Chloe Zhang
Plasticity is a term used in physics and materials science to describe the ability of a solid material to undergo permanent deformation, or a non-reversible change of shape, in response to applied forces. Brittle materials such as rock, concrete, and bone exhibit plasticity predominantly through slip at microcracks. This is caused by the rearrangement of adjacent grains or sliding motions relative to these cracks. At high temperatures and pressures, the plasticity of concrete can also be influenced by the motion of dislocations within its individual grains. Thus, the plasticity of concrete refers to its capacity to be moulded or altered through irreversible deformation under load.
| Characteristics | Values |
|---|---|
| Definition | Plasticity is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. |
| Other Names | Plastic deformation |
| Materials Exhibiting Plasticity | Metals, soils, rocks, concrete, foams, bone |
| Cause of Plasticity in Concrete | Predominantly by slip at microcracks |
| Cause of Plastic Behaviour in Soils | Rearrangement of clusters of adjacent grains |
| Cause of Inelastic Deformation in Rocks and Concrete | Formation of microcracks and sliding motions relative to these cracks |
| Factors Affecting Plastic Behaviour | Temperature, pressure, motion of dislocations in individual grains in the microstructure |
| Critical Stress | Tensile or compressive |
| Yield Criteria | Tresca, von Mises, and several others |
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What You'll Learn
Concrete is a brittle material
Concrete is made by mixing cement with an aggregate such as gravel and sand, and then adding water. The water causes a chemical reaction called hydration, which makes the material harden. Concrete became a structural material when French inventor Joseph Monier introduced the idea of reinforcing the brittle substance with steel wires in 1867.
Concrete's brittleness compromises its safety, durability and sustainability. This has led to the question of whether concrete can be made ductile, so that when stressed, it can stretch and bend without fracturing. However, concrete does not possess the mechanisms to dissipate the energy due to an applied load, so cracks result in an unstable fracture and rapid loss of load-bearing capacity.
Techniques have been developed to increase the strength of concrete in tension and compression, such as adding particles of microsilica to the mix, tightly packing the powder, and reducing the water content. However, these techniques tend to make the material even more brittle.
Fracture mechanics is the study of the propagation of cracks in materials. It uses methods of analytical solid mechanics to calculate the driving force on a crack. In ductile materials, a plastic zone develops at the tip of the crack, and as the applied load increases, this zone increases in size. In brittle materials such as concrete, plasticity is caused by slip at microcracks.
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Plastic deformation
In the context of physics and materials science, plasticity is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. This phenomenon is also referred to as plastic deformation. It is important to note that plasticity is distinct from elastic deformation, where a material returns to its original shape after the applied force is removed.
In crystalline materials, such as metals, plastic deformation occurs through two primary mechanisms: slip and twinning. Slip involves shear deformation, where planes of atoms within the crystal lattice slide past each other along specific directions, resulting in relative movements of atoms from their initial positions. Twinning, on the other hand, is a type of plastic deformation that occurs along two planes in response to applied forces.
The plasticity of concrete, specifically, can be influenced by various factors. For instance, the absence of formwork in 3D-printed concrete structures can make them more susceptible to plastic deformation during the early stages of construction. This is particularly evident in plastic shrinkage, where water loss leads to capillary pressure changes, causing the concrete to shrink and increasing the risk of cracking. Additionally, plastic settlement, which refers to vertical deformation caused by gravitational loads, can also contribute to plastic deformation in concrete.
Understanding the plastic deformation of concrete is crucial for ensuring the safety and durability of structures. By studying and modelling this behaviour, researchers can develop methods to predict and mitigate the effects of plastic deformation in construction and engineering applications.
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Concrete's propensity to undergo enduring deformation
Concrete is a common material used in mechanical and structural engineering. It is known for its ability to undergo significant volume changes due to alterations in pore water content, resulting in what is known as "creep and shrinkage". This behaviour contributes to concrete's plasticity, which refers to its propensity to undergo enduring deformation.
Plasticity, or plastic deformation, describes the capacity of a solid material to undergo irreversible changes in shape when subjected to external forces. Concrete, despite its rigid appearance, possesses this characteristic. Unlike elastic deformation, where the object returns to its original shape after the force is removed, plastic deformation results in permanent alterations.
In the context of concrete, plasticity is predominantly caused by the formation and slippage of microcracks within the material. As concrete experiences stress, microcracks can form and propagate, leading to sliding motions that result in enduring deformation. This is particularly evident in the behaviour of concrete under compressive loads, where its ability to sustain deformation is reflected in its compressive strength.
