Dimitri Feys

Design, Production and Placement of Self-Consolidating Concrete

The paper describes a new approach to determine the volume of the cement paste required to produce SCC irrespective of the granulometric characteristics of the aggregate. A test vessel has been designed which enables the determination of the void ratio of the aggregate and the necessary paste volume. The method has been applied to various aggregate (crushed quartz porphyry, crushed muschelkalk, gravel, fluvial sand). SCC mixtures have been designed according to the new approach, and it turned out that it has a good predictive potential as the mixture composition is concerned.

New Test Method to Evaluate Dynamic Stability of Self-consolidating Concrete

Stability is one of the crucial properties for self-consolidating concrete (SCC). Dynamic stability, which describes the stability during flow of concrete, remains, to date, one of the least investigated aspects of SCC workability. A novel dynamic stability test, tilting box, is proposed in this paper. The test involves the tilting of a rectangular box numerous times to simulate concrete flow and evaluating the homogeneity of the concrete with increasing flow distance. Results show that the test has an adequate repeatability and sensitivity to variations in dynamic stability stemming from changes in mixture design and flow conditions. Dynamic segregation is shown to increase as the flow distance increases or the flow velocity decreases. Workability characteristics are shown to have significant effect on dynamic segregation.

Understanding the pumping of conventional vibrated and self-compacting concrete

Abstract: This chapter describes the pumping of concrete from a more theoretical point of view. It is shown that friction causes significantly higher pressure losses and must be avoided at all times. Based on several full-scale experimental studies, it is found that less flowable concretes, such as conventional vibrated concrete (CVC), move as a full plug in a pipe surrounded by a lubrication layer, while for more flowable concretes, like self-compacting concrete (SCC), a part of the bulk concrete is also sheared. For CVC, the properties of the lubrication layer relate well to the pumping parameters, while for SCC, the required pumping pressure is dominated by the concrete viscosity. The influence of other parameters is briefly discussed.

Similarities and differences of pumping conventional and self-compacting concrete

In practice, self-compacting concrete (SCC) is considered as a simple extension of conventional vibrated concrete (CVC) when pumping is concerned. The same equipment, materials, pumping procedures and guidelines used for CVC are applied when pumping SCC. On the other hand, it has been clearly shown that the rheological properties and the mix design of SCC are different than CVC. Can the same pumping principles employed for CVC be applied for SCC? This paper compares the some published results of pumping of CVC with those for SCC. A first striking difference between pumping of CVC and SCC is the flow behaviour in the pipes. The flow of CVC is a plug, surrounded by a lubricating layer, while during the flow of SCC, part of the concrete volume itself is sheared inside the pipe. As a result, the importance of viscosity increases in case of SCC. Due to the low yield stress of SCC, the behaviour in bends is different, but quite complex to study. Due to the lower content of aggregate and better stability of SCC, as it is less prone to internal water migration, blocking is estimated to occur at lower frequency in case of SCC.

The rheology of self-compacting concrete made with Belgian materials

The rheology of fresh concrete and self compacting concrete (SCC) has been studied for several decades to better understand the flowing behaviour of these materials. This is especially important in cases the concrete is pumped into the formwork through pipes. Too stiff concretes, or concretes that segregate are not suitable for pumping operations. These concretes demand too high pressures or will cause blocking during pumping operations. Pumping SCC speeds up the casting process, because the concrete is more fluid and it does not need any compaction. In literature, the rheological behaviour of fresh concrete and fresh SCC is mainly described by the Bingham model. For traditional concrete, this behaviour has been confirmed. However, tests with SCC, made with Belgian materials, indicate in most cases shear thickening behaviour. The Bingham model can not be applied because it causes negative yield stresses. The Herschel-Bulkley equation ( n K γ τ τ & ⋅ + = 0 ) gives positive yield stresses, but it has some other problems. The dimension of the consistency factor “K” is dependent on the exponent “n”. As a result, this dimension is variable and “K” is inappropriate for physical interpretation. The Herschel-Bulkley curve is also forced mathematically to be horizontal for very low shear rates – in case of shear thickening – resulting in an overestimation of the yield stress. The modified Bingham model ( 2 0 γ γ μ τ τ & & ⋅ + ⋅ + = c ), i.e. the Bingham model extended with a quadratic term, correlates the data points as well as Herschel-Bulkley; has constant dimensions and gives a better fit for the yield stress. The degree of non-linearity, expressed by the parameter “n” in Herschel-Bulkley, can be examined with c/μ. This model is also a Taylor development of the second order of the Herschel-Bulkley equation. When examining the results from the rheometer tests, more shear thickening is observed when the slump flow increases. And when the w/p ratio decreases, c/μ increases dramatically.

