche limite. Les résultats du premier programme montrent que l’augmentation du volume de pâte, du rapport eau/ciment et du dosage en super-platifiant favorise la pompabilité du béton. L’augmentation du dosage en sablon substitué au sable de béton induit des effets négatifs sur les frottements et sur la pompabilité. Les résultats du deuxième programme prouvent que cette couche est constituée de l’eau, du ciment et des particules fines de sable du béton dont le diamètre est inférieur à 0,25 mm. Ils montrent également que la proportion volumique de l’eau et du ciment de la couche limite et du micro – béton (béton avec le plus grand diamètre des grains inférieur à 0,25 mm) sont presque identiques. L’enrichissement relatif en sable fin dans la couche limite diminue avec l’augmentation de la proportion volumique des fines de diamètre inférieur à 0,25 mm dans le sable béton.

Estimation of the Pumping Pressure from Concrete Composition Based on the Identified Tribological Parameters

A new method is proposed to estimate pumping pressure based on concrete composition without experimental measurements. Previous studies show that the pumping pressure depends on the interface friction between concrete and the wall of the pumping pipes. This friction is determined by the thickness and the rheology of the boundary layer formed at the interface. The latter is mainly formed by water, cement, and fine sand particles which come from concrete. Hence, interface parameters, which are the viscous constant and the interface yield stress, are directly related to concrete composition. In this work, at the first time the interface yield stress model is suggested and validated thanks to an experimental database also carried out in this study with a precision of around 13%. Then, the pressure estimation method is proposed using the two models to calculate the interface parameters. The validation of the method is carried out basing on the comparison with real measurements on the building site. This method enables the calculation of the pumping pressure with a precision of around 15%.

Application of Empirical Models to Optimizing Concrete Pumpabiltity

Pumbability of freshly-mixed concrete is a great challenge in the construction field; this attracts a growing number of researchers willing to study. This study pays attention on the use of empirical models that are established from the experimental data to evaluate the concrete pumpability. The purpose of the study is to reduce the amount of cost and time involved in the project construction. The models permit to directly estimate interface parameters (i.e., the viscous constant (η) and the interface yield stress (τ0t)) from the concrete formulation. These parameters directly affect the interface friction (τ) between the concrete and the wall of the pumping pipe, and thus the concrete-mix mobility (i.e., the pumping flow rate) can be improved by reducing the interface friction. The calculated interface parameters are then validated with measurement parameters using a tribometer. In this study, a parametric approach is also employed by varying formulation parameters such as water/cement ratio, cement paste volume, and aggregates/sand ratio of the concrete-mix. The obtained results demonstrates that the empirical model is reliable with a high precision. They permit to validate a theoretical approach to estimate and to optimize the pumping parameters and the concrete pumpability.