The plasticity of concrete also extends to its response to temperature changes. For instance, fly ash geopolymer concretes produced with higher molarity NaOH solutions exhibited a significant loss of compressive strength when exposed to elevated temperatures of 800 °C. This indicates that the concrete underwent substantial deformation under these thermal conditions.
Additionally, the chemical processes associated with Portland cement hydration can lead to autogenous shrinkage in concrete. This type of shrinkage occurs without moisture loss and is influenced by chemical volume changes and self-desiccation due to the loss of water consumed by the hydration reaction. While autogenous shrinkage is typically a small percentage of the overall drying shrinkage in normal concretes, it can become more significant in modern high-strength concretes with very low water-cement ratios.
In summary, concrete's propensity to undergo enduring deformation, or plasticity, is influenced by various factors, including stress, the formation of microcracks, temperature changes, and chemical processes. This behaviour has important implications in engineering applications, where understanding the deformation characteristics of concrete is crucial for designing and analysing structures.
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Plastic behaviour in concrete is caused by microcracks
In physics and materials science, plasticity refers to the ability of a solid material to undergo irreversible changes in response to applied forces. This is also known as plastic deformation. Plastic deformation is observed in most materials, including concrete.
The presence of microcracks in concrete can have several effects on its mechanical behaviour. For instance, the degree of tortuosity of the crack path in direct tension increases with larger maximum coarse aggregate sizes and lower ITZ strength. Additionally, the fracture energy and tensile strength of concrete are influenced by the ITZ strength and the maximum coarse aggregate size. High ITZ strength and smaller aggregate sizes result in higher fracture energy and tensile strength, while normal-strength concrete exhibits the opposite trend.
The formation of microcracks in concrete can also be influenced by the addition of reinforcement fibres. For example, the introduction of ribbon-shaped amorphous cast iron fibres can affect the workability and compressive strength of fibre-reinforced concrete (FRC). The addition of a superplasticizer helps maintain the water-to-cement ratio and improves the compactness of FRC. However, the orientation of fibres and microcracks should be carefully considered to optimize the mechanical properties of the concrete.
In summary, plastic behaviour in concrete is caused by microcracks that form due to various factors such as improper curing, rapid drying, and the presence of reinforcement fibres. These microcracks have a significant impact on the mechanical properties of concrete, including its fracture energy and tensile strength. Understanding the behaviour of microcracks is crucial for optimizing concrete mixtures and minimizing cracking in construction applications.
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Plastic deformation in concrete is irreversible
In physics and materials science, plasticity is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. This is also known as plastic deformation. Plastic deformation is observed in most materials, particularly metals, soils, rocks, concrete, and foams.
In the context of concrete, plasticity is predominantly caused by slip at microcracks. This means that cracks form in the concrete, and sliding motions relative to these cracks cause a permanent, irreversible change in the shape of the concrete. This is different from temporary deformation, also known as elastic deformation, where the deformation is recovered after removing the applied force.
The mathematical theory of plasticity, or flow plasticity theory, uses a set of non-linear, non-integrable equations to describe the changes in strain and stress with respect to a previous state and a small increase in deformation. If the stress exceeds a critical value, the material will undergo plastic or irreversible deformation. This critical stress can be tensile or compressive.
Plastic deformation in concrete can be understood through the concept of ductility. The amount of plastic deformation associated with a given stress can be determined by analysing the stress-strain curve. This curve illustrates the relationship between the stress applied to a material and the resulting deformation, with the horizontal distance from the origin to the point of intersection on the curve representing the plastic strain.
Additionally, plastic deformation in concrete can be influenced by factors such as temperature and pressure. At high temperatures and pressures, the motion of dislocations in individual grains in the microstructure can contribute to plastic behaviour. This understanding of the mechanical and microstructural properties of concrete is crucial for deformation processes in engineering applications.
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Frequently asked questions
Plasticity is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces.
In brittle materials such as concrete, plasticity is caused predominantly by slip at microcracks. Inelastic deformations of rocks and concrete are primarily caused by the formation of microcracks and sliding motions relative to these cracks.
Understanding the plasticity of concrete is crucial for ensuring the durability and safety of concrete structures. By studying how concrete behaves under different loads and stresses, engineers can design structures that can withstand specific environmental conditions and loads without failing or rupturing.