Influence of Thixotropy on Pressures Required during Pumping of Concrete

Concrete is a very popular construction material, which can be regarded as a suspension of solids of several orders of magnitude, suspended in water. From a rheological point of view, the fresh fluid concrete mostly obeys the Bingham law, showing a yield stress and a plastic viscosity. Due to the presence of cement, a permanent process of coagulation and dispersion occurs, causing the concrete to be thixotropic.This paper describes the influence of thixotropy on the measured pressure losses during high speed pumping. Apparently, the concrete becomes more fluid with increasing discharge, increasing the risk for the loss of internal stability, which can lead to a very low quality product.

Efficiency of waste tire rubber aggregate on the rheological properties and compressive strength of cementitious materials

A serial of experiments were carried out to investigate the efficiency of waste tire rubber on consistency and rheological behavior of cementitious materials at fresh state and on the mechanical strength at hardened state was evaluated. Results indicate that the addition of rubber aggregates significantly influences yield stress of the sample in the fresh state and the compressive strength at 28 days, depending on the grain size distribution of the rubber aggregate and its volume percentage in the mixture. The yield stress of the sample in fresh state is more influenced by an increasing volume of rubber compared to adding sand with equal grain size distribution. The percentage of reduction of compressive strength of cementitious materials caused by increasing the volume of rubber aggregates by 1% is about 4.5%.

Liposomal-delivery of phosphodiesterase 5 inhibitors augments UT-15C-stimulated ATP release from human erythrocytes

The use of liposomes to affect targeted delivery of pharmaceutical agents to specific sites may result in the reduction of side effects and an increase in drug efficacy. Since liposomes are delivered intravascularly, erythrocytes, which constitute almost half of the volume of blood, are ideal targets for liposomal drug delivery. In vivo, erythrocytes serve not only in the role of oxygen transport but also as participants in the regulation of vascular diameter through the regulated release of the potent vasodilator, adenosine triphosphate (ATP). Unfortunately, erythrocytes of humans with pulmonary arterial hypertension (PAH) do not release ATP in response to the physiological stimulus of exposure to increases in mechanical deformation as would occur when these cells traverse the pulmonary circulation. This defect in erythrocyte physiology has been suggested to contribute to pulmonary hypertension in these individuals. In contrast to deformation, both healthy human and PAH erythrocytes do release ATP in response to incubation with prostacyclin analogs via a well-characterized signaling pathway. Importantly, inhibitors of phosphodiesterase 5 (PDE5) have been shown to significantly increase prostacyclin analog-induced ATP release from human erythrocytes. Here we investigate the hypothesis that targeted delivery of PDE5 inhibitors to human erythrocytes, using a liposomal delivery system, potentiates prostacyclin analog- induced ATP release. The findings are consistent with the hypothesis that directed delivery of this class of drugs to erythrocytes could be a new and important method to augment prostacyclin analog-induced ATP release from these cells. Such an approach could significantly limit side effects of both classes of drugs without compromising their therapeutic effectiveness in diseases such as PAH.

Recent developments in evaluating pumping behavior of flowable and self-consolidating concretes

In this paper, recent developments in evaluating pumping behavior of flowable and self-consolidating concrete (SCC) are discussed. First, a brief overview of the underlying physics of pumping of concrete is given to introduce the concepts of friction, flow, hydrodynamic and hydrostatic pressure and to present the concept of the lubrication layer. Various methods to assess the lubrication layer properties are then discussed, including tribology and different visualization techniques for the velocity profile. Using tribology on the one hand, or by combining the rheological properties of the concrete and its constituent mortar and knowing the thickness of the lubrication layer on the other hand, it is shown that pressure losses during pumping of concrete can be successfully predicted. An overview is also provided to discuss the influence of flow rate, pipe diameter and critical SCC properties influencing the pressure during pumping.

Rheology and Pumping of Self-Compacting Concrete

Self-compacting concrete is a very flowable cementitious material, which does not need external vibration during casting. On the other hand, somewhat surprisingly, pumping of self-compacting concrete requires higher pumping pressures than traditional concrete. This paradox can be fundamentally explained by studying the rheological properties of self-compacting concrete and linking them to pumping operations. This paper describes full-scale pumping tests on self-compacting concrete. The first part deals with the influence of the rheological properties of the concrete on the pumping process, showing that viscosity and shear thickening have a major importance. The second part discusses the influence of pumping on the rheological properties of the concrete, clearly showing a decrease in viscosity due to pumping. Structural breakdown and air content change the rheological properties of the SCC. If structural breakdown dominates the effects of the air content, the yield stress and plastic viscosity will decrease, and the SCC will show a larger tendency to segregate. If the effects of the air content dominate, the yield stress of the SCC will increase, possibly leading to improper filling of the formwork.