Chloe Zhang
Plastic flow refers to the property of a material, such as concrete, that exhibits a sustained increase in deformation or strain under load or stress. In other words, it is the ability of a solid material, like concrete, to undergo permanent deformation or a non-reversible change of shape when subjected to applied forces. This phenomenon is known as plastic deformation and is observed in various materials, including metals, soils, rocks, concrete, and foams. Plastic concrete, for example, is a type of low-strength, low-stiffness concrete used in specific construction applications. The study of plastic flow helps understand the behaviour of materials, particularly their response to applied forces, which is crucial for structural performance and engineering design.
| Characteristics | Values |
|---|---|
| Plastic flow of concrete | A type of inelastic deformation |
| Cause | Formation of microcracks and sliding motions relative to these cracks |
| Other causes | Motion of dislocations in individual grains in the microstructure at high temperatures and pressures |
| Critical resolved shear stress (CRSS) | Defined by Schmid's law: τCRSS=σy/m, where σy is the yield strength of the single crystal and m is the Schmid factor |
| Schmid factor | Comprised of two variables λ and φ, defining the angle between the slip plane direction and the tensile force applied, and the angle between the slip plane normal and the tensile force applied, r |
| CRSS at low temperatures | Requires a high strain rate to achieve high τCRSS for plastic flow |
| CRSS at moderate temperatures | Thermal shear stress component τ* → 0, point defect impedance to dislocation migration is eliminated |
| CRSS at high temperatures | Remains low, but plastic flow still occurs due to thermally activated high-temperature time-dependent plastic deformation mechanisms |
| Plastic deformation in concrete | Observed at ordinary temperatures and any degree of stress |
| Plastic deformation in other materials | Observed in metals, soils, rocks, foams, and bone |
| Plasticity in metals | Usually a consequence of dislocations |
| Plasticity in rocks, concrete, and bone | Caused predominantly by slip at microcracks |
| Plasticity in cellular materials | A consequence of bubble or cell rearrangements |
| Plastic concrete | A low-strength, low-stiffness, impervious concrete used for cut-off wall construction in earthen dams |
| Compressive strength | Significantly slower development over time compared to ordinary concrete |
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What You'll Learn
Plastic deformation in concrete
Plastic deformation is the ability of a solid material to undergo a permanent, non-reversible change of shape in response to applied forces. In the context of concrete, plasticity is observed in 3D-printed concrete (3DPC) due to the absence of formwork, which can lead to plastic shrinkage and settlement. This formwork-free construction method offers advantages such as free-forming flexibility and high construction efficiency. However, it also makes 3DPC susceptible to plastic deformation at an early age.
The mechanisms causing plastic deformation in concrete can vary. In brittle materials like concrete, plasticity predominantly occurs due to slip at microcracks and the formation of these microcracks, along with sliding motions relative to them. At high temperatures and pressures, plastic behaviour can also be influenced by the motion of dislocations within the individual grains of the microstructure.
The behaviour of concrete under loading is essential to understanding its plastic deformation characteristics. When tensile loading is applied to ductile metals, they exhibit elastic behaviour, with each increment of load accompanied by a proportional increase in extension. However, once the load exceeds the yield strength, the extension increases more rapidly, and upon removing the load, some degree of extension remains, indicating plastic deformation.
Additionally, plastic concrete, a low-strength and low-stiffness concrete used in earthen dam construction, exhibits slower compressive strength development over time compared to ordinary concrete. This highlights the importance of considering the mechanical properties and deformation behaviour of different types of concrete in various applications.
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Plastic flow vs elastic behaviour
In the context of concrete, plasticity refers to the ability of the material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. This is also known as plastic deformation. In engineering, the transition from elastic behaviour to plastic behaviour is known as yielding.
Plastic deformation is observed in most materials, particularly metals, soils, rocks, concrete, and foams. However, the mechanisms that cause plastic deformation vary. In metals, plasticity is a result of dislocations, which are defects in the crystal structure of the material. In brittle materials like rock, concrete, and bone, plasticity is caused by slip at microcracks.
Elastic behaviour refers to the ability of a material to return to its original shape after the removal of applied forces. For example, when tensile loading is applied to ductile metals, each increment of load is accompanied by a proportional increment in extension. When the load is removed, the material returns to its original size. This behaviour is observed in the elastic region of a stress-strain curve, where the extension is proportional to the force applied.
Plastic flow, on the other hand, occurs when the load exceeds a threshold, known as the yield strength. Beyond this point, the extension increases more rapidly than in the elastic region, and when the load is removed, some degree of extension will remain. This behaviour is observed in the plastic region of a stress-strain curve, where the extension is no longer proportional to the applied force.
