Omar Khalid
The plastic viscosity of concrete is a critical concept in the concrete industry as it affects the placement and workability of fresh concrete. Rheological properties of fresh concrete, such as plastic viscosity and yield stress, influence the productivity and quality of concrete, including its mechanical properties and durability. Therefore, establishing a model for predicting the plastic viscosity of concrete based on its composition is extremely valuable for the concrete industry. Various methods and models have been proposed to predict and measure the plastic viscosity of concrete, including rheometers, regression analysis, and fundamental and phenomenological models. The accurate quantification of plastic viscosity is important for ensuring the sustainability and quality of concrete structures.
| Characteristics | Values |
|---|---|
| Rheological properties | Affect placement and workability, productivity and quality of concrete |
| Plastic viscosity measurements | Obtained using BTRHEOM, a parallel plate concrete rheometer |
| Plastic viscosity predictions | Based on composition, using fundamental and phenomenological rheological models |
| Plastic viscosity and yield stress | Defined by Bingham model |
| Plastic viscosity and superplasticizers | Superplasticizers can keep particles apart, reducing viscous dissipation |
| Plastic viscosity and particle size | Smaller particles lead to higher viscosity |
| Plastic viscosity and relative viscosity | Relative plastic viscosity is the ratio of plastic viscosity of suspension to viscosity of embedding fluid matrix |
| Plastic viscosity and particle concentration | Main influence on relative plastic viscosity is aggregate concentration |
| Plastic viscosity and slump | Slump is not a sufficient test method to characterize rheology of fresh concrete |
| Plastic viscosity and nano-materials | Addition of nano-materials enhances viscosity of cementitious suspensions |
| Plastic viscosity and shear rate | Plastic viscosity is the slope of the shear stress-shear rate plot |
| Plastic viscosity and vibration | Vibration testing can indicate plastic viscosity |
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What You'll Learn
Concrete rheology
Concrete is a mixture of coarse aggregates (rocks), fine aggregates (sand), cement, and water. One of the most important characteristics of concrete is its workability, or fluidity, which is the ability of fresh concrete to fill a given mould properly and with the desired amount of work (usually vibration) without reducing the concrete's quality. Workability depends on factors such as water content, aggregate shape and size, cement content, and age.
The rheological properties of fresh concrete, namely plastic viscosity and yield stress, are critical for the concrete industry because they affect placement and workability. These properties also influence the productivity and quality of concrete, including its mechanical properties and durability. Therefore, a proper characterisation of these properties is needed to control the quality of fresh concrete and ensure the sustainability of concrete structures.
Developing a quantitative characterisation of the rheological properties of fresh concrete is important for concrete mixture design and quality control. This involves establishing a quantitative measure for workability, which will help to prevent premature failure of concrete materials due to inadequate flow properties. It will also provide the tools to design concrete mixtures with the desired flow properties for transportation, pumping, casting, and vibration.
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Plastic viscosity vs. concrete yield stress
Concrete rheological properties, namely plastic viscosity and yield stress, are critical for the concrete industry because they affect placement and workability. Rheology is an effective and efficient indicator to characterize the flow properties of fresh concrete.
Plastic viscosity is essentially described as the resistance of a fluid to flow. It is used as an indicator of the size, shape, distribution, and quantity of solids, and the viscosity of the liquid phase. The plastic viscosity of fresh concrete is typically measured using a parallel plate concrete rheometer. The relative plastic viscosity of a concrete is the plastic viscosity of the concrete divided by the plastic viscosity of the mortar. It is assumed that the mortar used to determine the plastic viscosity of the matrix has the same composition as the mortar in the concrete.
Yield stress, on the other hand, is the stress needed to be applied to a material to initiate flow. For a liquid, the yield stress is equal to the intersection point on the stress axis. In the context of concrete, the yield stress is the stress required to overcome the concrete's resistance to deformation or flow. Once the yield stress is overcome, the concrete will start to flow and change shape under vibration.
The Bingham model is often used to describe the rheological behaviour of concrete, with plastic viscosity and yield stress as its defining factors. However, the special concrete combined with nano-materials requires certain modifications to the existing models. The rheological properties of fresh concrete, including plastic viscosity and yield stress, influence the productivity and quality of concrete, including mechanical properties and durability. Therefore, proper characterization of these properties is crucial for controlling the quality of fresh concrete and ensuring the sustainability of concrete structures.
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Plastic viscosity predictions
Plastic viscosity and yield stress are critical rheological properties of fresh concrete, affecting its placement and workability. These properties influence the productivity and quality of concrete, including its mechanical properties and durability. Therefore, a proper characterisation of these properties is necessary to control the quality of fresh concrete and ensure the sustainability of concrete structures.
Fundamental and phenomenological rheological models have been proposed in the literature to characterise the behaviour of fresh concrete. Developing a model to predict the plastic viscosity of concrete based on its composition would be extremely valuable for the concrete industry.
Several models have been proposed in the literature for predicting the plastic viscosity of concrete. However, these models have had limited success due to certain limitations. For instance, the proposed models did not consider particle interactions, which are crucial given the high concentration of particles in concrete. Additionally, these models require validation.
The input parameters for these models include the volumetric fraction of solid material, referred to as packing density, and the maximum packing density of the whole mixture. These input parameters are functions of concrete composition.
