Alexei Volkov
Plasticity is a property of materials that allows them to undergo irreversible deformation without any increase in stress or load. In the context of concrete, plasticity refers to its ability to deform without breaking, and it is influenced by factors such as temperature, pressure, and the microstructure of the material. Concrete is a brittle material, and its plasticity is primarily caused by microcracks and sliding motions relative to these cracks. The plasticity of concrete has been a subject of research, with studies focusing on its macro and micro approaches, the influence of aggregate properties, and the use of geopolymer binders as an alternative to ordinary Portland cement. Understanding the plasticity of concrete is essential for developing constitutive models and failure criteria, as well as improving its performance in various applications.
| Characteristics | Values |
|---|---|
| Plasticity in concrete | Caused by microcracks and sliding motions relative to these cracks |
| Plasticity in soils | Caused by the rearrangement of clusters of adjacent grains |
| Plasticity in metals | More plasticity when hot than when cold; lead is plastic at room temperature, cast iron is not |
| Plasticity in cellular materials | Caused by bubble or cell rearrangements |
| Plasticity in crystalline materials | Caused by slip and twinning |
| Plasticity in clay | Reduction in plasticity index results in decreased swell potential and swelling pressure |
| Plasticity in brittle materials | Caused by slip at microcracks |
| Perfect plasticity | Irreversible deformation without any increase in stresses or loads |
| Plastic deformation | Dependent on the deformation speed |
| Critical resolved shear stress for single crystals | Defined by Schmid's law |
| Post-peak portion of the stress-strain curve | Represents the strain-softening of concrete |
| Geopolymer binders | A promising alternative to ordinary Portland cement in concrete |
| Lightweight aggregate concrete (LWAC) | More brittle than normal weight concrete (NWC) |
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What You'll Learn
- Concrete plasticity is caused by microcracks and sliding motions
- Plasticity is influenced by temperature and pressure
- The plasticity of concrete is affected by its composition
- Concrete plasticity can be modelled using continuum damage mechanics
- Plasticity can be reduced by mixing concrete with clay and cement
Concrete plasticity is caused by microcracks and sliding motions
Concrete plasticity refers to the ability of concrete to undergo permanent deformation, a non-reversible change in shape in response to applied forces. It is a property exhibited by brittle materials such as rock, concrete, and bone. This plasticity is predominantly caused by slip at microcracks and the formation of these microcracks is a crucial factor in understanding concrete plasticity.
The microcracks in concrete form due to the contraction of the cement matrix, which creates tension that leads to rupture. This tension can also cause the formation of tensioned "granules" in the mortar, which eventually rupture and contribute to the overall microcracking. These microcracks can accumulate and lead to atrophy, or the degeneration of the concrete structure. The accumulation and stability of microcracks, instead of their growth into macrocracks, are explained by gradient models, particularly Poisson's model, which considers the measurable parameters influencing the behaviour of concrete under compression.
The formation of microcracks in concrete is a complex process influenced by various factors, including the differences in Poisson's ratio of its components. Additionally, the loading conditions, such as short-term loading, can affect the behaviour of concrete and the development of microcracks. The non-linear behaviour of concrete under loading is attributed to the formation and accumulation of microcracks, which contribute to atrophy and potential failure of the concrete structure.
Furthermore, the inelastic deformations of concrete are not only caused by microcracks but also by the sliding motions relative to these cracks. These sliding motions occur due to the shear deformation, known as "slip," which causes atoms to move through interatomic distances relative to their initial positions. This slip behaviour is one of the two primary modes of deformation in the crystal lattice, with the other being "twinning," which occurs along two planes due to applied forces.
Understanding the role of microcracks and sliding motions in concrete plasticity is essential for predicting and managing the behaviour of concrete structures under various loading conditions. The formation and accumulation of microcracks can lead to atrophy and potential failure, while the sliding motions contribute to the irreversible deformation of concrete. By studying these phenomena, engineers can design and construct more robust and durable concrete structures that can withstand applied forces and loads.
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Plasticity is influenced by temperature and pressure
The plasticity of concrete is predominantly caused by slip at microcracks. Inelastic deformations of concrete are caused by the formation of microcracks and sliding motions relative to these cracks.
There are three characteristic regions of the critical resolved shear stress as a function of temperature. In the low-temperature region (T ≤ 0.25Tm), a high strain rate is required to achieve high CRSS, which is necessary to initiate dislocation glide and plastic flow. The critical resolved shear stress in this region has two components: athermal (τa) and thermal (τ*) shear stresses.
The ductility and malleability of a material are directly proportional to its plasticity. Most metals exhibit greater plasticity when hot than when cold. For instance, lead demonstrates sufficient plasticity at room temperature, whereas cast iron does not achieve adequate plasticity for forging operations even when heated.
The rate of deformation is generally dependent on deformation speed, with higher stresses required to increase the rate of deformation. Materials exhibiting such behaviour are said to deform visco-plastically.
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The plasticity of concrete is affected by its composition
The plasticity of concrete refers to its ability to be moulded or shaped before it hardens. This property is essential in the construction industry as it allows concrete to be placed and consolidated into various forms, making it versatile and suitable for a wide range of applications. The plasticity of concrete is indeed influenced by its composition, and understanding these relationships is crucial for achieving the desired workability and strength in concrete structures.