The transition from elastic behaviour to plastic flow at the yield point is of exceptional significance. It represents the upper stress limit for structural performance and the lower limit for mechanical processing. In polymers, this transition is followed by an extensive region of plastic flow, where concepts such as pre-yield plasticity, yield drops, and work-hardening rate become relevant.
Plastic concrete, a type of low-strength and low-stiffness concrete, exhibits slower compressive strength development over time compared to ordinary concrete. This results in a lower elastic modulus, indicating that it deforms more easily under applied loads.
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Plastic instability
Plastic flow refers to the study of the flow of materials, primarily in a liquid state or as soft solids or solids under conditions in which they respond with plastic flow, rather than deforming elastically in response to an applied force. Plasticity, or plastic deformation, is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces.
Foamed concrete, for instance, is prone to instability at densities lower than 500 kg/m³, with instability becoming almost inevitable at very low densities of ≤300 kg/m³. This instability greatly limits the potential of foamed concrete for applications where mass is critical, such as weak soils or backfilling damaged structures. However, it has proven to be an effective alternative to granular fills and is widely used internationally in construction with precast blocks, especially in the development of lightweight materials for buildings to improve sustainability.
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Plastic concrete's compressive strength
Plastic concrete is a low-strength (fcm,28d ≤ 1.0 MPa), low-stiffness, impervious concrete used for cut-off wall construction in earthen dams. It has a slower compressive strength development over time compared to ordinary concrete. Compressive strength is a measure of a material's ability to resist failure under compressive loads, or "squeezing" forces. It is an essential test for quality control in the manufacturing process and for safety and performance evaluations of completed structures.
The compressive strength of concrete is derived from a combination of cement, water, aggregate, and air. The cement and water form a paste that coats and binds the aggregate together. The hydration process of the cement and water chemically reacts and produces calcium-silicate-hydrate and hydrates. This process continues for years, slowly gaining strength. The air content is typically around 6-8% and is required for proper curing. However, too much air will decrease the concrete's compressive strength.
The addition of plastic waste to concrete mixtures has been investigated, with some studies finding that it can reduce the compressive strength of concrete. For example, one study found that replacing the aggregate with plastic waste reduced the compressive strength by up to 33.8 MPa upon complete replacement. Another study found that a 5% replacement level of PET plastic waste resulted in a positive outcome due to a dense homogeneous mortar mix and improved workability.
In summary, plastic concrete has a slower development of compressive strength compared to ordinary concrete, and the addition of plastic waste to concrete mixtures can impact its compressive strength. Compressive strength testing is crucial for quality control and ensuring the safety and performance of structures.
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Plastic flow in portland cement concrete
In the context of concrete, plasticity refers to the ability of the material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. This is also known as plastic deformation. While plasticity can be observed in most materials, it is particularly common in metals, soils, rocks, concrete, and foams.
Portland cement is a type of cement that is commonly used in construction. It is manufactured through a process of inter-grinding with plasticizing agents, which enhance its desirable properties. Portland cement is often used in combination with aggregates to create concrete.
Plastic concrete is a specific type of concrete that exhibits plasticity. It is a low-strength, low-stiffness, and impervious concrete used for cut-off wall construction in earthen dams. Plastic concrete has a slower compressive strength development over time compared to ordinary concrete.
The plasticity or plastic flow of Portland cement concrete specifically refers to its ability to deform and change shape under applied forces. This is a desirable property in construction as it allows for the creation of various structures and the ability to fill in gaps, cracks, and holes. The addition of plasticizing agents during the manufacturing process enhances the plasticity of Portland cement, making it more workable and flexible when wet and drying to a durable finish.
The plastic flow of Portland cement concrete is an important consideration in engineering and construction. It allows for the material to be shaped and worked with before setting, but also has implications for the structural performance and mechanical processing of the final structure. Understanding the transition from elastic behaviour to plastic flow at the yield point is crucial for ensuring the safety and longevity of buildings and other structures.
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Frequently asked questions
Plastic flow is the property of a material that is evidenced by the continuance for a sustained period of time of a varying increase of deformation or strain under load or decrease of stress during sustained strain.
The plastic flow of concrete refers to the continuance of deformation or strain in concrete under sustained load or stress.
Plastic flow occurs in concrete when there is a decrease in stress during a sustained strain. This is often observed in concrete over time as strains or deformations continue to change even after the stresses have become constant.
Understanding the plastic flow of concrete is important in construction and engineering. For example, it is relevant in the use of concrete for cut-off wall construction in earthen dams, where the slow development of compressive strength over time is a key consideration.
The nature of plastic flow varies for different materials and their compositions. For example, in metals, plastic flow is the dominant process of crystal deformation due to dislocation mechanisms, whereas in concrete, it is caused by slip at microcracks.