One study evaluated eight prevailing models for predicting the plastic viscosity of concrete. It found that most of these models were based on theories not suitable for medium to high concentrations of suspended particles, such as concrete. Consequently, these models assume a low concentration of spherical solid particles and do not consider particle interactions. An exception to this is the Mahmoodzadeh and Chidiac model, which is based on the cell method and provides more accurate predictions.
Another study used machine learning techniques to predict the plastic viscosity and yield stress of self-compacting concrete. It employed four popular machine learning algorithms: Multiple Linear Regression (MLR), Random Forest (RF), Decision Tree (DT), and Support Sector Machine (SVM). The study used rheological parameters experimentally measured by empirical tests, such as slump flow, V-Funnel flow time, and the L-Box ratio, to predict the rheological behaviour of self-compacting concrete.
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Rheometers and concrete viscosity
Concrete rheological properties need to be properly measured and predicted to characterise the workability of fresh concrete. Rheology, a measure of flow behaviour, is an important way of determining concrete performance. Rheometers are used for complex material property investigations in the cement industry to determine relative viscosity, yield stress measurements, and dry times.
Rheometers are suited for applications such as relative viscosity determination, yield stress measurements, and time to harden and dry. They can analyse sample types, including mineral slurries, ceramic suspensions, construction materials, and mortars and grouts to determine qualities like flow behaviour, processability, processing time, and pumpability.
The BTRHEOM rheometer, developed at the Laboratoire Central des Ponts et Chaussées (LCPC) in France, consists of a bucket with a serrated bottom and a rotating top wheel resting on the concrete. The shear stress distribution allows for the direct calculation of viscosity and yield stress. The IBB Concrete Rheometer, developed at the Technical Research Centre of Finland, involves placing concrete between two concentric cylinders of different diameters. The outer cylinder rotates in an oscillatory mode, and the torque induced by the movement is measured in the inner cylinder. This configuration allows the operator to calculate the viscosity and yield stress of the concrete as a function of frequency.
A round-robin test in 2000 showed that different rheometer designs yielded different values of viscosity for the same concrete. While empirical correlations between different rheometers were possible, a new approach was needed to compare the results from various rheometers. This approach uses the relative plastic viscosity, which is defined as the ratio of the plastic viscosity of the whole suspension to the plastic viscosity of the embedding fluid matrix or medium.
The relative plastic viscosity of a concrete is the plastic viscosity of the concrete divided by the plastic viscosity of the mortar. It depends on the concentration of particles and their shape. By plotting the data on a graph of relative plastic viscosity versus particle concentration, it can be shown that the main influence on relative plastic viscosity is aggregate concentration. This allows for direct comparisons of data from all rheometers, which was not possible before.
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Plastic viscosity and concrete quality
The rheological properties of fresh concrete, such as plastic viscosity and yield stress, are critical factors in the concrete industry. They affect the placement and workability of fresh concrete, which forms the basis of concrete mixture design for quality control purposes.
Plastic viscosity is an important parameter for understanding the flow of fresh concrete. For instance, in the case of self-consolidating concrete (SCC), the viscosity of the mixture is necessary to avoid segregation of coarse aggregates from the cement paste. SCC must flow under its own weight and fill areas that are heavily congested with steel reinforcements without any segregation.
The ability to accurately predict the plastic viscosity of concrete based on its composition would be extremely valuable to the concrete industry. However, establishing a prediction equation for flow and deformation of fresh concrete due to various external forces such as dead weight, pressure, and vibration is challenging. The methods of measuring these physical values are generally complicated and unsuitable for practical purposes.
Several phenomenological and fundamental models have been proposed to predict the viscosity of concrete based on its composition, but few attempts have been successful due to the limited consideration of particle size and their interactions. The relative plastic viscosity of a concrete is defined as the ratio of the plastic viscosity of the whole suspension (concrete) to the plastic viscosity of the embedding fluid matrix or medium (mortar). This approach allows for direct comparisons of data from various rheometer designs and can be used to establish a quantitative measure for workability, which is essential for controlling the quality of fresh concrete.
In conclusion, plastic viscosity is a critical factor in the concrete industry as it affects the workability, placement, and quality of fresh concrete. Developing accurate models and prediction equations for plastic viscosity based on concrete composition is essential for ensuring the sustainability and durability of concrete structures.
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Frequently asked questions
Plastic viscosity is a critical rheological property of fresh concrete that affects its placement and workability. It is influenced by factors such as particle concentration, particle shape, and the use of superplasticizers.
Rheological properties like plastic viscosity and yield stress impact the productivity and quality of concrete, including its mechanical properties and durability. Properly characterizing these properties is essential for controlling the quality of fresh concrete and ensuring the sustainability of concrete structures.
Various methods and models have been proposed to measure and predict plastic viscosity in concrete. Common approaches include using rheometers, such as the BTRHEOM parallel plate concrete rheometer, and conducting tests like slump tests, remolding tests, and vibration tests.
One challenge is that different rheometers and models can yield varying viscosity values for the same concrete sample. Additionally, the methods for measuring plastic viscosity can be complex and impractical. Developing accurate prediction equations and models that consider particle size and interactions, and the effects of additives is an ongoing area of research.