One of the key factors affecting concrete plasticity is the type and proportion of aggregates used. The size, shape, and gradation of aggregates can impact the ease with which concrete can be moulded. Generally, a well-graded aggregate with a continuous gradation will yield more plastic concrete. The shape of the aggregates also plays a role – rounded aggregates tend to provide better flow and workability, resulting in improved plasticity, while angular aggregates may require more water to achieve the same level of workability.
The water-to-cement ratio is another critical factor. Within the range of workable concretes, an increase in the water-to-cement ratio generally increases plasticity. However, it is important to note that while a higher water-to-cement ratio may improve plasticity, it can also decrease strength and durability due to the increased porosity of the hardened concrete. Thus, finding the right balance is essential to achieving the desired plasticity without compromising other important concrete properties.
The use of admixtures can also significantly impact concrete plasticity. Admixtures are materials added to the concrete mix, other than water, cement, and aggregates, to enhance certain properties. Plasticizers, or water-reducing admixtures, are commonly used to improve workability and plasticity without requiring additional water. They disperse the cement particles, reducing the water demand for a given workability. Superplasticizers are a type of plasticizer that can be used to achieve high fluidity and workability, making them particularly useful for high-performance concrete applications or unique placement requirements.
Additionally, the type and quality of cement used can also influence plasticity. Different types of cement may exhibit varying levels of workability and plasticity due to differences in their chemical compositions and fineness. The rate at which cement hydrates can also impact plasticity, with slower-setting cements often providing more workable mixes. Finally, the presence of impurities or improper storage of cement can affect plasticity, as these factors may influence the hydration process and, consequently, the ease of placement and moulding.
In conclusion, the plasticity of concrete is strongly influenced by its composition, including the type and proportion of aggregates, water-to-cement ratio, the use of admixtures, and the type and quality of cement. Understanding these relationships allows engineers and contractors to design concrete mixes that possess the desired plasticity for specific construction requirements, ensuring efficient placement and consolidation while also meeting the necessary strength and durability standards.
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Concrete plasticity can be modelled using continuum damage mechanics
Concrete plasticity refers to the irreversible deformation of concrete without any increase in stress or load. This deformation is caused predominantly by slip at microcracks. Continuum damage mechanics has been used extensively in concrete modelling to predict behaviour, crack propagation, and microcrack coalescence.
Under uniaxial cyclic loading conditions, the degradation mechanisms are quite complex, involving the opening and closing of previously formed microcracks, as well as their interaction. The stiffness recovery effect, also known as the "unilateral effect," is an important aspect of concrete behaviour under cyclic loading. The concrete damaged plasticity model assumes that the reduction of the elastic modulus is given in terms of a scalar degradation variable.
Several studies have applied continuum damage mechanics to model concrete plasticity. For instance, Hillerborg, Modeer, and Petersson used fracture mechanics and finite elements to analyse crack formation and growth in concrete. Lee and Fenves developed a plastic-damage model for cyclic loading of concrete structures. Lubliner, Oliver, Oller, and other researchers have also contributed to this field.
Furthermore, numerical advancements in continuum damage mechanics have been presented in recent literature. These advancements include discussions on local, nonlocal, and rate-dependent models, numerical algorithms, element types, commercial software, iterative schemes, and explicit or implicit approaches to solving finite element equations.
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Plasticity can be reduced by mixing concrete with clay and cement
Concrete is a widely used building material in modern construction. However, it has some drawbacks, including the plasticity of the concrete mix. Plasticity is a property of materials that undergo irreversible deformation without any increase in stress or load. In the context of concrete, plasticity is caused by microcracks and sliding motions relative to these cracks.
Clay and cement are two common construction materials that can be mixed with concrete. The addition of clay to a concrete mix can increase its strength and decrease the amount of water needed for production, making it a more sustainable option. However, clay can also make the concrete weaker and more susceptible to cracks. Therefore, it is recommended to use small amounts of clay, with a maximum of 10% clay per volume of the total mixture content.
On the other hand, cement makes concrete stronger and less likely to crack. The inclusion of cement in a clay mix can reduce its plasticity and increase its strength. This is because cement decreases the water content of clay, limiting its plasticity. Additionally, the large particles in clay disrupt the chemical reaction necessary for cement to harden.
When mixing clay and cement with concrete, it is important to consider the type of clay and cement used, as well as the intended use of the mixture. For example, any type of clay and cement can be mixed for art or sculpture projects, while building projects must meet certain requirements set by building codes. Overall, by understanding the properties of clay, cement, and concrete, and by carefully controlling the mix ratios and curing conditions, the plasticity of concrete can be effectively reduced to create stronger and more durable structures.
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Frequently asked questions
Plasticity of concrete refers to its ability to undergo irreversible deformation without any increase in stress or load.
Plasticity in concrete is caused predominantly by slip at microcracks.
A better understanding of the plasticity of concrete can help in developing constitutive models or failure criteria. It can also help in improving the performance of concrete by increasing its strength, reducing its plasticity and swell potential, and increasing its resistance to moisture.
